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NBC HD and Dolby Digital

post #1 of 12
Thread Starter 
Does anyone else have issues with NBC HD when they have Dolby Digital enabled for a program that isn't necessarily Dolby Digital. Let me explain. It may just be my affiliate but when the Nightly News and Deal or No Deal is on Dolby Digital is enabled and I get an echo effect. I know both those programs aren't broadcast in HD, but the Tonight Show and Conan O'Brian both are in HD and aren't Dolby Digital, yet my receiver changes to Dolby Digital. Is this just my affiliate or an NBC problem. My local CBS does a great job of turning on and off Dolby Digital, so I don't think it is my receiver.
post #2 of 12
All stations broadcast in Dolby Digital 100% of the time. Are you talking about DD2.0 vs. DD5.1?
post #3 of 12
Thread Starter 
It might be DD 2.0. For some reason I get a low echo effect when Leno or Conan are on my Local NBC HD. Now my local CBS affiliate changes Letterman to Pro Logic and turns Dolby Digital off. However, there was a few days where it was put out in Dolby Digital and it sounded good, nothing like it sounds with Leno or Conan. I also notice that during the Nightly News it does the same echo effect, possibly DD 2.0, and then 2 minutes before 6 it kicks back to Pro Logic because my local news is getting ready to start. Might be the encoder my local affiliate is using.
post #4 of 12
As mentioned earlier, all TV stations use AC3 (aka Dolby Digital or DD) to transmit HD audio. The two common modes are 2/0 (2.0) and 3/2L (5.1). When your receiver detects 2/0 mode, it has the option of using a Pro Logic decoder.

Stations that broadcast in 5.1 use a Dolby DP569 AC3 encoder. Both NBC and CBS transmit network metadata to control the mode of a station's DP569 encoder. Whether this data is used or not is up to the station. Since the Olympics, NBC has kept the mode in constant 5.1 for all shows including the ones that are 2.0. CBS actively switches between 5.1 and 2.0. CBS's system works most of the time, but if something goes wrong it's supposed to default to 5.1 to make sure the dialog channel isn't lost. There's been some reports that Letterman, which is mixed as a 2.0, has shown 5.1 sometimes.
post #5 of 12
Thread Starter 
So most NBC stations throughout the US broadcast Leno and Conan with 5.1 even though it should be 2.0?
post #6 of 12
Thread Starter 
I just noticed tonight that during Leno that the audio is being broadcast correctly with Dolby Digital 2.0 (Pro Logic). Maybe NBC finally fixed their Dolby Digital encoder?
post #7 of 12
I know on my local NBC they sometimes broadcast in 5.1 but only sound comes out of the left and right channels(no center or surround) and i cant apply pro logic to it then as I can when they broadcast Dolby 2.0.
post #8 of 12
Seems odd this thread was moved to HD Reception Hardware as it's an NBC programming issue.
post #9 of 12
Quote:
Originally Posted by TVOD View Post

Seems odd this thread was moved to HD Reception Hardware as it's an NBC programming issue.

I agree. Itchy Moderator Trigger Finger, I fear...
post #10 of 12
I've been noticing that many shows on my NBC stations are in 3.0. Shows like the Earl, the Office, Crusoe and ER. Is this supposed to be this way, or has my local affiliate messed up the transmission?
post #11 of 12
Quote:
Originally Posted by Brian Glaeske View Post

I've been noticing that many shows on my NBC stations are in 3.0. Shows like the Earl, the Office, Crusoe and ER. Is this supposed to be this way, or has my local affiliate messed up the transmission?

Local issue. All the shows you mention are 5.1.
post #12 of 12
There is a paper knocking around on the Dolby site (I think) discussing this issue - of active switching sound-field modes or running in a fixed mode. It is accepted that the system is being used in a way not originally envisaged I think (such that the implementation spec doesn't mandate no glitches on format changes) because initially it was expected that early AC3 audio sources (Laserdiscs?) wouldn't need to format change during content, only before it.

BBC HD in the UK launched with sound-field switching - so stereo shows went out in DD2.0 and surround shows went out in DD5.1, with the playout area switching the metadata in the encoder. However this can cause some glitches with many DD receivers - usually a silent audio gap on the format switch which can cause clipped or lost audio at the start of the next show if the audio is not atually silent shortly after the switch - and in some cases the receivers make a large click as they switch audio feeds with relays (our Onkyo 606 does this)

They switched to permanent 5.1 broadcast - with stereo content having silence in the rears, centre and LFE channels - which stopped the gaps and clicks - but also meant that many receivers couldn't process stereo shows as stereo (so run in ProLogic mode on stereo movies with Dolby Stereo surround etc.)

After a bit of discussion they have returned to format switching - but always aim to do this on silence so you don't notice the missing audio. (Can't remember if they do the switch BEFORE the ident that introduces the content, or just have a second of silence on the transition)

Another issue - which is relevant to the US (but not the UK currently) is that if you decode and recode the HD network feed (so not Fox) - you have to ensure that network audio (whether delivered as compressed Dolby E or discrete PCM or MP2 compressed multichannel streams) metadata is preserved to drive the local station AC3 encoder to the right format.
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