[Discuss-gnuradio] Re: FUNCube dongle

[Patrick Strasser]

schrieb Moeller on 2011-02-25 17:36:
> On 24.02.2011 15:46, Patrick Strasser wrote:
>> Just like every USB sound interface it does not matter where the signal
>> comes, where it is going and how things behind the interface work. It
>> makes no difference to your application if you connect a converter via
>> cable to your sound interface in your computer or if you have the sound
>> interface built into your converter.
> 
> But I think it's a big difference in signal quality.

For sure. You ged rid of the quality decrease from all the long analog
lines.

Moreover it frees your computer audio interface for AF input and output.
Most computers are not shipped with two audio interfaces.

> Do you know the theoretical limits for the sample rate?

Common USB Audio adheres to USB Audio Class 1, which was specified for
USB 1.1. The data structures allow sample rates to over 4MHz, but USB
1.1 is the limiting factor with its bandwidth:
(12MiBit/second)/(16bit/Sample)/(2 Channels)=384kiSPS. That is without
protocol overhead, so you can theoretical expect 16bit 192kSPS stereo as
maximum. It turns out that not every OS driver supports this data rate,
rather 96kiSPS. Moreover you have to deal with different clock speeds
and other subtleties.
Just putting USB Audio Class 1 on USB 2.0 would not work, because blocks
are structured different between 1.1 and 2.0.

But back in 2005 USB Audio Class 2 (UAC2) was adopted, offering notably
higher sampling rates and bit depths. Apple introduced a UAC2 in Mac OSX
10.6, And Linux has good support since 2.6.34 or .35. Microsoft promised
to implement a UAC2 driver for its upcoming Windows incarnations, but
nothing on the horizon until now.

The SDR widget people sounded every possible combination for the
mentioned OSes, with lots of tricks to get the very best out of every
driver (especially Windows UAC1). They have different firmware for UAC1
and UAC2.

I just read some articles from the SDR widget list and had a
conversation with the Linux driver author, maybe I got some details wrong.

> Can it fill the full USB bandwidth or does it only accept
> "standard audio" sample rates?

You can set every integer sampling rate you like withing the capacity
limit, but most drivers expect the common sampling rates. The drivers
are the limiting factor.
And still synchronization of clocks is a big problem.

Patrick
-- 
Engineers motto: cheap, good, fast: choose any two
Patrick Strasser <patrick dot strasser at  tugraz dot at>
Student of Telematik, Techn. University Graz, Austria