Does LFE belong in a stereo downmix? - Page 2 - AVS Forum
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post #31 of 48 Old 07-23-2013, 02:44 PM
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I just confirmed using that particular LFE track on the AVIA disc that my simple SONY BD player drops the LFE channel when configured to output a 2-channel PCM downmix. No surprise, there, and exactly what I suspected. Makes perfect sense. It is an optical connection so I can't check what happens with multichannel PCM (but I already know what happens with multichannel PCM wink.gif). Have no idea about DTS or the lossless codecs as I have no way to test this but I would assume the same would happen; the LFE channel would be dropped by the source device if it is configured to output a 2-channel PCM downmix.

BTW, I have been running a subwoofer-less setup for over 2 years, now. Intitially it was still 5.0 but it has only been 2.0 for over a year, now. And I know what is happening to the LFE channel. At least with bitstreamed DD5.1 material. Easily confirmed with that AVIA track. My AVR routes it to the R/L speakers when it is configured as having no subwoofer connected. Always has. Always will.

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post #32 of 48 Old 07-23-2013, 07:24 PM
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Quote:
Originally Posted by Roger Dressler View Post

Let me see if I can recount the facts. First, we need to distinguish between downmixing and downmixing. One of them is a process conducted within the confines of the Dolby decoder algorithm, the other happens somewhere downstream.

When 2.0 downmixing is done in a Dolby Digital decoder, the LFE is omitted. For all other cases, including other Dolby decoders like DD+ and TrueHD, LFE is not discarded.

Bass management (redirecting bass) and speaker management (redirecting channels) all occur downstream of the decoder, and only function in a multi-channel environment (no downmixing in the audio codec). So it's a totally different paradigm than the case where a 2-ch source product like a DVD player or STB outputs only 2-ch audio.

When an AVR receives a DD bitstream, and the AVR is set to stereo mode (like for headphones or plain stereo listening), there is no downmixing in the audio codec. It outputs the full 5.1 of the source, and a secondary bass/speaker management process handles the formulation of the speaker feed signals.

Let me know if this fails to clarify the situation.
Thanks, Roger. Your insight is invaluable, as usual. A few follow up questions, if I may.

Are there any circumstances where the Dolby stereo downmix is used by a processor and are there any circumstances where it is not used by a source device such as a disc player or set top box when the output is stereo? What rules govern whether or not the stereo downmix is done by the decoder?

I am curious about TrueHD. You say LFE is not discarded. But, isn't TrueHD built on a 2 channel core that gets used for stereo playback, which would seem to suggest there's no LFE present to either discard or redirect?

One other curiosity. Your post says bass management happens downstream of the decoder while the Dolby 5.1 Production Guide quoted elsewhere in this thread says "The subwoofer signal is created in the decoder as needed for the particular speaker complement in use, using crossover filters. This signal is created using bass management, and all Dolby Digital decoders perform this function." Why the apparent discrepancy?

Once again, thanks for helping out here.
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post #33 of 48 Old 07-24-2013, 01:18 AM
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Originally Posted by BIslander View Post

Thanks, Roger. Your insight is invaluable, as usual. A few follow up questions, if I may.
Soitenly.
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Are there any circumstances where the Dolby stereo downmix is used by a processor and are there any circumstances where it is not used by a source device such as a disc player or set top box when the output is stereo? What rules govern whether or not the stereo downmix is done by the decoder?
I suspect you can find certain AVRs that use the DD downmixing -- I've learned to never say never. But I have not found one.

In a set top box (cable/sat) I have not seen anything but the barest of bones decoding. Cheap is the key factor, and downmixing to 2.0 in the DD chip is the cheapest solution by far.

As for disc players, it happens quite often that the DD decoder runs in full 5.1 mode and a separate downmixer is present. The original Panasonic DVD players had both 5.1 and 2.0 outputs running in parallel. Since players must deal with not only DD, but also PCM, they must have a separate downmixer anyway, so why not use it for all the codecs?

