Bass Effects on Downmixing DD to Stereo and Spectrum Labs information

[Ilkka]

Steve Callas started a similar thread over at AVS. Since I have some issues with AVS , I decided to start my own thread here. Here's a link to AVS thread: http://www.avsforum.com/avs-vb/showt...8&page=1&pp=30

Mark Seaton on AVS wrote:

When a DD soundtrack is downmixed to stereo or DPL, the LFE channel is thrown away. If you want to listen to DVD soundtracks with a powerful set of stereo main speakers you want to make sure you are in a DD mode and have the system set to yes or no to all of the appropriate speakers. While it should be obvious, if you don't have a center or a sub, be sure that you select that in the setup.

This is correct when strictly talking about downmixing. But almost every receiver/pre/pro has a some kind of bass management, which allows us to redirect the LFE channel into main channels (L&R) if there are only for example two speakers in the system. There are a few receivers that don't handle it perfectly, they usually attenuate the level of the LFE by 6 dB (for example some Marantz units). Disabling either the center or the surrounds when playing DD tracks will cause some problems like loss of dynamics and SPL. Check a few posts down for more information.

This is not the case with DVD players though. There are only a few DVD players which can redirect the LFE channel into main channels (for example Sony DVP-NS900). And of course then the player has to be connected either via digital output (coax/fibre) or via multichannel analog outputs.

If you connect a DVD player via analog stereo outputs (or via SCART to a TV), the LFE channel WILL get thrown away. Dolby specs don't allow LFE to be downmixed into analog stereo outputs.

AC3filter used with Spectrum Labs program does handle the LFE redirection perfectly. Nothing will be left out and no attenuation will be made. Though it requires that the program has correct settings in it. I've been doing these waterfall charts for a while now and I thought I had the correct settings, but today I started explore this matter a little bit more, and found out that the LFE channel level has been +6 dB in all the charts I've done (and in other people's charts too, since most of them were using my settings). Let me explain why.

The AC3filter uses a matrix chart to control the level of individual channels. I've been using the "auto matrix" option all the time, but it doesn't seem to work properly after all. The program correctly downmixes 5.1 channels into 2 channels, but it puts the LFE into both channels (L and R). This shouldn't be done since Spectrum Labs program adds both channels together for a mono signal, which it then monitors. Since the AC3 filter downmixes the LFE into both channels, we get this +6 dB error. But luckily there is an easy solution.

Turn off the "auto matrix". Place a zero in LFE/R cell. LFE/L cell must read 1. Now the LFE channel will be downmixed only to left channel and the relative level compared to redired bass will be correct.



I took some new charts from the WOWT with these new settings, though the results are still pretty much identical with the old ones, since the LFE channel hasn't got much material in it. As you can see, most of the bass is actually redirected bass, not LFE. These charts show the full lightning scene from the first strike to the last one. DTS track was used.

[Ilkka]

Ed Mullen on AVS wrote:

A great example is the plane barrel roll (about 4 continuous seconds at ~ 30 Hz) during the crash scene in the Flight Of The Phoenix. That bassy special effect is exclusive to the LFE channel.

Here are the charts which prove that Ed is telling the truth.

[eddthompson]

great stuff, perhaps this explains some of the issues chrissbee was having.

edd

[Exocer]

Hey Ilkka,
I ran a few tests to measure the differences in LFE output when using only 2.1 channels playing a DD 5.1 track. The mains were disconnected for both tests.

I watched the DD 5.1 track with only L/R and LFE enabled for the first test. At -15db on the AVR my SPL meter measured 102db max spl.

I then watched the same scene in DD 5.1 enabling all 5 channels at -15db on the AVR. Max SPL came in at 105-106 db.

[Ilkka]

J_Palmer_Cass on AVS wrote:

If you do not use a DD 5.1 DVD with a full speaker compliment , then DRC is used regardless of the DRC setting in your receiver.

The DRC value is determined by a specific DRC downmix value selected by the mixer in the DVD encoding process. It is not user adjustable, and it overrides the DRC setting that you have selected in your receiver.

For example, my test of Master and Commander told me that for M & C DRC is set to MAX if I set my center speaker to OFF. LFE is still there, but DRC squashes everything down. Low bass, low sound effects, and the like.

