Cisco Model 6109 6 MHz Off-Air Reference (NTSC) Fiche De Données
Definitions
NTSC VHF Off-Air - Any NTSC broadcast station with a channel assignment from Channel 2-13.
Broadcast Frequency Offsets - Frequency offsets of +10 kHz or -10 kHz may be assigned by the FCC to certain
broadcasters in overlapping coverage regions to minimize the potential for co-channel interference.
Co-Channel Interference - Interference seen as beats, bars, and other visible distortions caused by two NTSC
signals being received on the same channel.
Program IF - The IF signal that is being used with the modulator/upconverter to provide the actual RF output.
This signal may differ from the main composite IF signal due to EAS switching or other alternate programming
that could be temporarily switched into the RF output converter.
Broadcast Frequency Offsets - Frequency offsets of +10 kHz or -10 kHz may be assigned by the FCC to certain
broadcasters in overlapping coverage regions to minimize the potential for co-channel interference.
Co-Channel Interference - Interference seen as beats, bars, and other visible distortions caused by two NTSC
signals being received on the same channel.
Program IF - The IF signal that is being used with the modulator/upconverter to provide the actual RF output.
This signal may differ from the main composite IF signal due to EAS switching or other alternate programming
that could be temporarily switched into the RF output converter.
Phase Lock - Servo loop circuitry that forces two signals to maintain a stable phase relationship.
Ingress - The leakage of externally radiated signals into the CATV distribution system.
Ingress - The leakage of externally radiated signals into the CATV distribution system.
Technical Details
The Model 6109 Off-Air Reference provides a nominal 6.000 MHz, 1 volt peak-to-peak sine wave output that is
used to phase lock the modulator/upconverter RF output with the received NTSC video carrier.
The Model 6109 Off-Air Reference provides an automatic switching function between the 6.000 MHz reference
used by the modulator internally, and the 6.00 MHz controlled phase signal generated by the Model 6109 Off-Air
Reference. This function is useful in the event of input RF carrier loss. If the off-air phase lock source should
cease transmitting, the modulator will then switch over to its internal reference signal and operate normally.
A Model 6109 6 MHz Off-Air Reference is required for each VHF off-air channel (2-13) to be locked. The
modulator must accept a nominal 6 MHz reference input as its phase locking source. The Scientific-Atlanta
Continuum
used to phase lock the modulator/upconverter RF output with the received NTSC video carrier.
The Model 6109 Off-Air Reference provides an automatic switching function between the 6.000 MHz reference
used by the modulator internally, and the 6.00 MHz controlled phase signal generated by the Model 6109 Off-Air
Reference. This function is useful in the event of input RF carrier loss. If the off-air phase lock source should
cease transmitting, the modulator will then switch over to its internal reference signal and operate normally.
A Model 6109 6 MHz Off-Air Reference is required for each VHF off-air channel (2-13) to be locked. The
modulator must accept a nominal 6 MHz reference input as its phase locking source. The Scientific-Atlanta
Continuum
Models 9820/9821 Modulators, Models 9860/9861 Upconverters, and the Series VM2500 Modulators
satisfy this requirement.
Broadcast offsets (
Broadcast offsets (
±10 kHz) are handled by the 6 MHz Off-Air Reference.
The Model 6109 6 MHz Off-Air Reference does not include a demodulator section. If the program content on the
phase locked channel must be derived from an off-air source, an appropriate demodulator is required to provide
video and audio to the modulator or Composite IF to the upconverter.
On-Channel Carriage
If the off-air broadcast signal is carried on the same cable channel for convenience or franchise requirements,
then interference reduction should be good.
Alternate Channel Carriage
If different content than the broadcaster is carried on the same cable channel, then interference reduction is less
than for on-channel carriage, but better than a non-phase locked signal.