Worry about broadcast specs on LtRt/LoRo fold-downs?

Question

Alright, so this one is for my fellow TV-post fader jockeys out there. I've never really found a solid answer to this, not even from some of the networks we sell to (I wish we actually sold more to Discovery and NatGeo).

We're mixing to 5.0/5.1 for Dolby-E encoding as a delivery medium. For that specific final mix, we're using an internal spec of -24db LKFS/LUFS (infinite) with a maximum true peak of -9dB. You probably know that an E-encode includes an LtRt version of the full mix in channels 7 & 8 of the bitstream. The question is, should we be worrying about meeting the same spec for that LtRt like we do with the full surround mix? We've also taken to simply encoding a LoRo for a Mix-Minus when we're only providing stereo masters to a client network: complete main LtRt on 1&2, Mix-Minus LoRo on 3&4. So, I have concerns about the specs of these fold-downs beyond their inclusion in the Dolby-E bitstream.

In our current process, the -9dBTP goes out the window after LtRt encoding. It doesn't hit anywhere near zero (usually only going up to -8 or -7dB true peak), but it's still over. I haven't had time lately to run a full analysis of the LtRt to determine its "dialnorm," but I'm fairly confident that it's not sitting at -24. [We're going to need a new term for that in the future, I think.]

So, anyone out there got a definitive answer...idle speculation...?

Answer 1

Speaking specifically of NatGeo, yes, they are concerned with all audio (LtRt, LoRo, Dolby E) meeting spec.

Audio should be mixed such that average loudness (using LKFS) of the LtRt mix shall be equivalent to the measured average loudness (using LKFS) of the 5.1 mix, within, ±1dB

~Universal Tech Specs for NatGeo Channels & Nat Geo Wild

But your spec is just a bit off.

• Peak stereo mix audio are not to exceed -10dbfs for NTSC, -8dbfs for PAL using digital true-peak meter. Transient peaks up to -2dbfs are allowed on individual channels within a 5.1 mix.
• Program should be -24 LKFS (±1) averaged over each segment.

Additionally, we deliver on HDCAMSR (at least until the stock disappears completely) with a channel assignment as follows:

1. Stereo Left or Left Total (full mix)
2. Stereo Right or Right Total (full mix)
3. Stereo Left or Left Total (MDE - Undipped)
4. Stereo Right or Right Total (MDE - Undipped)
5. Dolby E 5.1+2 (full mix)
6. Dolby E 5.1+2 (full mix)
7. Dolby E 5.1+2 (MDE - Undipped)
8. Dolby E 5.1+2 (MDE - Undipped)

Having said all of that... shows do get through occasionally that should fail QC. But rather than levels, one of the worst offenders is failing to follow their 5.1 channel assignments:

• Left (Music, Ambience, Effects, Incidental dialog)
• Right (Music, Ambience, Effects, Incidental dialog)
• Center (Narration, Location dialog, Translation)
• Low Frequency Effects (Explosions/Music Effects)
• Left Surround (Music, Ambience, Effects)
• Right Surround (Music, Ambience, Effects)

The center channel must be free of all music and effects. It is reserved exclusively for Narration, Dialog and Translation. These center channel elements must not be diverged into the Left and Right channels.

~Universal Tech Specs for NatGeo Channels & Nat Geo Wild

It made me scratch my head until I recognized that the show isn't just going to air and get shelved. It's going to be translated into a number of different languages and re-broadcast all over the world. It's simply a money saving measure. Strip out the original Center channel, produce the new story in the language of your choice in Mono, add it to the 4.1 and, voilá you've reached a whole new market.†

I've mixed reversions and had to kick back shows that had narration in the LFE or music/fx in the center channel, because otherwise I'm the one passing the buck and the problem just compounds as it cascades. One or two of these shows had been produced over a year ago. Needless to say, it's a headache that you or your facility don't want to be remembered for originating.

†Although I'm still crusading for a 5.1 M&E + LCR Dialog delivery requirement which would nullify this. But alas, no progress yet.

Answer 2

Steve, your answer is completely correct.

We are forever making minor tweaks to our spec, trying to get to something that allows for maintaining the integrity of the mix, whilst still working for all the many and various end uses of the material.

We do try to make allowances in QC for special circumstances.

But... what are you putting narration in the LFE for anyway? I would kick that back for you being crazy, not necessarily beacuse it doesn't meet the spec.