I'd like to know Sample rate and bit rate range supported by USAC.
Lossy formats don't support a specific range of sample bits. Individual encoders might only accept certain sample sizes, but the format has no limit.
Sample rates should be the same as AAC as far as I know. It's backwards compatible.
It's backwards compatible.
No, it's not. The bitstream format ist different from previous versions of AAC. xHE-AAC use a more efficient type of entropy coding.
It's backwards compatible.No, it's not.
Pretty sure it is: https://hydrogenaud.io/index.php/topic,109486.msg956185.html#msg956185
IIRC, an existing decoder can still decode the underlying HE stream, but not the xHE extension.
Well, yes and no.
The answer is: There are two different profiles of xHE-ACC
One that uses the old bitstream format (Huffman-coding)
and the other one that uses the new, non-backwarts compatible bitstream coding (CBAC).