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

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From: <fons@...>
To: <linux-audio-user@...>
Date: Wednesday, December 2, 2009 - 4:59 pm

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

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

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,

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 ?
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Messages in current thread:
[LAU] USB audio interface support?, Ng Oon-Ee, (Wed Nov 25, 11:35 pm)
Re: [LAU] USB audio interface support?, Allan Wind, (Wed Nov 25, 11:58 pm)
Re: [LAU] USB audio interface support?, Ng Oon-Ee, (Thu Nov 26, 12:13 am)
Re: [LAU] USB audio interface support?, Clemens Ladisch, (Thu Nov 26, 8:21 am)
Re: [LAU] USB audio interface support?, Hans Wilmers, (Thu Nov 26, 9:28 am)
Re: [LAU] USB audio interface support?, Ng Oon-Ee, (Thu Nov 26, 9:36 am)
Re: [LAU] USB audio interface support?, Ng Oon-Ee, (Thu Nov 26, 9:09 am)
Re: [LAU] USB audio interface support?, Clemens Ladisch, (Thu Nov 26, 1:30 pm)
Re: [LAU] USB audio interface support?, , (Thu Nov 26, 10:54 am)
Re: [LAU] Measuring preamp performance (was: USB audio inter..., , (Wed Dec 2, 4:59 pm)
Re: [LAU] USB audio interface support?, Ng Oon-Ee, (Thu Nov 26, 11:00 am)
Re: [LAU] USB audio interface support?, , (Thu Nov 26, 11:23 am)
Re: [LAU] USB audio interface support?, Ng Oon-Ee, (Thu Nov 26, 11:30 am)
Re: [LAU] USB audio interface support?, Ronald Stewart, (Thu Nov 26, 6:50 pm)
Re: [LAU] USB audio interface support?, Ng Oon-Ee, (Thu Nov 26, 11:34 pm)
Re: [LAU] USB audio interface support?, Ralf Beck, (Thu Nov 26, 10:03 pm)
Re: [LAU] USB audio interface support?, , (Thu Nov 26, 10:28 pm)
Re: [LAU] USB audio interface support?, Ralf Beck, (Fri Nov 27, 3:18 pm)
Re: [LAU] USB audio interface support?, Allan Wind, (Thu Nov 26, 4:25 pm)
Re: [LAU] USB audio interface support?, , (Thu Nov 26, 12:14 pm)
Re: [LAU] USB audio interface support?, Lorenzo, (Thu Nov 26, 9:46 am)
Re: [LAU] USB audio interface support?, Ng Oon-Ee, (Thu Nov 26, 9:55 am)
Re: [LAU] USB audio interface support?, , (Thu Nov 26, 11:05 am)
Re: [LAU] USB audio interface support?, Ng Oon-Ee, (Thu Jan 7, 11:46 pm)
Re: [LAU] USB audio interface support?, Ricardus Vincente, (Fri Jan 8, 12:41 am)
Re: [LAU] USB audio interface support?, Ng Oon-Ee, (Fri Jan 8, 1:19 am)