Video resolution vs bit-rate

Q: I have a new iPod classic 160 GB that I’m loading with videos. I’ve read that a 640×480 file is the same MB size as a 320×176 file if encoded at the same kbps (bit-rate). Is this correct? Is there an advantage to the bigger size? Is there a “definitive” guide you can point me to that explains the relation between resolution and kbps clearly. Thanks in advance, and keep up the great work.

– Alex

A: As odd as it may sound, it is actually correct that a given video file’s size is based on the bit-rate of that file, expressed in kbps or kilobits per second, rather than on the resolution.

Much like audio files, the bit-rate defines how much data is stored for each second of playback. This means that a 1500 kbps video file has 1,500,000 bits of data stored for each second of playback time. In that sense, basic math will let you determine the size of a file relative to the bit-rate.

This may seem counterintuitive, since you expect a higher resolution file to occupy more space, in the same way that a higher resolution picture occupies more space. While this may be true in the case of an uncompressed video, the reality is that modern video compression algorithms are “lossy” by nature, and do not actually store a full image for each frame of video—instead, “reference” frames are stored, and then additional data is used to represent the changes between reference frames. These are interpreted during playback to produce relatively smooth motion video.

The catch is that there actually is a direct relationship between the bit-rate of a video and the resolution—or perhaps more accurately, between the resolution and an acceptable bit-rate. The larger the image, the more data is going to be required to accurately represent the motion within that image. For instance, although you may technically be able to encode a 640×480 and a 320×172 video file at the same bit-rate, you will either be creating an unnecessarily large 320×172 video (if the bit-rate is too high), or creating a very poor quality 640×480 video (if the bit-rate is too low). Generally, the bit-rate needs to be appropriate for the video resolution in order to produce an acceptable playback experience with reasonable quality. Too low of a bit-rate will produce a movie file that with visual problems such as “artifacting” and “macroblocking” and possibly even choppy playback (skipped frames).

As a rule, for acceptable playback when encoding video for the iPod using the H.264 codec, a 320×240 video file (encoded for the iPod screen), can generally be done with a bit-rate of approximately 750kbps, whereas a 640×480 file should use a bit-rate of approximately 1500kbps. On the other hand, videos encoded into standard MPEG-4 require slightly higher bit-rates, as the standard MPEG-4 codec is not as efficient.

You can find a much more detailed discussion about this, as well the suggested formats and display resolutions in our iPod 101 tutorial The Complete Guide to iPod, Apple TV and iPhone Video Formats

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