MODULE SET-UP
Chromadigm CHS/CHQ Transmitter – Quick Start Guide
4-1
MODULE SET-UP
4. Module Set-up
4.1 Setting RF Input Levels
The RF signals that are common to each of the four ITU lasers are connected to the BC input port. The common RF loading
into the BC RF input port should occupy the RF spectrum up to approximately 270 MHz to avoid SRS crosstalk degradation.
The Narrowcast (NC) signal for each individual ITU laser transmitter is connected to the corresponding laser transmitter
input port. Note that the NC input should be all QAM signals.
Determining the proper rear panel RF input level is important to ensure optimum transmitter performance. The following will
provide guidance in calculating the proper adjustment to the per channel RF level for your actual channel loading from the
reference channel loading level to ensure the proper total RF input power requirement is achieved.
Each CHS/CHQ transmitter is factory optimized for a rear panel total input power of +38 dBmV for the combined BC and
NC signals. This level provides 3-4 dB of reserve gain for the AGC operation. The reserve gain is the amount of headroom
above the +38 dBmV reference level.
Each CHQ transmitter is factory optimized for a rear panel total input power of +34 dBmV for the combined BC and NC
signals. This level provides 3-4 dB of reserve gain for AGC operation. The reserve gain is the amount of headroom above
the +34 dBmV reference level.
4.1.1 CHS All QAM Channels
The all QAM reference channel loading and per channel input level is 155 ITU-T J.83 Annex B QAM 256 channels between
54-1002 MHz set to 16 dBmV. The RF per channel level of 16 dBmV is calculated from the total power as follows:
Total Power - [10*log
10
(channel load)]
38 dBmV - [10*log
10
(155)] = 38 dBmV – 22 = 16 dBmV
Using the same formula to adjust for a lower channel loading such as a 750 MHz system with 110 channels:
38 dBmV - [10*log
10
(110)] = 38 dBmV – 20.4 = 17.6 dBmV
Therefore with 110 of all QAM channel loading, the correct rear panel RF input level is 17.6 dBmV at the input of both the
BC and NC ports.
4.1.2 CHS Mix of Analog & QAM Channels
The mix of analog and QAM reference channel loading and per channel input level is 80 NTSC analog (54-550 MHz) set
to 18 dBmV and 75 ITU-T J.83 Annex B QAM 256 channels (258-1002 MHz) set to 12 dBmV. The RF level per channel for
analog and QAM is calculated from the total power as follows:
Since there is a 6 dB power level difference between the QAM and analog channels, it is easier if we convert the QAM
channels to an equivalent analog power level in the formula below. The QAM RF level being 6 dB lower means the QAM
power level is 1/4
th
the analog level. As a point of reference, -3 dB is 1/2 power and -6 dB is 1/4
th
power.
Total Power - [10*log
10
(Analog channel load)+(QAM channel load/4)]
38 dBmV - [10*log
10
(80)+(75/4)] = 38 dBmV – 20 = 18 dBmV
Therefore the analog channels are set to 18 dBmV and the QAM channels are 6 dB lower or 12 dBmV.
Using the same formula to adjust for a decrease in the number of analog channels to 30 and increase the number of QAM
channels to 124 and calculate the correct input level for each:
38 dBmV - [10*log
10
(30)+(124/4)] = 38 dBmV – 17.8 = 20.2 dBmV
Therefore the analog channels are set to 20.2 dBmV and the QAM channels 6 dB lower or 14.2 dBmV.
4.1.3 CHQ All QAM Channels
The all QAM reference channel loading and per channel input level is 155 ITU-T J.83 Annex B QAM 256 channels between
54-1002 MHz set to 12 dBmV. The RF per channel level of 12 dBmV is calculated from the total power as follows:
Total Power – [10*log
10
(channel load)]
34 dBmV – [10*log
10
(155)] = 34 dBmV -22 = 12 dBmV
Using the same formula to adjust for a lower channel loading such as a 750 MHz system with 110 channels:
CHAPTER 4:
