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

[stwd4nder2]

Recently did a BD Sync for the Speed Racer Cinema DTS and I just kinda winged it with the LFE; left it alone in Foobar and instead went into Audacity, copied the surround channels and mixed them down to mono, put an 80hz 48dB rolloff LPF on the mixdown and corresponding HPF on the surrounds. Is this the way it's typically done here? Because I noticed the LFE sounded a bit different from the BD 5.1 this way.

I've always used the Foobar plugin to extract the LFE, then make the gain adjustments in a DAW.

[Frontino]

For the sake of science though, I did convert the Tenet R6 AUD to both 16-bit and 64-bit Float to see what'd happen, and both seem to report back with the exact same results in Reaper when doing a dry pass with the original sample rate preserved (resampling to 48000 causes it to peak by +0.2, so I'm guessing it's the r8brain algorithm to blame).

Hi.
Can you confirm if the UHD BD of Tenet has the same mix given to cinemas, besides surrounds and LFE mismatch?
Like, are the L, R, C channels identical?
Thanks.

[little-endian]

So because a +6 DB boost in the LFE for mixes like Tenet would clip the peaks, one could conservatively instead lower all other tracks by -6 DB to preserve the LFE as-is (turning the mix from 16-bit to 17.5 bit to account for the -9 DB in the surrounds).

Which technically - audible or not - is the better way of doing it. Clipping should be always avoided.

Would this do a better job?

Apparently, yes.

Welp, I just tried a sample of a track that had known clipping in the center when decoding straight to 16-bit.  I did 32-bit and it's not there.

Exactly my observation with the Cinema DTS of "Fight Club" with the notorious "everytime the plane banked too sharply for takeoff or landing, I prayed for a crash or a mid air collison" scene on reel 2 - the surround channels reverely clip when decoded at 0dB setting in the plugin and 16 bit output:



Attenuating it, doesn't change anything and it remainds clipped:



When using 32-bit floating point however, while it nominally clips as well:



Peaks can be nicely recovered, confirmed also by the fact that a reducation to nominally -5dBFs in this case leads to still a higher peak of about -1.1767 dBFS.

For the sake of science though, I did convert the Tenet R6 AUD to both 16-bit and 64-bit Float to see what'd happen, and both seem to report back with the exact same results in Reaper when doing a dry pass with the original sample rate preserved (resampling to 48000 causes it to peak by +0.2, so I'm guessing it's the r8brain algorithm to blame).

I guess with "blame" you simply mean "caused by" as level rises are to be expected with fundamental operations such as sample rate conversions or when dealing with lossy sources - which is exactly the reason why better DACs such as from RME or Benchmark nowadays attenuate the levels to create additional headroom for mangling around the audio before final conversion. It shall be meantioned though that the audibility is debated as in most such critical cases, the source material is already (too) hot to begin with and hopelessly clipped anyway.

Something to note, the DTS-HD MA on the UHD (which seems to be a direct port of the Cinema DTS) also contains clipping on the LFE channel:


Some other titles that are also close ports of the Cinema DTS show the same.

Indeed, however them having baked in clipping (too), doesn't mean it's right and one should follow their example. If there is an option to have less or no clipping, then this is technically the correct way to go. Or in other words: proper decoding and level adjustment of the Cinema DTS sources then gives us the chance to create a better version than the ones used for the (UHD) BDs.

Which is also the reason why I didn't decode the Fight Club Cinema DTS AUD files with +6dB for the LFE and -3 dB for the surrounds straight away as the LFE would already clip in the "mid air collision" scene as becoming about +2dBFS then, so I opted for lowering L,R,C by 2dB, raising the LFE by 4dB and the surrounds by -5 dB. No (additional) clipping this way, although the L,R,C channels for Fight Club look a little "hot" on their own, even when decoded to floating point which makes me wonder whether the APT-X100 is really using it for all channels (on the other hand, why not, right?).