[sdiy] NCO Jitter (was Large Numbers)

[Richie Burnett]

Should have said "Tim Stilson". Cursed auto-correction :-(

Sent from my Xperia SP on O2

---- Richie Burnett wrote ----

>With clocking speed of 10Mhz or more a raw NCO starts to sound pretty good over the full audio range. (Aliased harmonics are approximately 60dB down for a 10khz sawtooth generated at 10 Mhz.) Higher sampling rates are obviously better though. The NCO in the alpha juno used a 12 Mhz clock if I remember correctly.
>
>But you either need to band-limit and decimate this down to a lower sample rate before outputting it to a standard dac, or alternatively send the signal at the full sample rate of 10Mhz to a suitably fast dac. Otherwise you will incur aliasing as Eric said.
>
>-Richie, 
>
>Ps, I would encourage you to read up about BLIT, MinBLEP, polyBLEP, etc, if your're interested in digital synthesis of classic analog waveforms with reduced aliasing. These algorithms solve the problem of aliasing a more elegantly than just increasing the sample rate. They are a lot more efficient than simply burning CPU cycles by oversampling, which turns out to be a relatively poor method of achieving the end result. Papers by Tim Stilton and Eli Brandt are a good place to start.
>
>Sent from my Xperia SP on O2