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AUDIOPHILE AUDITION - web magazine for music, audio & home theater
 



April 2004, Part 4 [Pt. 1] [Pt. 2] [Pt. 3]


[If you thought bass management was a simple matter, have a look
at this article from the Boston Audio Society's Speaker newsletter...Ed.]

Lexicon’s Subwoofer Logic
by David J. Weinberg



[This was sent to Lexicon in the hope it might prove of value to them — DJW.]

With all my Lexicons (CP-1, DC-1, DC-2, MC-12) over the years, I have preferred a 7.1-channel configuration, non-THX. With my speakers I would prefer crossovers set at:

Left and Right Front: 40Hz highpass (HP);
Center: 80Hz highpass (the Lexicon feeds signal below this to both Left and Right Front outputs);
Left and Right Side: – 40Hz highpass;
Left and Right Rear: 60Hz highpass;with the single subwoofer set for a 120Hz lowpass (LP), so it handles the whole LFE signal plus all the seven channels’ audio below the above-listed highpass-crossover frequencies.

Of course this assumes that the processors feed the remainder (the signals below the seven channels’ HP crossover settings) to the subwoofer, regardless of the subwoofer’s LP crossover setting, as long as the frequencies are below that setting.

My assumption was that in no case would both a main channel speaker and the subwoofer radiate the same signal due to bandwidth overlap. From studying my MC-12 manual (which I found inadequate in this area), running some tests on the MC-12, and communicating with Henry Hecking of Lexicon support, who has been diligent in providing assistance, I have come to learn that the MC-12 doesn’t work this way.

Since I now have test equipment that enabled me to test the MC-12’s subwoofer logic, I found what I consider a problem with how the HP/LP filter system operates. The compromise solution in the following applies to the MC-12 only, and assumes that I have set the main channel crossovers as listed above.
If I have only one subwoofer, it clearly must handle the leftover LP signals from the seven main channels (below the lowest HP crossover setting among the main channels), plus the LFE signal.

However, as the MC-12 currently works, I find that if I feed that subwoofer from the Subwoofer 1 Output (with the MC-12 set for a single subwoofer and no LFE speaker connected) and set the subwoofer LP to lower than 120Hz, the upper portion of the dedicated 120Hz-bandwidth LFE signal, which is capable of being up to 10dB louder than the main channels, gets fed to the Left and Right Front speakers, which might not cleanly handle the extra level at those frequencies.
If I set the subwoofer crossover to 120Hz so as not to risk overloading the main channels with this LFE signal, then the difference between the main channel crossovers and the 120Hz LFE crossover setting is now radiated by too many speakers: the respective main channel(s) and the subwoofer, resulting in double output (in this case between 40/60Hz and 120Hz), which results in boosting that overlapped range above the correct level.

The only compromise solution I could find is to set the Left and Right Front to full bandwidth, so all the LP signal from the other channels is fed to the Left and Right Front (this also assumes that the Left and Right Rear bass frequencies are fed to the Left and Right Front, not to the side channels), and to feed the subwoofer from the LFE output, telling the MC-12 that I have no subwoofer, but only an LFE speaker. This stresses the main channels more than I want to.

It seems to me that the prodigious computational power and digital logic already in the Lexicon MC-12 would support the LF logic shown in the following diagram, which would allow any of three possibilities — one or two subwoofers, and even an additional dedicated LFE subwoofer — without overlapping output and without sending part of the LFE signal to the main speakers. The channel logic in this diagram is assumed to be such that once a channel’s HP crossover frequency is set, that channel’s LP is automatically set to match. I have also assumed that in the “No Subwoofer” configuration, the Left and Right Front speakers are set to full bandwidth (automatically causing zero output from those channels’ LP filters), and the “No Subwoofer” feed from the subwoofer channel back to the Left and Right Front is added into the signal before being fed to the respective main speakers.
Note that all four subwoofer possibilities (from NONE through three) are addressed by this logic. The diagram shows simplified logic around having one or two subwoofers, but the concept is shown.

Only if there is no subwoofer would the main channels have to handle any of the LFE, and, with more convoluted logic, perhaps it could be fed to all seven main channels in parallel, allowing each one to handle as much of the signal as possible, thus sharing the load (based on their HP settings). I believe I haven’t missed any configuration idiosyncrasies, but would be interested to hear if I have.

[This article was originally published in the January 2004 Boston Audio Society Speaker (USN 0195-0908) and is reprinted with permission of the author and the Boston Audio Society. Background, membership and dues information is available at www.BostonAudioSociety.org or by calling 603.899.5121. No part of this reprinted material may be further reprinted or otherwise reproduced without the written permission of the Boston Audio Society.]


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