Orban Optimod-TV 8382, Руководство по эксплуатации

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OPTIMOD-TV DIGITAL INTRODUCTION
1-17
Transmission Levels
The transmission engineer is primarily concerned with the peak level of a program
to prevent overloading or over-modulation of the transmission system. This peak
overload level is defined differently, system to system.
In FM modulation (FM / VHF radio and television broadcast, microwave or analog
satellite links), it is the maximum-permitted RF carrier frequency deviation. In AM
modulation, it is negative carrier pinch-off. In analog telephone / post / PTT trans-
mission, it is the level above which serious crosstalk into other channels occurs, or
the level at which the amplifiers in the channel overload. In digital, it is the largest
possible digital word.
For metering, the transmission engineer uses an oscilloscope, absolute peak-sensing
meter, calibrated peak-sensing LED indicator, or a modulation meter. A modulation
meter usually has two components—a semi-peak reading meter (like a PPM), and a
peak-indicating light, which is calibrated to turn on whenever the instantaneous
peak modulation exceeds the overmodulation threshold.
Line-Up Facilities
Metering of Levels
The meters on the 8382 show left/right input levels and left/right output modula-
tion. Left and right input level is shown on a VU-type scale

0 to –40dB), while the
metering indicates absolute instantaneous peak (much faster than a standard PPM
or VU meter). The input meter is scaled so that 0 dB corresponds to the absolute
maximum peak level that the 8382 can accept (+26 dBu). If you are using the AES3
digital input, the maximum digital word at the input corresponds to the 0 dB point
on the 8382’s input meter.
Built-in Calibrated Line-up Tones
To facilitate matching the output level of the 8382 to the transmission system that it
is driving, the 8382 contains an adjustable test tone oscillator that produces sine
waves at 8382’s (analog or digital) left and right outputs.
When the 8382’s left/right analog output is switched to F
LAT
, a de-emphasis filter is
inserted between output of the 8382’s audio processing and its line output. Thus, as
the frequency of the test tone is changed, the level at the 8382’s line output will fol-
low the selected de-emphasis curve. In most cases the pre-emphasis filter in the
driven equipment will undo the effect of the 8382’s internal de-emphasis, so the
8382’s output level should be adjusted such that the tone produces 100% modula-
tion of the transmission link as measured after the link’s pre-emphasis filter. At
100Hz, switching the de-emphasis out or in will have negligible effect on the level
appearing at the 8382’s left and right audio outputs.