[sdiy] Novation peak NCOs

[Richie Burnett]

Most modern audio DACs (last three decades?) actually work by converting the 16-bit audio input at say 48kHz sample rate into a *much* higher sample rate (MHz) but with a bit depth of just one or two bits. If it's done right the quality doesn't suffer much because all of the resulting noise and distortion is cleverly arranged to be outside the audio spectrum where you can't hear it.

There are a number of benefits from doing this seemingly quite complex operation. Primarily it is much easier and cheaper to make an accurate 1-bit DAC or 2-bit DAC (even though it has to work really fast,) than it is to make an accurate 16-bit one, heaven forbid a 24-bit one!!! Also because the sample rate of the audio was increased to something much higher (interpolation) the demands of the reconstruction filter that follows the DAC are way less in order to get a nice smooth analogue output devoid of steps.

In short, if you raise the sample rate high enough you can get away with just one digital output pin and a simple RC filter to produce sufficient quality audio. That's what Novation have done in their FPGA to save the space, cost, weight and I/O pins required for eight seperate 16-bit parallel input DACs.

The technique is like PWM but there's more to it in order to get high quality results: Dither, noise-shaping, guarding against limit-cycling etc.

It's interesting the limitations of using just a single bit that Brian mentioned. All I can think is that the presence of a bit of low-level noise, limit cycles and distortion are not really inhibiting for the application of generating hot raw oscillator signals that will subsequently be put through noisy analogue filters and VCAs anyway!?!? 

-Richie,

Sent from my Xperia SP on O2

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