Distorted DTS-HD to Stereo Downmix

Video Playback
1

Videos with DTS-HD audio tracks exhibit clipping/distortion when the Apple TV is configured to output Stereo L-PCM audio. The problem also occurs with playback to Bluetooth stereo headphones.

I rip my Blu-ray disc collection to MKV, keeping the original video and audio tracks (no re-encoding). When playing back these rips with Infuse, I’ve noticed that movies with DTS-HD audio, especially those with loud scenes (expositions, plane flyovers, etc), have distorted audio.

To replicate the problem I made a temporary testing setup to allow me to capture audio from the Apple TV onto my PC with Audacity. I used a HDfury DIVA, which lets me force the HDMI EDID to stereo audio. This device has SPDIF output, and I connected this to a SPDIF input on a USB audio interface connected to my PC.

For testing I used clips the following movies:

  • Interstellar
  • Sherlock Holmes
  • The Dark Knight Rises

These clips were created by first locating a section of the movie which exhibited the audio distortion. I the used FFmpeg to extract a short clip of the movie, starting at the problem timestamp. FFmpeg was set to copy the audio (no re-encoding). I then play these clips with Infuse and VLC on the Apple TV from a Samba share on my NAS.

My test capture setup is as follows:

  • Apple TV (model A2169, tvOS 16.2)

    • Settings > Video and Audio > Audio Format: Auto

  • Infuse Pro (v7.4.10 [4329])

    • Audio > Volume Boost: Disabled

  • Apple TV HDMI connected to a HDfury DIVA

    • This device lets me adjust the HDMI EDID and also has SPDIF output. It also shows detailed information about the HDMI signal.
    • EDID set to “1080P59.940 444 BT709 8b SDR 148MHz 2.2” video + “L-PCM 48kHz 2.0ch 24bit” audio.
    • Status screen shows that AppleTV is sending L-PCM 2.0ch audio.

  • HDfury DIVA SPDIF output connected to SPDIF input on USB audio capture interface.

  • HDfury HDMI output connected to monitor.

Here is an screenshot of Audacity with 3 captures of a clip from The Dark Knight Rises.
From top to bottom:

  • Infuse on Apple TV, captured via SPDIF
  • VLC on Apple TV, captured via SPDIF
  • FFmpeg on Linux (ffmpeg -i dkr.mkv -ac 2 dkr-2ch.wav)

You can clearly see that the volume of the Infuse capture is louder than the others, and this loudness results in lots of clipping. Listening to this capture, you can hear distortion, and voices and other sounds become difficult to hear.

DKR
DKR954×1065 38.3 KB

Similar for Interstellar:

interstellar
interstellar948×1061 39.2 KB

Here are the SPDIF captures of Infuse playback (converted to AAC for upload to the forum):

Interstellar:
The Dark Knight Rises:

And here is the audio extracted from the clips using FFmpeg with default downmix (e.g., ffmpeg -i dkr.mkv -ac 2 dkr-2ch.wav) (converted to AAC for upload to the forum):

Interstellar:
The Dark Knight Rises:

For the Infuse developers (and anyone else interested), I’ve uploaded the clips I used to test (with the video tracks re-encoded to a much lower bitrate to save space), and the WAV captures from Audacity.

https://drive.google.com/drive/folders/1xejwwJiW0Vc_p4dRjO5SMsagYpCvVlCT

14 Likes
2

Thanks for the super-detailed report! :heart_eyes:

We’ll look into this to see if we can uncover what is going on in this case.

2 Likes
3

I have the same problem except it’s not limited to DTS-HD, but is true for all multichannel audio and my conclusion is that how Apple TV handles stereo downmix is the cause. It boosts central channel and crushes the dynamic range of the soundtrack which may be ok for tv speakers but sounds awful on a hi-fi 2.0 setup.

Apple tv+ and basically every app that doesn’t have a downmix option or a dedicated stereo track has exactly the same problem. There are few exceptions, like Netflix which sounds great (i assume it provides a stereo track) or VLC.
Don’t remember vlc having a downmix option, maybe it can just extract stereo channel from multichannel audio and only send them.

It would be great if Infuse would have some options to solve this, because in every other aspect infuse is just flawless

2 Likes
4

I have noticed recently that the the audio quality of infuse appears to be significantly worse when decoding and down-mixing multichannel sources (Dolby/DTS or LPCM) vs other apps/sources.

I use a 2.1 setup, with the ATV decoding everything in 2.0 24/16bit 48KHz LPCM. Following the 7.3.4 Infuse release, the LFE track is also now mixed into the 2.0 stereo down-mix from multichannel tracks instead of being discarded.

It is hard to describe exactly the issue I am hearing and I have no way to do a direct digital recording to compare, but there appears to be some severe clipping and distortion in the LFE, along with a generally narrower sound stage and more “compressed” sound with reduced dynamic range.

