#1003481 - 11/17/06 09:05 AM
LFE calibration - various formats
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KMO
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Hello folks,
Looks like I've found a knowledgeable bunch here (except djwayne  - you can skip this question...)
I don't (yet) own a DVD-A or SACD player, but I'm trying to pin down correct (and actual) behaviour of recordings and consumer equipment. I've been putting together an explanation document at AVForums for my own benefit, and others.
Now, the LFE channel and its calibration is particularly troublesome.
I know that Dolby Digital (including HD variants) and DTS (including HD variants) all calibrate the LFE channel for 10dB extra headroom. Some early DTS music discs (CDs rather than DVDs?) got that wrong, so many receivers allow the 10dB boost to be disabled. And I wouldn't be surprised if some music mixers still got it wrong.
SACD on the other hand, doesn't use a 10dB boost (why oh why?); this is mandated by the Scarlet Book. But multi-format players tend to lower their analogue subwoofer output to compensate for this and make it match other formats.
But what about DVD-Audio MLP/PCM tracks? I've not seen a definitive answer on this - I've seen contradictory sources, including some studio equipment manuals that say DVD-Audio LFE is calibrated like SACD. My current belief is that a 10dB boost is actually assumed, but is this actually specified anywhere? Are real discs consistent?
At the consumer end there's much confusion, and the introduction of HDMI isn't helping. Just as everyone was beginning to get their head around analogue multi-channel interconnects; they've started cocking up digital ones.
From what I can see the HDMI spec doesn't directly mention LFE channel levels (unless the referenced CEA-861-D which I don't have discusses it). Which begins to cause problems; I'm aware of at least one receiver that won't boost LFE over HDMI PCM by 10dB. This then scuppers all hope of proper bass redirection.
And then what about SACD - now that HDMI 1.2 allows DSD transfer, should players be attenuating HDMI DSD LFE output by 10dB in the expectation that the receiver will always boost LFE by 10dB? Or should receivers "know" that DSD LFE doesn't need a boost. Has anyone specified this anywhere?
I suspect those final consumer kit questions might be outside the scope of this place, but hopefully someone can tell me definitively about DVD-Audio LFE.
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#1003482 - 11/23/06 04:15 AM
Re: LFE calibration - various formats
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Neil Wilkes
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Tricky Question.
The biggest problem with all these type of questions is not just one of end user calibration - you must also take into consideration who authored/mixed the DVD-A or SACD.
I have seen LFE content with frequency responses up to 2KHz and higher, which is an absurdity.
IMHO, all Surround mixing for music should be done as 5.0, leaving the 5 main channels full-range and allowing Bass Management to redirect what it needs to into the Sub (Which is not the same as an LFE)
Problem is usage of the LFE channel both in terms of content & levels is far from an "across the board" standard, with changes being quite dramatic not only from disc to disc, but label to label as well. LFE usage is the most misunderstoof function of multichannel, sadly.
It was designed as a Low Frequency Effects channel, to extend the bass response below what was usually achieved in the typical Home Cinema type setup where the 5 main channels are often far from full range.
Sadly - in the real world it has been made almost interchangeable between LFE & Subwoofer - which of course is wrong.
There are even a lot of DVD-A releases where there is actually zero LFE content on the disc itself, and any LFE you hear is from the Bass Management in the player/Amplifier.
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#1003483 - 11/23/06 07:48 AM
Re: LFE calibration - various formats
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KMO
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I agree the use of the LFE channel in music is an abomination, and just one more thing to get wrong at the consumer's end.
But does no-one here have any view on whether LFE should be mixed for +10dB or not? Surely someone must have some vague idea if they're using it?
I'm now beginning to suspect it shouldn't be mixed for a 10dB boost, but the DVD-Audio Book is $5000 out of my price range.
(That would then mean attenuating it by 10dB before feeding it into the DD/DTS encoder for the DVD-Video compatibility track.)
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#1003485 - 11/27/06 05:47 AM
Re: LFE calibration - various formats
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KMO
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Thanks for that reference Neil. Good guide. And my favourite bit is the advice not to use the LFE channel for normal bass. Yay!
But rather alarmingly, it slightly glosses over the LFE level issue. Its calibration guide assumes that the LFE has +10dB in-band gain, full stop.
In a sane world, that would be the way it should be (as per ITU-R BS.775-2 and BR.1384-1) for every format. But it's just wrong for Super Audio CD. Definitely. The most definitive reference I have is this Philips document , but every other source I've found (including BS.775) reinforces that. Even this Amazon review has reviewers castigating a disc for getting the LFE wrong, suggesting that this no-gain LFE level is universally expected.
So with that guide being wrong for SACD, it still leaves me in the dark over DVD-Audio. I really need to get my hands on the DVD-Audio Book, or someone who has it. Because DVD-Audio can include DD and DTS tracks, I'm pretty certain compatibility would have been considered. They'd have either specified
- DVD-Audio PCM/MLP tracks have +10dB LFE gain to match DD/DTS; or
- DVD-Audio PCM/MLP tracks have equal-level LFE, but the player has to make sure the difference from DD/DTS is accounted for on its line-level outputs, so it's not externally visible D
I started assuming the former, but my researches have now led me to favour the latter. For example, ITU-R BS.775-2 itself says that DVD-Audio doesn't have LFE gain, the same as Super Audio CD...
