technical question: why is the direct digital output via S/PDIF limited to 16 bit / 44.1 kHz?

[marcus]

dear people at brennan.

this question was already asked some weeks ago in another thread but never answered satisfyingly.  unfortunately.  sorry for starting a new thread on that but ...

the starting position: "The B2 was designed to rip and play Red Book standard CDs with 16 bits and 44.1kHz sampling." as paul stated in april.  "The DAC used in the B2 is the limiting factor and downsamples the digital stream from the Raspberry Pi to 16 bit 44.1kHz audio. The SPDIF output is also 16/44.1."

o.k.  (the S/PDIF optical output itself is not limited!  isn't it?)

a direct digital output via S/PDIF should mean the audio data is passed more or less untouched.  theoretical.  well ... for my understanding.  the audio stream is handled by an external DAC anyway.  the B2 should not care about any depth or rate.  even a multi-channel audio file should be transmitted unless the B2 isn't able to play it back.

but if the internal DAC downsamples the hires audio files - for some reasons i would really like to learn - i do not understand why the sound level of the "direct" digital out is not adjustable via UI on the other hand.  the audio data is not left untouched anyway.  ... and the B2 is fading between two tracks (adjusting the sound level of two different tracks at once) if my ears are not tricked by something very special.

my conclusion: as a customer i would expect the B2 to either really pass the audio stream directly (not involving the internal DAC) or to be able to adjust the volume my myself via UI.  i can accept that it's impossible to get both at a time!  =;^)  a former decision doesn't really answer the question why the digital stream is downsampled by design.

ober!schöne grüße,

marcus.

[Daniel Taylor]

I cannot answer for Brennan.  My (educated) guess is that the current design is easier, simpler, and cheaper.

Many times, when you choose a particular chip (integrated circuit) for a design, the data sheet for the chip includes examples of how the chip is intended to be used in a circuit.  Use the chip as intended and you are much less likely to have problems.  To deal with Hi-Res files in a more flexible way would mean using the cheap chip "off-label" so to speak, if that's even possible, or using a more expensive chip.

I'm sure there are others who can answer this question much better than I.  And I suspect you'd prefer a more in depth answer.  Please stay tuned...

[marcus]

hello daniel.

would you assume the internal DAC is part of the chip(set) which is used to provide the digital (optical) signal to the S/PDIF output?  i would ask to deal with hi-res files in a more rigid way: decode it, provide it without modifications via S/PDIF out and let the external DAC handle it =;^)

ober!schöne grüße,

marcus.

[PMB]

Hi Marcus,

The B2 uses the Burr Brown PCM2902C CODEC. Looking at the data sheet it looks like all digital processing is upstream of the DAC so the 16 bit 44.1kHz format is fixed by the PCM2902C.

Paul

Brennan Support.

[marcus]

dear paul.

thank you for the insight.  i wasn't aware of the fact that the DAC is involved generating the S/PDIF output too.  but this circumstance leads me to the "built-in digital volume controller" of the PCM2902C.  would it make any difference looking for audio quality to enable the volume control for S/PDIF output?  at least via override in the UI.  it's on my personal wishlist now =;^)

ober!schöne grüße,

marcus.