Playback of magnetic sound tracks on 70mm cinema prints at the Schauburg
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The 70mm Newsletter
|Written by: Clemens Scherer,
left to right the image shows one original Philips sound head, second an
early Winkler sound head with the milled slot still flat, followed by
three later Winkler sound heads that all got the full rework done.
In case, one is attempting to build up a sound system being able to play
back the recordings stored on magnetic sound tracks being found on 70 mm
cinema prints will face various problems. Some problems arise from using
devices built by the cinema industry decades ago. These devices were
developed to play back new struck prints, but already in 2005 we have
been looking back on archive prints being between one and five decades
Over the decades the prints suffer shrinkage. As the shrinkage effect
slows down after a couple of years, most prints from all decades show a
shrinkage of around 1%. When running these now smaller prints, we could
observe that they are swinging left and right in the guidance rollers
being placed before and after the sound head. Such a swinging cycle
takes only a few seconds. So for a short while the left side tracks are
well positioned to their pickups. Then the print is moving right. At the
very end only the right side tracks are well taken. Then the print is
moving left again. This continues in endless repeating.
At the Schauburg cinema in Karlsruhe the decision was made to have new
sound heads manufactured, not the usual ones, but with an updated
position of the pickups to reflect the average shrinkage of the prints.
Running over these sound heads, all the tracks are oscillating more
symmetrically to their pickups. One could consider in addition to have
new guidance rollers manufactured, optimized for the now smaller prints.
But the magnetic recording tracks proved to be wide enough to cover this
movement. And we receive a good abrasion image, no sharp edges appear.
We had these new heads manufactured in a one man factory, “Winkler
Magnetkopftechnik”. A "spin off" of the 60s, later taken over by the
son. Then manufacturing all kind of magnetic heads to stay in business.
Around 2010 business declined and the factory closed. Dietmar Zingl (Leo
Kino, Innsbruck) and Herbert Born (Schauburg, Karlsruhe) placed the
final orders. And so "Winkler MKT" ended its business with manufacturing
the original product, 10 track magnetic sound heads for cinema theatres.
When receiving the first new theatre head, we faced a major problem.
Neither normal nor excessive shielding could prevent humming to be
clearly heard in the auditorium while silent scenes. It turned out,
“Winkler MKT” had altered the internal construction of the pickups, due
to our request for them being wider than before. So with the pickups for
70 mm we withdrew this request and MKT made an effort for the pickup
construction having the best possible inherent immunity against magnetic
fields. After that was achieved, normal shielding proved to be
|More in 70mm reading:|
Schauburg Cinerama, Karlsruhe,
Germany. Home of The Todd-AO Festival
Teccon Precision Magnetic Recording
Why new magnetic sound
equipment for 35mm and 70mm prints?
Weekend Review by Clemens Scherer
auf das Karlsruher 70mm Wochenende 2005
image shows the main parts, a 10 track Winkler sound head consists of.
The high tech parts are the pickups inserted in three blocks of nickel
silver. These blocks will be held in position by two aluminium brackets.
Mounted together, this core will get a mounting plate fixed in an
electrically isolated way at the back side. Three Mu-metal sheets are
fixed at the other sides around, helping to improve shielding from
alternating magnetic fields.
There were still aspects to improve the overall construction, but after
the intricate and time-consuming optimization of the pickups we decided
for now to improve pure mechanical aspects by ourselves.
• 1. In the aluminium brackets the milled slot for the 35 mm film path
was completely flat and regarded too shallow, it had to be given proper
shape for being absolutely save from scratching prints, even if a print
tends to bend more than usual.
• 2. Front and especially back side of the assembled core had to be
trimmed for to get all must-be right angles really perfect.
• 3. The front side Mu-metal sheet had the edges to be cut und curved,
to allow film to be threaded in and out without getting scratched.
So at “Scherer Cinema Systems” the already delivered soundheads had to
be disassembled for to rework some parts.
In sum seven 10 track sound heads got reworked to ensure best
magnetic sound reproduction at the Schauburg (Karlsruhe) and at the
Leokino (Innsbruck) with many more
Todd-AO festivals and 70 mm special
presentations in the years to come. Then the design and construction of
the altered sound heads proved to be completed to satisfaction, but
unfortunately “Winkler MKT” was forced to close down and left the new
sound heads to be unique specimen.
image shows a reworked sound head. Well recognizable the 10 pickups and
some Mu-metal shields placed in between, all composed in three blocks of
Two blocks with 3 channels each (70 mm) divided by one block with 4
channels (35 mm).
The pickups for 35 mm film are now made extra wide to cover the slightly
different magnetic stripe and track locations of certain prints (first
with foxholes and later even with standard holes) together with the left
and right movement of the print due to shrinking. The resulting “excess
width scan” can be done with tracks on 35 mm film, as these are clearly
separated and each recording track covers nearly the whole magnetic
stripe. Picking up the full recording track width all the time avoids
loudness variation and improves the SNR significantly. At the Schauburg
this proved 35 mm magnetic playback to get close to the 70 mm sound
experience of the same movie.
To resign “excess width scan” with 70 mm has the advantage to avoid
reading additional noise from the unrecorded guard band between the
outer recording tracks, which share the same magnetic stripe and
avoiding the risk of crosstalk between these channels. Anyway with
current technology it turned out to be impossible to further reduce the
space between the directly neighboured pickups, due to the coils inside
already using up all space.
image shows the visible proof that the latest reassembled 10 track sound
head got the tracks positioned exactly like the already tested
In addition, new preamplifiers were built, optimized for these new
high impedance sound heads, allowing to remove the usual isolating
transformers from the playback chain. This measure is crucial to remove
another source for the typical humming with power frequency and other
noise from nearby magnetic fields.
In contrary to electric fields, there is no perfect shielding from
alternating magnetic fields. This circumstance makes transformers to be
more or less an antenna for alternating magnetic fields in the
projection booth – if not protected by an expensive design like the
soundhead itself. Especially in conjunction with such low level signals,
that have to be amplified by around 40 to 70 dB. In succession the
necessary electrical isolation had to be moved to the sound head and is
hence supported by the new design.
Further in the chain any equipment can be used, being able to process
the Dolby treatment for all the various sound formats one intends to
play. At the Schauburg an upgraded CP 200 does this job, supporting all
the sound formats ever defined for 70 mm prints.
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