Running BBC BASIC on the Sega Master System

Thursday, 22nd July 2021

I've recently been spending some time finding a way to run BBC BASIC on the Sega Master System, inspired by BASIC Month 6: The Mandelbaum Set on the RetroBattlestations Reddit community.

Photograph of the final setup with a Sega Master System running BBC BASIC and a Z88 computer acting as file store.

When this month's program was first announced I tried running it on my only unarguably retrobattlestation, my Cambridge Z88, but the screen's low (vertical) resolution didn't do the program much justice.

I thought this gave me two options:

  1. Control some external piece of retro tech to produce higher-resolution output (e.g. a printer or a plotter.
  2. Port a BASIC interpreter to another retro system with a higher resolution display and run the program on that.

Unfortunately, I don't own any old printers or plotters (or a Logo-like turtle!) so option 2 seemed my best option. I had some experience adapting Richard Russell's BBC BASIC (Z80) to run on the TI-83 Plus calculator so I thought I should pick the Sega Master System as that also has a Z80 CPU in it. Here are some rough specs:

  • 3.58(ish)MHz Z80 CPU.
  • 8KB work RAM.
  • TMS9918A-derived VDP for video with 16KB dedicated VRAM accessed via I/O port.
  • SN76489-derived PSG for sound.
  • Two controller ports with six input pins and two pins that could be configured as inputs or outputs.
  • Software loaded from ROM cartridge or card slot with a small BIOS ROM that detects whether a cartridge or card is inserted (and if not, runs its own built-in game).

There is some precedence to this endeavour with the computer version of Sega's SG-1000, the SC-3000, which came with a keyboard and had BASIC ROM cartridges.

An RS-232 serial adaptor and PS/2 keyboard adaptor for the Master System.

I wanted to try to keep this project as retro as possible, so no modern microcontrollers as I've been accused of cheating by using them in the past. I did have to make a couple of adaptors to allow me to plug in a keyboard and to give the Master System a serial port to load or save programs over – more about these later!

Loading BASIC onto the Master System

The first problem was getting BASIC onto the Master System at all. As my Master System doesn't have BASIC in ROM (it comes with Hang-On, which is perhaps more fun but less likely to handle the Mandelbaum set) I'd need to load the program onto a cartridge or card. Master System cartridges usually contain a mapper circuit to handle bank switching in the lower 48KB of the Z80's address space (the upper 16KB contains the 8KB work RAM, appearing twice) which is normally integrated directly into the ROM chip for the game, however there are a handful of games that have separate mapper chips and ROM chips and the ROM chips that Sega used have a pinout that is extremely close to that used by common EEPROMs (e.g. 29F010,
49F040) with only a couple of pins needing to be swapped around. After Burner is one such cartridge, so I've modified an old copy to let me plug in flash memory chips that I can program with the version of BASIC I'm working on. A switch at the top of the cartridge lets me switch back to the original pin configuration if I want to play the original copy of After Burner.

Typing commands into BASIC

With a flashable cartridge to hand I was able to assemble a version of Richard Russell's BBC BASIC (Z80) with some simple code stubs in place to direct text output to the screen. Output is useful but only half the story, we still need to be send commands to BASIC!

The Master System doesn't have its own keyboard, so I'd need to find some way to interface one to it. I have previously used PS/2 keyboards in a number of projects, as they are pretty simple to deal with. Electrically, they use two open collector I/O lines for bidirectional data transfer (one as clock, one as data), and fortunately each controller port of the Master System has two pins that can be configured as outputs which can therefore be used to interface with a keyboard (either left as inputs and pulled weakly high in their idle state, or driven low as outputs for their active state).

Showing the internal wiring of the PS/2 keyboard adaptor and controller passthrough.

It acts as a pass-through cable so you can still have a regular controller plugged in when using the keyboard.

The two pins on the Master System control port that can act as outputs are TH and TR; TH is normally used by light guns to latch the horizontal counter in the video chip so is unused with normal controllers so it's no great loss here, but TR is the right action button (marked "2") so by using this pass-through adaptor you do unfortunately lose one of the controller buttons. However, you do gain a hundred or so keyboard keys, so I don't think it's too bad a compromise...

For the software side of things I adapted my Emerson AT device library, previously written for the TI-83 Plus calculators, to the Sega Master System hardware. This library handles the low-level AT device protocol and also translates the raw keyboard scancode values to the corresponding characters.

Saving and loading programs

At this point I was able to type in BASIC programs and run them on the Master System, which was pretty neat! However, I was still working on adding new features (e.g. drawing commands for graphics) and having to type in the entire Mandelbaum program after every change was going to get pretty exhausting. I could bake it into the ROM but that seemed like cheating, so I thought I should try to find a way to load the file from an external source. A floppy disk or tape cassette would seem authentically retro but adding a floppy drive controller to the Master System would be a fairly complicated task and I don't have a suitable data cassette recorder to even attempt loading from tape so I thought some sort of file store accessible over a serial port would be a good option.

The Z88 has a serial port and can act as a remotely-controlled file store when running the PC Link software (with the protocol documented here. This seemed like a good choice, if not for the fact that the Sega Master System doesn't have a serial port of its own. To get around this, I added one, using a MAX232 chip to adapt the Master System's 5V logic levels to RS-232 compatible ones so I could plug in a null modem cable from the Z88 or my PC without accidentally frying the Master System with -12V.

Showing the insides of the RS-232 serial port adaptor.

