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Old 09-14-06, 01:16 PM   #1
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Bass Effects on Downmixing DD to Stereo and Spectrum Labs information


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

Quote:
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.







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Old 09-14-06, 02:13 PM   #2
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Re: Bass Effects on Downmixing DD to Stereo and Spectrum Labs information


Quote:
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.





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Old 09-14-06, 04:53 PM   #3
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Re: Bass Effects on Downmixing DD to Stereo and Spectrum Labs information


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

edd


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Old 09-14-06, 04:54 PM   #4
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Re: Bass Effects on Downmixing DD to Stereo and Spectrum Labs information


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.


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Old 09-14-06, 05:31 PM   #5
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Re: Bass Effects on Downmixing DD to Stereo and Spectrum Labs information


Quote:
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?! :yikes: 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.


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Old 09-14-06, 05:35 PM   #6
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Re: Bass Effects on Downmixing DD to Stereo and Spectrum Labs information


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


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Old 09-14-06, 05:47 PM   #7
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Re: Bass Effects on Downmixing DD to Stereo and Spectrum Labs information


Quote:
eddthompson wrote: View Post
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.


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Old 09-14-06, 06:11 PM   #8
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Re: Bass Effects on Downmixing DD to Stereo and Spectrum Labs information


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.

Quote:
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


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Old 09-14-06, 06:49 PM   #9
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Re: Bass Effects on Downmixing DD to Stereo and Spectrum Labs information


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.


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Old 09-15-06, 04:47 AM   #10
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Re: Bass Effects on Downmixing DD to Stereo and Spectrum Labs information


Quote:
Exocer wrote: View Post
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.

Quote:
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.


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