more detail about compressing technology.

2003-03-05 03:25:48

I have looked around for information on how MP3 and others work but have only found basic descriptions and no hardcore detail. Can anybody point me to the deep inside structure of the technology? For instance i hear people saying that it is good to cut out low frequencies because that will save space. That totally goes against my logic because low frequencies have much less waves per time unit and so need much less samples and bits to define an accurate waveform. To me the highest frequency present at any given moment will determine the amount of bits required to have a clear sounding signal. People think it's logical that when the sound is less intense it should use less bits but i fail to see why. I think it should be the frequency and complexity of the waveform and not the amplitude. With .wavs i take it that they choose a sample rate [44khz] that is twice the frequency we hear up to, and so that will be able to cover that upper end and anything below it with increasing definition. In mp3 technology it seems that frequency is divided into bands. I fail to see how that will save space when you now have to have seperate bits for each band. I can understand cutting out highfrequency band or lowering the sample rate when there is no audible sound present up there(since higher freqs require more bits for time for a given definition), but how would it benefit to cut out antthing lower(when ther are only highs present) when it could just ride on the same bits as the higher definition high freqs? My ideal compression system would be to vary the sample rate according to the highest frequence present in that frame and then to use the ham coding, or whatever, to do lossless compression on the sample positions or values. I don't really understand the point in cutting out mids or lows or even dividing into discreet bands in the first place. Maybe the cutoff lowpass should be continuously variable and no bands? But i have more to understand about the details before i can come up with what will really work. What do you think??