I wrote some code that bit-bangs the serial data over the controller port lines using the timing loop code I wrote for a previous BASIC Month (Crisps Tunes) to support rates between 19200 and 300 baud. 19200 baud is somewhat unreliable but that's OK because the Z88's 19200 baud is unreliable too, so the default 9600 baud speed does a good job. RTS/CTS handshaking has to be implemented, as there is no hardware serial support on the Master System and it needs to be actively polling the port to receive any data. In doing so I noticed one awkward fact about my PC's serial port - if you de-assert RTS it will continue sending data until its buffer is empty, presumably only checking the RTS line when it's about to top up the buffer. In practice this means that even if I change RTS virtually as soon as the start bit for the first byte is received, the PC will continue to send up to 256 bytes before stopping. To get around this I added a serial receive buffer that immediately checks for the next byte even after asserting RTS, and this seems to have done the trick.

The protocol used by PC Link requires acknowledgement after every single byte so is very slow but at least it's reliable. I plumbed the PC Link code into BASIC's LOAD and SAVE which makes loading and saving programs as transparent and easy as if you had a floppy disk in the system instead!

Pressed for size

The Master System has 8KB of RAM. Of this, 16 bytes are mapped to special hardware functions and BBC BASIC reserves 768 bytes for itself, so we're already down to 7,408 bytes. I initially reserved 256 bytes for my own needs (display settings, VDU command buffer, serial port status, keyboard status etc) bringing it down to 7,152 bytes. The 16KB of display memory is not directly accessible to the CPU, so it can't be used for additional work RAM, and having to access it indirectly via I/O ports is very slow but I can't afford to mirror parts of it in RAM for speed.

Initially the Mandelbaum program ran well enough by stripping out comments, but I then added sound support (with eight 13-byte envelope definitions, and four channels with their own state, copy of their active envelope and a command queue for 32 bytes per channel, adding around another 140 bytes of memory usage) and the program stopped running with a "No room" error during execution (performing a square root operation, of all things!) so I guess the tolerances were very tight. I went and combine more lines of code into single lines and replaced two-letter variable names with single-letter ones (no, really) and it was able to run again but I don't think 8KB is a particularly comfortable amount of RAM for a BASIC computer!

Further design considerations

The version of the BASIC host interface used here is very much a work-in-progress. I would need to extend it considerably to be useful, including:

  • Fuller support of different VDU commands, e.g. redefining character shapes, changing the text and graphics viewports, better colour handling (differentiating between logical palettes and physical palettes).
  • Better support of other modes (so far only TMS9918A "Text" and "Graphics II" modes are used, there is a Master System-specific "mode 4" but that lacks graphics support and only takes advantage of hardware scrolling for extremely fast program LISTing).
  • Implementation of more graphics commands - so far only PLOT 4 (MOVE) and PLOT 5 (DRAW) are implemented. They also use a non-standard coordinate system with (0,0) in the top left of the screen and a screen resolution of 256x192, whereas for standardisation with other BBC BASIC implementations this should move (0,0) to the bottom left and use a logical resolution of 1280x1024 or similar.
  • Support of other file systems rather than rely on a Z88 running PC Link, e.g. using I²C EEPROMs (as they use two open collector pins, so could be plugged into a controller port via a passive adaptor).
  • Support of extra RAM, either integrated directly on a custom cartridge or using the battery-backed SRAM supported by some other cartridge types.
  • Native controller support via BASIC ADVAL command (at the moment you can access the controller ports directly with GET(&DC))

This is before even getting into adding anything machine-specific (e.g. to take advantage of scrolling tilemaps or hardware sprites), but getting the BASICs down (and consistent!) is a very important starting point. Consistency is useful, after all I was able to get the original program running under BBC BASIC with very minimal changes to it.

But, in the short term, I have at least succeeded in what I set out to do, which was to run the Mandelbaum program on a retro "computer" wholly unsuited to it!

The above video provides another demonstration of the setup – playing the Cold Tea music demo, albeit with a heavily stripped-down version of the visuals as the Master System doesn't have anything that can match the capabilities of the BBC Micro's teletext mode 7.

Migrating SVN repositories to GitHub, maintaining history even on addition/removal of a trunk folder

Friday, 16th July 2021

Years ago I had a few projects hosted in Google Code's SVN repositories, but Google closed Code down in 2015 and though I backed the files up locally via svnsync to carry on working on the projects the code was never made publicly available again.

I decided it would be a good idea to move these repositories to GitHub, but quickly ran into the problem that during the initial migration from my local repository to Google Code's repository I had been forced to move all of the code into a trunk folder (I do not generally use SVN's branching or tagging features) and attempting to synchronise from SVN to Git lost all revision history prior to that change.

After scouring the Internet and many dead ends I came up with the below solution which has worked for me. As far as I can tell the Git tools used are designed for regular synchronisation of repositories and not a one-shot bulk migration. There are separate tools that claim to do a better job with less effort but getting them up and running on my Windows system probably takes longer than figuring out how to do it with Git's own tools...

First of all, you'll need to create a user list that maps your SVN user name(s) (left) to your name and email address (right) used on GitHub, like this:

Ben = Ben Ryves <benryves@benryves.com>
Benjamin = Ben Ryves <benryves@benryves.com>
benryves = Ben Ryves <benryves@benryves.com>
benryves@benryves.com = Ben Ryves <benryves@benryves.com>

Save this list in a text file in your current directory (e.g. users.txt). Next you'll need to need to find the revision number where the move to the trunk folder happened, e.g. by checking the SVN logs. For the sake of this example, let's pretend it was in revision 100. You can now use git svn clone to create a temp copy of the SVN repository. To continue the example, suppose the SVN repository was backed up to D:\SVN\Google\brass-assembler, then the command would look like this:
git svn clone --no-metadata --authors-file=users.txt -r 1:99 file:///D/SVN/Google/brass-assembler temp

There are some notable differences from the usual here, which I'll try to explain:
  • There is no --stdlayout parameter. This is because this makes the assumption that the repository follows the conventional trunk/tag/branch folder used in some SVN repositories, but this repository doesn't make use of such a structure (at least until revision 100!)
  • The -r 1:99 argument limits the operation to be between the revisions 1 and 99. This is when all the files were in the root of the repository, before they were moved in revision 100.
  • The SVN path looks like it's missing a colon, but this is intentional – at least on Windows the Git tools fail if the path contains a colon like it would when using the SVN tools for local paths, so remove that colon.

