==== 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.

{{av:dreamcast_da_chip.gif}} {{av:pcm1725.gif}}


^  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.

{{av:coaxial.gif}}



===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.

{{av:dreamcastda_chip.jpg}}

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: [[http://www.newark.com/jsp/Semiconductors+&+Prototyping/Interface+&+I/O+Controller+ICs/CIRRUS+LOGIC/CS8406-CSZ/displayProduct.jsp?sku=88H6505| Newark]]

SOIC to DIP adaptor board:
[[http://nkcelectronics.com/soic-to-dip-adapter-28p.html|NKCElectronics]]