iPod File Format and iPod Battery Life

Hi all,

I have a 5.5G 30GB iPod and love it. I've been ripping music in MP3 at 192kbps. I use MP3 because it makes it easier for me to share music with people who don't own iPods, and I prefer 192kbps because I like the sound quality.

But, lately, I've been wondering something. A lot of the music I listen (jazz and classical) consists of longer tracks, and so in some cases individual tracks are 12, 15, 18 MB in size. How do you think this will affect the battery life? Would I get more hours of battery life if I used the AAC format at a lower bitrate?

I've read that the 30GB iPod has about 32MB of RAM, and so some people recommend not playing large files much if your goal is to maximize battery life. But I don't know how much 32MB of RAM really allows for, how much of the RAM is used by the firmware just to run the iPod, how many songs the iPod will store in RAM, etc.

I haven't had time to run any serious file format/battery life tests, so I thought I'd post this message just to see what other people's experience might be. If it helps any, I've realized the following:

1) I'm willing to give up the MP3 format if I it means more battery life.
2) In running sound quality tests, I found that AAC at 160 kbps sounds very good to my ears. I'm not crazy about 128 AAC, though (but 128 AAC would create much smaller files).
3) I probably save about 1-2MB of space and up to 3MB of space for larger files for each song ripped at 160 AAC. But I'm wondering if that really makes a difference when it comes to the way the iPod uses RAM and how much it accesses the hard drive.

Any thoughts?

mad jew said:

Formatting aside, a lower bitrate will ensure less time the iPod must access the hard drive and therefore should in theory give better battery life. In practice, I reckon going from 192kbps to 128kbps would give negligible benefits in terms of battery life though.

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Yeah, I was sort of thinking along the same lines. I guess my main thought was, in the "real world," so to speak, if going from 192kbps to 128 (or 160) kbps would have a truly noticeable effect on battery life. Even though I haven't done any battery life tests, I don't get the sense that I'm getting the fully rated 14 hours, but that might have as much to do with how I use the iPod (jumping from one track to the next, selecting other artists and songs regularly) as it does with the bitrate. If I went with a lower bit rate, I might not notice much of a difference. I wonder if there would be a noticeable difference going, say, from 320kbsp to 128kbps.

Also, remember that if you reformat an MP3 into an AAC, you'll lose quite a substantial amount of quality since the files are both already compressed and use different algorithms to achieve their state of compression. You'd have to reimport your music from CDs or wherever they're stored as full uncompressed songs to avoid this.

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Luckily, I haven't ripped too many of my CDs into iTunes yet, so re-ripping them into AAC or into smaller bitrates wouldn't take too long. One thing I'm trying to do, though, is settle on a file format and bitrate and then consistently rip everything (instead of ripping music at various formats and bitrates).

I suppose I'm looking for the holy grail that many iPod users seek -- a balance between good sound quality, good bad battery life, and maximum use of storage space.

For what it's' worth, here's some differences purely in terms of file size that I noticed for some of my music:

Miles Davis, "Prince of Darkness"
MP3 at 192kbps: 9.1 MB
AAC at 128Kbps: 6.1 MB (big difference there)
AAC at 160kbps: 7.6 MB (sort of in-between)
AAC at 192kbps: 9.1 MB

Miles Davis, "Sid's Ahead"
MP3 at 192kbps: 18.3 MB
AAC at 128Kbps: 12.4 MB (even bigger difference there)
AAC at 160kbps: 15.4 MB
AAC at 192kbps: 18.4 MB

First movement of Beethoven's Fifth Symphony
MP3 at 192kbps: 10.4 MB
AAC at 128Kbps: 7.0 MB
AAC at 160kbps: 8.7 MB
AAC at 192kbps: 10.5 MB

The differences in file size aren't as substantial for some of the rock music I have. For example, one song at 192 MP3 is 4.7 MB. At 128 AAC, it's 3.2 MB. At 160 AAC, it's 4.0 MB. At 192 AAC, it's 4.8 MB.

I suspect if there's any impact in terms of better/worse battery life, it occurs with those larger files -- especially if I have a full album's worth of long tracks, which as you note requires more access to the hard drive.

Yeah, song changes, menu access -- and something else I forgot to mention, which is that I use the EQ too. That definitely affects battery life.

I did some searching over at iLounge, the dedicated iPod user site. A while ago, one of the forum members did a battery life test and found minimal difference in battery life between file formats and bitrates until he started ripping at above 192kbps. Keep in mind that these figures are for a 4G monochrome 40GB ipod, but in the very least they give a good indication of the minor differences in batterly life between 128 AAC, 160 AAC, 192 AAC, and 192 MP3.

Bitrate Format Battery Life (hours)
128kbps AAC 14 hours
160kbps AAC 14 hours
192kbps AAC 13.5 hours
192kbps VBR mp3 13.5 hours
256kbps VBR mp3 12 hours
320kbps mp3/AAC 10 hours
Lossless ALAC 8 hours

That's even better than I expected. Granted, it's not easy to extrapolate how these would translate to a 5.5G iPod, but I suspect the differences in battery life would be proportional.

At any rate, thought I would post that in case others were wondering, as I was, about format, bitrate, and battery life.

I think I won't worry about 192kbps MP3 versus lower AAC bitrates, though I'm giving some thought to encoding at 160kbps AAC because it would free up some hard drive space in the long run without diminishing sound quality.