Podcasts and bit-rates

Reader Gino Vogt’s question allows me to wear two hats—one as Macworld’s Mac 911 guy and the other as the host of Macworld’s podcast. He writes:

Your AAC version podcast is larger then the MP3 version. Isn’t the AAC version based on MPEG-4, which is supposed to provide the same quality but better compression? Are you using higher quality for the AAC version?

Gino, I’d like to believe that our podcast just reeks of quality, but my sense is that you’re not talking about the content. This compression/quality stuff can be confusing. Let me explain.

Bit-rate—the number of bits per second used to encode audio, which is represented in kilobits per second (kbps)—is the determining factor. The higher the bit-rate, the larger the file size and, generally, the better the quality. However…

Quality varies among encoders. Apple claims that an AAC file encoded at 128kbps sounds better than an MP3 file encoded at the same bit-rate (and as good as an MP3 file encoded at 160kbps). This explains why, when Apple changed iTunes’ default encoding format to AAC at 128kbps from MP3 at 160kbps, iPods were advertised to hold more music than they could previously. A 160kbps file—whether MP3 or AAC—consumes the same amount of hard drive space. But make iTunes rip at 128kbps and suddenly the size of a 4-minute track goes from 4.1MB to around 3.7MB.

Okay, so we get that a 128kbps AAC file and a 128kbps MP3 file are going to consume the same amount of space. In that case, what’s going on with our podcast files?

We’re using the same bit-rate for both the AAC and MP3 version—32kbps, mono. I wouldn’t dream of encoding music at such a low bit-rate, but for speech, it’s a perfectly fine compromise—particularly when you consider how long it would take to download 45 minutes of audio encoded at music-quality (and what it would cost to host such large files).

So what explains the slightly bulkier AAC version? It’s a touch larger because it’s an enhanced podcast—one that includes pictures, links, and chapters.

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