I was reading the information about ABR, that because of the bit reservoir, it can actually have frames which use as many as 320Kbps.

From the lame encoder, CBR also uses bit reservoir, does it mean that it can also have frames which use as many as 320Kbps.

I understand that if 128Kbps is set, it uses same number of bits in the whole file. Does the bit reservoir changes the bitrate if indeed there are some unsed bits in the reservoir whrn the encoder encouneter a diffucult passage.

Why is it that the bitrate does not change or does change?

Can somebody explain this.

Yes.

Yes, but only temporarily the ned bitrate will always be the same.

CBR is, essentially, a very restricted ABR. Over an entire file the encoder makes sure that it doesn't deviate more then a few kb and "padding" is added to the beginning and end of the file to ensure that the correct number of bits are used.

CBR will always use the frames of the specified size, but blocks of audio will be distributed across frames depending on the complexity. A particular block may return zero samples because its bits contribute toward a future frame. Or something like that. Don't take my word on this, feel free to look it up. I understand code better than I understand format technical documents.

This Word document may be of help to understand the concept of the bitreservoir.