SPDIF bitrate limitation

[DJ Bobo]

This question has been bugging me since a while now, but I couldn't find an explanation on the internet.
So here we go:

On almost any soundbar or AVR, you can tunnel 24-bit 96KHz Stereo PCM through the optical connection, which is 2x24x96 = 4,6 Mbit/s

Then why is it not possible to tunnel Dolby Digital Plus, which is limited to 1 Mbit/s on streaming, and 1,7 Mbps on Blu-ray?!

[huhn]

well it is part of the spec. now they need to care to add it. https://cdn.standards.iteh.ai/samples/22796/288d720389c94a57b648d29b2e6965a7/IEC-61937-3-2017.pdf

spdif is also fast enough to do 5.1 16 bit 48khz PCM it never made it into the spec and is only used by low end devices

[DJ Bobo]

Oh wow! Now I am shocked. It even says "Enhanced AC-3" in the second edition released in 2007 (!)
Can you name a single manufacturer that implemented this?!
Even the latest Panasonic soundbar, the SC-HTB600, released in 2020, accepts DD+ over ARC, but not over Toslink :O

[kurkosdr]

This question has been bugging me since a while now, but I couldn't find an explanation on the internet.
So here we go:

On almost any soundbar or AVR, you can tunnel 24-bit 96KHz Stereo PCM through the optical connection, which is 2x24x96 = 4,6 Mbit/s

Then why is it not possible to tunnel Dolby Digital Plus, which is limited to 1 Mbit/s on streaming, and 1,7 Mbps on Blu-ray?!
DTS has a bitrate of 1.5 Mbps (at full bitrate), and so does LPCM stereo at 16-bit/48Khz so Dolby Digital Plus (even with Atmos) can certainly fit in SPIDF from a bitrate perspective.

The problem is that if you feed an E-AC3 a bitstream to an old SPDIF receiver, you may get noise. In fact, it's a problem even with AC3 and DTS (since it's not certain that a SPDIF receiver is smart enough to detect and ignore AC3 and DTS instead of trying to play the bistream as LPCM stereo), so, I guess the industry has decided to not make this problem worse.

Also, Hollywood studios require Dolby Digital Atmos to be transported encrypted (which means HDCP), it's the reason we can't have USB-C (for example) replace SPIDF (as a dedicated audio cable from the source to the Dolby Atmos receiver) and we have to waste an HDMI port on the TV for ARC/eARC or have to pass the HDMI signal through an AVR receiver (with the lag and the limitations this introduces).

[DJ Bobo]

The problem is that if you feed an E-AC3 a bitstream to an old SPDIF receiver, you may get noise
Oh well, I tried that on an older Panasonic soundbar that can do Dolby Digital only, there was just no sound. So I would dare say, this is a problem only with some chinese made garbage?
Besides, the huge majority of external DACs on the market today do PCM only, so that really shouldn't be an issue.
I guess the industry has decided to not make this problem worse
Nobody wants to make money?! lol
According to some estimations, 8% of people have problems establishing ARC connections. 8% is a pretty good market niche I would say!
Hollywood studios require Dolby Digital Atmos to be transported encrypted
OMG... not those idiots again.
That would explain it better -.-

[huhn]

i only know the basic of the spdif spec so i cannot name manufactory that support e-ac3. i actually never used it in my life.

i also do not know much about e-ac3 it's an enhancment layer or something similar to dts HD whatever?

is DP over USB3 not encrypted? that should do the trick not?

BTW: the number of bit streamable audio codec is gigantic and barely supported maybe even wma made it on that list.

[DJ Bobo]

i also do not know much about e-ac3 it's an enhancment layer or something similar to dts HD whatever?
I was confused about it too. See this thread (https://forum.doom9.org/showthread.php?t=186907) for details.
Short answer: depends :D
the number of bit streamable audio codec is gigantic and barely supported maybe even wma made it on that list
Let me put it like this: I couldn't care less about other codecs used for stereo sound (especially AAC), as they can be decoded to PCM and pass just fine.
But streaming uses almost exclusively DD+ for surround sound, broadcast uses it a lot, and Blu-ray uses it to some extent for the secondary languages, so I am baffled at the non-existent support for it on SPDIF!

[kurkosdr]

I was confused about it too. See this thread (https://forum.doom9.org/showthread.php?t=186907) for details.
Short answer: depends :D[
E-AC3 unambiguously refers to a bitstream that is not compatible with AC3 (and by extension is not compatible with DVD-Video or Blu-ray).

The trademark "Dolby Digital Plus" may refer to either E-AC3 or AC3+additional channels encoded as a side-signal, so "Dolby Digital Plus" is ambiguous (aka "it depends").


But streaming uses almost exclusively DD+ for surround sound, broadcast uses it a lot, and Blu-ray uses it to some extent for the secondary languages, so I am baffled at the non-existent support for it on SPDIF!
Every streaming box or satellite/terrestrial receiver that supports E-AC3 playback is required by Dolby to also support E-AC3 -> AC3 conversion when outputting to SPDIF, so if it doesn't, it's a matter of looking at the settings. Please keep in mind though that this conversion is lossy.

If you are using a PC to watch those streaming services or are playing a file downloaded from those streaming services on a player that doesn't support E-AC3 or doesn't have a Dolby-licensed implementation, then you are on your own when it comes to E-AC3 -> AC3 conversion when outputting to SPDIF.

---

As an aside, what annoys me is that Dolby defined a (backwards-compatible) Atmos side-signal for E-AC3 (called E-AC3+JOC), but didn't define AC3+JOC, which would have enabled backwards compatible Atmos all the way down to DVD, or at least Blu-ray. But that's another rant for another day.

