Audacity Forum

When I use chain to speed up mp3 files I get a background echo. The greater the tempo change the greater the echo.

Some of the files are sampled at 44100Hz & 64kbps and for export Audacity needs them to be sampled down to lower frequency and 16kbps. I have tried 22050 Hz and 24000 Hz but the problem seems the same.

I think this is part of the problem although I sometimes seem to get better file by using tempo change from the menu rather than from chain.

Are there any suggestions please on how I might get around this as I wish to tempo change large numbers of files but need reasonable quality?

Are there other effects I can use to overcome this?

firebrick wrote:When I use chain to speed up mp3 files I get a background echo. The greater the tempo change the greater the echo.

Some of the files are sampled at 44100Hz & 64kbps and for export Audacity needs them to be sampled down to lower frequency and 16kbps. I have tried 22050 Hz and 24000 Hz but the problem seems the same.

I think this is part of the problem although I sometimes seem to get better file by using tempo change from the menu rather than from chain.

Are there any suggestions please on how I might get around this as I wish to tempo change large numbers of files but need reasonable quality?

Are there other effects I can use to overcome this?

I don't see any reason Change Tempo would give a different result in a Chain than applied via the Effect Menu, if Change Tempo is the only command in the chain besides export.

Change Tempo does produce more artifacts the more extreme the change because changing tempo should change the pitch - you are doing time stretching by trying to break that rule. MP3 is a lossy encoding (which means if you re-encode an MP3 even without applying any effects to it at all, you will still degrade it further).

If you have or can get other lossless (WAV or FLAC) copies of these files, that will help, even if you still need to export them as MP3. Keep the export bit rate as high as you can for the file size you want. 16 kbps will give you rubbish quality. Keep the sample rate at 44100 Hz and try at least 128 kbps which will take 1 MB of space per minute. Before exporting via the chain, click File > Export then the Options button and change Bit Rate Mode to "Variable". Compared to "Constant", that will tend to increase the quality for any achieved file size, by concentrating the bits on the harder to encode parts.

Also consider using Time Scale instead of Change Tempo. It will take longer to process but may give you a better result.



Gale

Thank you Gale.

I now certain that the problem relates to export sample rate.

A 15 min podcast I downloaded was 6.6 MB. Using the chained TempoChange it came down to 1.1 MB, and I expect that is impossibly low for the quality I need.

I found the settings you referred to when I process tempo from the menu and it gave me a good quality export 7.2 MB file.

These settings were not available when I chained, but I discovered that once I had set the variable option for a manual export it became the default for a chained process.

So, hurray, problem solved. And no more "Invalid sample rate" error messages either with their prompts to cut back the sample rate.

Thanks also for your suggestion to use Time Scale. It requires Nyquist commands and I will have to bone up on that. But it's useful to know about.

Thanks again for your help ... and thanks to developers for adding this terrific chain feature.

Firebrick

firebrick wrote:Thanks also for your suggestion to use Time Scale. It requires Nyquist commands and I will have to bone up on that.

Effect > Sliding Time Scale / Pitch Shift is a built-in effect, not Nyquist. Select it when you edit the chain by double-clicking TimeScale.



Gale

Thanks again Gale

Sounds so easy.

I started using Audacity about 5 years ago. I haven't moved on much since then but Audacity has!

In the last few days I just marvel at the extra capacity it gives me.

Firebrick