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I am curious about TrueHD. You say LFE is not discarded. But, isn't TrueHD built on a 2 channel core that gets used for stereo playback, which would seem to suggest there's no LFE present to either discard or redirect?
When TrueHD carries a 5.1 program, it is based on a 2.0 downmix and a 3.1 extension. That downmix has whatever and however much of the source 5.1 channels as the content maker decides, including LFE.
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One other curiosity. Your post says bass management happens downstream of the decoder while the Dolby 5.1 Production Guide quoted elsewhere in this thread says "The subwoofer signal is created in the decoder as needed for the particular speaker complement in use, using crossover filters. This signal is created using bass management, and all Dolby Digital decoders perform this function." Why the apparent discrepancy?
Because they are using the term Dolby Digital rather casually. The decoder being the metal box with the DD logo on it. They should have said "Dolby Digital decoder product." There never was any bass management in the decoder algorithm, per se, but it is in the decoder product.
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Once again, thanks for helping out here.
Always happy to help on these arcane matters of Dolby history.
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post #34 of 48 Old 07-24-2013, 07:17 AM
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Quote:
Originally Posted by Roger Dressler View Post

Let me see if I can recount the facts. First, we need to distinguish between downmixing and downmixing.


Huh?

I'm guessing 'downmixing and bass management'?
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post #35 of 48 Old 07-24-2013, 08:52 AM
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Quote:
Originally Posted by Roger Dressler View Post

...................we need to distinguish between downmixing and downmixing.
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Originally Posted by krabapple View Post

Huh?

I'm guessing 'downmixing and bass management'?

No, he meant exactly what he said. Sums it up quite well, actually. smile.gif

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post #36 of 48 Old 07-24-2013, 11:13 AM
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Quote:
Originally Posted by krabapple View Post

Huh?

I'm guessing 'downmixing and bass management'?
My attempt a coy humor was intended to say that downmixing can happen in two different places, but the circumstances are different. To be precise, the downstream AVR version is not bass management, but what I called "speaker management." No center speaker? Mix it into L/R. That sort of thing.
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post #37 of 48 Old 07-24-2013, 08:38 PM
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Roger, to button this up...

It looks like processors do not discard LFE, as sivadselim posted. And, while some source devices may downmix without LFE, others will fold it in, the same as processors.

Do you have any idea how DTS handles downmixing? We know DTS has a goal of matching the decoder output to the playback system's layout, leading to channel expansion by Essentials decoders and Speaker Re-mapping. But, does that philosophy extend to downmixing?
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post #38 of 48 Old 07-25-2013, 11:00 AM
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Quote:
Originally Posted by BIslander View Post

Do you have any idea how DTS handles downmixing? We know DTS has a goal of matching the decoder output to the playback system's layout, leading to channel expansion by Essentials decoders and Speaker Re-mapping. But, does that philosophy extend to downmixing?
I think you'll find that DTS is handled the same as Dolby and PCM.
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post #39 of 48 Old 07-27-2013, 02:33 PM
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This somewhat-out-of-date panel from the dts-hdma authoring process would seem to show that it is the author who determines how much of the LFE from the 7.1 mix is retained by the decoder in a subsequent 5.1 or 2.0 downmix. I'd guess the 2.0 downmix would have to contemplate playback on a "small" TV as a possible sink . . . but it seems likely that AVRs limit themselves to starting from the 5.1 or 7.1 mixes (for LFE determination), even if they are subsequently tasked to deliver just 2.0|2.1 output. cool.gif

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post #40 of 48 Old 07-27-2013, 08:59 PM
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^^ Yes, that's the same way TrueHD works.
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post #41 of 48 Old 07-29-2013, 11:57 AM
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For 2.0, I thought LFE was routed to the L/R speakers if they were large? At least that's the way it works in my system... For small, LFE is as shown.

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post #42 of 48 Old 10-14-2014, 06:02 PM
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Resurrecting this golden oldie...would it be correct to say that dolby 5.1 receivers ignore the dolby downmix metadata (center mix level, surround mix level)?
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post #43 of 48 Old 10-14-2014, 08:00 PM
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Reading up on dolby downmixing, I found out about a more important issue. Most dolby receivers engage dynamic range compression (regardless of settings) if either center or surround speakers are absent. See:

Bass Effects on Downmixing DD to Stereo and Electronics Rolloff

Another thread (read Roger Dressler's post):
Workaround to Dolby Digital DRC on systems with no center channel?

