[LAU] USB audio interfaces >= 8 channels

Fons Adriaensen fons at linuxaudio.org
Wed May 16 10:21:31 UTC 2012

On Wed, May 16, 2012 at 11:26:07AM +0200, Robin Gareus wrote:
> @Fons: I remember you were measuring preamps and AD/DA behaviour not too
> long ago (some RME card?!).
> Can one reproduce this using jnoisemeter or does one need special
> hardware equipment?
> Setting the output-volume to -0db, `jnoise` could be used to calibrate
> the input levels, even though it's bad practice to rely on the device
> itself, I suppose it'd work to measure the noise-floor and S/N, right?

It depends on what you want to measure...

For the S/N ratio of a line input all you need is jnoisemeter.

If you want to measure the mic input EIN you need a bit more:

* A passive, balanced and calibrated attenuator, 70..80 dB, and having
  a output impedance of 150 and/or +/- 30 ohm. In practice that means a
  small metal box with 2 or 3 XLR connectors and some resistors inside.
  Details on request. To calculate the attenuation you need a precise
  resistance meter, but only once.

* A calibrated truly balanced - antiphase signals of equal level - line
  level signal output. You could use two unbalanced outputs driven in
  antiphase for this.  You need a sound card with an exactly reproducable
  output gain (just setting it to max will do in most cases), and  measure
  it using real calibrated *audio* RMS meter. Provided you can reproduce 
  the output gains exactly this needs to be done only once, so you could
  borrow the real RMS meter from an electronics lab.

To measure AD/DA phase noise you need either a high quality analog sine
generator, or a soundcard that is at least as good as the one to be



A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)

More information about the Linux-audio-user mailing list