[ViperJPB]

My Reciever only supports optical input and so does not fully support higher HD tracks and uncompressed tracks. My video has both an uncompressed 5.1 track and a Dolby Digital 5.1 track. I Was able to setup XBMC to transcode the almost 7Mb uncompressed 5.1 track to AC3 5.1 by using the HDMI setting and only the AC3 capable setting on. During playback if I hit the "o" key to bring up the codec info overlay I can see that indeed the 5.1 AC3 track is playing back at a variable rate that peaks around 6880kbs which is exactly the uncompressed rate of the mix. Does this mean I am truly getting that bitrate or is XBMC lowering the AC3 live transcoded bitrate?

[jjd-uk]

Yes that's the rate you'll be truly getting.

[ViperJPB]

Sweet! Thanks for the reply. One more question. With these settings the XBMC volume functions. Is 100 considered a reference default for XBMC or will it be boosting and possibly clipping the source audio unless I turn this down?

[jjd-uk]

Yes 100 is the default for untouched audio.

Note that volume control is only done on formats that XBMC decodes internally and not passthrough formats.

[ViperJPB]

Thanks! That's what I assumed and yes I discovered the passthrough difference thx. Now in my case with a video file that has an uncompressed 5.1 and a Dolby Digital AC3 5.1 track, allowing XBMC to transcode the uncompressed track to a AC3 5.1 track is still better then just passing the 600 something k Dolby track through right? Higher bitrate higher quality.

[jjd-uk]

In theory yes, but whether there's any noticeable difference in practice not necessarily.

[ViperJPB]

Haha thx..totally get that. In this case comparing the two audio sources is obvious....other then the inherent volume difference, the dialogue is much less clippy than the compressed Dolby track, plus there is more atmosphere in the surround quality. Thx again for your quick and concise responses. When I couldn't get the uncompressed track to be properly recognized by my receiver I was really bummed out as I was looking forward to having a digital backup of my movies. But the quality snob in me did not like the idea of not being able to utilize the higher quality audio.

[jd17]

Hello - sorry for digging up this old thread but I figured why open a new one?

I read the initial post and first response and I was quite surprised by it.
I hope someone could help me understand by going a bit more into detail.

My situation:
I have a full-fledged 2160p / BT.2020 / 10bit / HDR10 / DV capable TV.
Recently I bought an S905X box and I am currently helping out a bit to get it to full 2160p / BT.2020 / 10bit / HDR10 output capability by testing and supporting the devs.
It's almost there.

You might already have guessed what's coming now...
I have an old 1080p-era AVR that I still like a lot (Onkyo TX-NR818, mainly because of Audyssey MultEQ XT32).
Of course it cannot pass those UHD signals, so I'll have to either...
a) buy a replacement AVR if I want to keep HDMI "HD-audio" or
b) just use the S905X optical out and listen to regular Dolby Digital or DTS.

When I encode my Blu-rays, I always keep a "S/PDIF compatibility" track, so either the usual 1,5mbit/s DTS core or the Dolby equivalent additional 640kbit/s AC3 track.
I always assumed those pre-compressed tracks would be superior in quality to Kodi live-transcoding to AC3...

However, the first response sounds to me like the exact opposite is the case?
Do I actually get a HD-audio quality via S/PDIF if Kodi transcodes live to Dolby Digital?
How is that possible? Or did I misunderstand the first answer?
Is AC3 not limited to 640kbit/s and also limited to the rather ancient compression (compared to the likes of opus or AAC-HE)?


Also, I recently decided to compress my HD-audio tracks (Dolby TrueHD / DTS-HD MA) to opus at 640kbit/s VBR (8 channel) and 480kbit/s VBR (6 channel), which is more than enough to be transparent to me.
If live-transcoding to AC3 is the "magic" I hope to have correctly understood it is, do the same rules apply for live-transcoding from opus, FLAC, HE-AAC and all the others too?

I'd really appreciate answers and explanations!

[ashlar]

I'm not sure I am understanding jjd-uk's answer correctly, but what I read seems definitely wrong. S/PDIF doesn't have the bandwidth to carry multichannel HD-Audio. Furthermore, AC3 peaks at 640kbps.

Also, regarding your last question, you should be mindful of lossy vs. lossless. FLAC is lossless, so transcoding to a lossy format (AC3) is good. Opus and HE-AAC are lossy, so transcoding to another lossy format introduces extra degradation in sound (whether the degradation is discernible it's a different matter, but it's there).

[jd17]

I'm not sure I am understanding jjd-uk's answer correctly, but what I read seems definitely wrong. S/PDIF doesn't have the bandwidth to carry multichannel HD-Audio. Furthermore, AC3 peaks at 640kbps.

Yes, that was always my understanding too.
I assumed any HD-audio track, FLAC, AAC, opus etc. would be decoded to multichannel PCM by Kodi and then either passed directly via HDMI or (in the case of S/PDIF output) live-transcoded to AC3 (I hope at 640kbit/s)?

From what I read here I just thought that there might be some kind of "loophole" when transcoding live...?
Maybe jjd-uk can shed some light on the matter.

Also, regarding your last question, you should be mindful of lossy vs. lossless. FLAC is lossless, so transcoding to a lossy format (AC3) is good. Opus and HE-AAC are lossy, so transcoding to another lossy format introduces extra degradation in sound (whether the degradation is discernible it's a different matter, but it's there).

Thanks, I know that of course.
I encode the opus tracks only from lossless DTS-HD or TrueHD, I would never encode from a lossy source.

[jjd-uk]

The answers in this thread no longer applies as the audio code has been completely changed since this thread was created.

How it works now, to have access to the transcoding option either a SPDIF device must be selected or if HDMI then Number of Channels must be set to 2.0, when transcoding is done it's now done at a fixed bitrate of 640kbps except Android where I think 384kps is used.

[jd17]

Thanks jjd-uk.
However, if it once applied in the past - how did it work?
And is there a way to bring it back?

It sounds like a dream come true to me.
No "pressure" to constantly feel the need to upgrade an AVR or something else in the HDMI chain due to the madness of ever changing standards...