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» Film-Tech Forum   » Operations   » Film Handlers' Forum   » Bypass and antiphased audio channel

Author Topic: Bypass and antiphased audio channel
Thomas Pitt
Master Film Handler

Posts: 265
From: Leeds, West Yorkshire, UK
Registered: May 2007

 - posted 09-29-2008 03:38 PM      Profile for Thomas Pitt   Email Thomas Pitt   Send New Private Message       Edit/Delete Post 
I went to see Death Race today at the Bradford Cineworld. Despite the DOLBY DIGITAL plaque on the auditorium doorway, when the film started up it appeared to be running in analogue optical. I saw a few lab splices cause a noticeable 'click' in the audio - I guess the Dolby unit was in bypass.

If you don't know about antiphase already (why are you on this forum? [Big Grin] ), it means that when one speaker is moving forward, the other will be moving backward and vice versa. Listeners hear a strange 'inside your head' effect, particularly when the sound is moving slightly between stereo channels.

What I'm wondering is if it's possible to accidentally wire up one of the stereo channels backwards, making the two channels out of phase? Or is it all 'plug and play', meaning the channels can't accidentally be wired wrong? The antiphase effect was also apparent during the adverts and trailers, and there didn't seem to be any sound coming from the surround speakers.

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John Walsh
Film God

Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999

 - posted 09-29-2008 06:17 PM      Profile for John Walsh   Email John Walsh   Send New Private Message       Edit/Delete Post 
A careless person can reverse the speaker wires anywhere. I have found them reversed in JBL 'new-out-of-the-box' speakers. I feel there is NO substite for phase checking all speakers at least just before EQ'ing for the first time.

Note that there are other reasons besides phasing that can cause that 'strangeness' in the sound you describe.

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