This IS the Avid L2: Re: [Avid-L2] OTish: Dolby Digital 5.1 vs PCM Center channel Matrix Errors on OTA signals in L.A.?

Tuesday, January 27, 2015

Re: [Avid-L2] OTish: Dolby Digital 5.1 vs PCM Center channel Matrix Errors on OTA signals in L.A.?

Job is correct here. The stream coming from OTA is Dolby E. That provides up to 5.1+2 channels of audio. Much of primetime programming is 5.1, so that's getting passed through the E stream. However, most of the commercials and promos are not mixed for 5.1, so the two channels (stereo) get encoded into the same E stream and passed through, coming out in stereo (no center channel or surround) from your receiver (since the receiver never has to switch decoding modes).

When you switch the TV into PCM, you're telling the TV to decode the E stream into two channels of PCM (likely a 5.1 downmix to Pro-Logic). Then your receiver is now decoding Pro Logic into LCRS, and performs that decode on everything. That decode is done in time and phase domains, so it's just math (not discrete) - therefore, something that is not surround can come out sounding like surround if the mix has certain time and phase relationships. An easy point to make here is that Pro Logic decoding always takes whatever is common phase and common level in L/R speakers and puts it out C channel. So if there's a stereo mix that has dialogue mixed into the phantom center, the Pro Logic decoder will attempt to isolate this and put it out the center channel speaker.

Ideally, I would leave the TV set to Auto, let your receiver decode the stream, and anything that's true surround will come out that way, and anything that's not will come out its appropriate channels. Then we can talk about how most promo and many commercials' post budgets do not accommodate for surround mixing.

On Tue, Jan 27, 2015 at 2:55 AM, 'Job ter Burg (L2B)' Job_L2@terburg.com [Avid-L2] <Avid-L2@yahoogroups.com> wrote:

You're confusing Dolby Digital encoding and actual channel use.

If you create a stereo mix, you end up with an L and an R channel. You can create a 5.1 DD signal from that with just the L and R channels addressed.

My bet is that some of those commercials were simply mixed in stereo, and do not use C, Lfe Ls or Rs.

J