Dolby Laboratories Information
Dolby and the double-D symbol are trademarks of Dolby Laboratories Licensing Corporation
Signal Processing and Noise Reduction Systems
Wootton Bassett
Wiltshire SN4 8QJ England
Telephone 01793-842100
Facsimile 01793-842101
100 Potrero Avenue
San Francisco, California 94103-4813
Telephone 415-558-0200
Facsimile 415-863-1373
“Phase Correctors” and Dolby Surround
By Jim Hilson
Surround Broadcast Engineer
Dolby Laboratories Licensing Corporation
Since the introduction of stereo broadcasting, devices designed to correct the phase of audio signals have
been in existence. Their original use was to be for replacing the azimuth adjustment on tape machines
used to “cart” commercials for radio stations. Since the phase on submitted tapes was and still is prone to
be different on each tape, a unit such as this could save countless hours of machine alignment. Using this
device would make the carts “sound better.”
As broadcasters became aware of these units, their place at the station took a new location. They started
appearing at the input to the transmitter as a “catch all” because in addition to their ability to correct small
phase errors found on tapes, they could correct and fix an out of phase condition. Using this device would
mean that any out of phase signal would be corrected and all the mono viewers would hear the
information.
Sounds like a good idea, but it isn’t. Unfortunately, the units aren’t smart enough to detect when signals
should and should not be out of phase. For example, signals that are present in a proper Dolby Surround
mix may contain conditions where there is more out of phase information than in phase information. A
perfect example is a front to back pan. As the effect passes the half way point it begins to contain more
out of phase information than in phase information. When doing live sporting events, a large crowd roar
with announcers talking over the ambience could suddenly have more out of phase than in phase
information if the announcers were to simply stop talking. When these conditions occur the unit will
momentarily flip the phase until the situation returns to “normal.” The return flip usually has a small delay
before taking place. Depending on how you are listening will determine what you actually hear.
In Dolby Surround, the center and surround channels will be reversed, the announcers will begin talking
from the surround speakers. In stereo, little will be noticed, although the mix will appear to have a “hole” in
the middle. In mono, the announcers will be totally gone. Of course, this will only last a second or two so
you will hear the signals bouncing around the room in surround or miss words if you are listening in mono.
These conditions usually prompt those irate phone calls from your viewers.
While the use of a phase corrector may make sense as an automatic azimuth adjuster, the use as a “catch
all” for improper engineering and/or attention to detail can and will actually cause more harm than good
where Dolby Surround encoded shows and material are being played.
Dolby Surround is now being used for more than just movies. Approximately 20 weekly or daily TV shows
are routinely using Dolby Surround as are several recording artists making CD’s and cassettes. Live
concert broadcasts are also being looked at favorably by radio stations and more and more commercials
are being produced, but not necessarily marked as Dolby Surround encoded.
Having the Dolby Surround encoded signals pass properly through your TV or radio station is our number
one concern. If you must have a phase corrector in the line, may we suggest that you make the phase flip
function an operator actuated function. Have the unit trigger an alarm when an out of phase condition
occurs and let the operator decide if the signal is truly out of phase or just has a large content of out of
phase material present.
S93/9792
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