[LAU] USB audio interface support?

[Ng Oon-Ee]

Hi fons, you're 'fons' in jack-devel, aren't you? Thanks =)

On Thu, 2009-11-26 at 11:50 +0100, fons at kokkinizita.net wrote:
> On Thu, Nov 26, 2009 at 05:07:16PM +0800, Ng Oon-Ee wrote:
> 
> > Sorry to sound like a broken record, but could anyone with knowledge in
> > these matters advise me on:-
> > 
> > 1) How much gain is needed by the Shure SM57 for comfortable recording
> > with a low noise floor?
> > 2) Whether the M-Audio Fast Track Pro, Tascam US-144, Alesis IO|2 or
> > Edirol UA-4FX are able to provide pre-amping with that much gain?
> 
> The question to ask is not how much gain would be required.
> All these interfaces will provide enough gain to the point
> that increasing it will not improve S/N ratio.
> 
> The important question is then: how much noise do we have
> in those conditions ?
> 
> Two values are important for this : the self noise of the mic,
> and the equivalent input noise of the preamp.
> 
> For the SM57 the self noise is easy to find: 
> 
> Sensitivity = 1.9 mV/Pa or -54 dBV/Pa
> The reference sound level is -94 dBPa, so the
> mic will produce -148 dBV at that level.
> 
> The SM57 is a passive microphone, so its self noise
> is determined by its impedance which is 310 ohm.
> This will produce a noise voltage of around -130 dBV
> in the audio range. 
> 
> Comparing these two numbers, the result is that the mic
> will produce the equivalent of around 18 dB of acoustic
> noise. For example if you record a sound of 78 dB SPL,
> the S/N ratio will be 60 dB.
> 
> This is a typical figure for a mic of this type, which
> is not really designed for low-noise use such as recording
> classical music or soft instruments. It's used mainly for
> solist voice, amplified instruments and drums. The best
> low-noise mics would be some 10 dB lower.
> 
> 
> The second parameter, equivalent input noise, is the
> noise of a preamp, measured in some operational condition,
> and assuming all that noise is generated by adding it at
> the input. For a good preamp this will be very close to
> reality. It will have the best EIN at max gain, and this
> level will remain almost constant over a large gain range.
> 
> None of the four manufacturers provide this information,
> nor any other data from which it could be computed. This
> is no coincidence: for most 'consumer' type preamps the
> EIN is not very good, and manufacturers want to avoid
> that you compare their performance to the competition.
> 
> For my Edirol UA-5 I just measured it. For a source (mic)
> impedance of 150 ohm it is around -120 dBV, at 310 ohm
> it would probably be around -117 dBV. Now if you compare
> this to the self noise of the mic, -130 dBV, the conclusion
> is that the preamp will produce 13 dB more noise than the
> mic. Not very good, but quite typical for any product in
> this price range. 
> 
> Combining all this, using the SM57 with the Edirol UA-5
> would produce a noise level that is equivalent to 31 dB
> of acoustic noise. This is, in the end, the only figure
> that matters, as it determines what you can record with
> a good S/N ratio and what not.
> 
> HTH,
> 

Thanks for the good write-up. As you mentioned, where I was frustrated
is the lack of information from manufacturers. I guess the old adage
that 'you get what you pay for' is, in the end, true.

I gather, then, that I should be seeking information comparing the ADCs
of the cards I listed, probably from side-by-side reviews. Prices are
competitive with each other (from 139 USD to 190 USD) and within my
budget, barely. Thanks for the help, again.