This may take a while to get started but eventually it should start synchronising the SVN repository to the new Git one in the temp folder up to (and including) the last revision where everything was in the root (99).

Once that has happened, you can perform the trick that makes this work. Open the file temp\.git\config in a text editor and in the [svn-remote "svn"] section you should find the line fetch = :refs/remotes/git-svn. As revisions after this point were moved into a trunk directory, we need to change this to fetch from trunk instead, so change this line to fetch = trunk:refs/remotes/git-svn:

[svn-remote "svn"]
	noMetadata = 1
	url = file:///D/SVN/Google/brass-assembler
	fetch = trunk:refs/remotes/git-svn
[svn]
	authorsfile = C:/Users/Ben/users.txt

Save the file, and then change your current working directory to your temporary repository, fetch the revisions from after the one that moved everything to trunk up to HEAD, then merge the changes (without the merge you'll only see up to the previously-cloned revision 99):
cd temp
git svn fetch -r 101:HEAD
git merge remotes/git-svn

At this point you can clone the temporary repository into a final one, so go up a level, clone the temp repository and delete it:
cd ..
git clone temp Brass3
rmdir /s /q temp

Finally, you can push this local repository to GitHub. When you add a new repository GitHub will provide a clone URL (in the form https://github.com/<username>/<reponame>.git) so change the remote origin for the local repository that was just created and push your changes:
cd Brass3
git remote rm origin
git remote add origin https://github.com/benryves/Brass3.git
git push origin master

Once that has completed you should be able to view your code on the GitHub site with its full history. The last thing I do is to delete the local Git repository and clone again from the remote one on GitHub using the GitHub Desktop program as I am more comfortable using a GUI tool to keep an eye on changes (and I find I'm less likely to mess something up by accidentally mistyping something!)

In case you couldn't tell from the above examples I've also been looking at the source code for my old Brass 3 assembler project. I've been working on a little Z80 assembly project: running BBC BASIC on the Sega Master System, which has involved a lot of my old projects – Brass 3 to assemble the code, Cogwheel to test it, Emerson to handle PS/2 keyboard input and of course my experiences with running BBC BASIC on the TI-83 Plus calculator.

One thing I realised during all this was that Brass 3 had some problems running on 64-bit versions of Windows – the help application is completely non-functional, for example, and crashes silently to desktop. I dug into the code to fix it, only to find out that I'd already done so two years ago, and even got as far as rebuilding the installer package but then just forgot to upload it to the Internet. So, I'm very sorry for the delay, but I have now uploaded "Beta 14" to the Brass 3 page.

Making your own Dreamcast MIDI Interface Cable

Saturday, 1st May 2021

I love an unusual accessory for a video game console or computer, and one such accessory is the Dreamcast MIDI Interface Cable, HKT-9200, which allows you to connect MIDI devices to the Dreamcast console's serial port. Only released in Japan and with only one piece of software released for it — the O・to・i・re (お・と・い・れ) sequencer – these are a somewhat hard to find accessory nowadays and prices for second-hand units are far beyond what I could hope to afford (at the time of writing there are two on eBay, both for over £300).

Fortunately, the user darcagn on the Obscure Gamers forum took some photos of the insides of the interface box and from that I could make a pretty good guess as to how the cable works.

MIDI uses a serial protocol running at 31.25Kbps (a speed that can be easily derived by dividing a 1MHz clock by 32). Rather than signal "0" or "1" bits with different voltage levels (as with a PC's RS-232 serial ports, for example, which commonly uses +12V for a "0" and -12V for a "1") it uses a current loop, with 5mA current on for a "0" and current switched off for a "1". To avoid ground loops, which are a big concern when working with audio as they can introduce intereference (e.g. a mains hum) on recordings, the two connected devices are electrically isolated with an optoisolator in the receiver.

At the very least I therefore expected to see some sort of optoisolator circuit on the adaptor's MIDI IN port and some sort of output buffer circuit on the adaptor's MIDI OUT port to convert between MIDI's current loop signalling and the Dreamcast's 3.3V logic on its serial I/O pins, and that is indeed what you can see from darcagn's photos. My worry was that there might be some additional Dreamcast-specific hardware inside the box, but fortunately there isn't – it's all off-the-shelf parts. My main concern was how everything was connected, as this can't be completely seen from the photos: would sending MIDI IN data to the console's serial port RX and relaying data from the console's serial port TX to MIDI OUT be enough? Some experimentation would be necessary!

Building a serial port connector

As mentioned above, the MIDI interface cable's box doesn't contain anything Dreamcast-specific, however this box is connected to the Dreamcast's serial port using a proprietary connector. To try anything out I'd need to find a way to connect a circuit to this port:

The serial port on the back of the Dreamcast

Fortunately, the port's contact pitch is the same as a PCI Express slot, and I was able to find a PCI Express slot for £5 which could be used to make multiple connectors! It will need to be cut down to size (and in half, as the Dreamcast's serial port only has contacts on one side rather than both sides of a PCI Express card) but with a bit of work will do the job.

