WASAPI and bit depth question

[Miska]

When you go from 16-bit to 24-bit (or 32-bit) by zero-padding, the result is still considered bit-perfect.

 

I don't know why foobar wouldn't allow selecting sampling rate with WASAPI, it is possible with both WASAPI (exclusive mode) and ASIO. It is correct that you cannot choose sample format (bit depth) with ASIO, because ASIO demands application to deliver samples in the format hardware supports, so application needs to adapt to the hardware and not vice versa.

 

With exclusive mode, the windows mixer setting shouldn't have any effect (with some drivers it may still have due to bugs).

 

So no, you are not ruining anything if the bit depth is increased.

 

[Miska]

6 hours ago, audiventory said:

 

In the digital domain it is bit perfect. But for DAC distortions it may be matter.

Because DAC may be buit by cascade scheme (group bits feed separate sigma delta modulators). As rule, it is fairly for any PCM DAC.

So sound may depend on zero-padding size. Though it is very subtle difference.

 

No it won't. The internal DSP pipeline on delta-sigma DAC chips is 24- or 32-bit, so if you input 16-bit it is zeropadded the same. In addition for PCM inputs you have digital filters first (unless you go at 352.8/384k input rates) that will produce 24/32-bit output before feeding the modulator. XMOS USB interfaces use 32-bit sample format anyway. Some others use 24-bit. There are only few that even allow sending 16-bit data over USB. S/PDIF is always 24-bit (24 bits of audio data and 8 bits of aux data).

 

If you can prove me wrong with measurement results, I'm happy to try to reproduce those. I have two different audio analyzers and pile of DACs for that purpose. I do a lot of DAC output measurements as part of my DSP development process.

 

[Miska]

On 10/7/2017 at 5:33 AM, yamamoto2002 said:

S/PDIF sends bit depth (word length) using channel status word. I checked RME Fireface 400 S/PDIF output and found word length == 24 even when playing audio with 16bit WASAPI device bit depth !

 

 

 

 

Yeah, commonly lot of the extra information is ignored by either end of S/PDIF link (or AES/EBU which has somewhat different meanings for some of the same bits). With I2S it becomes irrelevant anyway. With some of the older interfaces used with R2R ladders before I2S the information needed to be set correctly between digital filter chip and DAC chip so that the digital filter knew what bit depth to send to the R2R ladder. This didn't have impact on the input side.

 

[Miska]

On 10/7/2017 at 8:14 AM, audiventory said:

May be "multi level sigma-delta modulator" more correct term, than "cascase"?

 

In this case shifted 16 bit signal may don't touch lower levels of the modulator (DAC's cascades that generate the levels). I can suppose, that each level have own error.

 

Since those have at most 65-levels equivalent of 6-bits in two's complement it doesn't make sense in this scope.

 

But again, since there's a digital filter before the modulator that outputs 24/32-bit samples anyway, it doesn't matter. You wouldn't want to dither or truncate these interpolated samples back to 16-bit because it would partially make the process useless. The interpolation (digital filter) stage will produce information in the LSBs too, even if the original samples were just 16-bit.

 

On 10/7/2017 at 8:14 AM, audiventory said:

DSP and USB is not matter in context of the discussion.

 

It matters because the information about original word length is lost at USB interface level and latest at I2S level. So the DAC chip won't know...