[LAU] comparing audio recordings on the command-line?

Jörn Nettingsmeier nettings at stackingdwarves.net
Tue Sep 17 10:46:06 UTC 2013

On 09/17/2013 04:37 AM, Eric Wong wrote:
> Hi, I'd like to compare recording quality of portable recorders.
> I only care about the line-in input of these portable recorders.

if you have a "known good" signal source (that surpasses the quality of 
the unit under test in all aspects you care about), you can measure the 
line-in of your recorder from that.

your best option would be to measure the impulse response of the 
recorder by feeding it a sine sweep from, say, 20 hz to 22 khz (or more, 
if you care about extended frequency response and the device is running 
at above 48khz sampling rate), recording that, and generating an impulse 
response from it. fons adriaensen's "aliki" tool is useful for that, if 
somewhat tricky to use.

a look at the impulse reponse will reveal distortion, and a look at the 
raw sine sweep will give you a pretty good idea of the amplitude 
response over frequency.

then there's "jnoisemeter", also by fons, which can give you a pretty 
good measurement of the noise floor.

but all these measurements either rely on reference gear to produce the 
test signals, or _very_ careful thinking if the test signals might are 
already laden with their own problems.

there's an old saying in german which unfortunately doesn't translate 
well: "wer viel misst, misst mist.", literally, "s/he who measures a 
lot, will measure dung" (as in, will obtain false or misleading results).

there is no such thing as an audio diff, nor would it be meaningful. the 
only practical application of something resembling an audio diff is a 
dual fft that constantly compares the input of a P.A. system with its 
output as picked up by a measurement microphone in the room. you could 
do the same with your recorder, but it's compromised, because the 
program material will not be a good source signal for all aspects of 
audio quality. you can obtain better results with special test signals 
such as pink noise (which is obviously not possible during a show, 
except maybe for some fringe genres, hence the dual fft :)

Jörn Nettingsmeier
Lortzingstr. 11, 45128 Essen, Tel. +49 177 7937487

Meister für Veranstaltungstechnik (Bühne/Studio)
Tonmeister VDT


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