Cutting the PCI Express connector to make the serial port connector

The above photos show the process of cutting the PCI Express slot down to size. The Dreamcast serial port has 10 connectors in it, so a block is cut that is 12 connectors long using a cutting disc – as this is quite a rough process an extra sacrificial connector is left on each end as it doesn't matter if this gets mangled by the cutter. The block is then cut in half, leaving more of the support structure from the bottom of the slot on the side of the slot we're going to be using. The outer two connectors are then removed if they haven't already been damaged, leaving the central 10 connectors, and the outer plastic is brought to the final width and tidied up with some hand files. The fit of the connector should be tested against the Dreamcast's serial port:

Testing the fit of the connector inside the Dreamcast serial port

There shouldn't be too much side-to-side movement but the connector will be very loose without something to hold it down against the contacts. In my case I found some 2mm thick ABS plastic sheet was the perfect material to make the backing piece for the connector, though you may find your choice of material depends on the thickness of your PCI Express slot. It will need to be 13.5mm wide (about ½") and a decent enough length to fit inside the enclosure you're going to use for the plug – in my case 4cms was about right. The plastic can be cut by scoring it with a knife and then snapping it over the edge of a table.

Backing support piece to hold the connector against the serial port contacts

You should also drill some shallow holes in the plastic, with the centres of the holes being 3mm from the end and 3mm from the sides. The serial port has a couple of bumps stamped into the metal surround of the serial port and these matching holes in the plastic piece allow it to snap into place. A stripboard track-cutting drill is perfect for this task!

Groove filed into the end of the piece for the raised ridge on the PCI Express slot

In my case the PCI Express slot also has a slight lip that prevents it from sitting flush against the backing support. I could have filed this flat but the connector is quite fragile so I didn't want to risk damaging it so I ended up filing a corresponding channel into the bottom of the backing support piece.

Test fit of the connector and support piece

At this point make sure that everything fits. If it does you can start wiring up! The photos below show the process – heat-shrink tubing and strain reliefs are very useful, so don't forget to install them before soldering!

Soldering the wires to the connector

Putting a scrap piece of circuit board material between the two rows of pins also makes soldering much easier. In my case I only soldered the six pins required for this MIDI interface cable:

  • 1: +5V
  • 3: GND
  • 4: RX
  • 5: TX
  • 8: GND
  • 10: +3.3V

Pins are numbered from left to right when looking at the serial port at the rear of the console (if in doubt, you can check the voltages of the end pins against the console's metal chassis ground).

At this point you may wish to double check that your solder connections are made correctly and that nothing is shorted out – try with the cable plugged into the Dreamcast as well, and check that adjacent pins are not shorted together (the only two that should be shorted are pins 3 and 8, the two GNDs). If you're happy with that you can glue the connector onto the backing:

The connector is now glued to its supporting backing

You may notice that this is actually a different connector in the photo to the previous one, and that's because I accidentally got glue into the connector's springy contacts and jammed them so had to start again – definitely not a fun mistake to make, so be careful!

Fortunately, the second one went more successfully. The glued connector snaps in and out of the console with a nice reassuring click thanks to the two holes drilled into the top surface. Before switching on the Dreamcast I tried wiggling the cable around to ensure that even when treated roughly it woudn't short out adjacent pins. When I was happy this was the case I switched on the Dreamcast and ensured that I was getting a consistent +5V from pin 1 and +3.3V from pin 10 – as these pins are at the far end of the connector these are the ones that are more likely to have problems with crooked connectors. In my case I found there was an intermittent fault with pin 1's +5V. This was because I hadn't glued the connector on particularly straight and so pin 1's connector was slightly back from the edge of the backing piece. I very carefully filed the backing piece's edge so that it was flush with the slightly wonky PCI Express connector, after which pin 1 made reliable contact.

Potting the connector inside a small enclosure with hot glue

When you're completely happy with the connector, you can make it more robust by putting it inside an enclosure. I have some very small project boxes that are perfect for this sort of thing, it's a bit bulky when compared to the official Sega product but it does its job well here and doesn't bump into the power connector or AV port connector.
I cut a slot in one end of the box for the connector to stick out of and a notch in the other for the cable strain relief to clip into. I surrounded the console's serial port with a few of layers of masking tape to ensure that when the connector was inserted there was still a small gap between the case and the plug to make sure that it could always be fully inserted and not held back by interference from the case (it also protected the console shell from accidental strings of hot glue!) I then plugged the connector into the Dreamcast, made sure that everthing was neatly lined up, and secured the parts in place with copious amounts of hot glue. Once this had set I added more hot glue to the rear of the connector to make sure it was all held as securely as possible, and then screwed the enclosure shut.

The finished serial cable

With all the effort spent on the Dreamcast end of the cable, don't forget about the MIDI interface box end! I'm fond of JST-XH connectors so crimped one onto the end of the cable, ready to plug into the circuit board. The finished cable is seen above!

Building a prototype MIDI interface cable

With a Dreamcast serial port cable to hand I was able to experiment and see what happened when using the O・to・i・re (お・と・い・れ) sequencer. Here's the circuit I ended up building on a breadboard:

The prototype MIDI interface cable on a breadboard

On the very left is the first serial cable connector I tried constructing. It looks more yellow than the final version as I used a piece of pad board for the backing piece instead of the 2mm ABS plastic – I thought I was going to solder the PCI Express connector piece to a circuit board rather than directly to the cable's wires, this turned out to be a mistake. One advantage is that it does have eight wires soldered to it – I thought I'd see what RTS and CTS were doing, but I ended up not using them. The MIDI port to the left is for MIDI OUT and has a large black chip to act as a buffer, the port to the right is for MIDI IN and has a small white chip as an optoisolator. Here's the corresponding circuit diagram:

The schematic for the MIDI interface cable

The position of the ports is flipped in this diagram when compared to photo of the breadboard, but it otherwise matches up!