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#1003486 - 11/30/06 09:57 AM
Re: LFE calibration - various formats
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Neil Wilkes
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When you say that DVD-Audio can include DTS and DD streams, this is not quite as true as it seems.
Neither DTS or DD are natively supported in DVD-A, as the Audio for this format resides in the Audio_TS folder multiplexed into the AOB streams along with any ASV or ASVU supplied in Authoring.
These streams are either PCM or PPCM/MLP.
DD & DTS only come into the picture from an imported Video_TS folder for compatibility with non DVD-Audio players.
As such, I always treat the LFE for DVD-A in exactly the same manner as I do for the DD/DTS streams:
In fact, when I author these formats, the mix is exactly the same for all 3 streams (with the exception of the Dolby Digital which is always at 48KHz) from the same 24/96 Master Mix.
I've never had any complaints.....
I guess the point is that I use the same mixes, leaving LFE levels unchanged.
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#1003487 - 12/04/06 05:57 AM
Re: LFE calibration - various formats
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KMO
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Thanks for your info. It matches what I initially assumed - producers like you would likely be preparing DD and DTS versions for the DVD-V layer, so it would be natural to retain the same calibration for the DVD-A layer.
I'm going to keep picking away at this though, if you don't mind. One other item of suggestive evidence I've seen is in the demo of SurCode MLP, in its downmix set-up. The default downmix has
Lmix = Lf(-6dB) + C(-9dB) + Ls(-12dB) + LFE(-15dB)
Rmix = Rf(-6dB) + C(-9dB) + Rs(-12dB) + LFE(-15dB)
Is this downmix default a universal DVD-A concept, or just SurCode's own? The -15dB there troubles me, because it's not clear whether it allows for the alleged 10dB LFE gain or not.
If it means just mathematically -15dB on the channel data, and the LFE channel was calibrated for 10dB gain, then your LFE would almost vanish after downmixing, as it would effectively be at -25dB level.
So, one of the following is true:
1) the default downmix parameters deliberately shove the LFE way down (10dB below the already low surrounds), much as the default DD downmix omits it totally - this would certainly be bad for any producer using it as a general subwoofer channel;
2) the encoder/decoder silently allow for the 10dB LFE gain - it boosts by 10dB then attenuates by 15dB, so it's actually mathematically mixing at a factor of -5dB;
3) DVD-A tracks shouldn't be calibrated for 10dB LFE gain, so -15dB to both sides means the LFE is mixed in equal with the surround level;
4) SurCode's default is wrong - either it's not the correct default for DVD-A, or DVD-A doesn't specify a default.
I really haven't the foggiest which of those is more likely.
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#1003488 - 12/07/06 02:15 PM
Re: LFE calibration - various formats
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Neil Wilkes
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To Quote from the Surround "bible" -
When encoding for Dolby Digital, MLP, or WMA9, the Center channel downmix coefficient should be set to -3dB in relationship to the Left and Right front channels. However, the rear channel coefficients are largely program dependent.
Many of today's surround productions are mixed with an "in the band" perspective. (See section 3.3.1) In those instances, the rear channels
must be at the same level as the front left and right channels. However, the rear channels may be lowered -3dB if they contain only ambience or audience material. The stereo sum of the 5.1 mix will usually need to be lowered in level in
order to avoid creating signals so loud that they create digital "overs" (distortion).
While the Dolby Digital encoder/decoder chain can handle the occasional "over," the MLP encoder has no built in compressor or limiter, so a single "over" during the course of the entire musical program will cause the critical MLP verification
step to fail.
Typical settings for downmix coefficients are: Left Front = -6dB, Right Front = -6dB, Center = -9dB, LFE usually OFF or to taste (never more than -9dB in this scenario), Left Surround = -6dB, Right Surround = -6dB. Surround mixes should always be checked in this typical downmix configuration. The mix engineer should indicate to the mastering engineer whether these coefficients are suitable or whether any changes are necessary.
If surround sound material is Dolby Digital- or MLP-encoded, there is no way to completely avoid the possibility that a consumer might listen to a downmix instead of a true stereo mix. However, there are three ways to reduce the chances of that occurring. One is to ensure that a separate dedicated stereo mix is included on the disk; in the case of legacy material being repurposed, this
should preferably be the original stereo mix. The second is to specify to the mastering engineer that the disk be authored with the stereo mix as the default audio stream. This will, of course, require that the consumer proactively select
"5.1" if they want to hear the surround sound mix. However, if this is not done — if the disk is instead authored with the 5.1 mix as the default — then consumers who have only two speakers attached to their receiver will hear the downmix and
not the stereo mix, even if one exists.
Finally, in the case of DVD-Audio, when authoring in "pgc block" mode (where there is a dedicated stereo mix present but the 5.1 audio stream is the default), the MLP encoding should have the downmix function turned OFF. While there are some legacy DVD-Audio players that do not recognize this flag, its use is nonetheless recommended since all new and future machines are designed to conform to it.
