I have a collection of classical music. When i open these songs in a hex editor i see huge parts with 00 or FF. When i use zip on these files it cuts of 15-20%. How do i get rid of this ballast? What is it?

Are your files in wave format?

try mp3packer
it was designed to remove as much of the non-audio data as possible.

I believe that can be the MP3 padding. Are we talking about a CBR file??? If this is the case, that might be the beginning or the end, and the part contains silence or nearly silence, so it doesn't need lots of bits to encode. The rest, to conform with CBR is padded.

Sorry for not mention it. The files of course is mp3. mp3 320 CBR.
I tried mp3packer but it only removed a couple of KB.

Use VBR.

Perhaps it's a padded ID3v2 tag...

That would have to be one of the largest id3v2-tags i've ever seen ;-)

It must be the silent parts, which do not need more than 32 kbps to be encoded and are padded to fill in the full size of 320kbps.

it's a little strange that mp3repacker doesn't help to reduce the size very much.

Maybe zip can compress these files much better because zip can see that these padding sequences of FF and 00 are repeated throughout the file.

Actually, I think it's the same also with lossless audio comprssors. If you comress digital silence with FLAC and then compress flac file with ZIP the size will be reduced considerably. FLAC adds overhead for the framing and seeking etc.

Sorry for resurrecting this thread, but I'm kind of curious what it is too, and especially why mp3packer doesn't help (I wrote the program). Do you know what encoder was used? Could you put the first ~50KB somewhere on the net?

The mp3packer program is designed to get rid of stuff like this, but there are times when it doesn't work. One MP3 I encountered said that the data takes up the whole frame, and then fills the frame with fluff. This can't be repacked because it depends on the MP3 reporting the correct amount of data in each frame.