I sampled the drum sounds from the V50, and also dumped the waveform ROM. Like most of Yamaha’s sampled drums from that era (circa 1983-1989), the sound set is basically similar to the RX series drum machines. All sounds were recorded in stereo at 96 kHz with the default volume and pan settings.

YM3602 OPRW (1986): This drum IC was used only in the Electone HX models and the V50. It’s not especially flexible, but the sound quality is fairly good. Its advantage over the much more widely used YM2154 RYP4 is that it can use a larger waveform ROM, it uses a simpler serial interface, and its output can be digitally processed and mixed by other ICs. However, the output is stereo only and can’t be demultiplexed into individual channels. This also means that reverb or other effects can only be applied to all sounds globally.

The OPRW IC has 40 pins. The clock is 3.2 MHz, and the sample rate is 25 kHz, with the output being upsampled to 50 kHz using truncation. It’s 8 note polyphonic, with certain sounds assigned to monophonic groups that cut each other off. Unlike other ICs like the RYP4, most of the sounds can be played in any combination. It can address an external waveform ROM up to 512Ki x 8, which at the time was fairly large. The V50 and HX-1 each use a 512Ki x 8 ROM, and the HX-3 and HX-5 use 128Ki x 8. The HX series has mainly acoustic drum sounds, and the V50 uses a different ROM with some of the synthetic sounds from the RX5/RX7 sound set. The IC can apparently play up to 128 drum sounds, although in practice only 63 sounds are used at most (61 drum sounds in the V50, plus sine and square wave tones accessible via the test mode). There are 8 velocity levels, 16 volume levels and 7 pan positions (the volume and pan settings can be programmed for each sound). The IC can play looped waveforms (this is used for the sine and square wave tones only; none of the other sounds are looped), but it can’t transpose or reverse sounds. The sounds are enveloped, but I think this is controlled by the ROM, so there’s no way to change the decay time. Other than the sine and square tones, all sounds have a fixed duration with no dependence on gate time. There’s a single serial output that carries 2 channels. The output format is linear PCM up to 22 bits, but here the dynamic range seems to be limited to 17 bits at most (the YM3017, YM3028 and YM3032 DACs will clip everything above that). Yamaha used this format in most of their products at the time; usually the sync signal is labeled 31-62Y64 (this doesn’t identify the serial format; it just describes the timing of the sync signal). There’s also a serial input that can be used for daisy chaining other tone generator ICs. The serial input is added to the OPRW’s drum sounds and fed to the output. The IC is controlled entirely by Yamaha’s serial data bus, which was used by many different ICs at the time. The use of serial input, output and control signals allows the pin count to be fairly low, despite using many pins for the external ROM.

The ROM contains both waveform and header data. The waveform data is encoded in an unusual way. The first part of the sample is linear 8 bit PCM, but partway through the decay it switches to DPCM. The DPCM portion is scaled up in the ROM to increase the dynamic range. The switchover is controlled by the header data, and occurs after the peak of the derivative has dropped below what can be represented by the rescaled DPCM format. For sample playback, the linear portion of the waveform data is directly loaded into the most significant 8 bits of a 10 bit accumulator. After the switchover, the DPCM portion of the waveform is shifted right by 2 bits (multiplied by 0.25) and added to the current value of the accumulator for each sample. Not all samples are stored this way; some are entirely linear PCM. The resulting sound is then enveloped, but the original waveform data isn’t heavily compressed as it is in many other drum machines. This form of data compression seems to be quite effective. The transition from linear PCM to DPCM isn’t audible, and overall the sound is surprisingly clean, despite storing only 8 bit data.

There doesn’t seem to be much potential for modification or extension of the IC’s capabilities. Attempting to circuit bend the ROM would probably result in a mess, including DC offsets, since the ROM contains the header data used to address the desired portion of the waveform data, and to control the transition from linear PCM to DPCM. It would be possible replace the sounds, and a new ROM could be piggybacked on top of the old one without removing it, using a switch connected to the chip enable pins to select between the old ROM and the new one. But due to the relative complexity of the modification and the limited number of V50 and HX Electone users, this is likely not worth the effort. It’s not possible to get more outputs, and the pitch of the sounds can’t be modified by changing the master clock frequency, since the serial output data has to be synchronized with the other tone generator, mixing, effect and converter ICs.