On the left is the MIDI IN port. This uses an H11L1 optoisolator as they are still reasonably easy to get hold of today, are fast enough for use with MIDI and can be run directly from +3.3V. Its output on pin 4 is open collector so it needs the 270Ω pull-up resistor to the +3.3V rail. A +5V-demanding optoisolator could also have been used if it had an open collector output and the pull-up resistor on its output was still tied to +3.3V (we don't want to run +5V into the Dreamcast's +3.3V logic!) but this makes the wiring a little more complicated so sticking to a +3.3V-compatible part makes life easier.

The 36Ω resistor between the output of the MIDI IN circuit and the serial port's RX pin is there because it is in the official cable. The official cable also places ferrite beads on every I/O pin (and has a ferrite bead clipped onto the cable itself) which I have not replicated in my own cable, but they can't hurt and I suppose it could protect the console from certain direct shorts!

The MIDI OUT port uses a 74HC365N to convert from the serial port's +3.3V logic to +5V to drive the MIDI output. MIDI signals can be run from +3.3V (it's the flow of current that's more important than the selected voltage) but +5V seems more typical so I thought I'd stick with that, and as we need to buffer the signal anyway (I'm not sure how much current the Dreamcast's serial ports are designed to sink or source) using the +5V supply we have available to us made sense. The voltage threshold for a "high" input signal is probably a bit too high with the 74HC365N – +3.3V is pretty close to the recommended values in the datasheet, so the 74HCT365N version of the chip would give you more margin for error and would be a drop-in replacement. In my testing the 74HC365N does work well, though, and it's what I had available.

The cable used in this prototype has eight connections rather than the six in the final. This is because I did experiment with the RTS (pin 6) and CTS (pin 7) signals from the console, however as far as I can see these are just pulled low and high respectively and do not change from the moment the console boots, even when sending and receiving MIDI data within the O・to・i・re (お・と・い・れ) software. If they were planned to be used for some purpose then I'm not sure what, and with only one piece of software released to test with I'm not sure I'll find out.

In any case, with this circuit data from the MIDI IN port is translated to +3.3V logic levels suitable for the Dreamcast's serial port input (RX) and translated back from the serial port output (TX) to a current loop suitable to drive a device connected to the MIDI OUT port. In practice it seems this MIDI OUT port acts more like a MIDI THRU with the O・to・i・re (お・と・い・れ) sequencer – any data sent to the MIDI IN port comes straight back out of the MIDI OUT port, however if you record some notes in the sequencer and play them back afterwards they aren't played back out of MIDI OUT. At first I wondered if I'd made a wiring error (accidentally connecting MIDI IN straight to MIDI OUT) but the MIDI OUT is indeed under control of the software as it will stop relaying messages on certain screens.

Putting it all together in a nice box

I ended up using my usual pad board construction technique to build the final device:

The assembled circuit board for the adaptor

I followed the circuit diagram I'd drawn earlier (rather than copy the breadboard circuit) to ensure the diagram was correct. This was all assembled to fit in a ready-made ABS enclosure, into which I cut some holes. Historically I've had a hard time cutting neat round holes in plastic enclosures – I have a few hole saws and these are great for cutting through wood or acrylic but when trying to cut ABS they tend to bind and either rip the box out of the vice or just shatter it. I normally resort to drilling lots of very small holes around the perimeter of the circle and then try to file it to size, which is OK for buttons or sockets with overhanging parts to hide the inaccuracies but wouldn't do here! For this project I bought a very cheap step drill set on eBay (three bits for a fiver) with zero expectations but it did an excellent job, it kept the centre hole I'd started from and didn't need very much cleaning up. I wish I'd bought one sooner!

Using a step drill to make the MIDI port holes

The other enclosure challenge to deal with was labelling the two ports. I normally get away without labelling my projects because the function of each port can be guessed quite easily (e.g. inputs and outputs normally only plug in one way, or player 1 is on the left and player 2 is on the right) but in this case there's not much convention for where MIDI OUT and MIDI IN go (though the original Dreamcast MIDI interface cable puts MIDI OUT on the left, as does my M-Audio Midisport). For this I thought I'd try using some dry transfer lettering designed for model-making, and it seems to do a great job!

Dry transfer decal lettering marks the OUT and IN ports

I'm pretty happy with the final outcome – it's a bit of a pain making the serial cable for connection to the Dreamcast, but the end result works well and it saves spending an absolute fortune on the original Sega accessory.

The completed MIDI interface cable/box

Now I can get on with making music with my Dreamcast!

Connecting an old receiver/amplifier to an HDMI television, apropos achieving surround sound

Sunday, 16th August 2020

Now that the charity shops have reopened in the UK I enjoy hunting for good bargains and recently picked up a DAV-S400, a home cinema system originally sold back in 2002. I was not particularly interested in its DVD playback capabilities, especially as it only outputs composite video, but the integrated 5.1 surround sound amplifier and optical input made me think that I could use it to replace my current stereo amplifier for a better home cinema experience. The inclusion of SACD support was an added bonus as I'd bought myself a copy of The Dark Side of the Moon in that format back in 2004 and only been able to listen to it once, so if I couldn't get the home cinema side working at least the SACD part was self-contained and I'd be able to enjoy that.

