Film-Tech Forum ARCHIVE: Frequency response of matrixed surround channel?

This is topic Frequency response of matrixed surround channel? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Christopher Seo (Member # 7) on 01-05-2003, 10:15 PM:

I was reading a Dolby Pro Logic II brochure ("A Listener's Guide to Pro Logic II"), and came across this information:

quote:
The original Pro Logic features four channels-- front left, center, and front right, plus a mono surround channel that's usually split between two rear speakers. The surround channel is also "band-limited," reducing the treble frequencies. Pro Logic II further enhances the sound with a full-range stereo surround output...

How severe is this high-frequency limitation on the surround channel? Does it apply to the cinema version of Pro Logic as well? If so, what was the reason/purpose for the high-frequency rolloff?

Posted by Gordon McLeod (Member # 33) on 01-05-2003, 10:19 PM:

If memory serves it was very step around 7K and then on top of that was the Dolby NR.
Some of the reasons were
Noisey bucket brigade time delay chips
crosstalk of early matrix bleeding into the surround
the limitations of the old 4 track mag surround channel and the limitations of most of the old speaker that were already installed

Posted by Christos Mitsakis (Member # 143) on 01-06-2003, 04:29 AM:

You remember right Gordon. DPL surrounds are limited to 7Khz.
This begun from the cinema implementation and passed to the home theatre.
Lateral slit position in older soundheads and mainly azimuth problems create severe crosstalk problems where dialogue and high freq bleed to the surrounds. High limiting the surround channel was a "failsafe device" along with the delay and modified Dolby B, so in case of leak one can not hear and localise high sounds that were not supose to come fron the surrounds.
In the home theatre though, Hi-Fi VHS and laserdisks didn't have that amount of crosstalk, so - my opinion, I may be wrong - band limiting the surrounds isn't necessary.

Christos.