sdm wrote:
I'm wonder why Jaleco used this chip? NES/FC sound chip have DPCM channel for PCM samples
If you're using a lot of PCM, you generally want to put it in a separately switchable bank so that you don't need to freeze the game while it plays. From the perspective of the DPCM channel, there are three kinds of cartridge hardware:
- Boards that hardwire $C000-$FFFF, such as UNROM (Blades of Steel). To play waves outside the fixed bank, the program needs to stop everything and write to $4011 with timed code. Some games on more capable boards do this anyway to use more efficient codecs than 1-bit delta modulation (e.g. Big Bird's Hide and Speak, The Three Stooges).
- Boards that allow $C000-$FFFF to be switched, such as S*ROM, and boards that switch 32 KiB, such as A*ROM, B*ROM, G*ROM, and Color Dreams. Here, the DPCM area is switchable, but the code that runs during playback can't read another bank. The first two Bases Loaded games (USA equivalent of Moero!! Pro Yakyuu) used SFROM and SL3ROM.
- Boards that allow $C000-$DFFF and at least one other 8 KiB bank to be switched, such as T*ROM and some Sunsoft boards. Here, the code can switch a wave into $C000-$DFFF while reading other kinds of data from $A000-$BFFF. These came along later, and Bases Loaded 3 and 4 used TLROM.
(For the sake of simplicity, I'm ignoring the possibility of playing through the vectors and wrapping around to $8000.)