[linux-audio-user] USB audio capture line/mic: A/D quality, how set record levels?

[Mannr at uwaterloo.ca]

I am considering getting a USB audio interface.

My goal is to capture audio for making CDs.  I will capture from line level,
eg., from minidisc or phonograph, and from microphone inputs, eg., for
recording live shows to a laptop.  It should be completely USB powered for
maximum flexibility.  Digital I/O would be great, but is not required.

I am not looking for professional quality.  But I would like something pretty
good, eg., at the level of consumer minidisc recorders.  That is what I am
using to record now.  I don't need any mixing capability.  Plain capture
at 44.1kHz/16bit and optionally 48.0kHz would be fine.

I've seen lots of gear, eg., Edirol, M-audio, etc.  But I have several
questions:

- What is the A/D quality of some of the retail units?

  Retail units:
    Edirol UA-1A, UA-3D (edirol.com),
    MAudio Transit (m-audio.com),
    Soundblaster Extigy, Soundblaster MP3+

  Higher quality?
    Tascam U-122, Edirol UA-20?, MAudio MicPre,

  Professional level (>500USD)
    SoundDevices USBPre
    (http://www.sounddevices.com/products/usbpremaster.htm)

- Is the Mic preamp any good?  Ie., as good or better than the one
  in my consumer minidisc?

- What is the "native" capture mode of these units?

  16bits is OK, but I have heard reports that some, like the Soundblaster,
  capture at 48 or 96kHz and resample for 44.1kHz.  I am looking
  for true sampling at 44.1kHz.

- How can I control record level on these units?

  Some have level controls, but it is unclear what level these control, the
  mic preamp or the line level, or both.  The only one I can see a clear
  "block diagram" for is the USBPre.  This shows separate mic
  and line level gain controls, as it should have.  (See user guide PDF file.)

  If there are no knobs, can the level be set with software?
  Is this what the Linux "xmixer" program does?  The point is that
  I want to:
   - adjust the line level gain *before* A/D is done
   - adjust the mic level gain *within the mic preamp*, to avoid clipping
  So the unit must have internal gain control.


Here is what I have learned so far.

Please help...

  Richard

Encl: list

------------------------------------------------------------------------------
Product        Cost |USB  |LineI/O |MicI/O  |DigiI/O |Sample |Depth |Quality
                     self  level?   level?            (native) 16/24 Consumer/
           (street)  power control  control  exact-bit               Audioph/
                                |       |    copy?|                  Pro
                                    phantom
                                    power?|
------------------------------------------------------------------------------
Edirol UA-1A         Y     I/O, N   none     none     44.1k   16     Consumer
       UA-3D         Y     I/O, Y   I/O, YN  I/O, N   ?(1)    16     Consumer
       UA-20         Y     I/O, Y   none     I/O, N   44.1k   16/24  ?

Edirol UA-5          9VDC  I/O, Y   I/O, YY  none     44/48/96(2) 16/24  ???

Creative MP3+        Y     I/O, N   I/O, NN  I/O, N   48k(3)    16     Consumer
Creative Extigy      Y     I/O, Y   I/O, YN  I/O, N   48k(3)  16/24  Consumer

AFAICT all above units sample at 48/96K and resample for 44.1K.  So they
will add artifacts, and they will not make an accruate digital copy either!

(1) claims 32/44.1/48k.  Is this 48k native sample rate?
(2) "real time sample rate conversion", is this 96k native sample rate?
(3) rumored to be resampled.


MAudio Transit $100  Y     I/O, N   I/O, NN  I/O, ?   ?       16/24  Consumer
MAudio MicPre  $180  Y     I/O, ?   I/O, YY  none     44.1/48 16     ???
              (150?)

MAudio Duo     $350  9VAC! I/O, ?   I/O, YY  I/O, Y   44.1/48 16/24  Pro

Tascam US-122  $270  Y     I/O, ?   I/O, YY  none     44.1/48 16/24  Pro?
              (200?)

USBPre         $700  Y     I/O, Y   I/O, YY  I/O, Y   44.1/48 16/24  Pro


Possibilities:
-------------

MAudio Transit would be great if it had:
- reasonable quality A/D
- true level controls (from software)
- true hardware sampling at 44.1, 48k and 96k.

MAudio MobilePre looks good if I am happy with 16bit sample depth and
no digital I/O.

Tascam US-122 looks good as well, but is more expensive.