Are datarate and resolution related?

[gpattillo]

I've created several dvd clips using different framesizes and datarates.
I've noticed that the only factor in the filesize of the clip is the datarate,
which makes sense, since it is the number of mb/s.

But my question is, if I have a clip at 352x240 and a clip at 720x480 and they are both encoded at 8mb, since they are the same size, and one is lower resolution, where is the extra data?

And a related question, why would anyone use the lower resolutions if it takes the same amount of data to represent it, as it does the higher resolutions?

My thinking is if I'm sacrificing resolution within a given data rate, I should gain something back somewhere else.

Could someone please explain this?

[SaSi]

If you encode at constant bit rate (CBR), then file size is a function of movie length (time) and the bitrate (Bits/sec x secs ) / 8 = MB

A 352 x 288 clip (uncompressed) is 456 Mb / min.
A 720 x 480 clip (uncompressed) is 1.555 Mb / min.

If you encode them at 4000 Kbps (30 Mb/min) you are asking the encoder to make a 15:1 compression for the first and a 52:1 compression for the second.

The encoder will "loose" more information on the second clip to bring it down to the requested bitrate and size.

So the answer on your question "Where is the extra data?" is simple: it is lost.

In general, you need 3 times as much bitrate to encode the full frame compared to the half frame, using the same encoder and aiming for the same overall quality.