Repairing the amplifier's inability to play SACD

This ended up being quite the learning project for me and I had to solve quite a few problems along the way. I'm still not entirely sure if I've found the best solutions in places but I thought I'd write this post in case it helps anyone else hooking up an old receiver/amplifier to a modern HDMI system!

The proposed setup

I watch most media using my Windows PC as a source. The PC is in one room and the TV is in another and long HDMI and USB cables run between both under the floorboards. An HDMI splitter is also used near the PC so it can drive its main monitor and the TV in the other room simultaneously.

I was previously running analogue stereo audio from the headphone socket of the TV to a stereo amplifier with a line level "super woofer" output that ran to a separate amplified subwoofer for 2.1 sound.

I knew that HDMI could carry digital audio, and both my monitor and TV have digital outputs. As the DAV-S400 has a digital input, I could just connect that to the digital output from the TV (instead of the headphone output) and in my naivety assumed that's all it would take to have glorious multichannel digital audio. Of course, things are rarely that easy…

My TV only has coaxial digital audio out via an RCA connector and the receiver has an optical input via a TOSLINK socket. Fortunately, my PC monitor has an optical output so I connected that to the receiver for the sake of testing. When I played media on my PC sound came out of the speakers, however only in stereo, even from 5.1 sources. What was going on?

Capabilities of S/PDIF

The digital output from the monitor/TV and the digital input of the amplifier operate with S/PDIF digital audio. This can carry two channels of uncompressed PCM audio for regular stereo. To carry 5.1 multichannel audio the data needs to be compressed using Dolby Digital or DTS. The DAV-S400 supports both, but you need to find a way to transfer that compressed signal to the receiver first.

Foiled by the EDID

Here is the biggest fly in the ointment! Extended Display Identification Data (EDID) is metadata that an HDMI display provides to describe what sort of video resolutions and sound formats it supports. All three of my HDMI devices (the HDMI splitter and both displays) report that they support two-channel PCM audio but nothing else.

Sound properties showing that only PCM formats are supported

As a result, devices will only try to send stereo PCM audio to them and not Dolby Digital or DTS. If you have a sound card with an S/PDIF output you will notice that you can go in to its "Supported Formats" tab in Windows and tick which features the receiver supports – this is as the receiver can't tell the sound device which formats it supports automatically like an HDMI device can via its EDID. Unfortunately, Windows trusts the EDID and doesn't let you go in and enable support for features the display doesn't claim to provide. An easy solution here would be to just use my sound card's S/PDIF output but I didn't fancy taking the floorboards up again to run another cable…

A custom monitor driver to override the EDID

Fortunately you can create a custom monitor driver with a "fixed" EDID that advertises support for Dolby Digital and AC3 using a couple of easy tools.

The first piece of software you'll need to use is EnTech's Monitor Asset Manager – this will allow you to extract the current EDID to a file and will later let us turn the updated EDID back into an installable driver.

EnTech's Monitor Asset Manager

Run the software and find the monitor that is connected to the HDMI port that you are going to be outputting sound to from the list in the top left. In my case that's my HDMI splitter (which for some reason identifies itself as a DELL ST2320L). Save the EDID to a .bin file via the File→Save as menu item.

Now that you have your EDID data exported to a file, you'll need to edit it. A nice idiot-proof way to do so is with Analog Way's AW EDID Editor. Run the software and open the .bin file for the EDID file you just saved. Switch to the CEA Extension tab and you will probably see that it only reports support for stereo uncompressed PCM when you look at the "Audio" block.

AW EDID Editor

At this point, you can add audio descriptors according to the capabilities of your receiver. I'm not entirely sure what the best values to enter here are, especially when it comes to bitrate – as far as I'm aware DVDs usually use up to 448kbps for Dolby Digital AC-3 and up to 768kpbs for DTS but there's a more concrete limit of 640kbps for AC-3 and 1536kbps for DTS so these are the values I've used when setting up my EDID.

AW EDID Editor for AC-3
AW EDID Editor for DTS

You can also use the "Speaker allocation" block to specify which speakers your device has attached – in my case I've set this up to match my 5.1 setup. Save the EDID at this point, and you're ready to create the custom monitor driver!

AW EDID Editor for speaker allocation

Go back to EnTech's Monitor Asset Manager and open the saved .bin file for your new EDID. Click File→Create INF and save the INF to create your new monitor driver. Easy!

What's less easy now is installing that driver on a modern 64-bit version of Windows, as the driver is unsigned and Windows will not let you install unsigned drivers normally. To save headaches at this point I'd recommend looking up how to enable the installation of unsigned drivers, but at the time of writing this is how it's done in Windows 10 (this will involve restarting your computer):

  • Click Start→Power
  • Hold Shift, then click "Restart".
  • At the "Choose an option" screen, select "Troubleshoot".
  • At the "Troubleshoot" screen, select "Advanced options".
  • At the "Advanced options" screen, select "Start-up Settings".
  • At the "Start-up Settings" screen, click "Restart".
  • Once the computer has restarted, select the "Disable driver signature enforcement" option (F7).

When the computer has restarted, you should be able to install your unsigned monitor driver. To do this, go into Device Manager and find the entry for your HDMI device in the "Monitors" section.

  • Right-click your device and select "Update driver".
  • Select "Browse my computer for drivers".
  • Click "Let me pick from a list of available drivers on my computer".
  • Click the "Have Disk..." button.
  • Browse for the monitor.inf file you created previously, then click OK.
  • Select the EDID Override option that appears in the list and click "Next".
  • Click "Install this driver software anyway" when prompted by the unsigned driver warning dialog box.

