Studio Technologies 76 V.1.04 - Manual (Page 32)

Issue 4, September 2008 Model 76/77 User Guide
Page 32 Studio Technologies, Inc.
for Surround
Technical Notes
Downmix
The downmix functions are implemented in
the Model 76 Central Controller’s hardware
using analog circuitry. The surround to ste-
reo downmix function performs this action:
center (C) dropped in level by 6 dB and
routed to the left (L) and right (R) monitor
outputs; left surround (LS) dropped in level
by 3 dB and routed to the L monitor output;
right surround (RS) dropped in level by
3 dB and routed to the R monitor output;
low-frequency-enhancement (LFE) input
muted. A confi guration choice allows the
LFE channel to be dropped by 3 dB and
routed to the L and R monitor outputs. This
will be generally enabled only for special
applications.
The stereo to mono downmix function
performs this action: L dropped in level by
3 dB and routed to the C monitor output;
R dropped in level by 3 dB and routed to
the C monitor output; L, R, LFE, LS and
RS monitor outputs mute; C input muted.
Bandpass Filter
The bandpass fi lter associated with the
stereo to mono downmix mode is created
by cascading (connecting in series) a
high-pass and a low-pass active fi lter.
Each fi lter is a Sallen-Key type, with a
12 dB-per-octave response. The high-pass
fi lter has a nominal –3 dB point at 100 Hz;
the low-pass at 5 kHz. For a detailed de-
scription of the fi lters, refer to page 7 of
the Model 76 schematic diagram.
“Hot” Disconnection of
Model 77 Control Consoles
There’s no problem relocating a Model 77
Control Console while the StudioComm for
surround system is powered up and oper-
ating. You can disconnect the 9-pin inter-
connecting cable from a Model 77, move
the unit, and then re-connect without issue.
If only one Model 77 is present, discon-
necting it from the Model 76 will cause
the monitor outputs to mute. No clicks,
pops, or other noises will occur when the
Model 77 is again connected. The Model
77 will go through its standard power-up
sequence and then normal operation will
resume. If more than one Model 77 is con-
nected, disconnecting one unit won’t cause
any change to the system’s operation. Only
when there are no Model 77 units connect-
ed to a Model 76 will the outputs mute. Of
course, any time a Model 77 is connected
it will go through its standard power-up
sequence prior to continuing with normal
operation.
Monitor Output Transient
Protection
The Model 76 Central Controller contains a
power-up/power-down transient protection
feature. This limits the chance of damage
to the monitor loudspeakers during the
time when AC mains voltage is connected,
disconnected, or has changed signifi cantly
from nominal. A combination of hardware
and software is used to monitor the inter-
nal DC power supply “rails.” Until the DC
“rails” exceed their nominal operating volt-
age range, electromechanical relays main-
tain a short-circuit condition on the monitor
outputs. After the DC voltages are recog-
nized as valid, a short delay takes place
before the relays are allowed to function