In article ,
(peter) wrote:

After reading document of Ac3, I have some questions, which really
need your help.

why it is necessary to use this word? I think if the ac3 frame
length is fixed, we can get the next frame address easily without
syncword.

Not all AC3 streams are neatly stored on a disk; sometimes they come at
you through some kind of transmission channel over which you have no
control (digital TV on American cable uses AC3, for example).

How do you establish framing at the very beginning?

How do you find it if you "fast forward" into the bitstream?

What happens if there is a glitch and you lose some unknown amount of
data; how do you recover framing?

Answer to all three: By using the sync word.

What's more, why AC3 choose "0b77" as syncword? How to assure that
"0b77" is only used as syncword and will not exist in the audio data?

Doesn't matter. You just start looking, byte by byte. When you find one,
count exactly one whole frame's worth of bytes, and look again. If you
don't find another sync word right there, go back to looking. If you do
find one there, count another frame and look again. After about three or
four "hits" in a row, exactly one frame apart, you're virtually
guaranteed to be synched up.

Once you're in sync, you only look where you expect to find a sync word.
As long as you do, fine. If you don't, start looking all over again.

Isaac

hi,isaac:

Thank you very much for your post.
I agree with you completely.

Now I just wonder why using "0b77". It seems that in ac3 bitstream,
there are really very few "0b77" except the syncword.

Maybe it is due to ac3 encoder


Isaac Wingfield wrote in message ...
In article ,
(peter) wrote:

After reading document of Ac3, I have some questions, which really
need your help.

why it is necessary to use this word? I think if the ac3 frame
length is fixed, we can get the next frame address easily without
syncword.

Not all AC3 streams are neatly stored on a disk; sometimes they come at
you through some kind of transmission channel over which you have no
control (digital TV on American cable uses AC3, for example).

How do you establish framing at the very beginning?

How do you find it if you "fast forward" into the bitstream?

What happens if there is a glitch and you lose some unknown amount of
data; how do you recover framing?

Answer to all three: By using the sync word.

What's more, why AC3 choose "0b77" as syncword? How to assure that
"0b77" is only used as syncword and will not exist in the audio data?

Doesn't matter. You just start looking, byte by byte. When you find one,
count exactly one whole frame's worth of bytes, and look again. If you
don't find another sync word right there, go back to looking. If you do
find one there, count another frame and look again. After about three or
four "hits" in a row, exactly one frame apart, you're virtually
guaranteed to be synched up.

Once you're in sync, you only look where you expect to find a sync word.
As long as you do, fine. If you don't, start looking all over again.

Isaac