AV sync issue (AppleTV 4k)

i'm seeing a/v out of sync with my 4k appleTV with 1080i recordings from a particular channel.

the path is: appleTV -> hdmi -> samsung 4k TV -> optical output -> pioneer receiver

if i play this recording on my mac mini thru the same path (obviously different HDMI input on the tv, but otherwise the same), the av sync is much better (but not perfect)

i can trim one of these recordings and upload it somewhere. what tool would you recommend to trim? is ffmpeg ok?


Better to use dd to cut a sample, as ffmpeg will rewrite the timestamps by default.

Are you sure the sync issue is not due to your TV’s ARC? See if you can change it to bitstream audio mode.

Does the sync problem go away if you change your Apple TV audio settings to Stereo?

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the TV appears to be in bitstream mode. there does not seem to be a setting specifically for the ARC.

sure enough, switching to stereo fixes the sync. if the appleTV is in auto or 5.1 mode, there is a delay. so what does that mean? i’m pretty sure the desync is just on this one channel, so it seems like there’s something about the stream that’s wonky.

i can use dd to cut the file, i guess i was worried that i’d be cutting it in the middle of a packet but i guess that doesn’t matter given the nature of the beast.


This means something in your audio path is introducing a delay because it is slow at decoding surround sound. When you set the Apple TV to Stereo, our app outputs remixed stereo instead of the original surround sound track.

I can confirm that the audio is out of sync on my Samsung 6300 tv in bitstream mode going from optical to my receiver. It started when I started using this set up and it only seems to be affecting the Channels app. And it’s only on a couple of my ota channels, not all of them. Super annoying!

but what’s strange here is that other stations with 5.1 sound are totally fine sync-wise. and all of my movies with 5.1 sound are also fine… which makes me think this is stream-specific.

i realized that my VLC experiment was wrong - even though it was set to HDMI as the output, the particular mode seems to be decoding the audio and outputting it as stereo (and fixing the sync). the next audio output in the list is “HDMI (encoded output)” and i see the delay when using that output (and the receiver indicates 5.1 audio). but the even weirder thing is that the delay seems to only occur on PBS NewsHour, and only when a remote guest is speaking. the host’s a/v sync is reasonably good, but still a little off. this makes it seem like the delay is in the broadcast, but somehow when down mixing the sync is restored…

is it possible that somehow the PTSs in the stream are correcting for the delays but they are ignored by the receiver? meaning, the software decoding is able to fix up the sync while the hardware decoder in the receiver is not.


It is probably that your output is not bitstream and the TV is trying to decode the audio instead of your receiver. And the TV will prioritize the video which is why you notice the issue most on the channels that are 1080i with a high bitrate.

It would go away with the TV output on bitstream, but I don’t think your Samsung will do that.

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the TV is in bitstream mode, or at least the only setting that lets me set the audio mode is set to bitstream. as mentioned above i don’t know if that controls the optical output of the TV or not.

how could it be decoding the audio if my receiver is seeing a 5.1 stream on the SPDIF?


If you send me a sample I’ll see what the pts stream looks like.

totally flaked on this, i’ll try to make a snippet. the problem persists on KQED and really only during PBS news hour. all other recordings i have are fine, so i don’t think there are any intrinsic delays in the audio path.

it looks like KQED has fixed this problem - watching the broadcast now and the AV sync is fine.

(unless you guys made some change in a recent ATV client update recently)


well it’s 6 years later and this problem is back. i am not sure when it happened, but i have many terabytes of recordings from kqed 9.1 so i might be able to figure out if the problem is the fault of the broadcast or not. nothing has changed in my setup and the problem is still occurring only on 9.1.

are you guys still interested in a snippet of a problem recording?

I would use ARC from your TV to your receiver. A/V sync isnt mandatory over ARC like is with eARC but you will likely see a huge improvement going that route.

