[sdiy] ARM based digital synth.

[Seb Francis]

Eric Brombaugh wrote:
>
> Microchip dsPIC, such as those Seb Francis used in his 4xD have a 
> codec interface built in which can handle AC97 and I2S. I'm not sure 
> about cheap development tools though.
>

You can get ICD2 clones from about 50USD to program/debug dsPICs.  
Here's a link to the Olimex ones:
http://burnit.co.uk/sdiy/index.php?page=4xd
(In the "Use as a general audio-DSP platform" section at the bottom)

Then all you need is a breadboard (although you will find that it's 
impossible to get CODEC ICs in thru-hole, so you will need a surface 
mount to thru-hole adapter as well).

> SPI would be another avenue to check, but I haven't found any SPI DACs 
> that are good enough for audio quality while remaining inexpensive.
>

I looked around quite extensively and didn't find any audio DACs that 
used SPI.  There are high-quality (expensive) DACs that use SPI, but 
these are not specifically aimed at audio.  Also, I don't think SPI 
would be fast enough for multiple audio channels.

Audio CODECs on the other hand are inexpensive and you get the added 
bonus of audio inputs as well as outputs (e.g. for FM modulation, etc.)

Like Eric said, your microprocessor needs to have a hardware based CODEC 
interface (e.g. I2S or AC-Link)  And like you said, you can only use an 
I2S interface for 2 channels, so unless you have multiple CODEC 
interfaces (dsPICs don't) then you are left with AC-Link/AC-97 which has 
a 256 bit data frame: 1x 16bit control word followed by 12x 20-bit data 
slots (i.e. can handle up to 12 channels of audio.)

AC-97 CODECs are inexpensive and easy to find, but you will have trouble 
finding a package that can be breadboarded with a SMT-thru-hole adapter 
- probably you'll either have to make up a PCB or make a custom 
SMT-thru-hole adapter.

Seb