Why are mono audio files not half the size of stereo audio files ?

[vigi_lante]

For the same song and sampling rate & bit rate.

I guess that maybe is because the Stereo audio needs to share the bitrate between 2 channels. The mono file is encoded at twice the quality rate as the stereo file, since it only have 1 channel.

So, for example, a mono audio encoded at 128kbps will have the same quality of a 256kbps stereo file.

Is that right ?

[Mug Funky]

not quite right.

a stereo file will have high corellation between left and right channels (ie they sound pretty much the same most of the time). a clever encoder can exploit that and save tons of bitrate. how much it saves depends on the music.

most lossy audio coders use mid-side or some other technique, and give more bits to the "mid" part.

so a 256kbps stereo mp3 will sound quite a lot better than a 128kbps mono mp3 file from the same encoder, unless left and right are completely different (ie "independent channels", like english on one side and japanese on the other).

[GodofaGap]

I find it a bit hard to compare mono and stereo like that. As long as the encoder is not starved, for me a mono file will always be worse quality than a stereo file. The quality difference does only not come from using JS effectively.