AC-4 Audio?

ATSC 3.0 Forum
jakep82
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Re: AC-4 Audio?

Post by jakep82 »

peanutbutter71 wrote: Wed Oct 21, 2020 10:51 am I have been using a 2 tuner HDHomerun with Linux with great success for years. I didn't realize that when ATSC 3.0 came out that there was no support for AC-4. I can't see support in FFMPEG or VLC, and I think Kodi depends on FFMPEG.

I was kind of a late adopter of ATSC 1.0, but I suspect maybe some similar things happened for early adopters of ATSC 1.0?
This wasn't a big problem for the digital transition because AC-3 was already in use on DVDs. Looking through the source code for FFMPEG, the earliest copyright I see for AC-3 related files is 2000. The US digital transition was mandated in a 1996 law, and it wasn't completed until 2009. I'm not sure when the first ATSC stations actually went live, but it was probably after 2000.

Trip
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Re: AC-4 Audio?

Post by Trip »

The first ATSC 1.0 stations to operate commercially were in the 1997-1999 time frame, give or take, but receivers cost thousands of dollars at that point and signals were not widespread beyond the very largest markets. I got into it in 2003 with a Zenith HDV420 ($325 open box) and a Hauppauge WinTV-D ($250-ish used).

- Trip

NTC
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Re: AC-4 Audio?

Post by NTC »

Question. Is the audio portion of a TV broadcast, in AC4, the same as AC4 that is streamed OTT?

I ask because my Roku Ultra 2020, connected directly to my TV can easily play media files encoded in HEVC with Dolby AC4 (2.0 and 5.1) in an mp4 container, yet when I capture an ATSC 3.0 broadcast that's also HEVC/AC4, saving as a TS file, only the video is played. This happens regardless of the audio HDMI setting on the Roku. The mp4 files play flawlessly.

krv
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Re: AC-4 Audio?

Post by krv »

There are many software systems (Kodi, plex, VLC, handbrake, MCEBuddy...) that use FFmpeg, so the key to getting the ball rolling on various systems is to get AC-4 implemented in FFmpeg. There is an FFmpeg enhancement ticket that is currently marked as PRIORITY: WISH. if enough people logon to the ffmpeg forum and up-vote the ticket it might get some traction, but currently it only has two votes.

demonrik
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Re: AC-4 Audio?

Post by demonrik »

NTC wrote: Wed Oct 21, 2020 1:14 pm Question. Is the audio portion of a TV broadcast, in AC4, the same as AC4 that is streamed OTT?

I ask because my Roku Ultra 2020, connected directly to my TV can easily play media files encoded in HEVC with Dolby AC4 (2.0 and 5.1) in an mp4 container, yet when I capture an ATSC 3.0 broadcast that's also HEVC/AC4, saving as a TS file, only the video is played. This happens regardless of the audio HDMI setting on the Roku. The mp4 files play flawlessly.
AFAIK there is no AC-4 streamed audio by any OTT provider right now - happy to be told otherwise though.
Wonder if your confusing with AAC/MPEG4 audio?
That said - you are 100% accurate that Roku Ultra will play AC-4 from an MP4 file - or .MKV, .AC4... just not .TS.. So if you repackage the .TS from HDHR to .MP4 or .MKV you should be golden on the roku.
Quite a few latest android phones also support - Huawei, Samsung, Oppo

Hoping it's just a matter of DSP firmware updates for NV Shield or AVRs...
Little messy right now though

NTC
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Re: AC-4 Audio?

Post by NTC »

demonrik wrote: Wed Oct 21, 2020 3:00 pm
NTC wrote: Wed Oct 21, 2020 1:14 pm Question. Is the audio portion of a TV broadcast, in AC4, the same as AC4 that is streamed OTT?

I ask because my Roku Ultra 2020, connected directly to my TV can easily play media files encoded in HEVC with Dolby AC4 (2.0 and 5.1) in an mp4 container, yet when I capture an ATSC 3.0 broadcast that's also HEVC/AC4, saving as a TS file, only the video is played. This happens regardless of the audio HDMI setting on the Roku. The mp4 files play flawlessly.
AFAIK there is no AC-4 streamed audio by any OTT provider right now - happy to be told otherwise though.
Wonder if your confusing with AAC/MPEG4 audio?
That said - you are 100% accurate that Roku Ultra will play AC-4 from an MP4 file - or .MKV, .AC4... just not .TS.. So if you repackage the .TS from HDHR to .MP4 or .MKV you should be golden on the roku.
Quite a few latest android phones also support - Huawei, Samsung, Oppo

Hoping it's just a matter of DSP firmware updates for NV Shield or AVRs...
Little messy right now though
The MP4 files were from https://ott.dolby.com and are definitely AC4, confirmed with MediaInfo. I read Tidal started having audio files in AC4 for streaming but haven't check on that. I'll try to repackage them in another container. Thanks

Lost Dog
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Re: AC-4 Audio?

Post by Lost Dog »

krv wrote: Wed Oct 21, 2020 1:37 pm There are many software systems (Kodi, plex, VLC, handbrake, MCEBuddy...) that use FFmpeg, so the key to getting the ball rolling on various systems is to get AC-4 implemented in FFmpeg. There is an FFmpeg enhancement ticket that is currently marked as PRIORITY: WISH. if enough people logon to the ffmpeg forum and up-vote the ticket it might get some traction, but currently it only has two votes.
It has five now... I just added mine. For those of us in Linux world this will be critical. MythTV will need some overhaul for both audio and tuner handling.

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Re: AC-4 Audio?

