Sauraen

OPL3 Percussion Mode Map

3 posts in this topic

I've looked for a clear explanation of all the parameters in the OPL3's percussion mode, but I've never found one. So I sat down with my MIDIbox FM V2.0 Prototype and drew a diagram of what parameters influence what sound, with a vaguely accurate idea of how they do so. To be clear, these are the parameters for the two channels and four operators that are made into HH/SD/TT/CY within the OPL3, not any parameters specific to MIDIbox FM V2.0.

 

gallery_10357_214_131042.png


There has been much misinformation circulating on the 'net about these instruments, specifically which instruments' tuning can be controlled.

  • Vladimir Arnost's helpful breakdown of the OPL3 datasheet, the "Programmer's Guide to Yamaha YMF 262/OPL3 FM Music Synthesizer", states that "only Bass Drum, Snare Drum and Tom-Tom frequencies can be set. Cymbal and Hi-Hat frequencies are fixed."
  • TK states in the MIDIbox FM V1 user manual that "Due to OPL3 limitations it isn't possible to use independent frequencies on HiHat/Tom/Cymbal (Bass Drum has it's own frequency and the base frequency of the Snare Drum is static and can only be multiplied)."
  • The Adlib Tracker help page helpfully says "This mode is slightly hard to use (particularly the "SD/TT/TC/HH" tracks) so it is not recommended unless you know how it works or gain familiarity with it after several experiments :-)"

In contrast, I have found that all the percussion instruments' frequencies are modulatable, but the "metal noise" waveform which the HH and CY are based on is generated from some sort of modulation (sounds like ring mod) combining the two channels' frequencies (i.e. the frequencies that also apply to SD and TT respectively), so they are not independently modulatable.

 

This is why there's a picture! This fact is not the least of the oddities of this mode. However, one thing to note: the HH sound is in one way technically the most complex sound the OPL3 can generate, since nowhere else in the synth but in HH and CY is a sound modulatable by two frequencies independently (as opposed to the integer FMults), and HH has the addition of feedback.

Edited by Sauraen

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According to the YM2413 (OPLL) application manual, which is basically an OPL2 with some preset instruments:

 

Channels 7, 8 and 9 are used to synthesize percussion sounds. Five kinds of percussion sounds are generated by using these three channels and six slots. For the bass drum (BD), two slots are used to synthesize FM sounds. Therefore, bass drum sounds can be produced basically by the same procedure from (a) to ©. The other four kinds of percussion sounds (high hat, top cymbal, tom-tom, and snare drum) are described as follows.

 

OPLL includes a noise oscillator obtained by composing a white noise generator with several frequencies. This noise oscillator is specified by the frequency information (BLOCK, F-Number, Multiple) of the 8 and 9 channels. When composed with white noise, phase output suitable for percussion instruments is generated and given to the operator. Thus phases of four instruments are generated from two sets of frequency information. It is known empirically [?] that the optimum ratio of two frequencies is 3:1 (f7CH = 3×f8CH). Now phase data of individual instruments are obtained. Next, we multiply this output with envelope information...

[boldface and brackets are mine]

 

I'm not sure if this is only a characteristic of OPL3 or a misleading statement in the datasheet, but the white noise component of the "metal" sound in Hi-Hat is significantly affected by the choice of wave for the HH operator 33(14): choosing Abs-Sin will make the output only white noise, Sqr will make the output only "metal sound" with no noise, Sin gives a balanced mix of the two, and the other waveforms are somewhere in the middle. Thus, unless they specifically programmed the wave selection value to affect the mixing of a separate white noise generator with the "metal sound", I suspect the white noise is generated by feedback as shown above.

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  • The Adlib Tracker help page helpfully says "This mode is slightly hard to use (particularly the "SD/TT/TC/HH" tracks) so it is not recommended unless you know how it works or gain familiarity with it after several experiments :-)"

 

This is actually the best description which is suitable for most users ;-)

 

Best Regards, Thorsten.

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