Saving battery life on your iPod

Smart Playlists
One of the tips I was given when I got my iPod was to try and limit yourself to songs that are less than 9 minutes long.

Now, I realize that that tip is predicated on what bitrate the music is encoded at, but as I use the default 128bit AAC, I've created a smart playlist that helps with that.

Time is less than 9:00

Actually, as this is a random playlist, I also have a second line that excludes songs that are shorter than 1:00 as well.

by Unseelie on Nov 03 | 11:19 pm


okay, simply:

Time - is in the range - 1:00 - to - 9:00
Bit Rate - is less than - 128 - kbps

Good playlist idea!

As for you track time segment of your smart playlist....

I agree that the more times you access your iPod HD, the more energy is drained from the battery. But when you have playlists running *without interruption* doesn't the iPod software independently over-sample several minutes in advance, regardless to the track lengths?

So if you really want to save on battery life, isn't the key simply not to manually switch your tunes, just manage your tunes via playlist.

As for Bit Rate... will this add more than a few minutes on a 8 or 10 hour battery?

I'm very curious.

Anyone know how much power a 128 versus a 192kbps song will draw?

by nate on Nov 04 | 12:56 am

I would disagree with the statement.
Assuming all songs are equally encoded...
You are basically saying between that these two options:

*playing 2 songs of 9 minutes


*playing 6 songs of 3 minutes

the second one is more power efficient.

I would say the less songs to play the better. Hopefully the files are not fragmented all over the place in the HD. The less the HD heads are moved the better the battery performance is.

In the second option you have to retrieve 6 different songs in which there are no assumptions of how the files are stored in the HD.

Please correct me if I'm wrong.

by amarre on Nov 04 | 1:08 pm

Me? I just try to avoid songs longer than 9 minutes. I did some checking, and the actual cache size is 9mb. This holds, roughly, with 9 minutes at 128kbps as at 128kbps it's about a meg a minute.

I think that this will serve better as a smart playlist, knowing that.

Size is Less Than 9mb
Time is Greater Than 1:00

Again, the 'Greater than 1:00' is a personal preference.

by Unseelie on Nov 04 | 2:23 pm

amarre's comment:

In comparing more than one file equalling the playtime of one file. (with the same bit-rate and encoding)

I'd agree with you that the fewer songs you play the more battery life you will preserve.
(on a drive with low fragmentation and reasonable directory integrity)

Unseelie's comment:

I really don't know about how your playlist saves energy(the less than 9mb part). Can you explain?
If your iPod is playing off a playlist to begin with. Isn't it going to automatically read ahead 9mB and load the music into cache? So even as you are playing (let's say) a 120 minute track isn't continuing to load the song into cache 9mB in advance? What difference does it make? If it's 9 minutes or less isn't it always looking 9MB ahead?

by nate on Nov 04 | 3:48 pm

Well, I could be wrong... but Apple says the same thing, so I'm trusting them. However, a 21 minute song will have to hit the drive a minimum of three times. If nothing else, this will avoid skiping.

What I don't know, but what's implied by Apple's statement is that it's possible that the iPod caches up to 9mb at a time, but only one song, not 9mb of songs.

Perhaps this means that if a song is longer than 9mb, it doesn't cache at all...

Yes, I'm guessing... and relying on Apple's statement actually being true.

More homework may be required.

by Unseelie on Nov 04 | 4:06 pm

okay so I looked up Apple's tech notes:

According to article 93318-

iPod has built in skip protection

In addition to its hard drive, the iPod has a cache of solid-state memory (RAM). This memory has no moving parts and thus is not affected by movement of the device. Skip protection works by preloading this cache. iPod then plays music from the cache rather than directly from the hard drive.

The RAM cache represents about 25 minutes of playing time when used with a compressed file encoding such as MP3 or AAC at typical bit rates. But you would get dramatically less than 25 minutes when using AIFF format, for example, in which a song that is only a few minutes long can fill the entire cache. This means that the type of file encoding together with you usage habits determines whether the cache will represent three minutes, 20 minutes, or its maximum of 25 minutes of music.

Maximize skip protection

To get the maximum amount of time out the the skip protection, the songs on the iPod should be encoded so their file size is small. The more songs that the iPod can preload into the memory cache, the longer the skip protection. For example: Importing songs as AAC at 128 kilobits per second (kbit/s) rather then AAC at 192 kbit/s will increase the amount of songs the iPod will be able to store in the memory cache.

Minimizing disk access also will increase the amount of skip protection. Minimizing how many times you press the forward button will allow the iPod to play the songs that are already stored in the memory cache, instead of having to load a new song from the hard drive.

So I guess we should revise that playlist to:

Time - is less than - 00:25
Bit Rate - is less than - 128 - kbps

or something similar. I wish Apple would just give us the true data.

by nate on Nov 04 | 4:35 pm

I thought the cache was 32 MB, so that files larger than that would spin the drive until the entire file was cached and then go on the the next. But I can't find any info the Apple Site to support it! There's info on other retailer sites, like Amazon.

In Apple's tech note Article 61434, they suggest:

Use compressed songs
iPod's cache works best with songs of average file sizes (less than 9 MB). If your audio files are large, or uncompressed (including AIFF format), you may want to compress them, or use a different compression method, such as MP3, when importing them into iTunes or MusicMatch. Also, consider breaking very long songs or tracks into shorter tracks that have smaller file sizes.

by nate on Nov 04 | 4:46 pm

After reading Apple's notes I was afraid the hard drive might spin for long periods of time with long tracks, but that doesn't appear to be the case. The longest track I have is 35 MB, and the hard spins for the usual time and then stops (I just did it to double-check and that's exactly what it did). I'm assuming it loads the cache, makes a mental note of where it should pick up, and then loads the rest of the track when the cache has been drained enough to permit this.

I still cut most of my tracks down to some reasonable size--I figure 20 MB is a decent maximum--but sometimes you just want to keep a mix album in as few pieces as possible.

by pyramus on Nov 30 | 10:46 am

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