Bass Management with Stereo vs. Dolby PL II

mbtins

I have a question about my home theather set up that I hope you can answer....

I have just purchased the following components:

Onkyo TX-SR600 AV Receiver
Polk RM6600 Sat/Sub system

Per Polk's documentation, I have hooked the subwoofer up using the speaker level connections from the main front and left speaker outputs on the receiver. I then use the subwoofer speaker level outputs to send the signal on to the Left / Right Front speakers. The center channel, and the two surrounds are hooked up directly to the receiver.

Also, per Polk's documentation, I have made the following settings on my receiver:

Subwoofer: None (since I am NOT using the subwoofer line level output)
Front Speakers: Large
Center Speaker: Small
Surrounds: Small

I have the crossover on the receiver set to 120 Hz.

With this setup, bass from all channels below 120 Hz as well as any content encoded in the LFE channel should be directed the main Front / Left speakers. Since I have the front speakers hooked up through my sub, I use the crossover in the sub for blending the output.

When I play back two channel audio, such as a compact disk -- in Stereo mode -- I get more bass than when I play the same music CD using the Dolby Pro Logic II Music mode. If bass management is working correctly, I should not be losing any bass when I switch from two channel stereo to Dolby PL II, correct? However, it seems like when I spread the music out over all channels using Dolby PL II, not all of the bass that was there in Stereo mode is still getting redirected to the Fronts (and by virtue of my connection) the subs. Obviously, bass management is not an issue in Stereo mode since all of the signal is going to the front speakers through the sub.

Any insight you can offer into why I am losing bass (or at least I perceive I am losing bass) would be greatly appreciated.

Thanks

Michael B. Tinsley
Columbus, Indiana

scottvamp

Sounds like everything is hooked up correctly. It sounds as if that is just the way it is. I can not really give a technical explanation. But the PLII is a DSP mode and it will change the way the system sounds. Because that DSP mode probably does not send as much bass info directly to the mains the sub will not get as much bass info. I have always gone the LFE route. Perhaps if the sub has a gain on it u can pump that up a bit while listening in PLII.

Systems

Does your sub have a movie/music mode switch? If so, it may be that if listening to surround mode, even if it's music, you need to set the sub to "movie". Or maybe it's a subjective issue--with surround, you have 3 more non-sub spkrs coming at you, so maybe you notice the non-sub sounds more, even if a lab test would show that the bass output is the same or nearly so. Or maybe it's what I tend to think about surround music in general--it's in its infancy & they have a long way to go before they'll have a surround music system that sounds as good (dollar for dollar) as 2-ch. Sure, it's great if you're David Chesky, but I don't have & can't afford his rig(s).

mbtins

I don't have a movie / music switch on it or anything like that. I as talking to a tech support guy from polk on this and there are two other possible solutions we came up with together.

1. Having the rears turned on could be causing some bass cancellation with the output from the mains.

2. Everything is working fine, but having the rears turned on does send some bass output to the surrounds that they (RM2300) can't handle. The higest setting I have on the receivers bass management is 120 Hz. The sub's crossover is set to about 150 Hz -- which I think is more appropriate for the RM2300 speakers. When I am just listening to two channel, I don't have to rely on the receiver's x-over, it's all done by the sub (PSW350).

Mike T

scottvamp

Ya, just the way it be.
Movie and music settings on the sub.
There is no such thing. But it sounded good.:rolleyes:

Systems

There are subs with such a selector. There may not be many of them, but they do exist. My post did not state that the Polk has one, it was merely a suggestion regarding a possibility. Yes, there is a such a thing, though not perhaps on the Polk.

doowop02

I have the same setup with a Pioneer THX receiver and a Polk PSW250 Sub. Everything is fine when watching a movie, but I can only get decent bass out of the sub in stereo; sucks but guess that's just the way it is.

RBreak

My Velodyne CHT15 has an "audio/video" toggle switch on the back. According to the manual, the video setting adds 3db without changing anything else, as people sometimes want more bass when watching a movie than listening to music. If I remember correctly, the PSW650 I used to have had the same sort of switch also, adding db.
I use a Denon 2802 receiver, and it has good bass in 2 channel stereo, but it loses alot in PLII, I think thats just part of the PLII design but I have no idea why. PLII was one of the reasons I went for the Denon, but after having it for a while I am not that impressed with PLII, in fact I never even use it. It's a subjective thing, others may love it.

mbtins

I eventually wrote this very question to Dolby Labs. Here is their very detailed and thorough response. I must say I was very impressed with the quality of their answer. They actually went to Polks web site and read the book on the speaker set I have before they posted a reply.

Mike

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begin quoted material from Dolby Labs

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Dear Mr. Tinsley,

Thank you for the detailed email describing your situation. Based on your information, it is possible that you are getting less bass than you should in PLII Music mode. Assuming the Center Width control is somewhere in the middle of the range, one simple test is to adjust the Center Width control to full L/R position (no C output). If the bass seems to be restored, it is due to an interaction between the filter used in the bass management stage in and the wideband channels (L/R). The symptom seems more prevalent in PLII Music mode due to a higher degree of coherence between C and L/R channels (especially when the C Width control is set mid-way), but the same issue exists for any modes that use a center output. If the bass doesn't improve, let us know. That will require some extra thinking.

However, if we are on the right track, and adjusting C Width as above returned the bass to normal (same as stereo), we recommend a small setup change to fix the problem. We checked the Polk website and the RM6600 system, and read the manual. The reason for their somewhat strong recommendation to use the connection method specified is that not all receivers allow the crossover frequency to be much higher than 80 Hz (as specified by THX). With smaller satellites such as in the RM6600, it means they would be getting signals down to 80Hz. You would be losing about 1/2 octave of sound because these speakers roll off several dB below 100Hz. In your case, your receiver can set the crossover up to 120 Hz, which is an excellent match for this speaker system.

We therefore recommend connecting the L/R speakers directly to the receiver outputs (just like the other speakers), and then run a single RCA phono lead from the Sub output on the receiver to the "LFE" input on the sub. Tell the receiver that the L/R are "small" (all 5 should now be set to small). Set the sub's crossover filter control all the way up (highest frequency). This allows the receiver's crossover filter to control the response complementarily with the satellites.

You should then check the calibration of the sub's level, using either the sub's gain control or the sub output level trim in the receiver to get the best blend with the mains.

We believe you will find the bass more satisfying from PLII Music as well as other 5.1 modes.

Best regards,

Dolby Customer Support