First I didn't believe this could be true, but after I run some tests of my own (NAD T743 AV-receiver), I must admit that he's being right. It seems that the Dolby decoder has a will of its own, it will set on the DRC if one doesn't have all five speakers enabled. Also downmixing to stereo seems to have an exact effect. The amount of DRC probably varies between soundtracks.

I measured a few scenes with a professional SPL meter with four different configurations: DD 5.1, Downmixed 5.1 'stereo', DD 4.1 (center disabled) and Downmixed 4.1 'stereo' (center disabled). In reality I don't have a center speaker.

DD 5.1 always gave the highest SPL result, the other three were identical, but always much lower level.

FOTP - 'barrel roll': ~10 dB difference
M&C - 'first round': ~18 dB difference
WOTW - 'lightnings': ~9 dB difference

Those are pretty big differences, don't you think?! No wonder I have always preferred DTS tracks over DD (since normally my center IS disabled). DTS doesn't contain any DRC information!

If you don't have all five (5) speakers enabled, you will lose great amount of dynamics and max SPL when playing Dolby Digital soundtracks.

[eddthompson]

again, i belive chrissbee uses no centre, and has the .1 turned off, and hees been suffering dynamic range issues etc, a shame since he has an ib.

edd

[Ilkka]

eddthompson wrote:

again, i belive chrissbee uses no centre, and has the .1 turned off, and hees been suffering dynamic range issues etc, a shame since he has an ib.

edd

Yes, disabling center will cause dynamic range issues with DD tracks. Disabling subwoofer doesn't seem to have this same effect though.

[SteveCallas]

Finally I can get this page to load at home. Something about our internet servers at work didn't want to let me open this webpage for the past several hours. Anyway, that's neither here nor there.

Ilkka wrote:

First I didn't believe this could be true, but after I run some tests of my own (NAD T743 AV-receiver), I must admit that he's being right. It seems that the Dolby decoder has a will of its own, it will set on the DRC if one doesn't have all five speakers enabled. Also downmixing to stereo seems to have an exact effect. The amount of DRC probably varies between soundtracks.

I measured a few scenes with a professional SPL meter with four different configurations: DD 5.1, Downmixed 5.1 'stereo', DD 4.1 (center disabled) and Downmixed 4.1 'stereo' (center disabled). In reality I don't have a center speaker.

DD 5.1 always gave the highest SPL result, the other three were identical, but always much lower level.

FOTP - 'barrel roll': ~10 dB difference
M&C - 'first round': ~18 dB difference
WOTW - 'lightnings': ~9 dB difference

Those are pretty big differences, don't you think?! No wonder I have always preferred DTS tracks over DD (since normally my center IS disabled). DTS doesn't contain any DRC information!

If you don't have all five (5) speakers enabled, you will lose great amount of dynamics and max SPL when playing Dolby Digital soundtracks.

What, did you expect my "true story" was really a lie? Thanks for putting it to the test with measurements though - would you mind if I posted just this section:

FOTP - 'barrel roll': ~10 dB difference
M&C - 'first round': ~18 dB difference
WOTW - 'lightnings': ~9 dB difference

at AVS? More people need to be aware of this. Sounds like there is a guy on this forum with an IB that is making the same mistake. I know Jon was really happy after we discovered the problem....his 100 year old house wasn't too thrilled though

[Exocer]

I am wondering where a 3.1 channel system falls in the grid of things...Would it still pose as much of an SQ/SPL loss as downmixing to stereo? Its too late for me to run those tests here but i'll try tomorrow if there isn't already a definite answer.

[Ilkka]

Exocer wrote:

I am wondering where a 3.1 channel system falls in the grid of things...Would it still pose as much of an SQ/SPL loss as downmixing to stereo? Its too late for me to run those tests here but i'll try tomorrow if there isn't already a definite answer.

It did the testing also with this configuration and the result was exactly the same as when you disable the center. 4.1, 3.1, 2.1, 4.0, 3.0, 2.0 - same SPL loss compared to 5.1 or 5.0.

J_Palmer_Cass on AVS wrote:

Downmixing in the receiver activates the DD DRC downmix value (which is different for different DVD's). The DRC value selected by you via the receiver is ignored when you downmix.

