[linux-audio-user] [USB audio capture]: "standard" drivers, "mixers" and level controls

[Mannr at uwaterloo.ca]

Dear audio users:

Previously I asked about USB capture, eg., to laptop, but I still have
some questions:


1. Is there a "standard" USB interface for audio?  For example, will newer
   devices, like "Audiophile", "MobilePre", and "Transit" from Maudio work
   using some kind of standard interface as the "Duo" and "Quadro"?

   I've seen references to "ASIO" or somesuch.  Does that mean they
   will all function with standard drivers?


2. What do the [alsa|au|x]mixer controls do?

   In particular, if I adjust input gain on (analog) line input will that be
   done before or after the A/D?  Note, I may need to adjust line level if I
   am driving from different sources, like phonograph, cassette tape, or
   electret microphones.

   Similary, for mic in, is there a separate control for this, eg., before
   the mic preamp?

   I could not find this information, neither in docs from Maudio nor in alsa
   docs.  Please point me to the appropriate place if this is written down
   somewhere.  What I am looking for is a "block diagram" showing where
   the amps and level controls really are.


3. If mixers do allow level adjustments above, which USB devices have mixers?

   People have claimed that Edirol UA-1A doesn't have a mixer.
   How about Edirol UA-3D?
   What about Maudio Audiophile, MobilePre, Transit?

   Or, perhaps some devices have mixers, but USB control code is not written?


4. Is it usual to have stereo mic inputs on sound cards?
   Maudio "Revolution" (and most soundblasters?): *mono* input.
   Maudio "Transit": claims stereo input
   Maudio MobilePre, Duo, etc.  All naturally have stereo input.


5. Finally, I noticed that many of the Maudio USB products, like Audiophile,
   Quatro and Duo, need external power.  The funny thing is that they
   need 9 VAC.  AC voltage!  What's up with that?  I was expecting
   some standard DC voltage, eg., 9 or 12V, suitable for mobile use.

   Is there a simple way to power these from DC voltage, eg., batteries?  I am
   willing to open one up (and void the warantee) if necessary.  Perhaps there
   is a psu inside and I can just bypass that and give some standard voltage
   like 5V.

   Len at Core Sound made a portable Duo.  Perhaps he can tell us
   how he did it...


Thanks for all the help.  I'm really enjoying this list...

  Richard