changing play speed (audible quality loss ?)

[bomber1978]

Hey guys,

If I use Audacity to change the play speed of a WAVE file by about 5% and then at a later time change the speed back to the original speed is there going to be any "audible" quality loss ?

In the past I've corrected the play speed of old master casette tapes of live music, then decided at a later time to make some more adjustments when I no longer have the master cassettes and am just left with the digital transfer I already changed the speed of...

So I'm just wondering if I make a speed change by about 5% and at a later time decide to change it back to around the original speed, will there be any "audible" quality loss ?

Thanks

[steve]

The "Change Tempo" and "Change Pitch" effects cause a noticeable quality loss, but quality loss from the "Change Speed" effect should not be noticeable. There will be some loss in quality even if it is not noticeable, so for best quality I would keep a backup copy of the original (Exporting in FLAC format produces file about half the size of WAV files with no loss in quality, which makes it a good format for archive backups).

[bomber1978]

The "Change Tempo" and "Change Pitch" effects cause a noticeable quality loss, but quality loss from the "Change Speed" effect should not be noticeable. There will be some loss in quality even if it is not noticeable, so for best quality I would keep a backup copy of the original (Exporting in FLAC format produces file about half the size of WAV files with no loss in quality, which makes it a good format for archive backups).

Yeah it's just a pain to have to keep a "back up copy" when you know it's at wrong play speed because I actually have a lot of music... Even FLAC ends up taking up quite a bit of space! haha
Which is why I tend to delete the original once I have made what I think is a correct speed change.
But sometimes at a later time I realise I could make it a bit better, and have to change the speed of a file I have already changed once before...
Yeah I'm aware it's a lossy process, but wasn't sure if it was "audible", particularly when you do it a couple of times.
I know I could check myself, but thought there might be some kind of definitive technical answer as this group is specific to the Audacity programme.
So when you say it "should not be noticeable", do you mean it shouldn't be noticeable even if I change it a couple of times ?
Thanks

[steve]

I would very much doubt that you would hear the difference if you only changed it a couple of times. Why not try it?

[bomber1978]

I would very much doubt that you would hear the difference if you only changed it a couple of times. Why not try it?

Yeah I've been going to try when I have time..., I thought I'd ask the question anyway as I thought maybe an Audacity forum might have people posting who have done detailed analysis on how well Audacity performs this process...
As you know comparing on small computer speakers is different to comparing it on a good stereo system which I play most of my music on, so by the time I play a CD in the stereo, then change the CD to a speed altered version even if there's a significant audible difference it's very hard to make a comparison, especially if it's just a subtle difference...
You seem pretty sure that changing Tempo and Pitch definately results in an audible loss in quality, so I guess you've run through these tests to some degree before.., or at least have knowledge from others info...
Thanks

[steve]

Some more detail:

Yes, I do a lot of testing of Audacity. Yes Change Tempo and Change Pitch cause noticeable sound quality loss, particularly with large changes. Yes Change Speed causes a measurable distortion, but it is primarily "phase shift" which is virtually inaudible. There is also some slight degradation in "transients", but again it is virtually inaudible (to my ears, even using good monitoring equipment). The third thing is a loss of high frequencies - this becomes more noticeable as the amount that the speed is changed increases - for small changes (a few %) the difference is subtly noticeable with good monitoring equipment and good hearing. For larger changes (around 100%) the loss can be very obvious resulting in the sound becoming very dull.

(All three of these symptom essentially come down to a loss of bandwidth)

I believe that the difference between the Pitch/Tempo change and Speed Change is that the first two require "time stretching" (for which Audacity uses "SoundTouch") whereas for Change Speed all that is required is resampling. By default Audacity uses libresample, which is a high quality resampling library. If Audacity is built from the source code it can be built to use libsamplerate which is an even higher quality resampling library, but there are some licensing issues that prevent Audacity from being distributed with libsamplerate.

[bomber1978]

Thanks for the excellent info.
I just sped up a track +5.5% then slowed it -5.21% back to the original speed.
So far with my computer speakers I don't think I can notice any difference between the original and the changed file changed back to the original speed.
I think I'll burn both files onto a CD and try it on my stereo system.
But from the info you give it seems it's unlikely I'll hear anything, which is good to know !
Being able to just change the speed (for the 2nd time) of my final edited together piece of work will be much easier than transfering to digital, normalizing, track marking etc etc the whole thing all together again, which is what I would have to do if changing speed for the 2nd time would cause audible quality loss.
Thanks!

[bomber1978]

I just noticed I have Audacity 1.2.6 and this is the 1.3.x forum.
I guess though it doesn't change at all what we've discussed ?.....

[steve]

As far as I'm aware the same applies to Audacity 1.2.6, though I'd highly recommend upgrading to Audacity 1.3.12.

You can have both versions installed, but Audacity 1.2.6 cannot open projects created by Audacity 1.3.x

[bomber1978]

As far as I'm aware the same applies to Audacity 1.2.6, though I'd highly recommend upgrading to Audacity 1.3.12.

You can have both versions installed, but Audacity 1.2.6 cannot open projects created by Audacity 1.3.x

OK thanks. I think when I installed it some time ago I read that 1.2.6 was the "stable" version and 1.3.x was unfinished work in progress or something, so I thought at the time 1.2.6 was the more 'reliable' version.
Thanks