At this point, you will likely need to restart again but all being well after doing so you should now see that Dolby Digital and DTS Audio are now listed as supported encoded formats in the Sound control panel.

Sound properties showing that compressed formats are now supported

Passing through Dolby Digital and DTS without conversion to PCM

Getting Windows to believe that your receiver supports Dolby Digital and DTS over HDMI is the trickiest problem, but you still need to ensure that the compressed audio gets through every step of the way from the media to the receiver.

You will need to ensure that your media playback software is configured to pass through the audio directly rather than decoding it to PCM. If your software has an audio settings menu option, ensure that that is set to allow non-decoded audio to pass through.

Kodi audio configuration
PowerDVD audio configuration

You will also need to ensure that your display is configured to pass through the audio without decoding it to PCM – check its sound menus for the relevant options, and change any settings to "Auto" or the appropriate equivalent instead of "PCM".

Blaupunkt TV digital audio configuration   LG TV digital audio configuration

In my case I was testing with the receiver connected to my PC monitor via its optical output. Dolby Digital worked fine, however DTS was not working at all. I found that if I enabled the monitor's internal speaker it would output sound when fed Dolby Digital but not when fed DTS, however my TV would output sound when fed either. It seems that my PC monitor genuinely lacks support for DTS and won't even pass it out of the digital output port – fortunately when I acquired a coax to optical adaptor for my TV I found that it passed through DTS just fine!

Successfully decoding Dolby Digital AC-3
Successfully decoding DTS

The Windows 10 application version of Netflix also supports 5.1 audio over Dolby Digital and didn't need any further configuration. If you search the library for "Test" there's a useful selection of test patterns that can be used to ensure your speakers are set up correctly.

Netflix speaker test

Other notes

I have a separate Blu-ray player attached to my TV as my PC's video card displays corrupt video when decoding Blu-ray 3D unless I install very old versions of its drivers and I thought a separate Blu-ray player was an easier option than dual-booting into Windows Vista. Unfortunately, my Blu-ray player believes my TV's EDID and refuses to pass-through Dolby Digital or DTS over HDMI even when set to output "bitstream" audio. Fortunately it has a separate coaxial digital audio out that outputs Dolby Digital or DTS even when it's outputting PCM over HDMI but it does mean I need to add a switch box for the single digital input into the receiver which is slightly less elegant than running everything through the TV. There are standalone EDID spoofers that plug in between your device and the monitor that may be able to resolve this issue in the same way the custom PC driver did but these are not inexpensive so I have not been able to test this myself.

The DAV-S400 is extremely fussy about reading hybrid SACDs, and will often detect them as regular CDs. Ejecting and closing the tray persuades the machine to re-scan the disc, so if you have problems press the "DVD Display" button to ensure the OSD is visible and double-tap the eject button to quickly eject and close the tray until the indicator in the top right of the screen reads "SACD" instead of "CD".

If it does detect it as an SACD it will then play it absolutely flawlessly, the issue only seems to be with the initial format identification. I was able to improve the success rate with Dark Side of the Moon by carefully cleaning the drive lens, however I couldn't get Brothers in Arms to be detected at all until I went into the service menu and recalibrated the drive. You can do this by switching the machine on in DVD mode, holding the Stop+Display buttons on the unit together and rotating the volume wheel clockwise. From here go into "Drive Auto Adjustment". I initially tried calibrating with an SACD (labelled "LCD" in the menu) but this didn't help, so I tried the "All" menu option which prompts you to insert a single-layer DVD (DVD-SL), CD, then dual-layer DVD (DVD-DL). After this point it doesn't automatically prompt for an SACD so I left this in the default settings rather than calibrating and to my surprise found that Brothers in Arms now works (and, once detected, plays flawlessly). This inability to handle hybrid SACDs correctly seems to be a common flaw with Sony players of this vintage, unfortunately.

On a more practical note, I needed to find a way to attach my speakers to the amplifier, which had connectors that exposed a pair of pins around 6mm apart. Following a hunch I bought some Tamiya power connectors and found these to be a good fit once you cut off the retaining clip!

Speaker connectors made from Tamiya power connectors

I did however find that a couple of the audio channels were intermittent and when I took the case lid off was surprised to see that one of the electrolytic capacitors on the amplifier board was barely held in by any solder at all and when I touched it it let out a big fat noisy spark! I resoldered it in along with all of the audio transformers, connectors and relays on the amplifier PCB which fortunately restored all audio channels back to their former glory.

All in all this has been quite a learning experience, though having now watched Terminator 2 and Mad Max: Fury Road with 5.1 surround sound I can definitely say it was worth it.

The completed Light Phaser to Justifier adaptor

Wednesday, 29th April 2020

Since my previous post I've had a chance to test the Sega Light Phaser to Konami Justifier adaptor circuit with Lethal Enforcers II: Gun Fighters and it seems to work well throughout the game so I thought it was a good time to put the project into a neat box.

The Mega Drive uses common DE-9 connectors for its controller ports. You can easily find metal connectors with lugs for panel mounting however I wanted to use plastic connectors similar to the ones on the original console – not just for a consistent look, but to also protect the plastic connectors on the Sega Light Phasers from being scratched by the sharp edges of metal connectors. I'm using controller extension cables for the nice moulded connector and cable that plugs into the console, so thought I'd look at the controller socket end. Squeezing the bottom half of the connector splits the two halves apart and you can use a spudger or similar tool to pop the casing open and retrieve the connector – just what I needed!

