[sdiy] I2C DAC

[Olivier Gillet]

> Would I be correct in saying that these features are more important when
> playing back sampled material vs generating signals like a digital OSC?
> Oversampling a 12bit sine wave at x256 that is 48kHz and interpolating
> between steps and then outputting at the same sample rate 48K does not
> create any new data..or does it.

I am not sure what you refer to, but I don't know any conversion
device in which a signal is oversampled and then immediately
downsampled.
- Sigma-delta converters internally oversample (interpolate) the
signal in the digital domain to generate a high-speed 1-bit stream
which approximates the original signal once low-pass filtered.
- Some "traditional" (ie not 1 bit internally) converters might
oversample to get more resolution or to make reconstruction filtering
easier (the internal oversampling makes the steps finer and easier to
filter: you don't need a super steep filter to remove the aliasing).

I don't see how there would be a difference between synthesized audio
and samples read from a ROM. None of them are meant to contain data
above sr/2, so both of them will need a reconstruction filter.

However, if you're after the sound of early digital synths (PPG,
Prophet VS...) you'd better go with the rawest control/instrumentation
DACs - because those early machines did not have reconstruction
filters - which would have been tricky to implement I guess on devices
like the PPG wave in which sample transposition is achieved by
adjusting the clock rate rather than resampling (source:
http://www.vintagesynth.com/sci/birth.txt,
http://www.electricdruid.net/index.php?page=info.wavetableoscs). And
no, a SSM2044 doesn't count as a reconstruction filter ;)

> Would the sound be any better using an audio DAC vs a voltage out control
> type dac given the same sample rate and bit depth?

Might depend on what kind of "fidelity" (high fidelity to the
"theoretical" waveform, or high fidelity to classic digital synths)
you're after!

Another thing: more than DAC quality, something that will directly
impact the quality of your synthetic waveforms is whether they are
band-limited or not in the first place. If you work at 48kHz,
(square_output = phase_counter_16bits < 0x8000 ? 0 : 4095) is going to
sound ugly no matter what kind of DAC you put behind.

Olivier