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I really would love an open source, hackable FPGA DAC like the Chord DACs. What would be needed in addition to these cool looking boards for that to happen, I g

I really would love an open source, hackable FPGA DAC like the Chord DACs. What would be needed in addition to these cool looking boards for that to happen, I guess to code is pretty complicated for the USB interface to make it appear as a sound output ideally without drivers. Some kind of accurate clock and an output stage? Not sure if actually converting PCM to voltages over time is really that complicated...

IshKebab 6y ago
That's definitely possible. The actual DAC but it's trivial - the hard but is that you would need to implement the USB Audio device class and a USB stack. That's a hell of a lot of code. Also what would be the point?
- Kliment 6y ago
  Luckily there are wonderful USB stacks implemented in open FPGA toolchains these days - check out the toolchains described at https://workshop.fomu.im/en/latest/ https://workshop.fomu.im/en/latest/
  - IshKebab 6y ago
    I wondered if there might be, but did you post the right link? I can't see anything about open source USB device implementations there.
    - ohazi 6y ago
      Kate Temkin's Luna project is super cool, although I think it's still a WIP: https://github.com/greatscottgadgets/luna https://github.com/greatscottgadgets/luna It's a USB stack implemented as pure gateware - no hard/soft CPU needed, although I think you can hook one up if you want to.
    - Kliment 6y ago
      The code linked there is all open, and has a USB device softcore. It's the most approachable FPGA USB implementation I've played with. edit: Here is the actual USB core that's in there: https://github.com/im-tomu/valentyusb/ https://github.com/im-tomu/valentyusb/
- ranma42 6y ago
  > USB Audio device class and a USB stack. That's a hell of a lot of code. Its actually not that much for the bare minimum needed, though the USB docs are pretty daunting. If you go with USB1, you basically just need enough to answer the descriptor and set interface queries and set up the endpoints (in particular the isochronous endpoint for audio is a bit more complex). If you stick to exactly one supported format and sample rate and don't include any control units, you can basically skip all the audio class control messages. For USB1, in a pinch, you can use a cheap generic 24MHz usb logic analyzer and pulseview to decode the raw usb protocol for debugging (though with 24MHz you get some corruption in the capture from clock drift, you need at least 48MHz for more reliable signal sampling). I'd guess you can make do with ~1k LOC in this case.
pjc50 6y ago
> Not sure if actually converting PCM to voltages over time is really that complicated... ! As someone working in this area, the USB interface is the boring standard bit and the actual analogue bit is both difficult and determines quality. For what purpose do you want an open source DAC? Would you be happy with a chip doing USB to I2S for you and then doing the DAC yourself?
- jjoonathan 6y ago
  As someone skilled in the RF space but not the audiophile space -- why are audio DACs still a focus for innovation? The bandwidth, linearity, isolation, and jitter requirements all seem quite modest and distant from the edges of what's possible along every performance axis. Is it linearity at high power that's still a problem? With feedback techniques available, I wouldn't think so, but I could certainly be missing something. What's the sticking point? Or is this more of a marketing game of adding hiss / distortion / lowpass / other non-idealities that perceptually improve quality and convincing people to use your DAC to do it rather than software? It seems a bit unlikely. If I had to guess, it would be that the narrowly defined task of a DAC has been thoroughly solved and commodified while the high end of the space differentiates itself with industrially relevant features like failover, monitoring, durability, and compatibility. But then why the interest in a DIY DAC? Maybe I'm reading too much into this, it's all curiosity driven, and I shouldn't interpret a desire for DIY DACs as a statement that audio DACs aren't a solved problem.
  - nsb1 6y ago
    I would love to know the answer to this for all the same reasons. Is there some actual special sauce or is the world of 'audiophile DACs' just the new 'Monster Cable' FUD?
    - auxym 6y ago
      I don't think there's any special sauce, at least nothing that justifies stupidly high audiophile pricing. ESS "sabre32" DAC ICs are widely regarded as excellent and even integrated into high end audio products, yet they can be bought in small quantities for 12$ each: http://www.shawelectronics.com/es9018k2m.html http://www.shawelectronics.com/es9018k2m.html

IshKebab 6y ago

That's definitely possible. The actual DAC but it's trivial - the hard but is that you would need to implement the USB Audio device class and a USB stack. That's a hell of a lot of code. Also what would be the point?

