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Dolby Stereo Surround On 70mm | Read more at in70mm.com The 70mm Newsletter
|
Written by: John Iles,
England, Dolby Sound Consultant, formerly Dolby Laboratories, now retired | Date:
02.11.2012 |
Northpoint
Theatre in San Francisco. Early days of Dolby Stereo. Note the BIG logo on
the facade. Image from a Dolby brochure.
In early 1978 Gordon McCullum and his crew at
Pinewood were due to mix Dick Donner’s “Superman The Movie”. The release was
to be in Dolby Stereo 35mm optical sound and 70mm. Gordon or “Mack” as he
was known was a volatile character but at that time one of the few film
re-recorders that was willing to experiment and push the boundaries.
The Dolby 70mm format at that time was to use the two inner channels (Le &
Re) for low frequency effects only. Left, Centre, Right and the surround
channels were full range. The inner channels were not therefore fully
utilised, normally only having signals during action sequences where greater
low frequency output was required.
Gordon and Max Bell the Dolby Consultant assigned to the film discussed the
possibility of utilising the inner channels for a stereo surround track with
the production. As the production was already very late the mix shouldn’t
take any longer and to stand any chance of having prints available for
cinemas the format would also have to be compatible with existing Dolby 70mm
reproduction.
I built an encoder for the studio and David Watts built a decoder as Dolby
didn’t have any studio hardware available at that time. The idea was simply
to use the inner channels to carry the high frequency left and right
information whilst still carrying low frequency effects.
The surround
channel would be a sum of Left and right full range signals. The left and
right surrounds in the cinema would have low frequency added back in from
the mono surround channel. See figure below for the block diagram.
| More in 70mm reading:
"In 70mm and 6-track
Dolby Stereo"
All films presented in
70mm Dolby Stereo
70mm Blow Up List:
1978 |
1979 |
1982
Internet link:
|
Encoder block diagram.
As the inner channels were now effectively wide range, as were the other
channels, they would require A-type noise reduction encoding and decoding
when played in the stereo surround format. Fortunately, the low frequency
replay filters already in the CP100 contained low frequency expander
characteristics as per the lowest frequency band found in Dolby A thus
ensuring compatibility on standard installations. However, for the new
format an additional box would be required to add to the CP100.
I designed the encoding filters as 2nd order critically damped (12db per
octave slope) low pass and high pass. Tests showed this to provide
sufficient separation between the low frequency and surround channels. The
design was changed for “Apocalypse Now” the 2nd film to have a stereo
surround 70mm release. In the revised design the filters were made
complementary. Like most engineering decisions there are compromises to be
made and it’s about making the right compromise. If you ask an audio
engineer about loudspeaker crossover networks (effectively the same
requirements) you would find little agreement on what provided the best
sonic result!
The problem is that filters produce phase changes and low pass and high pass
filters produce opposite shifts in phase. If a signal is of equal amplitude
and summed after filtering it will be completely cancelled at the crossover
point. This sounds extreme but we are talking about a very narrow band and
only when signals are equal. In reality this is very hard to detect on a
before and after test with “golden ear” listeners. A complementary filter
which gets its characteristics by simply subtraction from the other filter
will have no cancellation. The problem with this I found when we came to
record “Pink Floyd the Wall”, the next stereo surround film also mixed at
Pinewood.
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A complementary filter can only have a 6dB per octave slope and at crossover
it has a significant peak. Panning a narrow spectrum guitar from left to
right in the surrounds it actually appeared right to left! However, such
narrow spectrum signals are unusual and as we always monitored through the
encode/decode chain we could correct any such abnormalities.
“Superman the Movie” had an outstanding soundtrack for its day thanks to the
enthusiasm of Gordon and his Crew. “Apocalypse Now” also broke new ground
and was another fine example of what you could achieve with the format.
Today almost all films are made with stereo-surrounds (5.1) although not all
Directors and re-recorders take full use of the possibilities.
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21-01-24 | |
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