iPod Video Sizes
October 25, 2005
While I don’t yet have a new iPod with video capabilities, I did get to check one out at work today. Before seeing one in person, I thought the size of the screen made the device look slightly out of proportion, but when you actually hold it in your hand, it’s just right. And now I really want one.
I’m torn between the two capacity versions, 30 GB and 60 GB, so I decided to do some math to figure out what I can expect my videos to compress to. My main experiment was with an episode of “24.” When the original DVD title is dumped to disk (look at me, I’m breaking the law!), it occupies just over 1 GB. When scaled down the the iPod’s screen width, the new dimensions will be 320x180 – which, in a simple Photoshop mock-up, doesn’t look too shabby. The iPod’s aspect ratio is 3:4, and my “24” episode is 16:9. Since you will want the video to always be as wide as possible, use the width as the one measurement of the iPod’s screen, and let the vertical changes fall where they may.
320 pixels/16 width = x pixels/9 height gives the scaled height of 180 pixels. Knowing the width will always be the same 320 pixels (unless you have a video that’s taller than it’s width, which is odd), you can put the two together and get 320x180 as your 16:9 iPod video dimensions.
I used HandBrake to handle the video conversion to a constant 750 Kbps. While the hour and a half long conversion was running, I decided to figure out what my completed file size will be. HandBrake reports at the title I’m encoding is 42 minutes and 41 seconds.
42 minutes * 60 seconds/minute is 2,520 seconds, plus the extra 41. So, there are 2,561 seconds in this title, and it will be encoded at a bitrate of 750 Kilobits per second. The “bits” part is an important distinction to make, and it’s one that I try to do to at all times. Abbreviated, Kilobits is “Kbps”, not “KBps,” as “KB” with a capital B means KiloBytes – the common metric for file downloading and uploading. Since a single byte is 8 bits, 1 KiloByte is equal to 8 Kilobits. Thus, we can take the 750 Kilobits and divide it by 8 to get the (smaller) number of KiloBytes. Remember, your unit of measurement is getting larger, so the associated number must get smaller. 750 Kilobits per second divided by 8 bits gives 93.75 KiloBytes per second. This is the speed the iPod will be processing the video at, in more “friendly” units (personally, I can’t think in Kilobit sizes, whereas KiloBytes I can easily quantify).
93.75 KB per second * 2,561 seconds yields 240,093.75 KB. Divide that by 1024 (1024 KB in a MegaByte) to get the final video size of about 234.5 MB. Compressed to 320x180, the video will occupy 234.5 MB. Don’t forget to factor in the audio, which runs at 128 Kbps (resulting in about 40 MB). To store the entire compressed title, you’ll need about 275 MB. A whole season of “24” will use just over 6.5 GB on the iPod.
Or you could just use a converter of some sort. I don’t expect to store more than 15 or 20 GB of music, so I think the new 30 GB iPod will be just fine for me. I don’t think I’ll ever be in a situation where I’ll need an entire TV season on my iPod at once. More space is always better, but having done the math, I think I’ve made my choice – one my wallet will appreciate. And hopefully, this will be the most math-oriented any of my posts will ever get!