Adding Digital Audio to the Sega Dreamcast
And on the heels of the Saturn digital audio mod here's one for the dreamcast.
The dreamcast generates digital audio internally and then passes it to a digital analog converter chip. As it uses an off the shelf d/a converter (PCM1725U) it's possible to interface the digital audio signals to an S/PDIF transmitter IC that will transform the digital audio into a form your amplifier will understand.
Here are the hookups for the Cirrus CS8406 chip I used here.
| Signal | CS8406 | PCM1725U |
|---|---|---|
| EMPH: emphasis - See Note!! | 3 | 12 |
| ILRCK: Word Clock | 12 | 1 |
| ISCLK: Bit Clock | 13 | 3 |
| SDIN: Audio Data | 14 | 2 |
| OMCK: Master Clock | 21 | 14 |
| GND | 7 | |
| +5v | 8 | |
| TXP: S/Pdif output | 26 |
You can hook up a toslink transmitter directly or wire up a coax connector with the same circuit as in the SNES mod.
Note
I looked at the data sheets again. The EMPH line on the cirrus chip is high for no pre-emphasis. Pin 12 on the PCM1725U is high for de-emphasis active. Therefore the signal should be inverted.
Construction
My first mod using the CS8406 was adding digital audio to the saturn. I wired everything directly to the chip. It worked but was a rather nerve wracking experience.
This time I used a soic to dip adaptor board and the soldering was a lot easier. The board adds a bit to the cost but it's worth it. You can't see it in the pic, but there is a small capacitor between +5V and Ground. The chip works without it but it seemed a good idea.
A few notes on installation: I soldered the +5v and GND leads to CF302 rather than the chip as access is a bit easier. When placing the toslink optical transmitter or RCA jack make sure you do a dry run and put the board in the case with the top and bottom rf shields in place. For instance I put the RCA jack around the corner from the expansion port and found that I needed to put the top shield in place before I soldered the orange TXP wire to the RCA jack.
Where to find the parts
CS8406 soic package: Newark
SOIC to DIP adaptor board: NKCElectronics