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

[fons at kokkinizita.net]

On Tue, Dec 01, 2009 at 08:53:55PM -0500, Rick Green wrote:

> 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?

No...

Do you want to measure a preamp with an analog output
or a soundcard which has mic inputs ?

> On a related note, what is the maximum dynamic range of a 24-bit a/d 
> converter?

Each bit adds 6 dB, but you don't start with 0. 
The RMS noise level, assuming a uniformly distributed
quantisation error, is 1 LSB / sqrt(12). The maximum
RMS value of a sine wave is 3 dB below peak. One bit
is used as a sign bit. Combining all this the S/N
ratio for N bits is 6 * N + 1.8 dB.

For a 24-bit converter this would give 145.8 dB, but
that is pure theory. First, the lower bits will not
be very accurate. Second, the digital S/N can't be
better than the analog one. For an ADC with a range
of +/- 1 V, and using top quality analog circuits, 
the S/N will be around 125 dB. Very few converters
reach this figure.

> I don't even know if audio is  represented by signed integers,
> or unsigned integers...

It doesn't matter for S/N ratio. 'VU' refers to an analog
level, in principle it has no meaning at all for digital.
If you use a 'real' (with the correct speed etc) VU on a
digital signal, the 0 VU mark should correspond to something
in between -10 and 20 dB below digital peak. There's no
standard, and the best value depends on the type of signal. 

Ciao,

-- 
FA

Wie der Mond heute Nacht aussieht !
Ist es nicht ein seltsames Bild ?