How to decode 6-track APTX-100 (cinema DTS) with the correct channel levels

[borisanddoris]

Surrounds universally need to be lowered -3dB since the B-chain always set them (in a properly aligned room) to 82dB vs. 85dB at home. This still holds true for a theatrical mix that is ported to the home today. If you have a full surround array, you could probably get away with 82dB.

The LFE is the more mysterious one for sure. I too have found the +3dB to work well for pre 1999 films except Jurassic Park. I’d like to get a hold of some other earlier DTS titles like Heart and Souls or Hard Target to check them out.

All the DTS6 install guides I’ve seen date well after 1993 and would be interested to see if there’s something different, even slightly, in older documentation. Albeit I’ve spoken to many folks who ran Jurassic Park and the 88dB SPL for LFE was a thing with no special considerations to the contrary.

[2001CluelessDianogas]

So, here's what the measurements should apparently be:

L, C, and R, measured with an SPL meter, playing internal pink noise or pink noise from DTS Setup Disc or Empirical Disc:
85 dBC

Ls and Rs, measured with an SPL meter, playing internal pink noise or pink noise from DTS Setup Disc or Empirical Disc:
82 dBC (-3 dB compared to screen channels)

Subwoofer, measured with an SPL meter, playing pink noise from DTS Setup Disc or Empirical Disc:
91 dBC (+6 dB compared to screen channels, but pink noise is only ~20-80 Hz because of DTS processing)

Subwoofer, measured with an SPL meter, playing pink noise from the internal pink noise generator of the DTS-6AD:
85 dBC (same as screen channels, for some reason, which I'll get to in a sec)

Right, so this brings me back to one of the things that was baffling to me before: why is the DTS-6AD's internally generated pink noise for the subwoofer apparently 6 dB quieter than the pink noise off a DTS Setup Disc or Empirical Disc?

When @schorman calibrated his DTS-6AD completely flat (no gain adjustments on any channels) then recorded some soundtracks through it from films released in or after 1999 (thus complying with the new SMPTE RP200:1999 subwoofer level standard), it seemed to output the subwoofer channel at a level that matches other releases of theatrical audio on LD and DVD releases of the same films, but the surrounds still needed the -3 dB cut. The foobar2000 plugin, by contrast, has the subwoofer channel 6 dB quieter than the DTS-6AD, so it would seem to be the case that the DTS-6AD automatically handles the +6 dB subwoofer boost for SMPTE RP200:1999 films but the foobar2000 plugin defaults it to 6 dB lower than it should be unless you manually change the setting. Curiously, neither the DTS-6AD nor foobar2000 plugin seem to apply the -3 dB cut to the surrounds: in both cases, the surrounds seem to be 3 dB louder than the LD and DVD releases of the theatrical audio mixes.

Based on the measurements of the DTS-6AD's subwoofer output, I'm wondering if this -6 dB measurement discrepancy (85 dBC for internal pink noise, 91 dBC for pink noise from setup disc) maybe has something to do with the unit already adjusting the subwoofer output by +6 dB when it's decoding off a disc.

Yeah the docs are a confusing mess. And it gets confusing about when they are talking about home the speakers are set, how the data is stored, when this or that is on, whether they are comparing full or partial bandwidth, at what stage, etc. the mess where home theaters set subwoofer to same as regular channels but theaters have the subwoofer dialed up +10dB and how most cinemas have full bandwidth mains and most home cinemas re-route bass from mains to subwoofer and don't use full range mains. For home theaters bass re-routed from mains to sub needs to be played at same level as mains, but sound from LFE channel from movies and properly LFE calibrated music discs needs to be then boosted +10dB at last stage to sub to account for movie theaters having sub that plays the LFE +10dB to mains. If you just dial up home subwoofer +10dB then you get the mess that re-routed bass from mains is way too hot. And most A/V/ receivers have weak and confusing documentation as to what they do, especially if using analog inputs, etc. And same for much PC playback softare and crossover sotware, little of describes what it does, does it boost LFE or not? how does it handle crossed over vs LFE volume differences, etc. More of a mess if you use analog output from soundcard. It is easy to end up with way boosted rerouted bass from mains for music or movies or with way too soft LFE effects or only one or the other correct at one time, etc.

Anyway, the surrounds need to be lowered -3dB by foobar when you decode because home theater designates that you set L,C,R,Ls,Rs to all read the same volume for the same input but in cinemas their Ls and Rs each put out -3dB less volume for the same input than L,C,R so they are calibrated -3dB compared to for home theater. So when listening to CinemaDTS mixes you either play it as is and then set -3dB for each surround or simply leave your home cinema speaker levels alone (safest, since not everyone will know to adjust) and then simply lower the signal -3dB. The cinemas put their Ls and Rs -3dB lower each than L,C,R as an old artifact from when surround was mono and for easy auto compatability if it dropped back to some optical mono surround or whatnot since that gets fed then to Ls and Rs they'd have mono surround volume doubled otherwise. So the Cinema DTS decoder doesn't do any -3dB for surrounds since they don't have to, the cinema surround speakers are already each calibrated -3dB lower unlike for home setups.

[2001CluelessDianogas]

Thanks so much for all of this information. I'm in the process of doing some Cinema DTS syncs at the moment and I'd like to make sure I'm setting the levels correctly. How certain is it that all post-1999 movies need an LFE boost of +6db?

