FYI, if you can raise the PWM rate on the arduino output, you can use modulation tricks to increase the resolution. There are a lot of limitations here, but there is probably some degree of improvement possible. Virtually all modern codecs do this to the extreme -- they sample really fast, like 12 MHz, but only use 1 bit of resolution. Modern 24-bit codecs are really only 1 bit on the business end, but really fast. Search for delta-sigma or sigma-delta modulators if you're not familiar.

If you can transmit, say, 10 bits and run your sample rate at ~4x your intended rate, you can probably get about 10-bits of resolution on the output. It requires a lowpass (antialiasing) filter though -- which gets really nasty at sample rates below 40khz -- because then they need to be sharp so you don't hear the aliasing. Modern codecs manage this by shoving all the aliasing noise up at a few MHz, so a simple RC filter is enough to eliminate aliasing issues. And not even necessary if driving speakers/headphones directly.

Anyway, 9-bits is going to be much better than 8, so if you can run the arduino at 88.2k, you could probably get roughly 9 bits of resolution. You'd probably do the upsampling on the linux side -- since the AVR is not exactly bursting with MIPS.

Hmmm, or another idea: what if you use 2 PWM channels and mix the outputs together with an op-amp? You could potentially get much improved resolution that way. Basically, use a summing node on your op-amp with one signal at unity, and the other signal at 1/256*unity. I doubt that you'd ever get near 96 dB SNR (the ideal for 16-bits), but you'd probably do better than 1 channel PWM at 8-bits could do (theoretical best is 48 dB).

Of course, if you're going through the trouble to add an op-amp, you might as well skip the arduino all together and put a USB <-> audio chip in. :-)

-caleb

On Mon, May 30, 2011 at 1:37 PM, Smilen Dimitrov sd@imi.aau.dk wrote:

Hi Sid,

...

Are there any possibilities of extending it beyond 8-bit / 44.1 kHz?

The driver currently exposes a stereo, 16-bit, 44.1 kHz on the ALSA side, since that is the lowest quality with which Audacity works in the playback direction.

However, Arduino Duemillanove's ATMega328 is not really capable of 16-bit reproduction in this context: it has one 16-bit and two 8-bit counters, and the 16-bit one is used to derive the 44.1 kHz (PCM) rate, so the only thing available for PWM (analog) reproduction are the 8-bit counters (there is a bit more on this in the paper on the webpage).

So what happens during stereo, 16-bit, 44.1 kHz playback, is that the driver simply extracts the MSB of the left channel, and sends it as an 8-bit sample representation to the Arduino for PWM reproduction.

So in brief - as far as ALSA interfaces are concerned, I believe it is possible to specify any rate you want (which is supported) - however, "by default", it would be easy to only get 8-bit resolution (hardware) reproduction on the Arduino (however, higher rates should in principle be possible; as I think there should still some of bandwidth left, since a duplex operation can be demonstrated at 2Mbps).

Hope this helps, Cheers!

...

Regards Sid.

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