Note: This is the second post of a two-part series.  Part one explained the benefits of changing the tempo of your spoken-word audio (audiobooks and podcasts) to listen to more content in less time.  This post will explain how to use Audacity to change the tempo of your audio files.

Audacity is a free, open-source audio file editor, and yet is one of the most powerful tools out there for recording and editing audio.  The software itself has too many features and options for me to really go into in detail in a single post, so I'll focus only on one specific feature that relates to this series of posts – the "Change Tempo" feature.

All of the below assume you've already downloaded and installed Audacity.  Although there is a way to change the tempo of a single file in the version 1.2.6, this is unwieldy and requires a lot of user interaction.  For the method I describe below, you MUST have the latest Beta version (currently 1.3.6 as of this post).

Install LAME (if necessary)
Although you may have installed Audacity, you may not have installed the LAME codec, which comes separately from Audacity (due to licensing restrictions).  Audacity has instructions on how to install this codec here – once you've got this installed, you'll be able to export audio files to new .mp3 files.  Optionally, you may choose to set up your file characteristics for the .mp3 exports (see next section).

Set Up MP3 Export Options (optional)
Export options can only be configured when you are exporting a file.  Once you set them up, that configuration will remain until you change it, so it is worth setting up your export options the first time, and then you can leave them as-is from then on.

1) To be able to export a file, you first have to have a file loaded.  You can open a file from somewhere on your computer using the File->Open option, or just drag and drop to the window.  Choose a short file (less than 5 minutes) for your first configuration to save time.

2) Once the file is open, use the File->Export menu option to bring up the export dialog.  From the "Save As" drop-down menu, choose "MP3 Files" option.  Then click the "Options" button to get the configuration dialog.  This will let you choose details like bit-rate, stereo/mono, etc for your audio files.  Most spoken word audio does fine at 64 kbps, but you can bump that up to 128 kbps if you prefer.  Remember, the higher the bit-rate, the larger the resulting files will be.

3) Click OK, and export the file to a temporary location (like your desktop).  Once you've done this, the settings you've chosen will be used for all mp3 exports from then on, unless you go back and change it again.  You can now delete the file you exported to your desktop.

Set up Chains
Batching processing in Audacity is done by "Applying a chain".  A chain is simply a set of instructions (much like an Audacity-specific script) that you want Audacity to perform on the files you select.  In this case, the chain consists of the following actions:

1) Import a file into Audacity
2) Change the tempo by X%
3) Export to a new .mp3 file

When applying a chain in Audacity, importing the file is done by default.  The rest, however, need to be set up in a chain.  A chain only needs to be created once, and then can be used any number of times.

You can manually set up chains via the "Edit Chains" menu option under the File menu, or you can simply copy pre-written files into the Audacity data folder and the software will make them available for use automatically.  I've pre-created 10 tempo-change chain scripts for you and located them in this .zip file here.  Just download to your computer and extract to the Audacity data folder to make them available to use.  The folder, on Windows, is usually located at:

C:Documents and Settings[your windows profile name here]Application DataAudacityChains

These files are called "SpeedUp_10_Percent" through "SpeedUp_100_Percent" – each changes the tempo by a corresponding percentage.  To get an idea of scale:

10% increase means you listen in 91% of the time
40% increase means you listen in 70% of the time
50% increase means you listen in 66% (2/3) of the time
100% increase means you listen in 50% of the time

I personally like the 40% tempo increase – it makes it quick enough that I save time (about 1 hour for every 4 of original audio), but still slow enough that I can catch everything that is said.  However, your mileage will vary, and you'll want to experiment with different rates for different podcasts until you find the speeds you like.  Now that you have the chains available, you just need to apply a specific chain to some files in order to get your sped-up audio ready to listen to!

Apply Chains to Speed Up Audio

Now that you've got the chains all set up, you can do the processing on the audio files:

1) From the File menu, choose "Apply Chain" and select one of the "SpeedUp_X_Percent" chains.  Click the "Apply to files" button.  This will bring up a dialog box asking which files you want to speed up.

2) Navigate to where your files are stored.  Select the files you want to speed up.  You can select more than one file to speed up in one batch in the same way you select multiple files through your operating system (e.g. hold down the control key for Windows, Command key for the Mac, etc).  The chain will be applied to each file in turn, so you'll end up with all the files sped up by the same amount when you're done.

3) When you click the "Open" button on the dialog, Audacity will begin processing the files, one at a time.  If you watch the progress bars, you will see Audacity import a file, speed it up, and then export the file, then repeat the process with the next file.  Each file that is exported is saved in a folder called "Cleaned" that is located in the same directory as the original file location.  It will have the same name and ID3 tag information as the original file.

NOTE: Audacity creates some very large temporary files while processing.  These files only get deleted after you close the program down.  This means you COULD run out of disk space if you try to apply a chain to too many files at once – I recommend applying the chain to about 2-3 hours worth of audio at a time (the number of files will vary depending on their length).  After that, close Audacity (say "no" to saving the temporary or project files when prompted) and re-open and repeat the Apply Chain process with a new batch of files.

That's pretty much all there is to it.  At this point, if you know the file is in good shape, you can delete the originals and load the new files into your audio player or library.  If you want to reprocess the file at another speed, be aware that Audacity will overwrite the file in the "cleaned" directory if you apply a chain to a file with the same name.  To avoid this problem, either rename the original file prior to reprocessing, or rename (or move) the file in the "cleaned" directory.

I usually set up files to process in the background while I'm doing other things.  For me, this is definitely a worthwhile effort.  I invest about 20 minutes time in the file manipulation over the course of an afternoon of Audacity processing, thereby saving about 6 hours worth of listening time for every 24 hours of podcasts and audiobooks I listen to.  If you can stomach speeding up the audio even more, you can get even bigger time savings.  Pretty soon you won't ever want to listen to spoken word audio at the normal rate, ever again!

[NaBloPoMo 2008 – #24.2/30]

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