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(yes, I'm writing this article on a Monday. But the title sounded better this way. Whatever.)

As you may know, I'm interested in old computers and sound and music. Recently I was reminded by a youtube video about the Amstrad CKX100, a toy-synthetizer from 1988. I had already heard about it but never had an opportunity to disassemble one, so I didn't know how it was built. Well, this video did just that and got me curious about the soundchip used, the ST M114S. I had not heard about it before and was curious how it worked and if I could possibly connect it to a computer to play some music.

So, a few web searches later, I didn't find a lot, but I found what I was looking for: a datasheet and a research paper from the chip creators, both explaining in some detail how the chip operates.

The research paper is dated from 1985 and mentions that the chip was developped in cooperation with GEM, another synth keyboard manufacturer (who went on to use it in several of their keyboards). It was designed by SGS before merging in ST, so it was the opportunity for me to learn a little more about the Italian side of ST Microelectronics. Previously I had known them mostly for the EF9345 (used in the Matra Alice and the Minitel) and some of the chips used in the Thomson TO8 and MO6 machines, mainly the palette circuit that was developped for the TO9 but later became an off-the-shelf component in ST databooks.

The chip uses an 8KB ROM where sound samples are stored. The samples are in power of two sizes from 16 to 2048 bytes, and are played in a loop to sustain the sound. They use delta coding, which means they encode the derivative of the actual sound. An analogue integrator is used to turn this back into the original signal, which has the advantage of providing some level of smoothing of the output.

The chip does a bit more than just playing the table, however, it can smoothly transition from one sample to another by mixing them together. And it can also play the sample at various speeds by using only one out of two or one out of four bytes (but this risks having a not perfectly balanced signal, which will drive the integrator into saturation).

Moreover, it is indeed designed to be driven by a CPU. It is not a "keyboard-on-a-chip" system that would handle keyboard scanning directly and generate the sounds from there. Instead there is an interface for a normal CPU bus where you can send commands to trigger notes. The commands are made of 8 6-bit words. This may seem a bit strange, 6 8-bit words may have made more sense (especially as most of the values transmitted are indeed 8 bits), but I guess it allowed to save some pins and fit the thing in a 40 pin DIP package.

As a result, this chip was apparently used in some pinball and arcade machines. However, MAME doesn't have an emulation for it yet.

During my search for the Datasheet, I of course looked at ST databooks, and found that there is a larger version called M114A. This one has a 48 pin package, which allows to address 265K of memory instead of 8K. Surely this must allow for much more complex sounds.

Interestingly, the datasheet for the M114S shows a clever hack to increase the ROM space to 16K instead of 8. The chip has an "output enable" pin to signal the ROM when it wants to read a byte. This signal will always trigger twice within a short time since the chip will always read a byte from two samples (remember, it then mixes them together). So, they suggest connecting a flip-flop to this output and resetting it with an RC circuit tuned to a longer delay. The output is then used as an extra address bit, and as a result, the first sample will be from the first half of the ROM, and the second sample from the second half. This simulates the behavior of one of the pins of the M114A, which probably does the same without needing an RC circuit as it knows about the chip internal timing.

Of course I am interested in wiring the chip to RAM. The datasheet says it's possible, but some arbitration will be needed to make sure the M114 and CPU don't try to both access the RAM at the same time. Should I go with dual port RAM? Can I simply block the M114 from accessing the bus when the CPU wants to use it? Maybe I can do some fun tricks where the CPU manages to synchronize itself with the chip and feed it data without using the RAM at all? And, first of all, can I even get one of these chips? There seem to be someone selling them ("used") online, but will they be real chips, or will they be some random other chip being relabelled?

I'm also considering writing an emulator for this chip, just to hear how it could sound like. However, to do anything like that, I think I need ROM dumps from one or more of the original devices using it. At least to make sure my understanding of the storage format is correct. There is a dump from one of the pinball machines using it, but it is using mainly or only PCM samples, and not delta encoding.

Finally I am surprised that this chip was not more popular, for example in home computers. Ok, maybe needing a whole 8K of ROM may be the cause. But it sounds quite nicely, and it would have been interesting to see it alongside the AY3, the SID, and the various options from Yamaha, don't you think?

Programming notes

This information is directly from the datasheet, but here in easier to read HTML format.

All communication with the chip is made with 8 byte commands (only 6 bits in each byte). The bytes must be no more than 128us apart, otherwise the chip considers itself out of sync and the command is aborted.

The command layout is as follows:

Byte	Bit		5	4	3	2	1	0
1			Attenuation
2			Output		T1 address MSB		T2 address MSB
3			T2 address LSB
4			T1 address LSB
5			T1 length			Mode
6			Interpolation				Immediate	Octave Divider
7			Channel				Tuning LSB
7			Note				Tuning MSB

• Attenuation: higher values for lower volume
• Output: route the channel to one of the four outputs
• Table addresses: bits A12-A4 of the ROM address to read from
• Length: number of bytes in T1 sample, from 16 to 2048
• Mode: various ways to access the two samples, see below
• Interpolation: Mixing value K. The generated output is S1 * (K + 1)/16 + S2 * (15 - K)/16. A value of 15 plays only table 1, and lower values add more and more of the second sample to the mix.
• Immediate: apply the new attenuation immediately (otherwise a ramp from the previous value is generated)
• Octave divider: divide the frequency by two, to play an octave lower
• Channel: channel on which the command must be run
• Tuning: finetune the selected note, see below.
• Note: from 0 to E, 15 possible notes a semitone apart, covering a bit more than an octave. F is reserved for special commands (See below)

The possible values for the tuning:

• 0 to 5: detune by -6/12 to -1/12
• 6: detune by -2/1000
• 7: detune by -1/1000
• 8: exact note
• 9: detune by 1/1000
• A: detune by 2/1000
• B to F: detune by +1/12 to +5/12

The special commands are not related to a channel. They are globally applied

• F8: ROM ID mode. The chip will generate addresses 0 to 8 in a loop and send them to the ROM.
• F9: SSG, enable synchronous mode globally. In synchronous mode, frequency changes are delayed until the next loop point in the sample
• FB: RSG, disable synchronous mode. Frequency changes happen immediately.
• FA: RSS, invert the state of synchronous mode just for the next command.
• FC: Keep the existing frequency from a previously played note.
• FF: Immediately stop the channel playing.

TODO: SSG and RSG are swapped in another part of the datasheet. Check which one is correct.

And finally the modes affect how the waveforms are accessed:

Mode	T1 speed	T2 speed	T2 length
000	/2	/2	=T1
001	1	1	=T1
010	/4	/4	=T1
011	1	1	max(T1/2, 16)
100	1	/2	T1/2
101	1	1	max(T1/4, 16)
110	1	/4	T1/4
111	1	1	max(T1/8, 16)

Emulation

It turns out someone already wrote an emulator!

Next steps

I'm looking for ROM dumps of the Amstrad CKX100 (both the samples and the CPU code) so I can learn more about how it uses the chip. Anyone has one and is willing to dump the ROMs?

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