8 Subwoofer s
SDDS Print Master Guidelines, Page 23 of 26
8.7
Setting t he digit al subwoofer in the cinema.
In a s i milar way as on the dubbing s tage i n Step 3, the l evel of the digi tal subwoofer is
set by using a mul t i - channel real time analys er and adj us ti ng the electrical gain of the
monitor system to achieve 10dB of in-band acoustical gain, relative to a s cr een
speaker. The cons equence of this adjustment is that signals which were r ecor ded
appr ox i matel y 10dB lower for the s ubwoofer will now play back in the cinema at the
same acous ti cal level as they did when the dubbi ng engineer recorded them on the
dubbi ng s tage, because the pl ayback conditions have been acoustically matched
between the dubbi ng s tage and the ci nema ( as closely as variations in rooms and
loudspeakers will allow).
8.8
Optical subwoof er defined.
T he opti cal subwoofer has a di fferent function than the digi tal subwoofer . It serves to
enhance the bass response of the opti cal playback. It is totally artificial, synthesized in
the ci nema pr ocessor by combining the L , C ( -3dB), and R signals from the decoded
optical Lt,Rt and s endi ng the s um through a l ow pass filter. The cut off frequency of
this filter may be 50Hz, 80Hz, 100Hz, 120Hz or some other frequency, depending on
the ci nema pr ocessor and the i ns taller’s judgement. A hi gh pass filter may also be
applied, if it is not already a component of the opti cal preamplifier, to r educe l ow
frequency artefacts caused by ground noi s e ti mi ng (GNR) errors and s tr eaki ng nois e
on the pr i nt. The end r es ul t is a s i gnal containing fr equency components which overlap
those being s ent to, though not necessarily reproduced by, the s cr een speakers. In
Hollywood, the dubbi ng engineer may not have listened to this signal on the dubbing
stage (i n part because the DS 4 without a cat . 160 card does not create it) and s o
made no ar ti s tic deci s i on s based on it. It is merely an enhancement created i n the
cinema t o give the effect of more bass extension in the s cr een speakers and has no
correspondence to any discrete s i gnal on the dub s tage. Note that it also has a much
lower dynamic r ange than the di gital subwoofer signal. The digi tal subwoofer signal, in
contrast, was creatively recorded on its own discrete channel (Sub or LFE) of the pr i nt
master and i s used for specific l ow fr equency sound effects.
8.9
Setting t he opt ical subwoof er .
T he opti cal (analogue) subwoofer is adjusted to match the opti cal screen speakers and
effectively extend thei r low fr equency response, again using a mul t i - channel real time
analyser to compar e bands in the pass band of a s cr een speaker and the pass band of
the s ubwoofer , and s etting in-band acoustical levels accor d i ng l y . In this case, there i s
no in-band acous ti cal gain difference, as the s ubwoofer signal is just the l ow fr equency
components of the s cr een speaker signals.
