Work In Progress: Q-Sound

(Español abajo)

This has been my first week working on Q-Sound. CAPCOM abandoned the YM2151 FM chip for the last few CPS titles and started using a PCM-based solution. PCM is the main sound source used in NeoGeo titles too, although the NeoGeo can do FM and PSG sounds too. CAPCOM had already abandoned PSG sounds when entering CPS, and the transition to PCM was sharp too: FM was left behind.

The PCM solution used by CAPCOM was not based on a hard wired ADPCM controller, like that of NeoGeo. Instead they chose a system called Q-Sound, which was implemented inside a special type of processor: a DSP. Note that Q-Sound could have been implemented as a regular digital chip, but they preferred the processor way. So CAPCOM's Q-Sound boards have two processors: a Z80 and a WE-DSP16A. The later is the DSP.

A DSP is a special type of processor. It is specialized in signal processing. It can do general processing too, but it's rather slow at doing it. However, at signal processing, it is very fast. Signal processing can be summarized in two verbs: multiply and add. DSP was a new device back at the time and companies were starting to replace analogue and custom digital chips with these new devices.

My version of WE-DSP16A for FPGA is ready for the first simulations of actual instruction execution. But only a small subset. The smallest subset of the chip that I have been able to isolate for test: RAM and ROM arithmetic units plus a prototype of the control unit. If this goes well, I will go on to add the other system units.

This will still take some time to complete and once completed I still have to link the rest of the Q-Sound board elements (the Z80, the memories)... but the FPGA QSound age is getting closer now.

Español

Esta ha sido la primera semana con el Q-Sound. Este sistema de sonido se basa en PCM, y no en FM o PSG. Representa una ruptura abrupta con el historial previo de CAPCOM. Suena algo parecido a la NeoGeo, que también se basa en gran medida en PCM. Pero la NeoGeo sí puede tirar de FM o PSG a voluntad. CAPCOM los abandonó del todo al final de la CPS y ya no volvería a esa tecnología.

Este chip PCM no está implementado en silicio, con un chip a posta. Si no que usa un DSP. Los DSP son un tipo de procesador especializado en señales digitales. No son buenos para uso general -como CPU de un ordenador- pero para machacar unos y ceros son lo más rápido que hay. Los DSP comenzaron a utilizarse a finales de los 80 para sustituir filtros analógicos y chips especializados y tuvieron un crecimiento brutal. A lo mejor recordáis haber leído DSP en el nombre de una Sound Blaster de la época.

Lo que llevo hecho de este DSP, el WE-DSP16A, son las unidades aritméticas de RAM, ROM y un prototipo del control. Ya lo tengo corriendo las primeras simulaciones y pinta bien. Ahora a ir añadiendo lo que falta poco a poco.

Aun hay trabajo para un tiempo. Y luego tengo que montar el resto del sistema Q-Sound (el Z80, las memorias...) pero todo progresa adecuadamente.