Really fast (50x) playback speed possible?

[ttcheng]

Hi,

I'm using Audacity for listening to really long (24 hour) EEG brain activity files. I'd like to listen to them at very fast playback speeds, like 50x, without using "resample" and "change speed". The "play-at-speed" and "playback speed" on the transcription toolbar are what I'm looking for, but the maximum playback speed is 3x. Is there any way to achieve a faster playback speed? Any help would be appreciated.

Tom

[kozikowski]

you can get 'fake' speed change by changing the sampling rate the "wrong" way.

Black down arrow to the left of the track > Set Sample Rate.

If that works, I think you can export the track, import it and do it again.

Koz

[steve]

without using "resample" and "change speed"

Why those constraints?
The way to do it is to use the Change Speed effect.

[ttcheng]

A couple of reasons.

1. It takes a long time to resample, change speed, export, import, etc. I'd like to change the speed quickly and on the fly. For example, I'd like to listen to the file at 10, 20, 30, 40, and 50 times normal, and switch between them. Scan through at high speed, then replay at lower speeds when I find something interesting.

2. If I resample and change speed, my time offsets within the file change. I'm marking events using labels, exporting the labels as a csv, then importing the labels into excel. I'd like the time offsets to be in real hours, minutes and seconds.

[steve]

I see. No Audacity can't do that.

What's the difference between "something interesting" in the recording and the rest of the recording?
Is it anything that will show up on either the waveform or the Spectrogram?

[ttcheng]

"Interesting" things are visible in the both the waveform and the spectrogram, but scanning by ear is much faster than doing it visually. I'll zoom in to corrobroate events that I detect by ear.

[steve]

You know that you can click on the time line (above the audio track) and playback will immediately start from the position that you click?
You know that you can zoom out so that the entire waveform fits in the Audacity window so that you can see all of it at once?

If you have a wheel mouse, Ctrl+mouse wheel will zoom in/out. Shift + Mouse wheel will scroll right/left (if you're zoomed in).

[ttcheng]

Yep. Got all that. Remember we're talking about 10-50x zoom levels and 24 hour recordings. It takes a very long time, like a few hours, to review the file visually at a useful zoom level, which is about 90 seconds wide. By ear at 50x, a first pass can be done in a matter of minutes.

Should I think about modifying the transcription toolbar and playback speed code?

[steve]

Should I think about modifying the transcription toolbar and playback speed code?

That's one of the beauties of open source software. If you know a bit about programming you're free to modify it to suit your own requirements.
Unfortunately I don't think that I'll be able to help with that as I'm not a programmer, but if you do decide to do this it might be nice if you posted something here on the forum about what you do as it could be helpful for anyone else that wants to build the same feature.

There's some information about building Audacity from the source code here: http://wiki.audacityteam.org/wiki/Developing_On_Windows

One thing that occurs to me about your proposal is that the data throughput from hard drive to sound card will need to be 50 x faster than normal, so I hope that your computer is running efficiently.

[steve]

2. If I resample and change speed, my time offsets within the file change. I'm marking events using labels, exporting the labels as a csv, then importing the labels into excel. I'd like the time offsets to be in real hours, minutes and seconds.

Another thought - if you know how much you have speeded up the playback, then it would be a simple matter to convert the label times from a sped up track to real hh:mm:ss in excel. This could then be converted back to text and imported back into Audacity as a new label track (that should then line up correctly with the original track).