Is it allowed to switch error protection bit (indicating whether CRC is
used) within one song?
No, it is not allowed.
To my knowledge, there is nothing technically prohibiting it.
This is the source of the confusion that protection bit + XING-VBR header ...
The Xing issue I think has only to do with the ability to detect whether a stream was encoded with CRC protection by looking at the first frame. Such an extrapolation is merely a hint, for the very reason that protection applies independently to each frame and could therefore change from one frame to the next.
Of course, if some frames have CRC protection and others do not, you lose some of the benefit of that protection, and it might be considered reasonable for an implementation to ignore frames without protection on the theory that the frame header itself is corrupt. I don't think ISO/IEC 11172-3 ever suggests this behavior, however.
-- Rob Leslie [EMAIL PROTECTED]
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