Note: The following post assumes that you're trying to get multichannel audio over a single S/PDIF link (either AC3 or DTS). If not, disregard the rest of this.
Unfortunately, multichannel is beyond the reach of almost any DIYer. It can be done with multiple sources (like a PC and multiple sound cards), but decoding multichannel from e.g. a DVD is prohibitively difficult. Here's why:
- AC3 and DTS are both mired in patents and licensing schemes. As a result, any dedicated decoder ICs require a valid license ($1000s) and are difficult/impossible to source and get data on
- As a result of ^^, any DSPs or similar devices aren't going to come with AC3/DTS decoder firmware. Writing your own is going to be beyond the scope of all but a small few DSP and audio encoding experts
- Small microcontrollers that are easily accessible to DIYers aren't powerful enough to decode AC3/DTS or be an I2S source. Faster microcontrollers can probably handle it (ARM7+), but aren't easy to work with on a DIY scale.
If you did want to pursue this, you do have a couple of options that are slightly easier. You may be able to find a commercial DVD product where the decoder board is separate from the transport and DAC boards and could be easily removed and used in your project. Otherwise, you may be able to figure out how to isolate the decoder section and use it for your purposes. This isn't terribly likely in modern players though, since most use all-in-one chips these days.
Finally, if you have some experience with software development you might be able to source a single-board computer or eval/development board for a fast microcontroller/microprocessor and integrate existing decoding software (rockbox, a52dec etc.).
Long story short, it's not nearly as easy as stringing the chips together properly like most DIY projects. If you already have code you want to load into the DSP, the easiest way is usually to use an EEPROM programmer and have the DSP load the code from the EEPROM at startup. Most DSPs should support such a mode.