I use Dolby Reference Player v3.1.0 to listen to music in Dolby Atmos. By recording its output with Loopback, I found that AC-4 files played back through DRP lack a lot in the higher frequency range; they cut off at around 18.2 kHz. This happens in all downmix modes.
Same songs in E-AC-3 are fine (they cut off at 20.5 kHz, but that's probably because the Dolby versions are mixed with a 44.1 kHz master).
Here's a gif:
(https://i.ibb.co/fQyQTp7/anigif.gif)
I also played a random .ts sample (this (https://drive.google.com/file/d/1xpEyd59cD4NgLtfYsWuBOrMjovzCO7xj/view) tv broadcast recording from here (https://trac.ffmpeg.org/ticket/8349)) with a single audio AC-4 stream in DRP to the same effect:
(https://i.ibb.co/BT0P79Q/random-ts-sample.jpg)
Does anybody have an idea why that could happen? Might this version of DRP be faulty? :'(
update: v3.2.0 on Windows has the same problem...
(https://i.ibb.co/Jx2xdtc/Screenshot-2023-04-03-at-7-51-52-PM.png)
Man, you're using a window size of 256 in the 2 images u posted initially... am actually surprised that it is showing some differences across the entire spectrum at all... cuz you cannot see anything in window size that small...
Increase it to at least 4096 and Hann Window in the dropdowns there. Then see if you still cannot spot stuff above 18khz...
darkalex thanks for your suggestion! A window of 256 is really small, isn't it... I don't look at spectrums on a regular basis, but this time I could hear that there are less highs and I needed evidence :)
Here's a gif with a 4096 window, the problem seems to persist:
(https://i.ibb.co/K6TTZz2/anigif2.gif)
I asked Dolby the same question (https://professionalsupport.dolby.com/s/question/0D54u00009UX2t9CAD/ac4-played-back-in-drp-loses-all-highs-after-18-khz-why) and an employee replied "duh, it's a lossy codec with psychoacoustics". So... it's MP3 all over again and my question was stupid 🤡