Downmixing means disabling the center, or the surrounds, or both together. Anything except 5.1 or 5.0 is considered downmixing.

Downmixing does not mean selecting the subwoofer as off or on.

Downmixing in a DVD player is different again (LFE is dropped), but the DD DRC downmix value is used in the DVD player also.

The reason this DRC downmix value exists is that when you downmix you can overload the DAC's in the receiver if this downmix DRC is not applied. A DTS DVD DECODER has a similar downmix overload feature (compression) as I recall.

Remember that these DECODERS do things that are not obvious!!!

Just like he says, only the DTS part is incorrect. I tested it too and DTS doesn't behave the same. There is no loss if you disable the center or surrounds. DTS doesn't carry any DRC information. Surprisingly DD 5.1 was only around 2 dB behind DTS at the same MV setting (Master and Commander).

So do remember that this happens only when playing DD tracks, DTS doesn't have this problem.

[Ilkka]

SteveCallas wrote:

Finally I can get this page to load at home. Something about our internet servers at work didn't want to let me open this webpage for the past several hours. Anyway, that's neither here nor there.

What, did you expect my "true story" was really a lie? Thanks for putting it to the test with measurements though - would you mind if I posted just this section:

FOTP - 'barrel roll': ~10 dB difference
M&C - 'first round': ~18 dB difference
WOTW - 'lightnings': ~9 dB difference

at AVS? More people need to be aware of this. Sounds like there is a guy on this forum with an IB that is making the same mistake. I know Jon was really happy after we discovered the problem....his 100 year old house wasn't too thrilled though

No, I didn't think you were lying, I just thought there may have been some other reason. You can post the numbers, but don't post the whole thing. AVS doesn't deserve it.

[SteveCallas]

Lol, ok, no worries, I won't post any, as this thread is linked to over there. As bad as AVS is, that other place with AV in the name is even worse Of course the Shack beats them all anyway...

[Chrisbee]

Bwaaah!

Thanks for this timely thread Ilkka.

I'm using a NAD T533 DVDP to feed my Naim stereo pre-power + EP2500 pro-amp driving the IB.

I'm using an active crossover at 80Hz to split between front speakers and the IB.

I use the DVDP's multichannel analogue connections to an old Yamaha DSP E800 for my rear speakers only.

The Yamaha SQ is not good enough to use as the surround processor in my system. It displays all the symptoms of most of the affordable surround kit that I have heard so far. (Poor dynamics, poor dialogue clarity, scratchy and tinny ambient effects with emphasis on the HF)

In discussion with NAD they warned me that the T533 is not a surround processor.

I still believe the T533 can redirect bass from Small Rear speaker settings to Large Fronts.

I confirmed this using "The Calibrator" test DVD on (spoken voice) channel ID. The subwoofer pink noise test produced no redirection to the Mains.

Centre was redirected to L&R Mains when Centre set to None. Rears sounded out of phase coming from the Mains but the signal was redirected to Mains when rears were set to Small and muted at the rear.

My Centre is normally set to None in the DVDP. LFE doesn't exist since I'm only using the Front Mains = Large signal. So LFE must be dumped by the DVD as you suggest.

I tried my SVS on the centre channel and found loads of deep bass in there when set to Large. But I really don't want the SVS running with the IB and certainly not as a centre speaker when I get such excellent results from my main speakers.

How do I keep my present SQ while getting the benefit of my missing LFE? (and any other non-redirected bass)

I don't get any sound out of my DVDP from the analogue sockets if I select the optional DTS instead of DD5.1 in a film menu. Without an AV receiver I can't use digital connections.

[SteveCallas]

Buy a third main to use as your center speaker. I run three vertical towers myself.

Please don't anyone take this the wrong way, I honestly mean no offense by it, but did you guys never question the bass or dynamics you were getting in movies when compared to the impressions or graphs other members were posting? As I've stated, not only do you lose bass, but you lose top end clarity as well due to clipped peaks. I remember Jon being quite disappointed over the phone after watching some movie scenes in 2.1 after all the hoopla people had made about subwoofers and certain bass scenes - and even the hoopla I made about the LLT design

[J_Palmer_Cass]

Ilkka wrote:

It did the testing also with this configuration and the result was exactly the same as when you disable the center. 4.1, 3.1, 2.1, 4.0, 3.0, 2.0 - same SPL loss compared to 5.1 or 5.0.


