A new MPU-401 solution: The Music Quest clone

This is a draft that I have kept around since March 16, 2019, so it is about time I finally finish and publish it.

This story started somewhere in 2015, when a user named Keropi posted a thread on the Vogons forum, about cloning a Music Quest card. What is a Music Quest card, you may ask? It is a clone of the Roland MPU-401 MIDI interface. Keropi had collected a lot of information on it in a previous thread. But I suppose that begs the question: what exactly is a Roland MPU-401?

I would gather that most people know MPU-401 as a standard for MIDI on the PC, and then usually they are familiar with the UART-mode aka ‘dumb’ mode, since that is the simple interface, which most clones, including many Sound Blasters, support, and which was also used for the later Wave Blaster standard.

An actual MPU-401 is somewhat different though. For starters, strictly speaking, the MPU-401 is not a PC device. It is an external module, and it looks like this:

As you can see, it has a connector labeled “To Computer”. So what do you connect this to? Well, this explains why Roland designed it like this: the MPU-401 is the “MIDI Processing Unit”. This unit contains a Z80 processor and some support chips, and does the actual MIDI handling:

So technically this is like a computer on its own. Roland decided to make this part generic, so they only had to design a simple interface for the various computer systems around in the 80s, and use the same MPU-401 unit for all of them. For the PC, the interface was a simple ISA card, known as MIF-IPC:

So for the PC, a Roland MPU-401 interface was actually a setup of both the ISA card and the MPU-401 unit.

Intelligent Design

Okay, so we mentioned the ‘dumb’ mode. But what is ‘intelligent mode’? This is why the MPU-401 is such a complex design: it is actually a basic hardware MIDI sequencer (Roland calls it a ‘Conductor’ in its manual). That is: you can send MIDI data to it in advance, and queue it up with timestamps, and the device will send out the MIDI data at the correct time by itself. Then it will signal the host with an interrupt that it is ready to receive new data.

Why did they make MIDI so complex back in the day? These were early days. MIDI was introduced somewhere around 1983, and the Roland MPU-401 is to my knowledge the first available MIDI interface for home/personal computers, and was introduced in 1984, and supported computers such as the Commodore 64, Apple II and MSX, and of course the IBM PC.

So we are talking early 80s 8-bit machines, with limited memory and processing power. Sure, these machines could play music, but this was generally synchronized to the display refresh rate, so you had an update rate of 50 or 60 Hz. This severely limited you in terms of tempo. MIDI allowed for much higher resolutions, as it was aimed at professional musicians in a studio setting.

Now, while I have developed a MIDI player myself a few years ago, and explained how you could get the high resolution of MIDI even on an original IBM PC or PCjr, there are two obvious limitations here:

1. My player requires preprocessing of the data, so you cannot play MIDI files as-is. It can only be output as a single data-stream, no separate MIDI tracks, as you would have in a sequencer.
2. Playing a MIDI file is possible, but recording and editing of MIDI will not work in realtime with this approach, as you would need to convert between MIDI and preprocessed data.

So for various reasons it made a whole lot of sense to use an MPU-401, which has hardware that is custom-designed to record and play back MIDI data in realtime, with its own clock (we have seen that the IBM Music Feature Card also has its own clock that is designed to handle MIDI timings, although it is not as advanced as the MPU-401 is). You could easily guarantee that your MIDI data was played at the correct time, without having to worry about what the CPU was doing. And editing MIDI data was also simpler, as you wouldn’t need to convert to some internal system clock that was not even remotely similar to the timestamps that MIDI uses (which are based on a 1 MHz clock).

So in short, ‘intelligent mode’ is where you offload playback to the MPU-401, rather than just outputting the MIDI data in realtime, where you need to make sure that your CPU output routine is accurately timing each byte. The MPU-401 has a total of 8 ‘tracks’ internally, where you can queue up timestamped data for each of these tracks in a fire-and-forget fashion.

Who cares?

One of the problems with the MPU-401 was that it was very expensive. Aside from requiring the ISA board and the MPU-401 unit, you’d also need an actual synthesizer. In 1987, Roland introduced the MT-32, a somewhat affordable semi-professional MIDI module (a synthesizer without a keyboard attached, basically), which became somewhat of a de facto standard, as Sierra and various other game developers started to support MPU-401+MT-32 as an audio device.

But by the time MIDI started to gain traction, we were approaching the 90s, and PCs had become much more powerful. On the one hand, playing MIDI in realtime wasn’t such a big problem anymore. On the other hand, games generally had MIDI music that was already quantized to a relatively low resolution anyway (somewhere in the 140-270 Hz range for example) that ‘dumb’ UART mode output was good enough. And since the PC has always been the platform of the lowest common denominator, and most cheap sound cards only supported the UART mode of the MPU-401, most games won’t require ‘intelligent mode’ compatibility.

For that small selection of software that does require a full MPU-401 though, your options were limited. There is a software solution known as SoftMPU, which emulates the intelligent stuff on the CPU-side, so you can upgrade your UART MPU-401 to a full intelligent MPU-401 with this TSR. Downside is that it requires a 386 CPU or better. So for your 286 or older machine, you still need a hardware solution for true MPU-401 compatibility.

But we can anyway

As technology progressed, it became simpler and cheaper to integrate both the host interface and the MPU-401 on a single ISA card. Roland introduced its own MPU-IPC, where the external module was now a simple breakout box:

And later still, they introduced the MPU-401AT, which had mini DIN connectors on the card itself, and a Wave Blaster interface to plug a MIDI module directly on the card:

And of course there were other parties that made their clones of the concept. The original MPU-401 setup was rather complex and expensive to clone. But when one can integrate it all on a single card with off-the-shelf parts, it becomes feasible to try and make a cheaper alternative to the Roland offerings. This is where the Music Quest comes in. It was one of these clones, and it was a good one, as with the later v010 firmware, it is considered to be 100% compatible with Roland:

So, after this lengthy introduction, we can now finally get to the actual card this blog is about:

Because well, if a card is made mostly from off-the-shelf parts, like a Z80 and an EPROM, in this day and age, it is not that difficult for a single skilled individual to design and build their own PCB, dump the roms, and clone the clone.

And this is what was done here. What’s great about this, is that it is a good clone of a good clone: people can now get their hands on reliable MPU-401-compatible hardware at relatively low cost. I received mine a few years ago, and wanted to write about it, as I was quite happy with the product. Better late than never, I suppose.

I received a somewhat later revision, which also includes a Wave Blaster interface:

And another revision has been developed since, which uses a higher level of integration on the PCB:

So, if you want a good MPU-401 interface for your retro DOS machine, either for running MIDI software, or developing your own MPU-401 routines, I can recommend this card. For some more information, software, and a link to the order form, you can go to the Serdashop store page for this card.