Originally Posted by RobLee
No doubt I am wrong (I usually am) so then by all means, PLEASE tell me what is the proper term that I should be using to describe the signal which triggers our AVR to display "DOLBY DIGITAL" on the front panel, so we can get back to what I had originally intended to discuss, which was which networks broadcast 720p video and which broadcast 1080i.
Depends on your amp and possibly also your digital source.
All US OTA digital TV, as I understand it, uses Dolby Digital (aka AC3) audio encoding. This is true whether the audio is 2.0 (aka Stereo) or 5.1 (aka 5.1 surround) I believe the same is true for cable - though I'm not sure about satellite. (In the US MP2 audio wasn't mandated as part of the ATSC spec - so the only audio encoding format used is DD)
Some amps may only display a "DD" or "Dolby Digital" symbol when receiving a Dolby Digital 5.1 signal and don't display it when receiving a Dolby Digital 2.0 (These amps effectively display "Dolby Digital" only when receiving a 5.1 signal - so the "DD" or "Dolby Digital" indicator doesn't indicate the presence of a Dolby Digital source, it indicates the presence of a Dolby Digital 5.1 source)
Some sources may also decode Dolby Digital 2.0 signals to PCM 2.0 (meaning no lights come on on amps when receiving stereo as the source may deliver the stereo as PCM not DD), whilst leaving Dolby Digital 5.1 signals as pass-through. (Not sure how widespread it is - but I've seen it on other gear like DVD players that handle DD 2.0 and DD 5.1 content)
Other amps - like my Onkyo - will display a Dolby Digital logo whatever format of Dolby Digital audio they receive - displaying "Dolby Digital" whether a DD 2.0 or a DD 5.1 source, and will actually tell you whether the source is 2.0 or 5.1 on their front display. (So I see "Dolby Digital 2.0" or "Dolby Digital 5.1" on my amp display when watching DVDs with multiple audio tracks - some 2.0, some 5.1)
Thanks, yes I knew about KRC... and I know I've seen this on several other channels as well. Some, I think maybe Spike and Palladia (could be wrong) trigger the "DOLBY DIGITAL" display (for lack of the proper term) on the AVR, but then fall back to PLII for commercials or station promos and such. Others remain in DD mode fulltime.
That sounds as if your amp only displays "Dolby Digital" when receiving a DD 5.1 input, and doesn't display "Dolby Digital" when receiving a DD 2.0 input (though the 2.0 signal may then be decoded using "Dolby Pro Logic" decoding - which is used to extract the surround content from Stereo matrix encoded sources)
Broadcasters that broadcast 5.1 and 2.0 content generally do this in two ways.
1. Permanently broadcast a DD 5.1 signal, but encode silence 3.1 of the channels (i.e. centre, rear surround and sub channels) during stereo 2.0 content. This avoids having to send metadata to alter the Dolby Encoder at the station, but means amps can't use ProLogic or other effects on stereo 2.0 content (as the amp is being told it is 5.1 not 2.0) It also means that amps that tell you whether a source is 5.1 or 2.0 constantly report a 5.1 signal - even if the broadcaster is only sending audio in 2.0 of the channels.
2. Dynamically switch between a DD 5.1 signal (when the source is 5.1) and a DD 2.0 signal (when the source is stereo 2.0 - or even mono with the same content in both channels) This is probably the better way of doing things, as your amp can properly process 2.0 sources properly (using Pro Logic or other decoding) AND if your amp displays it, it will properly indicate whether the broadcast is 5.1 or 2.0. (In fact it is possible to send signals other than 5.1 or 2.0 using Dolby Digital - I've seen 4.0 used on occasion).
The downside is that it requires broadcasters to manage metadata - and if this goes wrong you could end up with 5.1 programmes being broadcast in 2.0 mode (and as a lot of dialogue is in the centre channel you could end up with dialogue-free drama...) Also mode switching between 2.0 and 5.1 dynamically can cause large-clicks in amps with relays, and can cause a short drop-out as the amp switches modes - so it is best if the audio mode change happens during broadcast silence.
I would expect that dynamically switching between DD 2.0 and DD 5.1 is much easier for a station that has no local affiliates to route through (or uses technology like Fox's splicer)