[LAU] Measuring preamp performance (was: USB audio interface support)

[Rick Green]

On Thu, 26 Nov 2009, fons at kokkinizita.net wrote:

> Two values are important for this : the self noise of the mic,
> and the equivalent input noise of the preamp.
>
  /snip/
> For my Edirol UA-5 I just measured it. For a source (mic)
> impedance of 150 ohm it is around -120 dBV,
   /snip/

Fons, thank you for your clear explanation!  I got quite an education from 
that one post alone.  Makes me want to go out and measure all the preamps 
in my environment.
   So how do I do it?

   Just off the top of my head, I'm thinking of something as simple as 
placing a 150ohm resistor across the input of the preamp, turning the gain 
all the way up, and measurnig the p-p voltage at the output. Comparing 
that to the '0dB' reference level would give the EIN.  Am I even close?

   On a related note, what is the maximum dynamic range of a 24-bit a/d 
converter?  Each 'bit' of precision represents a doubling of the maximum 
input voltage, and I remember being told somewhere along the line that 3dB 
represents a doubling, but I don't remember if that's voltage or power. 
3*24 is 72dB, but that sounds too low.  I remember being told that 16-bit 
CD audio is capable of something like 100dB dynamic range, so I would 
expect 24bit to be capable of something more like 124dB.  And the '0dB 
reference level' isn't the full-scale 24-bit clipping level, is it?  So 
the 'working' dynamic range would be somewhat less.

   I realize I'm babbling on... I guess the ultimate question is how good 
is good enough?  WHat is the EIN figure that would be unmeasurable with a 
24-bit A/D converter?  What level (expressed in dB wrt '0VU' is required 
to make the transition from 0x000000 to 0x000001?

   I'm displaying my total ignorance here.  I don't even know if audio is 
represented by signed integers, or unsigned integers.  Is a shorted input 
represented by 0x000000, and a minimal signal would oscillate between 
0x000001 and 0xFFFFFF, or is it represented unsigned, where a shorted 
input would be 0xEFFFFF, and a maximum signal would oscillate between 
0x000000 and 0xFFFFFF?  What's the binary representation of the '0VU' 
reference level?

-- 
Rick Green

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