Just like he says, only the DTS part is incorrect. I tested it too and DTS doesn't behave the same. There is no loss if you disable the center or surrounds. DTS doesn't carry any DRC information. Surprisingly DD 5.1 was only around 2 dB behind DTS at the same MV setting (Master and Commander).

So do remember that this happens only when playing DD tracks, DTS doesn't have this problem.

Actually, DTS is different than DD in that DD uses DIALNORM, and DTS does not do so. DIALNORM reduces volume by an amount that varies from DVD to DVD. There is no volume reduction when the value of DIALNORM is -31.

If a DD DVD has a DIALNORM value of -31 (like Air Force 1), then DD and DTS will playback at the same volume level.

The Master and Commander DD DVD uses a DIALNORM value of -27, so volume is reduced by 4 dB (-31 -(-27) = -4) as compared with the DTS version. DTS and DD sound just about the same if you set the DTS version 4 dB lower than the DD version.

The War of the Worlds DD DVD uses a DIALNORM value of -23, so volume is reduced by 8 dB (-31 -(-23) = -8) as compared with the DTS version. DTS and DD sound just about the same if you set the DTS version 8 dB lower than the DD version.


My receiver reads out the DIALNORM value that is ENCODED in the DVD, so I can read the DIALNORM value directly on my receiver. Does anyone else have this feature?


As far as downmixing DTS is concerned, I tried downmixing M & C DTS last night. If you downmix the DTS version of M & C, it does not sound the same as the 5.1 version. It sounds like a bit of overload compression is used, but not anything like the drastic DD DRC. Perhaps it is just the electronic downmixing of signals that changes the sound.


Anyhow, everyone should use a full 5.0 or 5.1 system (even with crappy speakers). Some DVD's just sound BAD when you downmix, DD versions in particular!

[Ayreonaut]

I don't have a center channel right now.
My receiver is the Denon AVR-1905.
How should I test to see if it is compressing DD dynamics with "Center Sp." set to "None"?

BTW, this is a real problem that Dolby needs to fix immediately.

[Ayreonaut]

I found this in the user manual for the Dolby Model DP564 Multichannel Audio Decoder

Compression
The Compression buttons can apply the dynamic range control profiles available to consumers when decoding Dolby Digital audio. Only one of these modes is active at a time; to disable compression, press the active button.

RF applies the strongest profile available, equivalent to an RF connection to a TV set or small PC speakers. Line applies what appears on some consumer decoders as “light” compression; this is usually the default setting in DVD players and set-top boxes. Custom allows scaling of the Line profile. The factory setting for Custom is 0 percent. At that setting, selecting Custom turns off dynamic range compression. To access the display screen to adjust the percentage setting for custom scaling, hold down Custom for two seconds.

Note: When downmixing, the DP564 automatically applies compression at peak moments to prevent potential signal overload from combining multiple channels, regardless of whether a compression mode is selected.

[SteveCallas]

J_Palmer wrote:

My receiver reads out the DIALNORM value that is ENCODED in the DVD, so I can read the DIALNORM value directly on my receiver. Does anyone else have this feature?

Yes, my Yamaha HTR 5890 does the same thing at the start of a movie. I always get a little perturbed when I see that **** "Dial Norm. -4db" pop up on some movies In fact, it even comes up on a DD intro demo disc I have, you know, the ones with the DD helicopter, train, canyon, pyramid, etc.

Ayreonaut wrote:

How should I test to see if it is compressing DD dynamics with "Center Sp." set to "None"?

Play a bassy movie scene with your setup as it is now. Then tell your receiver you DO have a center channel and surrounds (even if you don't have anything connected) and play the scene again. You should notice more bass. If you really did have a center channel connected, you'd notice more clarity too. I never realized how many people weren't using center channels

[brucek]

I never realized how many people weren't using center channels

Neither did I.

Why aren't they? Is it financial or some preference for phantom, etc ?

brucek

[eddthompson]

people have the notion that stereo equipment is better


edd