I've found this document:
http://www.rmworkshop.com/dvd_info/related_info/ac3hdr.html
According to it, 0x0B77 is the beginning of an AC3 frame. I opened up my
AC3 file in hexedit and, lo and behold, there were 0x0B77 strings at
regular intervals in the file.
Using dd, I cut a chunk of the file from the beginning until right
before the second occurrence of 0x0B77. Apparently, that chunk is a
single AC3 frame and a valid AC3 stream on its own.
Using cat and a bit of shell scripting, I concatenated this chunk 69
times at the beginning of the existing file. The result, according to
a52dec, mplayer and mplex, was a valid AC3 file with just the right
amount of delay at the beginning (69 times the duration of one frame was
what I determined experimentally to be needed).
--
Florin Andrei
http://florin.myip.org/