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Some back-of-the-envelope calculations later I'm looking at about an 8-10khz playback rate. At 10khz I'll be needing to feed the DAC a new sample about every 100uS. Each byte transfer over the SPI bus takes up to 40uS. By breaking up the transfer process into interleavable chunks we should be able to achieve the required rate - but waaiiiit a minute. The DAC requires a 16bit transfer. So I guess I'm looking at 8khz. I know I could get better by using serial EEPROM, FRAM, SRAM, BLUEJAM or whatever. But that isn't the point! I always like to just have a go and make what I've got work.

If you're familiar with transferring data from MMC you'll know that there are command blocks involved in the process. These are 48bit monsters. It might be that with the latency involved in sending the commands and the MMCs reply delay we are looking at even less than the 8khz as a replay rate. We'd need to be pushing a sample out as fast as we can read them which means the time taken for sending command blocks would cause glitches.

I'll have to do some more sums and see how long we can sustain continuous playback with some buffering. With the Arduino's 2k of SRAM we could possibly hold 1024 pre-loaded samples whilst reading a new sector from the card... My head is saying about 3.5 seconds - roughly 50 sectors of 512 bytes.