Open-Source Dolby/DTS decoder?

[kipphebel3]

I find it odd that no-one seems to have created an open-source implementation of a Dolby/DTS decoder? The patents for the original implementation have expired, right?

 

[ojg]

There have been open-source Dolby Digital/AC3 codecs out for many years. Search for ac3dec and libavcodec.

BTW, I think it's the first time I've seen the flag of Åland in here. Welcome!

 

[mushroommunk]

As ojg said, they do exist. The two I know of are ffmpeg and ac3filter.

ffmpeg also "kind of" supports Dolby Digital+, but all it really seems to do is pull the original Dolby Digital signal out.

 

[kipphebel3]

Thanks. Will have a look. Wonder how much work it would be to port to a suitable small DSP?

 

[mushroommunk]

Not that much work if you know C++. I already looked at doing it myself. I just don't have time at the moment due to a different DSP project.

 

[daemonsgr]

I see many implementations
I just checked github:
GitHub - foo86/dcadec: DTS Coherent Acoustics decoder with support for HD extensions

 

[kipphebel3]

"Not that much work if you know C++"

That I do Wonder if you even need a DSP if all you need to do is to split up 5.1 into separate channels.

Ie, cannot ALSA decode Dolby Digital/AC3 without extra hardware?

 

[mushroommunk]

It depends on what you're trying to do. I don't know too much about ALSA but I wouldn't be surprised at all if it supports the codec. ffmpeg has already been put on a raspberry pi and it has HDMI so you could look at that as a possibility for decoding streams (Like TV HDMI ARC out, which is what I want, so I can process it for my soundbar).

If you just want to re-encode dvd's then yeah you don't need extra hardware for sure.

 

[kipphebel3]

Background: I have an (CM6206-based) USB soundcard that can do 5.1 or 7.1. It has analogue outputs and one S/PDIF output. The chip allows me to route individual channel pairs to the S/PDIF, but when doing so it mutes front L/R.

What I would like to do is to have front L/R and center+Sub output on the analogue outputs and L/R Surround on S/PDIF. If needed from a different card.

 

[mushroommunk]

I took a glance at the documentation and see no hardware reason for that muting, weird.

Anyways, you could definitely do something like decode on a pc and send the audio over two USB connections, then you're just using the two soundcards as DACs really. Syncing with video will be your pain in that scenario depending on how fast you can decode the audio.

 

[kipphebel3]

No, I see no reason for that either. I tried mailing the manufacturer, but they did not care to reply...

Other than dropping L/R, it works nicely. Wrote a small program to wiggle the bits and it works, even if it is playing. Ie I get the desired channels out on S/PDIF where I pipe it to a DIR9100.

My idea was to extend the S/PDIF over IR, then receiver, DIT9100 and D-class amp for the rear speakers.

 

[kipphebel3]

Correction: It does not drop L/R. It seems that L/R and the S/PDIF outputs are interconnected. If I request for the side channels to go to S/PDIF, then I get them on L/R as well

 

[kipphebel3]

I now realise that I have been barking up the wrong tree. As Kodi decodes it for me, all I need is an USB card with multiple outputs.