Studio Technologies 761 - Manual (Page 16)

Issue 1, March 2008 Model 761/771 User Guide
Page 16 Studio Technologies, Inc.
for Surround
time in tens of years. Note that the actual
memory integrated circuit is located in the
Model 761 Central Controller, rather than
in the Model 771 Control Console.
Digital Input Sample Rate
The Model 761’s digital audio inputs are
compatible with signals that have sample
rates spanning the professional digital
audio range of 32 to 192 kHz. A confi gura-
tion setting allows the performance of the
digital audio receiver integrated circuits to
be optimized for three sample rate zones
within the overall range. The choices are
32 to 48 kHz, 88.2 to 96 kHz, and 176.4
to 192 kHz. Note that selecting the 88.2
to 96 kHz setting will also allow signals
with a sample rate of 32 to 48 kHz to be
received. But audio performance of those
lower-rate signals may be compromised by
clock rate confusion in the receiver cir-
cuitry. This situation also holds true for the
176.4 to 192 kHz setting which will allow
signals with sample rates all the way down
to 32 kHz to be received. But again the
audio performance of the lower-rate sig-
nals could suffer from clocking errors. Note
that the inputs are confi gured as a group;
no individual receivers can be confi gured
to a range different from the others. This is
generally not an issue as a single sample
rate is typically selected for each facility.
Digital Input Reference Level
Confi guring the digital input reference level
to match that of a specifi c installation is
an important step toward achieving optimal
audio performance. This ensures that the
Model 761’s digital audio receiver integrat-
ed circuits are set to match the digital
input source’s “0 VU” or average point.
The digital input reference level can be
selected from four choices: –20.0, –18.0,
–16.0, and –14.0 dBFS. SMPTE recom-
mends a reference level of –20 dBFS,
a value which is also extensively used
in professional audio applications. The
recommended digital reference level for
PAL broadcast applications is –18 dBFS.
The other two levels were included be-
cause… well, we can’t remember why! But
it seemed like a good idea at the time. If
you end up using the –16.0 or –14.0 dBFS
levels please let us know why.
Monitor Output Nominal Level
The nominal level of the six monitor output
channels can be confi gured, as a group, to
match the requirements of an installation.
This setting defi nes the analog output level
when a digital input source, at its refer-
ence level, is connected and the monitor
output level control is at its maximum. The
choices are 0.0 dBu and +4.0 dBu. Most
applications will fi nd the +4.0 dBu setting
correct as it meets the dominant worldwide
audio standards. For broadcast applica-
tions that follow the PAL standards the
0.0 dBu setting may be appropriate.
While sometimes not fully understood,
using the term dBu is more in line with
contemporary audio applications than the
outdated dBm and the “semi-pro” dBV
references; dBu refers to audio levels
without regard to their load impedance,
typical of situations where an output has a
low source impedance and is connected to
a high-impedance input. An analog audio
signal with a level of +4 dBu has an RMS
level of 1.228 volts. A 0 dBu signal will
have an RMS level of 0.775 volts.
A fi nal review of the selected setting might
now be in order. The most common input
and output combination will have a digital
input reference level of –20 dBFS and a
monitor output nominal level of +4 dBu.