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

On Thu, 26 Nov 2009, fons@email-addr-hidden 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
"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."
                                   -Benjamin Franklin
"As for our common defense, we reject as false the choice between our
safety and our ideals."
                                -President Barack Obama 20 Jan 2009
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