Orban OPTIMOD 6300, Руководство по эксплуатации

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OPTIMOD 6300 DIGITAL INTRODUCTION
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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 channels, 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 6300 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 6300 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 6300’s input meter.
Built-in Calibrated Line-up Tones
To facilitate matching the output level of the 6300 to the transmission system that it
is driving, the 6300 contains an adjustable test tone oscillator that produces sine
waves at 6300’s (analog or digital) left and right outputs.
When the 6300’s left/right analog output is switched to F
LAT
, a deemphasis filter is
inserted between output of the 6300’s audio processing and its line output. Thus, as
the frequency of the test tone is changed, the level at the 6300’s line output will fol-
low the selected deemphasis curve. In most cases the preemphasis filter in the driven
equipment will undo the effect of the 6300’s internal deemphasis, so the 6300’s out-
put level should be adjusted such that the tone produces 100% modulation of the
transmission link as measured after the link’s preemphasis filter. At 100Hz, switching
the deemphasis out or in will have negligible effect on the level appearing at the
6300’s left and right audio outputs.