Kliment 6y ago
Luckily there are wonderful USB stacks implemented in open FPGA toolchains these days - check out the toolchains described at https://workshop.fomu.im/en/latest/ https://workshop.fomu.im/en/latest/
- IshKebab 6y ago
  I wondered if there might be, but did you post the right link? I can't see anything about open source USB device implementations there.
  - ohazi 6y ago
    Kate Temkin's Luna project is super cool, although I think it's still a WIP: https://github.com/greatscottgadgets/luna https://github.com/greatscottgadgets/luna It's a USB stack implemented as pure gateware - no hard/soft CPU needed, although I think you can hook one up if you want to.
  - Kliment 6y ago
    The code linked there is all open, and has a USB device softcore. It's the most approachable FPGA USB implementation I've played with. edit: Here is the actual USB core that's in there: https://github.com/im-tomu/valentyusb/ https://github.com/im-tomu/valentyusb/
ranma42 6y ago
> USB Audio device class and a USB stack. That's a hell of a lot of code. Its actually not that much for the bare minimum needed, though the USB docs are pretty daunting. If you go with USB1, you basically just need enough to answer the descriptor and set interface queries and set up the endpoints (in particular the isochronous endpoint for audio is a bit more complex). If you stick to exactly one supported format and sample rate and don't include any control units, you can basically skip all the audio class control messages. For USB1, in a pinch, you can use a cheap generic 24MHz usb logic analyzer and pulseview to decode the raw usb protocol for debugging (though with 24MHz you get some corruption in the capture from clock drift, you need at least 48MHz for more reliable signal sampling). I'd guess you can make do with ~1k LOC in this case.

pjc50 6y ago

> Not sure if actually converting PCM to voltages over time is really that complicated... ! As someone working in this area, the USB interface is the boring standard bit and the actual analogue bit is both difficult and determines quality. For what purpose do you want an open source DAC? Would you be happy with a chip doing USB to I2S for you and then doing the DAC yourself?

jjoonathan 6y ago
As someone skilled in the RF space but not the audiophile space -- why are audio DACs still a focus for innovation? The bandwidth, linearity, isolation, and jitter requirements all seem quite modest and distant from the edges of what's possible along every performance axis. Is it linearity at high power that's still a problem? With feedback techniques available, I wouldn't think so, but I could certainly be missing something. What's the sticking point? Or is this more of a marketing game of adding hiss / distortion / lowpass / other non-idealities that perceptually improve quality and convincing people to use your DAC to do it rather than software? It seems a bit unlikely. If I had to guess, it would be that the narrowly defined task of a DAC has been thoroughly solved and commodified while the high end of the space differentiates itself with industrially relevant features like failover, monitoring, durability, and compatibility. But then why the interest in a DIY DAC? Maybe I'm reading too much into this, it's all curiosity driven, and I shouldn't interpret a desire for DIY DACs as a statement that audio DACs aren't a solved problem.
- nsb1 6y ago
  I would love to know the answer to this for all the same reasons. Is there some actual special sauce or is the world of 'audiophile DACs' just the new 'Monster Cable' FUD?
  - auxym 6y ago
    I don't think there's any special sauce, at least nothing that justifies stupidly high audiophile pricing. ESS "sabre32" DAC ICs are widely regarded as excellent and even integrated into high end audio products, yet they can be bought in small quantities for 12$ each: http://www.shawelectronics.com/es9018k2m.html http://www.shawelectronics.com/es9018k2m.html