How can I change the size of .AAC or .M4A (iTunes) files exported from Narrator?
Narrator exports presentations as two types of files: .AAC and .(for iTunes exports) M4A. These are audio (sound) files, which use AAC (lossy) compression to encode audio.
Two factors affect the amount of space an audio file occupies on a disk:
- Track Time: The amount of time it takes to play the track.
- Audio Quality: The quality of the sound in an audio file.
Adjusting either of these factors, then, can decrease the amount of disk space required by a file.
Altering Track Time
The simplest way to change the track time of a sound file is to increase a character's speech rate in Narrator.
To change a character's speech rate in Narrator:
- Select the character from the character list.
- Slide the slide adjuster for Rate to the right in order to increase the speed the character reads at.
Changing Audio Quality
The files Narrator exports to iTunes are AAC-encoded with a bitrate of 128kbps with a sample rate of 44.100 kHz. Because AAC is a lossy format, reducing the quality of the file (by manipulating the bitrate and the sample rate) will reduce the amount of hard drive space a file requires--at the sacrifice of the file's sound quality.
In iTunes, it is possible to reduce both the bitrate and the sample rate of a file exported by Narrator by:
- Changing the default import settings for AAC imports in iTunes. This can be accomplished by manipulating the settings under iTunes > Preferences > General > Import Settings.
- Creating a copy of the file using the new setting the file by control-clicking the existing file in your iTunes library and choosing "Convert to AAC" from the contextual menu.