My new iPod – part II:
Video format, bitrate, and battery life

Star Trek II: The Wrath of Khan - iPod compression

As I mentioned in Part I of the story, I’ve begun the process of ripping my DVDs into iPod ready movies. I’m using Handbrake, a great little open source (free) piece of software for Mac and Linux. I know there are Windows apps that can do this too, but I haven’t researched those. Mark Pilgrim has put together a nice tutorial on how to use Handbrake for this procedure so I won’t go over the specifics of that part of the process. I’m going to concentrate on file format, choosing a bitrate, and iPod battery life.

Let’s start with the official specs from Apple for video played on an iPod:

“Video formats supported: H.264 video up to 768 Kbps, 320 x 240, 30 frames per sec., Baseline Profile up to Level 1.3 with AAC-LC up to 160 Kbps, 48 KHz, stereo audio in .m4v, .mp4 and .mov file formats; MPEG-4 video up to 2.5 Mbps, 480 x 480, 30 frames per sec., Simple Profile with AAC-LC up to 160 Kbps, 48 KHz, stereo audio in .m4v, .mp4 and .mov file formats”

Choosing a format:

As you can see above, we can use H.264 and MPEG-4 movie formats. I would recommend using MPEG-4, at least when converting using Handbrake. Handbrake will do H.264, but it takes twice as long to convert, and there isn’t any substantial quality or size difference, at least not in the current (beta) version of Handbrake. Also, for some strange reason the iPod doesn’t always like the H.264 movies generated by Handbrake.

Choosing a size:

Another thing you will notice from the specs above is, within MPEG-4 you can go up to 480 pixels wide. You have a decision to make at this point. If you intend to watch these movies on your computer, as well as your iPod, then 480 wide may be a good solution… but only for your computer screen. On your computer, 480 is easier to watch because it is bigger and more clear. On your iPod (or a connected TV) the extra pixels are virtually useless, and won’t give you a better picture. If you are primarily going to watch the movies on your iPod (or connected TV) than you should stick with 320 pixels wide.

Choosing a bitrate:

The last thing you should notice in the specs is the ability to play up to 768 kbps. Here is where most people make their biggest mistake, they automatically choose the highest bitrate possible. My biggest tip for you, is don’t under compress your movies. That’s right, I said UNDER compress. At 320 wide, 768 is way overkill for most movies. The result will be a file that is larger than it needs to be (taking up valuable drive space), and diminished battery performance (I’ll address that later on).

Here is a handy formula for determining the proper bitrate for an MPEG-4 movie… Are you ready?… It’s…

"(W x H x FPS) / 4800" or Width x Height x Frames per second, devided by 4800.

That formula will generally give you good quality at a reasonable file size. Some movies can even go below that rate, while others may need a little more, if they have a lot of extreme motion throughout the movie. Here is an example: I did some compression tests on one of my favorite movies… Star Trek II: The Wrath of Khan. The movie is widescreen, so at 320 wide, the height is 144 (not all widescreen movies are the same). Using the formula above, we would get…

(320 x 144 x 24) / 4800 = 230 kbps

note: most DVD movies are 24 fps (frames per second). TV shows on DVD may be 24 or 30 fps.

230 kbps would be the best starting point for compressing this size movie. That being said, I found that 230 kbps was actually a bit low for this particular movie. I saw just a little bit more compression in some of the scenes than I wanted, so I bumped mine up 350 kbps. 300 may have been fine, but I was tired of compressing it.

I did several versions so you could see how the birate would affect file size. Star Trek II is one hour and 52 minutes long, and these all have an audio bitrate of 128.

320 x 144 @ 230 kbps = 291 MB
320 x 144 @ 350 kbps = 388 MB
320 x 144 @ 600 kbps = 550 MB
480 x 208 @ 600 kbps = 777 MB

There is no noticeable difference between the 350k version and the 600k version when played on the iPod screen or a connected TV. But, as you can see, the file size is half that of the 600k (480×208) version. If I had gone higher than 350, I would be wasting space and battery.

Battery Life:

Although the 60GB iPod will play up to 20 hrs of music on a single charge, I was surprised to learn that Apple rates the battery at a mere 3 hrs for video playback (2 hrs on the 30GB iPod). Videos take a huge toll on the iPod’s battery. If your movies are bigger than they need to be, that battery drain will go even faster. The iPod tries to save power by loading as much of a video as it can into a buffer, that way it can spin down the hard drive. The larger the movie, the less it can load into the buffer, and the iPod has to spin the hard drive up more often. So, it is in your best interest NOT to UNDER compress your movie. Make them as small as possible, while maintaining acceptable quality. note: the same holds true for music… keep that in mind when you rip that CD at 320 kbps.

14 thoughts on “My new iPod – part II:<br />Video format, bitrate, and battery life”

  1. From my observations, the battery life drops for movies because the BACKLIGHT STAYS ON!!! It really has nothing to do with drive activity, or any of the other stuff I read.

