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Old 17th January 2019, 12:47   #1  |  Link
hajj_3
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Dolby AC-4 decoding support?

It seems as though vlc player and mpc-be do not support Dolby AC-4. From reading Dolby's own website (https://dolbyac4.com/uk/) it appears that U.K free-to-air satellite platform 'Freesat' has picked AC-4 as the exclusive next gen audio format for their 3rd generation satellite boxes that will be released this year. Therefore AC-4 broadcasts will likely happen this year not just a trial. It would be nice if FFmpeg/vlc/mpc-be added support to decode this audio codec. Does anyone know if work is being done in any of these to add support?
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Old 27th January 2019, 21:23   #2  |  Link
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I found this, not sure if it will be helpful for us: https://github.com/DolbyLaboratories/dlb_mp4base
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Old 28th January 2019, 07:55   #3  |  Link
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I found this, not sure if it will be helpful for us: https://github.com/DolbyLaboratories/dlb_mp4base
Not helpful. That's an MP4 muxer, not an AC-4 decoder. This PDF file might be useful, assuming there exist capable developers interested in transforming the specs into a functional decoder :-/
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Old 20th February 2019, 21:37   #4  |  Link
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Some work for FFmpeg to support AC-4 decoding has begun. When it'll be complete and/or usable .. we'll see.
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Old 21st February 2019, 00:20   #5  |  Link
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Some work for FFmpeg to support AC-4 decoding has begun. When it'll be complete and/or usable .. we'll see.
nice

I can't seem to find any .AC4 sample audio files on the internet though which is annoying.
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Old 28th February 2019, 17:53   #6  |  Link
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If anyone has any AC-4 samples, that would be helpful for development. Preferably real-world samples, the commonly found ones in the DASH test suite etc, we already got.
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Old 2nd March 2019, 01:19   #7  |  Link
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Some dvb broadcaster are sending ac4:
https://www.dvbviewer.tv/forum/topic...4-sound-track/

I can ask for a sample if it helps.
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Old 2nd March 2019, 08:23   #8  |  Link
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Already got some from the DVBViewer people, was hoping for more alternative samples to cover as much as possible, but thanks!
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Old 4th March 2019, 11:27   #9  |  Link
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Already got some from the DVBViewer people, was hoping for more alternative samples to cover as much as possible, but thanks!
Please send all of your AC-4 samples to the developer of mediainfo, he wants some to add detection support: jerome@mediaarea.net
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Old 13th September 2019, 09:19   #10  |  Link
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MediaInfo 19.09 changelog:

AC-4: basic detection, raw, in MP4 or TS
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Old 13th September 2019, 17:40   #11  |  Link
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AC3 has been a good and reliable codec for ages but it needed an high enough bitrate, something like 320-384 kbit/s to sound really good and now there's AC4... How is it gonna be compared to AAC at no so high bitrates like 128-192 kbit/s, I wonder...
Well, I think we're about to find out in the near future.
Unfortunately, I do not have a sample since nobody ever sent us an AC4 audio file, otherwise I would have shared it...
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Old 14th September 2019, 23:00   #12  |  Link
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AC3 has been a good and reliable codec for ages but it needed an high enough bitrate, something like 320-384 kbit/s to sound really good and now there's AC4... How is it gonna be compared to AAC at no so high bitrates like 128-192 kbit/s, I wonder...
Well, I think we're about to find out in the near future.
Unfortunately, I do not have a sample since nobody ever sent us an AC4 audio file, otherwise I would have shared it...
Most likely around opus levels... I doubt that ac4 will be better at very low bitrates.
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Old 15th September 2019, 14:09   #13  |  Link
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Most likely around opus levels... I doubt that ac4 will be better at very low bitrates.
I see.
I never really bothered to monkey around with opus to be fair, but that's only 'cause opus was born as a low-bitrate speech-optimized codec which has been extended to support other profiles for general contents encoding and because of that, its adoption has never been very widespread for general purpose contents and therefore many devices don't officially support it. For this very reason, I've been using AAC to encode audio files in lossy. It's pretty much the same reason why, whenever I gotta keep an audio file lossless, I still keep the original PCM in .wav instead of re-encoding it to FLAC: the megabytes it saves are negligible compared to the fact that almost nothing will play it.
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Old 15th September 2019, 14:57   #14  |  Link
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Originally Posted by FranceBB View Post
AC3 has been a good and reliable codec for ages but it needed an high enough bitrate, something like 320-384 kbit/s to sound really good and now there's AC4... How is it gonna be compared to AAC at no so high bitrates like 128-192 kbit/s...
If I had to guess, I reckon it's more likely that AC4 coding technology is derived from E-AC-3, which works much more efficiently at lower bit-rates than AC-3...

From Dolby's blog [2017]: Hearing Is Believing: Comparing Dolby AC-4 to Dolby AC-3
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Last edited by SeeMoreDigital; 15th September 2019 at 15:07.
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Old 15th September 2019, 23:09   #15  |  Link
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I see.
I never really bothered to monkey around with opus to be fair, but that's only 'cause opus was born as a low-bitrate speech-optimized codec which has been extended to support other profiles for general contents encoding and because of that, its adoption has never been very widespread for general purpose contents and therefore many devices don't officially support it. For this very reason, I've been using AAC to encode audio files in lossy. It's pretty much the same reason why, whenever I gotta keep an audio file lossless, I still keep the original PCM in .wav instead of re-encoding it to FLAC: the megabytes it saves are negligible compared to the fact that almost nothing will play it.
Opus is a standard audio codec on YouTube so every android device (Smart TV/smartphone) must support IT.
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Old 16th September 2019, 17:29   #16  |  Link
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... I still keep the original PCM in .wav instead of re-encoding it to FLAC: the megabytes it saves are negligible compared to the fact that almost nothing will play it.
Eh???

I would hardly call an average 50% reduction in a Flac file size over PCM file size "negligible"...

Also in my case... my OPPO's, my Onkyo AVR, my Android mobile phone, my Android tablet and my LG televisions (via HDD) can play Flac files.
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Last edited by SeeMoreDigital; 16th September 2019 at 22:08.
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