Kodi Community Forum

Hello,

I have a Pi4 connected via HDMI to my TV.  My TV is connected to my avr via SP/DIF.  My TV is set to passthrough sound to the avr.

With Kodi set to passthrough mode for audio, the avr receives and decodes the audio (Dolby Digital and DTS) correctly.  However, I notice 'flickers' of sound while watching a movie (The Matrix in this case), and it isn't long before the sound is out of sync.  If I stop playback, go back 10sec and play again, everything is fine, until it loses sync again in a short time.

However, I read HERE that I shouldn't use passthrough and should allow Kodi to do the decoding.  That's fine, I thought by allowing the avr to do the decoding, it would be reducing the load on the Pi, but whatever.

Problem is, when I turn off passthrough in Kodi, I only get Stero from the avr.  Setting the "Number of channels" in Kodi doesn't seem to have any effect.  And I can't find any other relevent settings.

It's possible that the movies I'm trying to play back also have a stero sound track, but why would Kodi choose that one when there is DTS available?  If that's even what's happening.

So two things:

1.  Why when using passthrough does the audio end up out of sync?
2.  More importantly, how can I get Kodi to decode the Dolby/DTS and send it to the avr?

Really appreciate any help.

I used htop to check how the Pi4 was performing.  I thought it might not be able to handle 4k content or something.

Looking at the four cores, they are all minimal use, so (seemingly) no issue there.

The memory was yellow and 350MB/3.5GB.  That doesn't look like an issue, by why was it yellow?  Could that be the problem?  Not enough memory allocated

I can't tell if the issue is a video playback one, or an audio one, but it makes movies unwatchable.

Really need some help here.

SPDIF only supports two channels, but can pass compressed data through those two channels (AC3/DTS).
If a HDMI connection to receiver is available then that is preferable and will support multichannel PCM.

If not, you might want to try settings number of channels to 2, enable AC3 (but not DTS) and enabled AC3 transcode.
I think that may allow you to enable "sync playback to display" (which can improve audio sync in some cases) while still having multichannel audio.

(2020-10-09, 12:13)popcornmix Wrote: [ -> ]SPDIF only supports two channels, but can pass compressed data through those two channels (AC3/DTS).

HDMI has long surpassed TOSLink...and as u/popcornmix has suggested, we should be using it for both Audio and Video transmission wherever possible. 

TOSLink interfaces are basically 4x the 2 channel SPDIF optical format, and they're capable of 8 channels of unidirectional 48kHz 16Bit throughput.

By the time affordable 1080 HDTV/displays became the de facto norm in the market, I'm pretty sure the TOSLink optical audio interface was inexpensive enough to include on just about every display. 
The spec is a good match for multichannel audio - to a point: there were sync issues between pairs of channels back in the day that this was a prosumer audio format; it was simply a stopgap to give HDMI time to develop/catch up in the digital audio world.

If memory serves, the latest HDMI spec does at least 7.1audio at 192kHz/24bits AND 4k video...quite a bit of data....bidirectionally.

NOW - ATSC3.0 is coming, and it can go up to something like 28 (or more!) channels of immersive audio and 8k Video (or higher)...and they're thinking about putting 5G mobile chipsets in TVs to compliment the tuner...it's getting ridiculous .  is your home theatre ready?