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

[FoLLgoTT]

Would that filtering be an inherent part of the mix, or just something introduced in the DTS encoding process?

It is always the result of the Cinema DTS format, because of the missing dedicated LFE channel. I analyzed many home releases in the past and none of them had a high pass at 80 Hz in the surrounds.

[zoidberg]

DTS' official specs are/were 20Hz-20kHz for L-C-R, 80Hz-20kHz for L/R surrounds and 20Hz-80Hz for subwoofer. Any information below 80Hz that was originally in the surround channels will be sent to the subwoofer (but low frequencies are non-directional). In the theatrical environment the subwoofer level is set to +10dB in-band gain using a RTA, home codecs electronically add +10dB gain to the LFE channel, it wouldn't surprise me if an RTA is needed to 100% get the levels correct.

[FoLLgoTT]

it wouldn't surprise me if an RTA is needed to 100% get the levels correct.

Room EQ Wizard has an RTA that can load WAV files. I threw in the band limited pink noise which contains the Center and the LFE (Reel 10) from the DS3 test disc. With a low pass at 80 Hz of 4th order the +6 dB produces a difference of 1 dB when measured with dB C. So, this seems to be correct (there are always small differences).

The spectrum of the pink noise (not band limited) of Reel 1 is interesting. The bass of the surrounds are attenuated by -10 dB. This seems to be correct, because after the routing to the LFE the level will be raised by +10 dB. But, there is a large dip in the range of about 60 Hz. Maybe during the mix the bass was separated by a low pass, attenuated and mixed back. I don't know if this can be reconstructed properly or if it is by design.

If anyone has captures from a DTS-6D/DTS-6AD this could help to find the filter function for LFE extraction. Pink noise from DS3 test disc would be helpful.

[FoLLgoTT]

The whole LFE level thing is much more trivial than we thought. The +6 dB is only necessary, because the foobar plugin multiplies both surrounds with 0.5 before mixing them. SoX does the same. This is necessary to avoid clipping. The +6 dB is just a compensation of that. It has nothing to do with the calibration of the speakers.
There is a test signal on the DS3 test disc that proofs that -20 dB on the Center equals -30 dB on the LFE (like in home environment). And this can be reached by a simple sum of the surrounds and a low pass. No correction factor is needed. The correction factor is only needed when using the foobar plugin, because it halves the level before summing.

The bass for the surround channel itself is attenuated by -10 dB as stated in my last post, because it will be played with +10 dB by the subwoofer. We can calculate the following maximum level for the summed and low passed LFE channel:

-6 dBFS (LFE/2) -6 dBFS (LFE/2) -10 dBFS (Surround left) -10 dBFS (Surround right) = +2.4 dBFS

This explains why some movies show clipping in the extracted LFE. A good master should never exceed -6 dBFS in the bass of the surrounds before the encoding. It is interesting that this is violated by real mixes.

Maybe the hardware decoder handle that in some way by attenuating all channels by -2.4 dB before extracting the LFE. That has to be validated...

Of course all this don't explain why some early movies (e.g., "Jurassic Park") seems to have a too low LFE. But it explains the +6 dB.


PS: Audacity doesn't apply 0.5 when mixing channels.