Can Someone Explain Bitrates To Me Please?

[Lee]

Im having some trouble understanding some of this bitrate stuff, i get that a higher bitrate means a larger file size but, what im not understanding is how you actually choose the bitrate you need to use for a specific type of video player/device.

Are there a set of 'standards' that should be applied when choosing a bitrate when it comes to the type of media file, player or dimension of the media file or is it all pretty much just 'guess work' as to what will look/sound best for any given type of filesize?

For example, i just took a 41mb movie file, encoded it at 320x240,
1248kbps for an MP4 and after encoding it came out to over 200mb in size, is this normal?

Regards,

Lee

[rawTOP]

Bitrate is literally the number of bits per second of video. Hence the bit rate times the length of the video should be the approximate size of the file, but I think you know that...

Where you're probably confused is that the same bitrate can yield different results. This depends on the type of compression being used (the codec). Use an efficient codec and you can get great quality at relatively low bitrates. Choose an old/bad codec and you can have really crappy video at the same bitrate.

So you can't really discuss standard bitrates unless you specify what codec your talking about. H.264 is becoming very popular, but it's a bit processor intensive. On2's VP6 is one of the standards for FLVs - Sorenson used to be the standard for FLVs.

You also can't discuss standard bitrates without discussing the dimensions of the video.

Just for a bit of reference - I encode promo FLVs at 640x320 at a bitrate of 800 or 850 kbps using VP6-E. When I do 1080i MPEG-4 with H.264 compression I find I need at least 4000 - 4500 kbps.