I was wondering what bitrate ATSC broadcasters commonly use for Dolby Digital. I know DVD is limited to 448 kbps, while Blu ray and the ATSC spec (http://www.atsc.org/standards/a_52b.pdf) allow for up to 640 kbps, but I don't know what's used in practice.

Can anyone point me to data or anecdotes about what's commonly used? Is it generally a decision implemented at the network-feed level, or are stations free to encode and allocate bits as they see fit?

I seem to remember from WAY BACK that the rate was set at 384 kbps in the defining stages of the ATSC standard. This may have been in Joel Brinkley's book Defining Visions which by the way is a very good history of the ATSC program.

My pre-pro can display the bitrate. I have noticed that it is 384 kbps very often. I just don't remember whether it's always that or if I ever see anything else. I'll keep an eye on it and let you know.

Ed

FOX has a bitrate of 448kbs, on the main audio, which is passed through at the local level. The secondary audio, used for SAP, is 192 kbps mono.

Most other network affiliates do the encoding at the local level and that is where the bitrate is chosen. 384kbps is pretty common, but I have seen lower rates.

DD 5.1 is usually either 384 or 448. Stereo is often 192. 640 is not allowed for broadcast, but PS3 and XBox games use it a lot.

640 is not allowed for broadcast...
Are you sure? From p.20 of the spec:
The AC-3 digital compression algorithm specified in this document can encode from 1 to 5.1 channels of source audio from a PCM representation into a serial bit stream at data rates ranging from 32 kbps to 640 kbps.
Is it that AC-3 is capable of 640k per the A52 spec, but another part of the ATSC spec throttles it back to 448k max? Do you know where in the ATSC spec a 448k limit is set?

Are you sure?

Quite. The data rate of a "complete main" stream (most typical audio streams) is limited to 448 kb/s

Is it that AC-3 is capable of 640k per the A52 spec, but another part of the ATSC spec throttles it back to 448k max? Do you know where in the ATSC spec a 448k limit is set?

Yup. A/53, Part 5:2007 lists "Constraints With Respect to Standard A/52." I believe the reason for these constraints was the amount of memory available in the DTV decoder. Memory was still expensive when the ATSC system was developed in the 1990's.

Yup. A/53, Part 5:2007 lists "Constraints With Respect to Standard A/52."

Thanks for the pointer.

Here's an interesting thread about a fellow that bought a new Sharp HDTV with sound problems and how kqed and kcsm in the SF bay area solved it..... It was interesting to see both were having audio buffer overruns in their encoders...

Both links go to the same thread
http://www.avsforum.com/avs-vb/showthread.php?p=13660319#post13660319
OTA audio dropout update
The engineer at KQED (9.1) found a workaround that stopped the audio dropouts on my Sharp. He turned off the stat mux on their encoder. Maybe the set is not at fault after all...

http://www.avsforum.com/avs-vb/showthread.php?p=13661851#post13661851
yet another OTA audio dropout update
Today the other channel I was having trouble with, KCSM, finally looked at their StreamScope and saw audio buffer problems at 384k. They reduced their audio bit rate to 192k and they stopped seeing problems on their scope. And my audio dropouts disappeared.

P.S. I never had a sound problem on these stations.

DD 5.1 is usually either 384 or 448. Stereo is often 192. 640 is not allowed for broadcast, but PS3 and XBox games use it a lot.

384 kb/s works fine for 5.1. 192 is good for stereo. Yet switching between the two data rates causes a brief silent period. Therfore stations usually use the same data rate for both stereo and 5.1.

384 kb/s works fine for 5.1. 192 is good for stereo. Yet switching between the two data rates causes a brief silent period. Therfore stations usually use the same data rate for both stereo and 5.1.
Stations may not wish to change audio data rates for fear of upsetting the multiplexer portions of their encoders, but cable providers change these modes all the time in mid-stream, without incident.

I see the data rate and coding mode change regularly every time a local commercial is inserted into a cable program. A 5.1-channel 384 kbps cable channel will often be interrupted with a 192 kbps or 128 kbps 2-channel commercial. The results are seamless, although it is difficult to notice a one-frame mute between commercials.

384 kb/s works fine for 5.1. 192 is good for stereo. Yet switching between the two data rates causes a brief silent period. Therfore stations usually use the same data rate for both stereo and 5.1.

Not if:

1) the affiliate encoding system is properly designed,genlocked and timed.
2) the consumers dolby decoder fully adheres to dolby specs.

Neither of which occur very often in the real world at this time.