After deciding on creating a wavetable oscillator for the XM8, I've thought quite a bit about the digital oscillator structure. Here is my current solution
Two physical DACs
First of all, I will add a new PCM5102 I2S stereo DAC. This has built-in oversampling etc to prevent aliasing-noise. However, in old synths there were a lot of aliasing, which is part of what we now recognize as the vintage sound. I don't think the PCM5102 can recreate this.
However, my initial idea was to use the DAC8830 on the DCO boards for wave duties as well, and have the DCO MCU do the heavy lifting when it comes to wavetable morphing and other interpolation. The DCO DAC was then followed by an analog reconstruction filter.
The DCO as a DAC proxy
I now have a new idea. Let's instead use the DCO MCU as a simple DAC proxy. By copying the incoming SPI signal to the output whenever the DCO is in "direct to DAC" mode, the MCU doesn't have to do any work. We could probably speed up the SPI bus to max and easily get 44.1kHz (or 48kHz) sample rate on the DAC. The only thing left to solve is when to interpret the DCO incoming data as bytes directly for the DAC - we could increase bit length to 24 and use the first as command, but perhaps that makes it harder to use Teensy Audio. It should be possible to use custom SPI DACs with Teensy audio so let's hope this works. If not, there is still an available utility pin on the DCO that can be used for mode switching.
We would also have to send the chip select for the DAC, but only after two received bytes, not 1 as on the PIC16F SPI.
We can also get rid of the recon filter, after all the only reason we do it this way is to get the aliasing. Also, it would be possible to reduce the bit depth to 12bit if we want etc.
DAC routing
The analog board already has an input for digital waves per DAC, and a switch to select between that and the waveshaper output.
We can reuse that - and in place of the current reconstruction filter, we'll put a second switch selecting either PCM5102 or DAC8830. We also need the circuit to center and amplify the output from the DAC8830. Currently it's at 0 to 3v3. Finally, we need two available pins on the Teensy for controlling the switch.
Modifying the v1.0 Digital voice controller
This requires the use of two additional pins on the recon filter socket - one for the PCM5102 input and one from the Teensy to control the switch. We should also cut the paths from the PCM5102 to the ext input (cut on the digital voice card side).
Summary
It will be possible to use one of three modes per oscillator: DCO, high quality audio DAC or aliasing basic DAC. It won't be possible to combine DCO with the other two at the same time on the same oscillator. I think this is a fair limitation that makes operation of the oscillators simpler - what you see is what you get, if you switch oscillator mode you also switch its output.