Splitting the DE-9 connector open

I nearly always put my projects in off-the-shelf enclosures, cutting and drilling holes in them where required. In this project's case the most awkward holes to cut will be for the controller connectors as they do not have an external lip to mask any poorly-cut holes. As such, I thought I'd start here:

Cutting the holes for the DE-9 controller connectors

My process here is a bit tedious, but gets the job done – I cover the enclosure in masking tape, then mark out where the holes should appear. I use a drill to make several holes inside the shape I wish to cut out, then use a router bit or sharp knife to cut between the holes to open up the hole properly. A set of hand files is then used to bring the hole to its final shape and size, checking against the connector along the way to ensure an accurate fit.

Cutting the holes for the buttons

The circular holes for the buttons are cut in a similar fashion, except that as the buttons have outer lips that cover the borders of the hole the outer edges don't need to be quite as precise and a larger router bit and drum sander can speed up the job considerably rather than having to rely on careful hand filing.

A test fit of the parts and circuit board

A rectangular notch was cut in the rear centre of the case for the cable to the console to exit through and the parts were then loosely installed for a test fit. A piece of pad board was cut to size by scoring it and snapping it over the edge of a table. I wanted it to be as large as possible to make it easier to fit all of the components and wires in, but could the case be closed with the cables and connectors in place too? To make sure it would I thought I'd start with the bulkiest of cables, the one that goes to the console.

Creating an internal connector for the controller cable with crimp connectors

I attached a nine-pin crimp connector to the end of the controller cable to plug into the pad board and experimented with the placement of the destination connector – I found that four rows down was about the closest it could go to the top edge of the board whilst still providing enough room for the cable coming in to bend neatly out of the way.

Rough component placement prior to soldering

The various parts for the console output connector, light gun input connectors, start button connectors and multiplexer chip DIL sockets were placed on the pad board without soldering to get a rough idea of the final layout and component spacing, ensuring there was enough space around each part to install other components or wires. Once a rough idea was in mind, the main soldering work could begin!

The stages of assembling the circuit

Not much appears to happen between the first and final stages, as most of the work takes place in the wiring on the bottom of the board. Connections that can be made in straight lines without crossing other connections are soldered first, followed by straight connections that cross other connections that can be insulated by slipping on a piece of insulation stripped from a reel of wire. When all of the easy connections are made in this way the outstanding connections are made by soldering lengths of thin wire wrap wire between points on the underside of the board. As each connection is made it is crossed off the circuit diagram – this allows me to check that the circuit diagram is correct, as if I can replicate the circuit from this diagram it's a pretty good indication that it's safe for other people to use to make their own versions of this project.

The DE-9 sockets used for the Light Phaser inputs have all nine pins soldered to the cable by default. We only care about four of them (VCC, GND, TL and TH) and as we don't have enough room for all nine connections to be soldered to the board four-pin connectors are used instead. The old wires are unsoldered and new wires and crimp connectors are attached in their place. This let me check that the circuit board was otherwise working fine, but I still need to figure out how to mount these connectors securely inside the enclosure. I was initially thinking of just holding them in place with hot glue, but this is not very elegant and makes future maintenance much harder! The DE-9 sockets do have a flat surface at the top that is only slightly more than 5mm from the top surface of the enclosure, and I do have some 5mm PCB standoffs that could be handy…

Mounting tabs for the DE-9 connectors

I cut a 9mm wide strip of acrylic from 2mm thick sheet which fits neatly into the top channel of the DE-9 connectors. I then cut it into short sections that could be glued together into Z-shaped brackets. The long section of the "Z" mounts to the DE-9 connector and the short section has a hole drilled through it so it can be secured to the PCB stand-off with a screw. The arrangement is then test fit inside the enclosure using double-sided sticky tape to check that a solid mount is possible – the screw to the PCB stand-off accounts for one part of that mount, and the tight fit of the connector inside the D-shaped hole in the enslosure accounts for the other part.

When it seems like a good fit is possible the Z-shaped brackets are glued to their DE-9 connectors. The DE-9 connectors are then installed in the enclosure with some glue on the bottom of the PCB stand-offs – when everything is lined up neatly this glue secures the stand-offs in the right position, albeit not very strongly.

Mounting the PCB stand-offs inside the enclosure with two-part epoxy

Once the glue had set the screws were removed and the DE-9 sockets were carefully removed. Two-part epoxy was then applied liberally around the PCB stand-offs to provide a bit of extra support. Unfortunately, the stand-offs are very close to the nuts used to secure the start buttons to the enclosure and as such not much epoxy could be placed on that side but hopefully the rest should provide enough support.

Exploded view of all of the component parts of the adaptor

Whilst the epoxy was curing the final parts of the project could be assembled – a pair of start buttons had wires soldered to them with crimp connectors on the other end and a couple of rubber strips were cut to provide a bit of extra grip to the bottom of the adaptor. The various component parts of the project can be seen in the photo above.

The various component parts assembled inside the enclosure

Everything does fit fairly neatly inside the enclosure, fortunately! After everything is assembled inside the bottom of the enclosure was screwed on and the rubber grip strips attached with double-sided sticky tape.

The finished adaptor, screwed together

I still haven't been able to test this adaptor with any Mega CD games however it does work well in both Lethal Enforcers and Lethal Enforcers II: Gun Fighters on the stock Mega Drive so hopefully it will also work well in Mega CD games.

The finished adaptor, plugged into a Mega Drive and all ready to play

I'm pretty happy with how this project turned out. If you would like to assemble the adaptor yourself, you can find the schematic in the previous journal entry, and if you do build it I'd love to hear how you got on!

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