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Last modified on 11 April 2025, at 08:21

Willy

Willy - MIDI and OPL3 Soundcard adapter for CPC

It is the spiritual successor of the CPCISA project.

Willy1.jpg Willy2.jpg


Demonstrations


Weblinks


Compatible Serdaco soundcards

Note: Like the MIDI synth functionality of the RSF3, the S2P soundcard natively supports Wavetable MIDI. It doesn't require a software translation layer like the AMSDAP OPL4 & YRW801 ROM solution.


I/O Ports

2 ports are used to control the MIDI soundcard: &FEAC for data, and &FEAD for control and status. Sadly, that means it is not compatible with the EMR MIDI Interface which is located at ports &F8F2 and &F8F3.

3 ports are used to control the OPL3 soundcard: &FEBC and &FEBE are register select ports, and &FEBD is the data port. Again, we can regret it doesn't use the same ports as the AMSDAP OPL4.


OPL3 sound chip

The OPL3, released by Yamaha in 1990, is an upgraded version of the OPL2, which came out in 1985.

OPL3 is register compatible with OPL2, with twice the number of signal sources, 4 new operator modes, 4 new selectable waveforms, and stereo output.

OPL3 offers 18 FM 2-op chans in its standard mode, but that can be configured:

  • One setting converts 3 of the FM chans into a 5-chan FM percussion set
  • Another setting pairs 12 of the FM chans into 6 FM 4-op chans
  • Both settings can be used at the same time, to obtain 6 FM 4-op chans + 3 FM 2-op chans + 5-chan FM percussion set


Basic Components

The most basic elements of an OPL FM synthesiser are:

  • envelope generators, which vary volume over time;
  • phase generators, which produce a waveform such as a sine wave at a programmatic rate;
  • operators, which combine an envelope generator and a phase generator;
  • a global low-frequency oscillator which can provide additional vibrato (i.e. back-and-forth modification of frequency) and tremolo (like vibrato, but for volume) to operators;
  • channels, which combine operators to produce a single tone output;
  • a distinct rhythm section, which incorporates a noise generator with some of the parts above to provide a few fixed percussion noises.


Operators

On an OPL1 or 2, each channel consists of two operators. On an OPL3 it may also consist of four. In either case, the basic FM synthesis operation is the same:

The output of one operator is used to modulate the current phase of another. The former is known as the modulator, the latter as the carrier.

In two-operator frequency modulation mode, the output of the carrier, after modulation, is the audio output.

In four-operator mode there are some more complicated options involving routing outputs between operators.


Algorithm

The OPL chips generate sound efficiently, without using any multiplication or other costly arithmetic operations. Instead, they use two look-up tables (exp and logsin) each containing 256 entries.

The tables are used in this formula for 2-op "frequency modulated" (actually phase modulated) synthesis: 

out = exp(logsin(phase2 + exp(logsin(phase1) + gain1)) + gain2)

Exponential table

x = 0..255, y = round((power(2, x/256)-1)*1024)

When such a table is used for calculation of the exponential, the table is read at the position given by the 8 LSB's of the input. The value + 1024 (the hidden bit) is then the significand of the floating point output and the yet unused MSB's of the input are the exponent of the floating point output.

Logsin table

x = 0..255, y = round(-log(sin((x+0.5)*pi/256/2))/log(2)*256)

This is the first (rising) quarter of sine wave. The rest can be constructed by flipping all the bits of x and/or by changing the sign of the samples.


Waveforms

The original OPL only has access to the sine wave.

From the OPL2 onwards other waves are also available, specifically a half sine (i.e. the positive part only, silence for the second half of the period), rectified sine (the positive part repeated twice per cycle) or quarter sine (just the first ramp from 0 to 1, repeated twice, providing harmonics a lot like a sawtooth).

The OPL3 goes even further, adding some extra sine options plus a square wave and a logarithmic sawtooth.

Waveforms OPL3.jpg


FM Percussion Set

FM synthesis does a great job relative to its hardware requirements at reproducing melodic instruments but isn't especially useful for harsh percussion noises like drums. Yamaha provides separate drum noises for this purpose. The programmer has to forfeit three channels of regular FM synthesis, freeing up six operators for five rhythm instruments.

A global random level generator is implemented for this mode; it's a standard LFSR just like the noise generator on simple chips like the AY-3-8910 or SN76489, so it provides a one-bit semi-random output. Source

The OPL can then generate:

  • a bass drum, which is just another FM noise;
  • a tom tom, which is the unmodulated output of a single operator;
  • a snare drum, which combines the phase of a single operator with the random noise bit;
  • a cymbal which XORs six bits from the phase of two operators;
  • a high-hat, which uses the same XOR as the cymbal and combines it with the random noise bit.


OPL3 undeserved reputation

The OPL3 soundchip has been underused and unfairly criticized, largely due to 2 factors:

  • As OPL3 is compatible with OPL2, PC game producers did not make the effort to specifically exploit the OPL3 soundchip capabilities, instead they relied on inferior OPL2 tunes. These are the equivalent of the Speccy ports that gave the Amstrad CPC a bad reputation.
  • Western PC game developers in the early 90's also tended to produce music by using a master MIDI track on Roland MT-32, and then downgrading it for the various PC soundchips. The result was a cheesy so-called "MIDI" music. That was unfair for OPL3 (but also for MIDI, as Wavetable MIDI sounds way better than that).

Examples of proper OPL3 music: MoonDriver Paul the Penguin Zen Challenge


Links