Compressed video not like compressed audio

[Jimmykicker]

Maybe someone can explain this to me:

Why is the quality of compressed video not like that of compressed audio? For example, if I take a mpeg-2 stream and convert it to a divx compatible mpeg-4 stream, why is there quality reduction? I can take a song file off of a CD and compress that to mpeg-3 and still keep the full quality. I don't really understand it....

[MpegEncoder]

The audio that you speak of only seems to be the same quality. That depends on the encoder, bitrate, etc. you use. Try encoding a WAV to mp3 at 64kbps and then come back.

[DJRumpy]

There is a loss in quality, you just don't notice it. MP3 compression is designed that way, removing audio frequencies your ears can't hear. At higher bitrates (say 224kbps+), you probably won't notice any change, but it's there.

Your eyes can register thousands of colors and shades. Your ears on the other hand register a very narrow range of sound frequencies.

In addition to that, mp3 audio requires very little bitrate before it gets to the point where you no longer notice compression artifacts (somewhere around 190-224 kbps). Go below that bitrate, and it starts removing information that is in the audible spectrum of hearing.

MPEG-4 video improved drastically over the old MPEG-2 compression scheme for video, dropping the necessary bitrate per second into the high hundreds for full D1 video, but it still requires far more bitrate than audio, and probably always will do to it's inherent large size.

[mkelley]

Your eyes can register thousands of colors and shades.

Millions. Easiest way to see this is to view a blue sky -- just a blue sky -- on a high color monitor (not true color). You will see banding, because high color only can show 65 thousand colors. Only true color will look right to us (blue sky is actually the litmus test, since there are very subtle graduations in it most people don't even think about).

It's actually quite amazing we accept as much as we do -- movies don't nearly have the range of Real Life, and video doesn't nearly have the range of movies. Eventually it breaks down (can you say VCD?).

[DJRumpy]

Sorry, I wasn't going for accuracy . The human eye can differentiate approximately 7,000,000 to 10,000,000 million colors, give or take. Far more information than is available in the audible spectrum for a human.