Why use speed option instead of tempo -s in SoX for speed perturbation

[Alim Misbullah]

Hi, 

I would like to know why Kaldi developer consider to use speed option instead of tempo -s in SoX to do speed perturbation.

For example,

tempo option:

sox -t wav input.wav -t wav output.tempo0.9.wav tempo -s 0.9

speed option:

sox -t wav input.wav -t wav output.speed0.9.wav speed 0.9

If we use speed option like above, the pitch of audio also will be changed, but the tempo option does not change the pitch.

I tried to use smaller factor value like 0.5 or 0.6 to see the different between speed and tempo option.

Maybe do you have any consideration why we should use speed instead of tempo option.

Thanks,

Alim 

[Daniel Povey]

When we originally developed this method Tom Ko did a bunch of
experiments using different combinations of those kinds of options,
and he found that just changing the speed was the best approach. I'm
sure 'tempo' was one of the things he tried.

Dan

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[Alim Misbullah]

Hi Dan,

Thank a lot for your reply.

I think it is depend on training data. 

I just would like to know:

-   Does the changed pitch of audio will affect the performance?

    Based on my experiment, it will affect the performance.

For information, tempo -s option will not change the pitch of audio even we use small factor value to change the speed of audio.

Thanks,

Alim

[Daniel Povey]

Yes, it will affect the performance, but that's kind of the point-- it helps.