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not sure my stack supports that, but i will look. the TV and receiver are ~2014 and currently the tv is outputting the audio from the apple tv via spdif (optical)

edit: ok, both do support it but i'm using the ARC-capable port on the TV to connect to a computer. moving it is possible but because i'm using the logitech harmony hub it's kind of a hassle to reprogram everything, not to mention i've cut the harmony off from the internet to avoid surprise firmware updates like the one that killed homebridge support. i can certainly try ARC though just to see if it solves the problem.

from a technical standpoint, i'm confused by this though. the audio and video streams should have PTS markers in them to establish the sync, right? since the TV is receiving the muxed video/audio from the appleTV i'd think that the TV would fix the sync before passing the audio signal to the receiver. or is this impossible since the signal is still in its digital form? i'm not sure if this TV even has speakers, but it might be interesting to see if the sync is OK when the TV does the D/A conversion.

You can allow harmony to get to the internet. They arent sending any firmware updates to it anymore. Spdif is a bad idea in general as it is really old school. It can absolutely effect latency and sync. Appletv is outputting HDMI to your TV and then your TV is sending its digital audio feed to your receiver. There is no sync feature with Spdif at all so it kind of is what it is... now with regular ARC sync is optional but it will likely work better.

All of this is just my guess. If your souce is jacked up then all of this is moot lol.

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we will see. the thing that doesn’t make sense to me is that it’s only one channel that is doing this. so i guess there is something different about the broadcast. perhaps it is still in spec though.

i’m going to see what happens if i use different software to watch one of the bad ts files, since vlc now exists for the apple tv.

another data point... it seems that if i pause the video, back up a little bit and replay that the sync fixes itself for a while. i think perhaps this is a thing that just gets worse and worse the longer the program goes on.

if i play the file in VLC the sync seems correct, however, it could be caused by scrubbing around in the file in the same way as in Channels. i guess i will just have to watch 10 minutes of it or so in VLC and see if sync is maintained.

also, as expected, telling the appleTV to output stereo fixes the sync problem. unfortunately telling it to change audio formats to 5.1 does not solve the problem.

i haven't been able to try ARC yet.

i do have a snippet of a program that exhibited the problem, but i wanted to play it in Channels to see if the problem could be reproduced on a short file. however, i can't seem to add a .ts file to my Personal Media source. is this to be expected @tmm1? i guess i could temporarily replace a file for an actual recording if adding a transport stream to the personal media is impossible.

You can add files to the Imports folder

ok - i've tried to connect the receiver and TV with an HDMI cable on the appropriate ports to support ARC. the receiver has a soft switch to control whether audio comes from the spdif or the hdmi port. unfortunately there's no sound at all when i tell the receiver to use the ARC hdmi port. i fiddled around with the sound settings on the TV to try all the different audio formats and see if it liked any one in particular but no luck. so ARC seems to be out. i think the equipment here is just too old.

in messing with the TV's sound settings while trying to get ARC to work, i had to reboot the TV once (switching digital output between Bitstream and PCM). this seems to have fixed the problem - almost - on my problem broadcasts. i also fiddled with the fixed audio delay in the TV's settings and it seems to me that in order to fix this problem i'd actually need a negative audio delay. that's pretty counterintuitive and i guess it could point to a problem with the broadcast.

anyway i will keep an eye on it and if it starts happening again i'll just try power cycling the TV to see if that fixes the problem.

Using ATV4K for many months. Never ran into lip-sync until a few months ago. Running Channels DVR from Synology NAS wired into switch feeding to ATV4K (running the Channels client) as well as HDHomeRun for OTA channels, and finally into Lumagen Radiance Pro which sends video via HDMI to JVC projector and separately sends audio to Trinnov A16 audio processor. Have to use the Lumangen's facility to alter video delay by about 100ms (about 2 frames) and it's still not perfect.

However, if I do the same with my Zidoo player as the source running the Channels client I do not have to alter the video delay from 0. And there is no audio/video delay of any kind for watching channels in this situation, nor accessing video files from the same NAS.

Maybe it's a difference in the Channels apps on the ATV and Zidoo? Any other ideas?