If I apply a +6dB LFE boost to any of the LOTR movies then it results in clipping; +4dB is about as loud as it can go which incidentally puts LFE at the same level as the BD has it.

JPIII can be boosted +6dB without clipping but this makes the LFE +2dB louder than the DTS DVD track (all other channel levels of this track match the theatrical). Again +4dB would match the DVD.

I'm converting with the foobar2000 plugin v0.1.4. Also these movies were released in DTS-ES 6.1 with the additional channel matrixed into the surrounds. I don't know if this makes a difference. I'd be grateful for any insight.

I have not done a DTS-ES 6.1 yet, not sure if that affects things.

If not and for the others:
First when using foobar turn off it's auto LFE creation and all of it's +dB processing for any channel, let it do nothing at all. I seem to recall that you set it to do LFE and adjust dB of anything that the dB amounbt adjusted seems different numbers than when doing it manually so not sure what it is doing under the hood. And use the most recent version.
Take the Ls,Rs and combine the two tracks and then do a lowpass at 80Hz filter on them and then save that out as the LFE and then do a +3dB gain to it (already have a +3dB from combining the signal stored in both Ls AND Rs). For pre-1999 I think forget the +3dB stage. Smetimes you need to do the 80Hz lowpass filter first before combinging them as the upper frequency power might clip stuff, in those cases, 80Hz filter first, then combine the two channels, then +3dB if post start of 1999 release. Actually it's been a while, I forget, I think maybe in the end I did NO +3dB for the early 1999 title actually and decided it maybe just needed nothing done since they had recorded it in studio with LFE speaker already set to account for+3dB and then they stored it once in each channel and combining gives another +3dB. So maybe it is actually low pass, combine and then -3dB for pre-1999 and as is for 1999+? I forget, will check. I know that whatever I did, in audactity, my LFE looked about the same as what Schorman got for AOTC and all I did was either lowpass each and then combine Ls and Rs and then leave them be or at most then do a +3dB at that point and did no +6dB or +9dB and such for sure.

Then undo those actions on the Ls and Rs and then apply -3dB to each and then apply a high pass filter to them at 80Hz.

I was doing 48dB octave passes in Audacity although that seems to remove more stray high frquency from LFE than what the CinemaDTS box does looking at Schorman ouput so that box maybe does something more like 24dB per octave but it also seems to use a different filter method too so it won't be quite the same whatever way.

My software method in Audacity left a bit less instances of clipping than the Schroman DTS box output, neither had many clips at all, but the official box output had a few more. Or perhaps it came from when he resampled it from 44kHz to 48kHz? Not sure if resampling can produce clipping, have to think, maybe. I know applying low and high pass filters can since even though you are removing total energy you can realign some stuff to re-enforce and can phase shift some frequencies relative to others and they can add in new ways and they can also produce peak artifacts near cut frequency, etc. As even his center for one disc had the part that also clipped one mine (apparently clipping in the master!) clip just a bit more, maybe that was from his 44->48Hz resmapling stuff.

To best avoid clipping you may need to play around with the order of steps and balance clipping vs. maintaing max SNR:

For one title (using Audacity and whatever algorithms and methods it uses) reel 1:
I have to first -3dB before high-pass to create surrounds if I don't want any clipping in them.
I can straight add the two and then lowpass and make LFE with no clipping.

for reel 2:
I can just high-pass the surrounds first without clipping and then give them their -3dB.
I can straight add the two and then lowpass and make LFE with no clipping.

for reel 3:
I can just high-pass the surrounds first without clipping and then give them their -3dB. EDIT: nope can only do that for left surround, right surround need to first -3dB and then high-pass to avoid clipping.
I can't just straight add the two surrounds and then lowpass and make the LFE without it clipping. Even doing -3dB first to each surround and adding them still clips. I found I could first low-pass them and then add them as is without clipping though.

etc.

I wonder how the DTS processor does it. For safety does it just first -3dB the surrounds and then high-pass and then raise them back up again 3dB (remember the theater speakers have surrounds set -3dB) and then also add the two -3dB adjusted (EDIT: nope this is not enough to stop clipping for some of them when starting to make the LFE) together to make the LFE and then low pass that and then raise that back up again 3dB? Does it just apply all the stuff and the adding as they are and you end up with some clipping? In which case these manual methods with care may give more distortion free output than the theatrical processor. Maybe it does -3dB to each surround and then high-pass filters them to produce the surrounds. And then it goes back and looks at the original combo surround data and does a low pass filter on them and then adds them together.

[2001CluelessDianogas]

One thing to keep in mind is how you are outputting this stuff and whether your PC is doing the +10dB (+6dB depending how you measure stuff) boost for LFE channel to your sub and your sub at home is normally set +0dB and not +10(+6dB) like in a theater, in a theater the sub is the LFE while in most home theaters the sub is the LFE signal (which needs to be +10dB) and the re-reouted deep stuff from all other channels (which needs to be +0dB). Etc.

How does your A/V receiver handle .1 input from analog vs PCM digital inputs vs DTS/DD etc.

[maksnew]

Perhaps I just didn’t see this in various comments, but most likely this is not the case, when changing the sample from 44.1 to 48kHz in most modern editors, a frequency cut occurs and quality is lost by 20k in the frequency range, which already becomes acceptable only for DTS but not as not for DTS HD, I certainly found a way out, but the result of the deterioration was very unexpected for me.