On 27.03.2013 06:50, Andreas Koch wrote:

Hi Daniel

...

Ok. So we could define 3 DSD formats, for 8-bit, 16-bit and 32-bit. On that three formats, arbitrary samples can be transported, and the mapping between the configured sample rate and the resulting DSD rates would be as follows (please tell me if I'm correct):

    352.8kHz       705.6KHz    1411.2KHz

8-bit 2.8MHz 5.6MHz 11.2MHz 16-bit 5.6MHz 11.2MHz 32-bit 11.2MHz

So driver would mark a stream as DSD capable (8, 16, 32bit, depending on the USB descriptors), and then leave it up to the application to do the sample frequency determination according to the above table, and also as stated in the DoP document. Other drivers for more sound cards (presumable on other bus types) might follow.

I think it's ok to handle it like this, because after all, what an application sets as sample rate for the stream is the number of actual words that are transported, where as the DSD rate is referring to bits/s.

Jussi, for your application, that means that you'd have to add these 3 new formats, and teach your application that they can be used for DSD right away.

The question is whether all DSD-capable USB DACs behave correctly in order to make this logic work. Andreas?

If I understand this correctly then your driver would send the DSD data in whatever format the DAC requests via its descriptors. The DSD rate would either control the word length (8,16,32) or the underlying sample rate (1x, 2x, 4x 352.8kHz). That would be the most flexible implementation and I don't see why any hardware implementation would have a problem with that.

The question is whether all DACs would assume the same DSD rate as in the table above (ie, 8-bit, 352.8kHz == 2.8MHz DSD).

And do we actually need a 32-bit format at all or is 8 and 16 enough?

The only potential fly in the ointment is that we assume that the raw data format means DSD. In other words, if a DAC uses raw data for any other purpose with similar wordlength and/or sample rate spec. then we are in trouble. The chance for that is small though.

As there's nothing the UAC2 spec helps us with, we have to live with quirks for that.

Daniel