AAC Versus MP3 for Podcasts

Podcasters have to decide which audio encoding format to use for their podcast. There are two main options, MP3 or AAC. At CCNMTL, we have chosen the AAC format. There is much debate on the merits of each format -- many of the arguments relate to their music fidelity -- but for podcasts, some different issues matter. Below, we explain some differences and the reasons for our choice.

Both formats originate from the standards group MPEG (Moving Pictures Experts Group). The MPEG-1 standard, defined in 1991, had an audio component called MPEG-1 Layer 3, hence the confusing contractions to MP3. The Advance Audio Coding (AAC) definition is part of the MPEG-4 standard from around 1999 -- which is why AAC is also referred to as MP4. For a white paper on the formats, see MP3 and AAC Explained by Karlheinz Brandenburg of the Fraunhofer Institute, one of the inventors of the MP3 format.

The MP3 format has a wide acceptance. It is available on numerous devices and systems, making it the current standard. AAC is the newer format. It has recently become much more widespread because it is the format of choice by the iTunes Music Store and the iPod. (It is important to note that the iPod is also compatible with the MP3 format, among other formats. But the iTunes store only sells music in the AAC format.)

From an acceptance point of view, the MP3 format is more prevalent, but you have take into account that iPods hold around 80 percent of the player market and many of the remaining 20 percent digital music players can also play AAC-encoded podcasts using the cross-platform iTunes software. So, let's look at the features of the AAC format that sway many podcasters.

Smaller File Size
AAC encoded files can be smaller than MP3 encoded files. This is important because podcasts are typically downloaded to the user's computer. The quality of the audio files is usually expressed in the number of bits per second used to store audio information. For example, if your acceptable podcast audio quality requires 128 kilobits per second (kbps) using MP3, you can probably settle on a 96kbps AAC encoding without loss in quality, thereby saving about 1/4 in file size. An hour-long podcast at 128kbps would be about 56MB. The same podcast at 96kbps would be about 42MB -- a considerable savings in size and therefore, download time.

Enhanced Podcasts
The AAC format begins to shine when creating enhanced podcasts. If you are a frequent listener of podcasts, you know the value of an enhanced podcast. An enhanced podcast adds markers to the track timeline, making it easier to locate interesting or specific segments. These "chapter" markers can include a caption and an image, which are both accessible from an iPod or iTunes. On an iPod, the chapter markers are graphically displayed on the timeline itself. (Those same markers are not shown in the iTunes window, but instead, iTunes displays an extra menu called Chapters, which allows quick navigation to points in the podcast.)

Speeding Up Podcasts
A simple trick with AAC files allows your iPod listeners to change the speed of the podcasts without changing the audio pitch. This handy feature was originally created for audiobooks, but you can use it for any AAC podcast by simply changing the filetype of your audiofile from m4a to m4b. On the iPod, the audiobooks setting has three speeds: slower, normal, and faster. The faster setting is about 25 percent faster than normal, giving you more time to listen to more podcasts. Obviously, changing the speed is not useful for podcasts that contain time sensitive material, like music.

There are other simple production steps we take to simplify things. For example, we convert most podcasts to mono rather than stereo to save audio bandwidth.

Thinking about the above options will help you create podcasts with your listeners in mind -- and they will appreciate the smaller file-sizes, shorter download times, and a better, more engaging experience with content that can enhance the value of your audio files.