    To prove this, I played my iPod at the office with the backlight on constantly. I get to the red battery level after 3 hours.

  2. Gordy said…

    “From my observations, the battery life drops for movies because the BACKLIGHT STAYS ON!!! It really has nothing to do with drive activity, or any of the other stuff I read.

    To prove this, I played my iPod at the office with the backlight on constantly. I get to the red battery level after 3 hours.”

    I am assuming you have the 60GB model, right? The 30GB model is only rated at 2hrs of video playback. I won’t disagree with you about the backlight, but there are more factors involved. Your test was slightly flawed… when the backlight is on during music playback, almost every pixel is at full brightness. This wouldn’y happen during most movies, especially if you are watching a widescreen movie in letterbox format, many of the pixels would be dark. It’s also hard to say what effect your specific music data rate had on your battery life. Is your music ripped at 128kbps or 320kbps… it makes a difference.

    While there are many factors that can affect battery life, drive activity is without a doubt one of them. The less often your iPod has to spin up that drive to load more data into the buffer, the longer your battery will last.

    I also heard a report yesterday that MPEG-4 video is easier on the battery then H.264. H.264 is a very processor-intensive codec to decode.

  3. paul, i’m curious, can the new the ipod play videos at any other frame rate other than 30fps? i have found that on 320×240 videos at 15fps is virtually indistingishable from 30fps and are of course significantly smaller in size. i’m hoping to pick one up this weekend for my paris trip next week.

  4. i’m curious, can the new the ipod play videos at any other frame rate other than 30fps?

    It will definitely play movies that are less than 30fps. All of mine are at 24fps (including ‘Lost’, bought from Apple), and they look great. As far as I know, there are no problems with using 15fps. Congratulations on the new iPod… and the trip to paris!

  5. i just noticed that 15fps looks really chopy when playing double size or full screen. so i’ll stick to the original 24fps or 30fps. 2-pass encoding yields better compression but man does it take a long time!

  6. Battery life is not related to size. Even playing a Video in MPEG-4 at 720 x 320 – HD loads 64 MB into Flash – and stops using Lossless, or 320K AAC.

    The CPU on say a Mac, which encoding Video really takes all you can throw at it, and still want more, MPEG-2 optimization for iDVD for example, started aweful! Later we got real-time, then 2 x realtime. This is an issue of optimizing these complex calculations for a CPU which is not even part of say DVD hardware. ( h.264 will improve – but still a lot of choices on how to keep quality say for editing feature film, vs. speed for less important purposes.)

    MPEG-2 is a great illustration in this case. Ripping a DVD is at least 90 minutes from DVD. Yet you can fast foreword at 32 X on a DVD player that sold for $9.99 @ Wallmart! ( WTF??)

    iPod is similar. It’s small CPU, (ARM 7 @ 80MHz max – Treo 650 has 317MHz ARM ) It uses hardware to decode – Wolfson chip encodes/decodes MP3 – CPU does zero… Video/ Photo/ Audio – these hard coded chips make use of their spec’s – so it doesn’t “Slow down” using 720 x 304 – it processes it’s 230,400 the same – with no regard for your preferences.
    The HD has a RISC controller which prefetches with 95% accuracy – one simple guess is you will likely listen to the rest of this song/movie file next – 99.999% of the time.

    Backlight is the killer, on laptops, iPods, nothing comes close to hogging more battery life. ( If the light is off, say using TV – it should do far better -and TV should use hardware which is more efficient & that is a hardware reality- )

    You do have a good point on file size though. MPEG-4 Movies should fit on a CD, with 720 x320, 544 x 416, AAC 128K – and be indistinguishable from DVD – but pixels reduced from 720 to 320 wide – will be aweful! YMMV – everyone notices pixel count blur ( even ON the iPod screen ) and tweaks can keep file sizes down like dual pass –

    I’m aware of the issues of Video size on a HD. Quickly adds up… So, if 60 hours of video on iPod is too low, you’ll need to adjust some things….
    ( If it’s a TV show I’ll never watch again, maybe different than favorite film ) The reason MP3 is so popular is “most” don’t notice, but can fit 150 songs on 1 CD – so convenient… ( Interesting to watch it happen to Video now…)

  7. Wow! I have no idea what that last comment is trying to say… exactly. There are a few sentences here and there that make sense, but the rest is kind of all over the place.

  8. I’m a caveman. Is it possible to load a dvd from my imac to my ipod? or do I have to buy movies exclusively from itunes kolchose store.

  9. Me the same problem as Sean.
    Does anybody has a tool, just like Handbrake, for Windows? Ive some DVD’s here I’d like to put on my iPod Movie 30 GB …
    I dont want to buy ”Quicktime 7… (PRO)” for 30,- , but it seems the only way to make my mpeg3 files mpeg4 files!


  10. Penny – no, having a lot of videos downloaded / stored on your iPod won’t effect battery life. They’re only using battery when they are playing.

  11. what a relif i was worried because i have like 500 videos stored,

    so how long should my battery last if im watching vids?


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