Audacity Forum

Input: wave file direcly imported from a CD
Change tempo: 15%

Result: weird low frequency "beats".

Example: http://thierry.nkaoua.free.fr/Audacity/
100.wav is a piece of the original wave file
115.wav has just "change tempo" applied (15%)

Do I do somthing wrong? Do I have something badly configured?

Best
Thierry

I just tried the same on a windows vista machine, same result

file 115win.wav on http://thierry.nkaoua.free.fr/Audacity/

Nothing "wrong" - it's just an unfortunate consequence of stretching.
The way that stretching works is to cut the music in tiny pieces, space the pieces out as required (duplicating pieces as required) and merging all the bits back together again. This is most definitely not a "lossless" process. The results are better on some types of sound than on others, but generally it gets worse the more you stretch.

Actually, the original (100.WAV) sounds a bit "iffy" to me. Apart from anything else there appears to be a low farting sound, mostly on the left channel - perhaps using a low frequency high pass filter before stretching may help improve the stretching a bit.

Thanks a lot for a so fast answer. I am a bit disappointed, because I use CD recordings of orchestras without the soloist (Mozart Piano Concertos, Music Minus One collection) for training, and sometimes the tempo is really not the one I want.
And effectively, I noticed that this "bad" effect mainly appears when there are "low frequency" instruments (double bass here)
The result is really bad as it is amplified as I play those files on a HiFi system

Could you please give me any hint on how I could use "a low frequency high pass filter"? I am not really an expert in sound editing.

TNK wrote:Thanks a lot for a so fast answer. I am a bit disappointed, because I use CD recordings of orchestras without the soloist (Mozart Piano Concertos, Music Minus One collection) for training, and sometimes the tempo is really not the one I want.
And effectively, I noticed that this "bad" effect mainly appears when there are "low frequency" instruments (double bass here)
The result is really bad as it is amplified as I play those files on a HiFi system

Could you please give me any hint on how I could use "a low frequency high pass filter"? I am not really an expert in sound editing.

It sounds like there might be a bit of vibrato on the double bass, and when you increase the tempo, the rate of the vibrato increases as well. It could be that the tempo change and the vibrato rate are interacting. Or it could just be the that Change Tempo effect is not good at dealing with very low frequencies.

Anyway, select a portion of the track, go to Effect > High Pass Filter. Set Rolloff to 48 dB and Cutoff Frequency to 100 Hz. Listen to the result. Too much bass cut out? Undo and try again with Cutoff Frequency at 80 Hz. Continue until you find something you can live with. Undo then select the entire track then apply the High Pass Filter one more time. Now try changing tempo.

No guarantee that this will work, though.

-- Bill

Thanks for your help!!

I started doing some tests, but I tried something different meanwhile....
I changed first the speed (so the pitch gets up), and the result is good... so the bass vibrato is accelerated but nothing weird happens.
If then, I get down the pitch, then the bad things appear again.

So maybe, it is not the vibrato bass speeding up that is the cause of my problem... I made these tests on the 100.wav file.

TNK wrote: So maybe, it is not the vibrato bass speeding up that is the cause of my problem... I made these tests on the 100.wav file.

Nope, it's not the vibrato bass. It's a limitation of Change Tempo. I tried this on a 50 Hz pure sine wave, change tempo +15%. Note all the non-harmonic distortion, above and below the tone. You can hear the "vibrato" effect when you play back the tone.

All these distortion products are missing when you try this with a 1000 Hz tone.

-- Bill

Thanks for all your work on my problem.
Unhappily, it seems that I have no solution, except renting an orchestra to play my tempo