[sdiy] ARM based digital synth.

[Eric Brombaugh]

Alwyn Lloyd wrote:
> Hi all,
> 
> Whilst browsing around the internet at interesting components (as you do)
> i came accross
> http://www.futurlec.com/ET-ARM_Stamp.shtml
> 
> Its a ~58MHz ARM on a carrier board with a nice feature set, such as spi,
> i2c and 2 uarts..
> oh, and its cheap :)

Looks like a nice development system! Lots of useful I/O, fast and 
definitely inexpensive. You can get cheap/free compilers for ARM7 too.

> after seeing the projects based around the AVR's such as the AVR-synth and
> AVR-X, this obviously there is a fair bit of power avaliable.
> I have done an overview here:
> http://zarquin.ucc.asn.au/files/ARMsynthoverview.jpg

Nice drawing.

> What remains in my overview planning stage is how to connect the
> audioCodec to the MicroController.  I'd really like to have multiple
> dac's, though most chips require multiple i2s like interfaces to do this.
> 
> One option i have thought about is using an AC97 codec to do this.  the
> ac97 spec has up to 6 dac's as part of the specification, so thats cool.

There are a lot of decent quality and inexpensive AC97 codecs available. 
The big problem you'll run into is that you really can't bit-bang the 
AC97 interface in software - you'll need some sort of customized 
hardware to talk to it. Might I suggest an FPGA or CPLD?

The Analog Devices ADuC70XX family of ARM7 processors has high-quality 
12-bit ADCs and DACs built in (full parallel access), but 12-bits is not 
audio quality and you'll not find and inexpensive development system for 
those chips like the Futurlec one mentioned above. The max clock speeds 
aren't as good as the Philips parts either.

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.

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.

It sounds like a neat project - I'd like to see what you come up with.

Eric