Bit Rate minimums sure reduce file size but still keeps quality

[johnharlin]

I have been converting some 20 minute movies that I only expect to watch on a computer for the purpose of reducing file size yet still keeping a minimum of quality. I have been converting movies to 640 by 360 avi format. What I have noticed is that when

I keep original frame
Use MP3
44100Hz at 16bit
128 kbps
Stereo
1500 bit rate
and reducing it to 640 by 360 I get about 180 mbytes or more in file size

However if I reduce the bit rate to 430 with all the same settings as before, I end up with 70 mbytes
That's a huge difference yet I don't notice much of a change in quality
and the movie appears to play fine.

So what does the bit rate do anyway and how low can you take it
before quality goes real bad.

[johns0]

It's all up to the user as to what quality they can see and live with,i would never re-encode a movie to the specs you stated,my minimum quality coming from a good clean source(1920x1080) would be 1280x720 at 4gb files size with 640 5.1 ac3 .Play your file on a large hdtv and you will see the quality.

[hello_hello]

Bitrate is the average video bitrate.
Bitrate x duration = video file size
The lower the bitrate the lower the quality (although there's a point where the bitrate is high enough and increasing it further doesn't give you much of a visual quality increase. No two movies compress they same so they require different bitrates for a given quality.

If you're playing the files on a computer consider using the x264 encoder instead of Xvid. It'll produce better quality than Xvid at a given bitrate.

Instead of using 2 pass encoding you could try a single pass constant quantizer encode (I assume you're encoding with Xvid). A quantizer of 2 is maximum quality, or you can increase it to 3 for smaller file sizes and lower quality (the quality might suffer if you increase it too much). The bitrate will be different for each video as you're encoding to a specific quality. If your encoding program can't do it, try AutoGK (a single pass 75% quality encode for AutoGK is a constant quantizer encode of 2.67, if memory serves me correctly.
Even if you select a bitrate, AutoGK might be worth a try. If you leave it in auto mode it'll make adjustments (including the resolution if you let it) to give you the best quality it can for the specified bitrate.