Basically put, Downmixing really is not advisable unless there is no option.
LFE levels are "to taste", and in a music mix, ideally not there at all - unless you really want to. Remember that the 5 main channels are always mixed as full-range for DVD-A and all music mixing and as such, in an ideal world there would not be any LFE content, and all that would ever be present is what gets routed there by Bass Management.
DVD-Video, on the other hand, changes all the rules as the LFE there is for Low Frequency Effects.
Again - levels are down to the mixer, and assume the same calibration.
The only differences in calibration are reducing the Ls/Rs channels by -3dB when encoding for Cinema.
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#1003489 - 12/22/06 12:02 PM
Re: LFE calibration - various formats
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stuart
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http://www.surroundassociates.com/newformats.html
This article originally appeared in the June 1999 issue of EQ Magazine.
SMART Content
One of the more interesting aspects of DVD-A is a new feature known as SMART (System Managed Audio Resource Technique) Content. SMART Content is an auto downmix provision that lets a consumer with only a stereo system have the multichannel mix automatically downmixed to that format. In other words, the 6 channel 5.1 mix automatically becomes a stereo mix if there's only two channels in the playback system. While on the surface it might seem like a scary thing having that great multichannel mix automatically fold down to stereo, SMART Content actually gives the producer a choice in the way this downmix will take place by allowing the producer to select one of 16 downmix coefficients that get stored along with the audio data. SMART Content has the added byproduct of potentially eliminating the need to include a separate stereo mix on the disc, thereby freeing up space for either higher quality audio or additional data information. While this looks good on paper, it's still too early to tell what it actually sounds like in practice.
I think that Bobby Owsinski probably has much more to say on this.
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#1003490 - 12/24/06 10:45 AM
Re: LFE calibration - various formats
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Neil Wilkes
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No Downmix is going to be as good as a dedicated stereo mix.
SMART is okay as far as it goes - if enabled in the disc or the player - but it is actually, if a downmix must be run, better to set it up when encoding to the MLP file, as you get control over every channel in increments of 0.1dB/channel.
If downmixing is used, or even a dedicated stereo mix, you can - software permitting - use PGC Blocks to embed the stereo mix in with the 5.1 mix, saving a considerable amount of space and also avoiding a separate stereo group in authoring. Playback will be according to the way the player is set up.
Downside?
You cannot access the stereo mix if using a multichannel system.
I prefer to include a dedicated stereo mix myself, as it seems somehow wrong to assume all are in 5.1 - there is no requirement for DVD-A to be in 5.1, and I think it loses a lot of sales because so many people assume it is surround based only. Could be wrong.
Anyway, I feel it is better to give the end user the option. I know I prefer multichannel, but occasionally I like to play the stereo streams as well.
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#1003491 - 12/26/06 12:19 PM
Re: LFE calibration - various formats
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stuart
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Hi Neil,
Thanks for the info. Yes I agree that a separate stereo mix is much better than allowing a simple fold down of the 5.1 mix.
Originally posted by Neil Wilkes:
... you can - software permitting - use PGC Blocks to embed the stereo mix in with the 5.1 mix, saving a considerable amount of space and also avoiding a separate stereo group in authoring. Playback will be according to the way the player is set up.
Downside?
You cannot access the stereo mix if using a multichannel system.
Originally posted by Neil Wilkes:
I prefer to include a dedicated stereo mix myself, as it seems somehow wrong to assume all are in 5.1 - there is no requirement for DVD-A to be in 5.1, and I think it loses a lot of sales because so many people assume it is surround based only. Could be wrong.
Thanks again,
Stuart
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#1003492 - 12/29/06 05:37 AM
Re: LFE calibration - various formats
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Neil Wilkes
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Stuart.
Embedding a Stereo file is not done when encoding to MLP, it is done in the Authoring.
If using Chrome, it is called a PGC Block, and is added in the main project window.
When the surround track has been added to the group, right-click it and select the option to add stream.
The stereo file must be exactly the same length as the Surround one.
As far as stereo goes, I have a series of 5 DVD-A DualDisc, where the DVD-A side is stereo only (the multitracks were no longer in existence, and thankfully Rhino did not use the godawful "Unwrap" Pseudosurround and left it as Stereo in 24/96 resolution. Quality is stunning, and leaves CD floundering in it's wake.
Getting (almost) back on topic, the LFE channel in DVD-A is not bandwidth limited, either.
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#1003493 - 12/29/06 12:36 PM
Re: LFE calibration - various formats
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stuart
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Hi Neil,
Thanks for the info. I'll give it a try.
I also don't place anything in the LFE channel and haven't found to many options for creating 5.1 from stereo that sound very good. I'd rather listen to it in stereo but that's just me.
Thanks again,
Stuart
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#1003494 - 12/30/06 09:36 AM
Re: LFE calibration - various formats
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Neil Wilkes
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Stuart.
One word of a caveat - using this method will give playback according to the player setup, and will not allow independant selection of Stereo or Surround.
If the player is set up as a stereo, it will output stereo. Ditto surround.
If a choice of either surround or stereo is required, the material must be in it's own group.
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