— 35 —
Normalized
(Flat) FM Output
The
FM-Flat
output characteristic is similar to
FM-Pre
, ex-
cept that the signal has been ‘normalized’ with a de-emphasis
network to restore a flat frequency characteristic. This output
would be the proper feed to an FM exciter that does its own
pre-emphasis. Although the audio response from this output is
flat, it still includes the 15kHz low-pass filter and independent
HF limiting to protect the spectrum and the stereo pilot, plus it
will also have the HD Radio delay if it is enabled.
Output Level
Controls
The
Digital Level
and
Analog Level
output sliders vary the limited
program output over a broad range. Both outputs are available
simultaneously and may be programmed independently as ap-
propriate.
The
Digital Level
is variable between 0dBFS and –20dBFS. FS re-
fers to digital-full-scale, or as high as the digital signal can go.
This means that audio peaks in the program signal, which have
been limited to 100% modulation, can assume any level between
digital-full-scale and a value 20dB below this. In digital signal
distribution it is customary to maintain the
average
level of the
program audio (“zero” on a VU meter) a comfortable value be-
low digital full scale to allow for program peaks. This ‘head-
room offset’ is historically on the order of 20dB for raw, unpro-
cessed audio, but since the output of the DAVID IV is tightly
peak-limited there is no headroom issue even when this is set
wide-open at
0dB
. Nevertheless, running program peaks to
0dBFS is not a good idea because of what’s called “zero dBFS
plus,” a phenomenon leading to overshoots, clipping and dis-
tortion in the eventual D-to-A converter somewhere down-
stream. Try an Internet search for:
dBFS+
to learn more about
this.
The
Analog Level
may be varied b24dBu and –10dBu for
program peak values. The dBu numbers shown below the slider
are based on traditional dBm voltage values, the ‘u’ referring to
unloaded, open-circuit volts (0dBu = 0.775V r.m.s), rather than
one milliwatt in a 600-ohm circuit (0dBm).
NOTE:
The output level in dBm (feeding a 600-ohm termina-
tion) may be set by adding 2.5dB to the numbers shown below
the slider. (e.g.: +6.5dBu = +4dBm when the output is loaded.)
The range of the
Analog Level
control covers traditional studio
line levels (e.g.: “Plus Four”) and the lower levels common to
semi-pro audio gear. This lower output range is useful when,
for example, the DAVID IV feeds the input of a conventional
computer sound card. As explained earlier (Page 13), when a
single-ended unbalanced output is taken from the DAVID IV,
only one side of the line-drive stage is used. The actual dBu
output level is 6dB lower than shown below the
Analog Level
slider in the case of an unbalanced output.
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