I was wondering why my 40GB iPod doesn't hold 10,000 songs like Apple says it should.

One reason is I have some hour-long DJ mixes on it that count as single songs. Another is that the "40GB" iPod has only 37GB of useable disk space.

But the main reason is that lots of my music files were compressed at very high bitrates that waste disk space. These files (maybe 60-70% of my collection) were 2 to 10 times as big as they need to be.

You can compress these files to a lower bitrate and save disk space:

In iTunes, go to Edit -- Preferences -- Importing.

Select "Import Using MP3 Encoder", and set Setting to the bitrate of your choice.

(Note: 192 and 160 are crazy high quality. 128 is near-CD quality. On an iPod, you can't tell the difference. On a high-end audio system in ideal listening conditions, you might be able to tell. I certainly can't tell the difference in sound between my 128 kbps files and my higher-bitrate files.)

In your music library, view the Bit Rate information for your songs and sort them by Bit Rate. You may see some songs have bit rates of 320 kbps or even as high as 1400+ kbps (which is essentially uncompressed).

Select the high bit rate songs (say, everything over 192 kbps, for example).

Then go to Advanced -- Convert Selection to MP3.

iTunes will create new versions of the selected files at the bitrate you chose. If you are doing thousands of songs, this will take a very long time. Let the computer work on it overnight.

Finally, delete the original high bit rate versions of the songs and you are left with your new, compressed files.

This all adds up to massive disk space savings. By compressing all my higher-than-192 kbps files down to 192, I saved about 21GB on my 104 GB library. If I compressed everything above 128 kbps down to 128, I would save about another 17 GB. My music collection would take up only 60% as much disk space as it did before.

It all adds up to more songs fitting on your hard disk and your iPod, as well as faster file transfer times (because the files are smaller). Check it out.