What a disaster! I thought dolby digital sometimes sounded compressed, but I didn't know what the cause was.

DTS doesn't have this problem, but all broadcast stuff is dolby digital.

Last edited by Luke M; 10-14-2014 at 08:17 PM.
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post #44 of 48 Old 10-14-2014, 09:47 PM
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Quote:
Originally Posted by Luke M View Post
Reading up on dolby downmixing, I found out about a more important issue. Most dolby receivers engage dynamic range compression (regardless of settings) if either center or surround speakers are absent. See:

Bass Effects on Downmixing DD to Stereo and Electronics Rolloff

Another thread (read Roger Dressler's post):
Workaround to Dolby Digital DRC on systems with no center channel?

What a disaster! I thought dolby digital sometimes sounded compressed, but I didn't know what the cause was.

DTS doesn't have this problem, but all broadcast stuff is dolby digital.
Most AVR's do not apply DRC when downmixing. Maybe you have one that does so, but that's not the norm. And if the AVR has HDMI, and the source is DVD or BD, then you can ask the player to do the decoding, then there will be no DRC even when the AVR is downmixing.
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post #45 of 48 Old 10-15-2014, 01:21 AM
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Originally Posted by Roger Dressler View Post
Most AVR's do not apply DRC when downmixing. Maybe you have one that does so, but that's not the norm.
I'm using a Sony from 1998. The DD source is a cable box (well, a Tivo actually). It's worse if I use PCM from the Tivo, because LFE is completely dropped and the surround channels are Dolby Surround encoded so you get a weird phasey sound.

Besides not forcing compression, I'd like a receiver that gave me control over LFE/center/surround levels (- and + for each), whether corresponding speakers are present or not. Just trying to get a clean 2.0 or 4.0 output with some user controls that can't be overridden.
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post #46 of 48 Old 10-15-2014, 09:12 AM
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Originally Posted by Luke M View Post
I'm using a Sony from 1998.
If you have explored all the options in the AVR, and cannot decode a 5.1 DD bitstream and downmix the Center channel without activating DRC, then you'll need a new decoder.

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The DD source is a cable box (well, a Tivo actually). It's worse if I use PCM from the Tivo, because LFE is completely dropped and the surround channels are Dolby Surround encoded so you get a weird phasey sound.
Does not the cable box offer 2 options for the stereo output: Surround and Stereo? Stereo will not sound phasey, but DRC will be on and LFE will be gone.

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Besides not forcing compression, I'd like a receiver that gave me control over LFE/center/surround levels (- and + for each), whether corresponding speakers are present or not. Just trying to get a clean 2.0 or 4.0 output with some user controls that can't be overridden.
A new decoder will give you a downmix with LFE and no compression, but no mix controls for the missing speakers. There are some that offer LFE mix, but it's rare, and you can always adjust the sub level as a last resort.

If your Sony has line outputs, you could have it decode the 5.1 and use an audio mixer to make a custom stereo output any way you like.
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post #47 of 48 Old 10-15-2014, 10:55 AM
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Originally Posted by Roger Dressler View Post
Does not the cable box offer 2 options for the stereo output: Surround and Stereo? Stereo will not sound phasey, but DRC will be on and LFE will be gone.
Nope. The only options are "Dolby Digital" and "Dolby Digital to PCM".

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Originally Posted by Roger Dressler View Post
A new decoder will give you a downmix with LFE and no compression, but no mix controls for the missing speakers. There are some that offer LFE mix, but it's rare, and you can always adjust the sub level as a last resort.
Is the center/surround mix level fixed or does it use the downmix metadata?
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post #48 of 48 Old 10-15-2014, 11:05 AM
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Originally Posted by Luke M View Post
Is the center/surround mix level fixed or does it use the downmix metadata?
When a Dolby Digital decoder performs the downmix, it applies the Cmix and Smix metadata. When the downmix is after the decoder, the metadata is not used.

I would not say there's any real difference, as the default settings are the same as a standard downmix. I doubt the metadata is ever changed.
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