[linux-audio-user] Re: 192kHz

fons adriaensen fons.adriaensen at skynet.be
Sun Jan 29 09:36:31 EST 2006

On Sun, Jan 29, 2006 at 01:40:46PM +0100, Wolfgang Woehl wrote:

> I make 2 audio files with 400 and 500 hz sine tones and play 
> them along (out of ardour). There is clearly audible beat 
> frequency, a 100 hz hum and harmonics of that. Checked with 
> jaaa: Adding a 99 hz tone to the mix results in another 1 hz 
> beat frequency.

Did you send the ardour output directly to jaaa or did the
signal go via some analog path ? I just tried the same
with two jaaa instances, one generating 400 Hz and the
other 500, summed via jack and measured in either jaaa.
No traces of 100 Hz.

(Side remark: if your signals go through any analog hardware,
it's better to avoid 100 Hz as a beat frequency. The mains
frequency is 50 Hz in Europe, and I still have to see the
first audio card that doesn't show up traces of 50, 100, 150,
... Hz in jaaa.)

There are several different mechanisms at work here that
can make you hear 100 Hz with your test.

First, in order to hear anything at all, your signal has to be
reproduced by a speaker, and these *do* produce intermodulation
distortion. This can be quite noticeable in such a test, and in
fact that's how it is measured. The sound card and amplifier
will distort as well, but normally much less than any speaker. 

Second, you have chosen two frequencies that have a simple
ratio (4/5). Human hearing will try to 'decode' them as
a single signal, in this case as the harmonics of an 100
Hz one. So you could *hear* 100 Hz even if it isn't present
physically. The effect has been kwown for ages by organ 
builders, who use it to make 'fake' 32" stops.



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