[DJ Bobo]

Every streaming box or satellite/terrestrial receiver that supports E-AC3 playback is required by Dolby to also support E-AC3 -> AC3 conversion when outputting to SPDIF ... Please keep in mind though that this conversion is lossy
That's exactly my point. Now it is clear that the interface is supposed to be able to handle DD+ natively according to the document shared by Mr. huhn, yet we have to go through that conversion, that is not only lossy, but also not transparent (I just found out that the PS5 applies a low-pass filter at 18KHz, even though it is using the full 640kbps bitrate - seems to be an artificial limit imposed by Dolby for consumer products, as the software version used by Pegasys has the exact same cut-off frequency).
I analyzed the original E-AC3 stream coming from Amazon, it definitely has the full spectrum up to 20KHz.

[kurkosdr]

That's exactly my point. Now it is clear that the interface is supposed to be able to handle DD+ natively according to the document shared by Mr. huhn, yet we have to go through that conversion, that is not only lossy, but also not transparent (I just found out that the PS5 applies a low-pass filter at 18KHz, even though it is using the full 640kbps bitrate - seems to be an artificial limit imposed by Dolby for consumer products, as the software version used by Pegasys has the exact same cut-off frequency).
I analyzed the original E-AC3 stream coming from Amazon, it definitely has the full spectrum up to 20KHz.
In the world of consumer electronics, what matters is not what is defined in the document, but what is reliably supported by most devices. For example, the Divx Home Theater profile defines support for 4:3 and 16:9 anamorphic (for the DVD-Video resolutions), but half of the Divx-certfied devices out there (the ones with the crappy ESS Vibrato chips) don't support it.

Similarly, E-AC3 over SPDIF is not reliably supported, deal with it.

[DJ Bobo]

E-AC3 over SPDIF is not reliably supported
Do you happen to know a soundbar that does???
One of my setups is suffering from the ARC incompatibilities and has to work with SPDIF :-3

[kurkosdr]

Do you happen to know a soundbar that does???
One of my setups is suffering from the ARC incompatibilities and has to work with SPDIF :-3
No, and it doesn't matter anyway. You need a soundbar that can take E-AC3 over SPDIF and source devices that are happy with sending E-AC3 over SPDIF. That's two levels of "very-hard-to-impossible", forget about it.

One of my setups is suffering from the ARC incompatibilities and has to work with SPDIF :-3
Since you are looking for a new soundbar, can you try with a new soundbar and return it if it also has ARC incompatibilities?

[DJ Bobo]

can you try with a new soundbar and return it if it also has ARC incompatibilities
It is a very good soundbar. I am very satisfied with it. Good old Panasonic engineering (re-watched Aquaman yesterday, almost cried, so good the sound!). It's the f**king TV (Hisense is probably the worst when it comes to ARC compatibility -.-)
The only reason I would consider changing the soundbar is, if it checks all the boxes AND can do DD+ over SPDIF (the Hisense sound format menu displays DD+ as one of the available options - at least in theory).
I don't trust Hisense to have a wide compatibility with non-Hisense soundbars (according to what I see on Youtube and Reddit), and I will probably never buy Hisense again; not that other brands don't have their issues with ARC and what not, just saying... :-3

[kurkosdr]

It is a very good soundbar. I am very satisfied with it. Good old Panasonic engineering (re-watched Aquaman yesterday, almost cried, so good the sound!). It's the f**king TV (Hisense is probably the worst when it comes to ARC compatibility -.-)
The only reason I would consider changing the soundbar is, if it checks all the boxes AND can do DD+ over SPDIF (the Hisense sound format menu displays DD+ as one of the available options - at least in theory).
I don't trust Hisense to have a wide compatibility with non-Hisense soundbars (according to what I see on Youtube and Reddit), and I will probably never buy Hisense again; not that other brands don't have their issues with ARC and what not, just saying... :-3
Anyway, you aren't getting E-AC3 over SPDIF, not with standard consumer electronics equipment, deal with it.

What you do with your ARC setup (whether you replace your soundbar or your TV or just leave it as-is and tolerate the E-AC3 -> AC3 lossy conversion), is your business.

[DJ Bobo]

is your business
and the business of the other 8% (or so) that have problems with ARC in particular and HDMI-CEC in general (aren't you in a good mood today? :eek: )
#MeToo
https://www.youtube.com/watch?v=GVJdqyRKtHk

[kurkosdr]

and the business of the other 8% (or so) that have problems with ARC in particular and HDMI-CEC in general (aren't you in a good mood today? :eek: )
#MeToo
https://www.youtube.com/watch?v=GVJdqyRKtHk
I am in a mostly good mood, but I also want to make my point clear: Trying to make E-AC3 over SPDIF happen with consumer equipment is like trying to flog a dead horse, no matter how hard you want it to happen, it won't happen.

Maybe with a PC with custom drivers or some special settings in the software player and an FPGA board acting as a receiver, but not with consumer equipment.

If you must avoid the lossy E-AC3 -> AC3 conversion, you are better off trying to find a combination of equipment that don't have ARC incompatibilities with each other, since this has some chance of working.

[DJ Bobo]

If you must avoid the lossy E-AC3 -> AC3 conversion
I am using the E-AC3 to DTS conversion at the moment.
According to the EBU study Tech3324, it is almost transparent (only 2% quality loss on average)
Now I understand why Toshiba used to transcode to DTS instead of DD in the HD-DVD days (which used almost exclusively DD+), saying that it delivered higher quality sound. They were probably referring to the 18KHz lowpass of standard Dolby transcoders (and I had to find out the hard way in 2026... 20 years after the fact! lol)