MP3 is not the only format for digital audio. There's AAC, Ogg, WAV, WMA, AIFF, and lots more.

MP3 is the most popular format for audio podcasts. It makes for a fairly small file size and is compatible with all web browsers and portable audio players. AAC is a similar format championed by Apple and iTunes. AAC produces higher quality sound at the same files size as MP3. Additionally, AAC allows for chapter markers, changing artwork, and hyperlinks. Unfortunately, AAC encoded audio is not able to be played as universally as MP3.

GarageBand 3 (part of iLife '06 suite of software), Macintosh software for creating music and podcasts, allows for exporting as AAC but not MP3. Of course other programs like iTunes could be used to convert the AAC file to MP3. That extra step is an inconvenience for those who want to publish their podcast as an MP3.

Apple's new GarageBand '08 (version 4 and part iLife '08), has some improvements for podcasters. One of them is the ability to export directly to MP3. By clicking Export Podcast to Disk... from GarageBand's Share menu, podcasters can choose to compress using either AAC Encoder or MP3 Encoder. If your podcast is not enhanced with the extra features of AAC, you might as well use the more compatible MP3 format.

For both the AAC and MP3 encoders, GarageBand offers four preset compression settings: Mono, Spoken, Musical, and Higher Quality.

Below is the information GarageBand displays when selecting the various MP3 settings. I've included the size of a sample podcast for comparison. Notice that a sound file in the Higher Quality Encoder setting is six times larger than the same file exported using Mono Encoder.

Mono Optimized for spoken podcasts. Produces small file size which download fast. 32kbps, mono, optimized for voice. Size: 2.9MB	Spoken Optimized for spoken podcasts. Produces small file size which downloads fast. 64kbps, stereo, optimized for voice. 5.9MB
Musical Ideal for podcasts with voice and music. Download times are moderate. 128kbps, stereo, optimized for music and complex audio. Size: 11.7MB	Higher Quality Ideal for podcasts where music is the focus and even higher quality is desired. 192kbps, stereo, optimized for music and complex audio. Size: 17.6MB.

Shouldn't everyone choose the highest quality sound they can get? Well, the higher the quality the large the file size. Larger file sizes take longer to download and take up valuable megabytes on file servers, in iTunes, and on portable players. If you use a hosting service with limits on storage and bandwidth, larger episode files might use up your monthly allotment.

GarageBand's presets make a really handy reference. In fact, podcasters using Audacity and other recording programs could use these same settings when encoding audio. You'll have to manually input the settings after choosing Preferences > File Formats. Under MP3 Export Setup, change the Bit Rate to match GarageBand's preset. For example, set the Bit Rate to 64 for a Spoken podcast.

Whatever software you use to compress your podcast's audio, I suggest exporting the same sound file with different settings. Have a listen to each one and balance sound quality with file size when settling on a compression setting.

Labels: podcasting