Post by demonrik »

NTC wrote: Wed Oct 21, 2020 3:20 pm The MP4 files were from https://ott.dolby.com and are definitely AC4, confirmed with MediaInfo. I read Tidal started having audio files in AC4 for streaming but haven't check on that. I'll try to repackage them in another container. Thanks
Well - that link isn't an OTT streamer - I thought you meant a Netflix, Sling or similar ;)
But - yes - definitely AC-4 from there.

Is not clear to me if Tidal streams Dolby Atmos, or AC-4 or both.
They say service plays to all dolby atmos devices - which doesn't mean AC-4. Probably depends on device.

NTC
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Re: AC-4 Audio?

Post by NTC »

demonrik wrote: Wed Oct 21, 2020 3:56 pm
NTC wrote: Wed Oct 21, 2020 3:20 pm The MP4 files were from https://ott.dolby.com and are definitely AC4, confirmed with MediaInfo. I read Tidal started having audio files in AC4 for streaming but haven't check on that. I'll try to repackage them in another container. Thanks
Well - that link isn't an OTT streamer - I thought you meant a Netflix, Sling or similar ;)
But - yes - definitely AC-4 from there.

Is not clear to me if Tidal streams Dolby Atmos, or AC-4 or both.
They say service plays to all dolby atmos devices - which doesn't mean AC-4. Probably depends on device.
From Dolby on Tidal and AC4,
https://professional.dolby.com/music/tidal-ac4-atmos/

swmarks
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Re: AC-4 Audio?

Post by swmarks »

mooninite wrote: Wed Oct 21, 2020 12:29 pm I wonder how much Dolby paid the ATSC group and/or broadcasters to force AC-4. There was no reason to use a new codec when there's a million of them out there. The best one is FOSS and Royalty-Free - Opus.
I bet that Dolby provided a turnkey package that made it stupid simple to implement.

HTBruceM
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Re: AC-4 Audio?

Post by HTBruceM »

Anyone know of an AC-4 package to install on a Windows Plex server that would work with Plex?
Or am I getting the cart before the horse?
I suppose Plex Server has to be aware of and know how to use the API for it...
I was hoping that we would be able to use a Plex Server to transcode the audio to AC3 or something compatible with the Plex client environment.

I see a package from Dolby for PCs, called Dolby Access. It advertises ATMOS support for Headphones/receivers. It's $15, but has a 7 day trial.

kyl416
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Re: AC-4 Audio?

Post by kyl416 »

Plex uses FFmpeg for their transcode function, until FFmpeg gets AC-4 support, it wouldn't know what to do with the audio format.

HTBruceM
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Re: AC-4 Audio?

Post by HTBruceM »

Ah.... OK that makes sense. It also puts many of us into a box for using/testing the new tuner. I really wish I'd have known about the AC-4 gap before I signed on to the Kickstarter program. Sorta makes it pointless.

I have only one scenario that might work... My Sony H900 TV (AndroidTV) is supposed to have (at some point??) ATSC3 tuner capability. So "in theory", this platform SHOULD have an AC-4 decoder built in. I have an ATMOS-capable Denon receiver connected to it via eARC. So far, no app on that TV will play any audio from the HDHomeRun ATSC3 tuners, with either the TV speakers OR the Denon receiver being set as the audio output device. Not the HDHomeRun app, not the Channels app, not Plex.

I was hoping that since the TV is "capable", that the Plex Client would communicate this back to the Plex Server, so that it could simply "Pass through" the AC-4 stream. After all, the TV includes the audio codec. Perhaps my TV firmware doesn't have it enabled yet. I just took a FW upgrade that enabled eARC, but maybe the ATSC3 stuff isn't turned on yet.

I hope the HDHomeRun app will be updated for the Sony TV platform soon, like it has for the LG CX OLED platform. After all, this is ONE OF THE FIRST TVs to have a built-in ATSC3 tuner. I'll try testing that tuner out next.


Update 10/22: It looks like the Sony X900H TV tuner is working with ATSC3. I suspect I have some reception issues on the tower/freq being used in Portland for ATSC3, so it's hit/miss - I don't see ALL the channels but I did receive KGW (8.1) pretty well. Moreover, the audio is decoded correctly. It plays through the TV speakers, and it also plays through my Denon via eARC on HDMI3. The Denon just says "Dolby Digital".

One note... With the Sony onscreen guide, the ATSC3 channels are shown with a trailing 0 on the virtual channel.
8.1 is the ATSC1 channel on VHF-H 8.
8.10 is the ATSC3 channel on UHF 30, 569MHZ.
The trailing 0 seems to screw up the guide data, since no programming information is present on those channels.
But at least I was able to tune into each channel separately.
This might be a solution for HDHomeRun - that way the user can make a choice which tuner is used.
Last edited by HTBruceM on Thu Oct 22, 2020 9:40 am, edited 1 time in total.

jasonl
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Re: AC-4 Audio?

Post by jasonl »

The AC-4 spec appears to be freely available as ETSI TS 103 190-1 and 190-2, so I don't think there's any technical reason for a decoder not existing, just a matter of one or more people sitting down and actually writing it. Of course, once it does exist, then it's fine to distribute it as source, but beware the army of lawyers once it gets compiled.

And the answer to why AC-4 would have been chosen is because good old fashioned back-scratching and back-stabbing. All of these decisions are made by the companies who directly stand the benefit from their technologies being chosen, so they all try to cram as much in as they can so they get more licensing. There's a lot of deals where one company that has a technology in one area will make a deal with a company that has tech in a different area to support each other over their respective competitors. And when all else fails, Dolby has been known to "offer financial enticement" to get committee members to change their votes to support Dolby: a guy from MIT on the decision-making committee in the Grand Alliance ended up making $8 million after switching his vote from Philip's system to AC-3 in the original recommendation that was given to the FCC.

peanutbutter71
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Re: AC-4 Audio?

Post by peanutbutter71 »

Should we start a gofundme to get someone to write it for FFMPEG?

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