An obvious example of the issues described is with the Dolby Amaze demo (available here) using either the standard DD or True-HD Atmos version. At around 00:36 there is a bass rumble, which when played in Infuse sounds terrible with significant clipping and distortion. Also around 00:20 there is a sound that pans left and right, which in Infuse sounds very narrow.

If the same file is played in MrMC (which can also mix the LFE on down mix) the audio quality is noticeably more dynamic, with no clipping in the bass and the full dynamic range maintained. Given this is an app on the same ATV with the same settings then the problem is with how Infuse is handling the decoding and down-mix. Even using VLC (which drops the LFE) the quality is much better and more dynamic than Infuse.

I have no way to tell if similar issues occur when the ATV is outputting 5.1 or 7.1 LPCM and a down mix is not involved, but presumably as the LFE is not being mixed my assumption is that it do not.

There is no reason why MrMC should sound significantly better than Infuse, they are decoding and outputting the same signal on the same hardware. It would also be nice if Infuse provided the option to adjust the LFE mix volume, as well as disable it entirely in the same way as MrMC.

Versions
Infuse: 7.3.10 (tvOS)
tvOS: 15.5

Relevant AppleTV settings:
Audio Format: Stereo (makes no difference set to stereo or auto as the display EDID is forcing 2.0 PCM)
Reduce Loud sounds: Off
Audio mode: Auto

2 Likes
5

Bump as this audio issue is still a problem on 7.4

2 Likes
6

Bump, issue remains as of 7.4.2 tvOS

2 Likes
7

I had the same sound quality issue as yours.

However, I found this problem while using the Airpods Max to watch a video on my iPhone.

This video is the Dolby Amaze demo you mentioned. And, right around 00:36 you mentioned there is a bass rumble, which when played in Infuse sounds terrible.

Also, this is an audio quality issue that only appeared after my iPhone was upgraded to iOS 15.7. Before this, my iPhone was iOS 15.4.1, and I watched the Dolby Amaze demo countless times under this version, but it never had bad bass.

Have you solved this problem now?
best wishes

Versions
Infuse: 7.4.5 (iOS)
iOS: 15.7 (iPhone 13 Pro Max)

1 Like
8

No nothing has changed, the audio issue persists in the current release.

No response, no investigation, just ignored. Even raised directly as a support ticket got nowhere.

Surprised more people dont notice, but if you care about audio quality then take your money elsewhere.

I no longer use infuse as my primary device as a result.

2 Likes
9

Hi, I have same problem with downmix to stereo. I also try another app (vlc, plex) and there is it much better. So this must be a Infuse app problem. Any reaction from developer?

3 Likes
10

See above, situation has not changed and no reaction from the Devs.

There have been a number of posts regarding poor audio quality in Infuse vs other software that are not just related to downmixing.

I am not aware that any are being investigated and have given up chasing as it is clear it is never going to be fixed.

3 Likes
11

Bump.
The issue remains

3 Likes
12

It’s still not fixed. Especially loud sfx are distorted. The center channel volume is too loud as well.

3 Likes
13

Bump

The issue has not been fixed yet. Loud parts of movies are distorted.

3 Likes
14

This isn’t the only problem. The soundstage also feels very compressed and narrow. It feels like there is a lot of information missing in the downmix. Mrmc has the function to include the Lfe as well and manages to mix all channels correctly. It sounds so much wider and more open, sfx are rendered correctly and at the right volume level. So I don’t know why infuse shouldn’t be able to do this.

3 Likes
15

+1 noticing this problem. It’s especially noticeable for me using “audiophile” headphones, the images provided in the first post translate exactly what I feel I’m listening to. Granted, I have to bump the volume by a lot using Plex, for instance, but the dynamic range doesn’t get lost

3 Likes
16

I suggest that we all fill a request.

It’s not acceptable that this issue is still here today after being exposed in a very detailed way by @twistedpair in January…

Infuse team, please resolve this issue ASAP. Distorted/clipping sound is not fun at all!

3 Likes
17

Do we all have to submit a request individually or does only one person have to do it?

18

Don’t know about you guys, but I noticed a difference in the last 7.5.2 update.

The volume of downmixed files is lower; not as low as MrMc for example, but I didn’t hear any audible distortion or clipping.

However, the sound presentation is completely different compared to MrMc. Infuse seem to focus greatly on the center channel.

Let me know what you think of this update.

19

I don’t think any audio changes were in 7.5.2. Maybe it was related to tvOS update? Who knows what they put in there

20

Still getting distortion; I’ve been using the same scenes to test it and it’s still there. The first ~10min of Blade Runner 2049 have some good examples - like the ship landing.
Using a Remux version of the Blu-Ray, TrueHD 7.1 downmixed to stereo.

1 Like