Strange issue with PSX SCPH-100x

Hello !

I have a PSX with a very strange failure. All system is in good working condition. Games booting fine, FMV playback is good and loading time is reasonable for this early model (PU-8 mobo).

Except there is no sound with FMV or streamed BGM. Sequenced BGM and sound fx works, redbook audio too but all games using streamed audio don't works.

Examples :

- In Biohazard : no sound during FMV sequence, and no character voice during game. Only sfx and sequenced BGM.
- Soulblade : no sound during FMV and only sfx during game.
- Parodius : no music during game only sfx

I think there is an issue with MDEC, but I always believed that the SPU who produces sound, even for FMV or audio stream...

Any idea ?

 

My guess is that the CXD1815 is (partially) bad - the ADPCM decoder is embedded in that, and from the description this is what's not working.

It's definitely not MDEC - that only deals with uncompressing the video macroblocks and isn't involved in audio at all. The SPU seems to be OK, since the same interface carries both redbook audio and post-decompression ADPCM, and if redbook works then the SPU side of things must be working.

That basically only leaves the ADPCM decoder and it's associated sample rate converter - and they are both part of the CXD1815 CD-Interface chip. It's a bit of a surprising diagnosis, since I've never seen one of them go bad, but it's the only thing that seems to match the symptoms.

 

Thanks TriMesh
Surprising indeed ! There is a standalone ADPCM decoder beside SPU ? SPU does not decode audio when it's in his streaming form ?

 

What basically happened was that Sony decided to use CD-ROM XA as the basic CD format for PlayStation, and that had a ADPCM decoder already defined as part of the format. Since XA was a generalized format that might be used in machines that didn't have a specific audio DSP, the decode logic for it was integrated into the CD decoder. As the silicon was there already, they decided to use it.