[Ned Scott]

How is this not the exact purpose that passthrough was created for? You don't want the inferior dac in the pc to do the decoding so we use passthrough to let the dac/processor do the decoding of the untouched signal. The two signals are very different. Both analog signal or a digital one is sent out of the hdmi, depending on weather passthrough is checked or not. Usb signal to the dac seems to follow this same pattern.

Passthrough was created for a few reasons, actually. For example, some audio receivers can only accept a multichannel signal through DTS or AC3, because they only have SPDIF. The easiest way to get the signal to the AVR is to just use passthrough, assuming the video file has a DTS or AC3 track. It might not have anything to do with quality.

There is no DAC involved, at all, on the Pi when it sends a signal to the USB DAC. You cannot send an analog signal over USB. Data, and only digital data, is sent to the USB DAC/sound card.

USB is simply how the Pi is interfacing with the DAC/soundcard. It's like how a desktop might interface with a soundcard via PCIe.

Maybe I'm way off base and it's more like pcm vs analog in the usb, the point is it doesn't sound like the dac until I enable passthrough. The whole point of passthrough is to pass the unadulterated signal through and let you dac/ processor handle all decoding. I'm just trying to help the guy stick with kodi. The difference is huge. Not all digital signals are equal.

With no passtrough, my processor has only pcm lights on. When you enable it, dts or whatever kind of audio it is lights come on. There is plenty of difference in digital signals.

That has nothing to do with what we are talking about. No matter what setting you use, the USB DAC on a Pi is always the device doing the work, period. It is the device that produces an analog signal. I don't know why your DAC doesn't sound as good if you don't press passthrough. Maybe there's some bug or something weird is going on. All I know is there's no DAC involved between the Pi and the USB DAC, and there's no analog signal that goes over USB. It just doesn't work like that.

Of course, you will see different lights come on when you use passthrough or not. In those cases you aren't even uses the DAC as a "DAC". You are using it as a signal converter, and it is also passing the data through (passthrough), so that your AVR's internal DAC is the device that does the actual digital to analog conversion. That has nothing to do with how the USB DAC performs when it convert a signal to analog.

[behinson]

Just wanted to post in case someone runs across this old thread.  I had the same question as the original poster, whether to use the S/PDIF or Analog output device.  Unfortunately there's some incorrect and misleading information in this thread.

First off, there is a such thing as USB Audio, and it's not the same as USB digital for peripheral devices:

https://en.wikipedia.org/wiki/USB#Audio_streaming

So to say all USB is digital is true, but doesn't tell the whole story because USB Audio is a separate specification which carries signals closer in practice to how PCM works.

So now back to the original question of which output device to use.  ALSA (which is the Linux sound subsystem) is creating 2 devices for the USB DAC when it's attached.  The S/PDIF device is passed through the software device driver and *downsampled* to S/PDIF standards.  The Analog device *bypasses* the ALSA framework, thus providing a bit-for-bit unaltered multi-channel signal.  The S/PDIF is more compatible since it's limited to 48 kHz sample rate, where USB Audio can carry 192 kHz audio.  If your DAC can support it, it's much better to use the "Analog" USB option, as this provides a much better signal.  This is why others have mentioned hearing a difference, and I confirm I hear the difference with my external DAC too.