I have finished an application note for a four channel numerically controlled oscillator (NCO). The documentation, library and sketch are at http://tinyurl.com/ydmz2su
The code runs on the Arduino compatible Wiblocks NB1A which has an 168P/328P and a TI TLV5620 four channel DAC. A timer interrupt is used to trigger periodic updates of the DAC channels.
The code documentation is sparse but I believe that the application note is complete. I document how the phase accumulator works and the relationship between the phase increment and frequency. I did not go into the details of the SNR calculations. The two references I used are listed at the bottom of the webpage.
Also, I just prototyped an NB1A MIDI interface that includes a midi-in, midi-out and four two-pole filters. The filters connect to the DAC outputs to smooth the stair-steps of the NCO.
(* jcl *)