AC3 Passthrough


I’ve been using my Tablo Dual Lite for less than a month, in this time I’ve been quite happily surprised at how well it performs, how dependable and easy to use it is, etc.

I have Surround Sound enabled, and it seems to be saving the Dolby Digital audio without issue, but the Tablo client apps are mixing it down to PCM. The same devices can playback AC3 tracks locally as well as over my sound system using other sources. I’ve also checked the files themselves and the codec info showed it was an AC3 track as well.

Any ideas? Reception is good, I always receive Dolby Audio when watching the shows OTA via direct antenna input, so I’m somewhat at a loss…

Here’s a rundown of my setup:

Tablo - Dual Lite, firmware 2.2.30
Apple TV 4K - tvOS 14.3 (latest), App 1.9.2
iPhone XR - iOS 14.3, App 1.9.2
iPad Air - iOS 14.3, App 1.9.2

General playback remotely and locally is good, recordings are good quality, etc without stutters, skips, or other playback issues and the Tablo does not fail to record the program either.

If you have any input, please let me know, and hopefully someone from support is able to weigh in. I filed a ticket 12/27, got a super dissatisfying (and very delayed) response yesterday 1/8, I replied and have not even gotten a “thanks we will look into it” type confirmation.

This sounds like a system settings issue on your Apple TV. The Apple TV should be able to pass through AC3 audio via HDMI.

I have my Roku Premiere+ Model 4630 directly connected to my HDTV via HDMI and it passes through AC3 audio to the HDTV. In the Audio settings on the Roku you can set the HDMI to “Stereo” or “Dolby Digital (DD)”. Roku models cannot decode AC3 audio to PCM so when you set the system Audio settings to Stereo, the Tablo app just tells you the Roku cannot playback the recording for example.

If I didn’t have flawless AC3 passthrough from iTunes/Apple TV app, Netflix, Disney+, and every other service I’d be inclined to agree…this also is replicable across multiple devices.

How do you expect to achieve AC3 passthrough with these devices? The iPhone and iPad will decode the AC3 audio so you can actually hear it through the devices speakers or connected headphones.

For the mobile devices, it’s a little different, but I’d like to retain AC3 vs PCM for these two use cases:

  1. AirPlay to other devices
  2. Playback on device in native AC3 to use the Spatial Audio Processing feature

If you search the forums, people have posted this works on an iPad and iPhone.

What’s interesting is that the mobile devices seem to be mixing down the track to PCM at the client app level, so the feature doesn’t engage.

I’ve reviewed settings, and also filed a ticket with Apple for good measure. They concluded the same.

As I have an Apple Developer account, I can review more device data/analytics, but at the moment I’m watching the Buffalo Bills in the playoffs so that’ll have to wait :joy:

I just wanted to give a quick shoutout to @TabloSupport!

I couldn’t discern anything of note from my analytics data, and support could not reproduce the issue, nor did they uncover a problem thus far (that they could pinpoint at least).

Even though my ticket has not been resolved, I’ve received communication from support management, and they seem to be doing their best; that’s all I can really ask for!

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Any update on this? It’s odd you can’t get AC3 passthrough from the Apple TV via the HDMI cable. Do you have the HDMI cable from the Apple TV directly connected to your AVR?

No update, lol. If I hardcode all output to AC3 in the Apple TV settings, it obviously works, but otherwise it’s mixed down when set to Auto.

I’ve tried different cables, as well as direct cabling vs HDMI ARC. No difference. It’s something I’ve learned to live with, and for the most part, I just bypass the issue by downloading the shows and movies to my file server and watching them with another app.

What’s wrong with manually setting the Audio settings on the Apple TV to what your AVR can support? I assume it can support DD (AC3), DD+, DTS, etc.

So, hardcoding down to DD 5.1 is not ideal, because then anything with a TrueHD or DTS track is converted to the lower fidelity format. My AVR supports all major codecs.

For comparison, the Netflix, Disney+, Hulu etc apps don’t have this problem, and of course the Apple apps don’t either.

And what you say hard code, you’re not actually altering any code? Cause it’s Apple and I’m sure that code is locked down tighter than a ___ (whatever analogy you want) lol

Haha no, hardcode is an industry term meaning ‘set manually’, as opposed to auto-configuration.