Down Rating Guide
On power rating of amplifiers with only DTT signals
Now that the great majority of domestic TV signal amplifiers are handling only DTT signals (importantly, no analogue signals are present) there is a strong case for moving from the well-established two-tone method of power rating an amplifier.
Reasons for this include:
- Analogue TV signals concentrated their energy principally at or around the vision carrier, so inter-modulation products tended also to be concentrated at discrete frequencies.
- Analogue TV signals were particularly sensitive to interfering tones, which would normally produce highly visible pattern on the picture.
- DTT signals spread their energy evenly across an 8 MHz channel, so intermodulation products tend also to be spread and noise-like
- DTT signals are not as sensitive to co-channel interference as analogue signals.
Work by Mandercom for the CAI, also involving the DTG, has established that if a single DTT channel is carried in an amplifier at a power level that generates a given value of MER, then provided the total power of a number of equal level DTT channels does not exceed the power of the single channel signal, then the same value of MER will be achieved for all the multiplexes.
This means that it is possible to measure the output power of an amplifier carrying a single multiplex at a given MER value, and then simply calculate the power of each of N channels by the formula:
(power per multiplex) = (power of single multiplex) - (10 x log(N)).
The output power vs MER characteristic of a proMHD14R masthead amplifier was measured with a DTT signal from a modulator.
The red line connects the measured data points, and the black line shows the trend.
The CAI CoP (Code of Practise) for systems suggests a value of 30dB for C/N (=MER) of a DTT signal at the aerial.
Using this figure, we can see that the amplifier degrades the test signal to 30dB MER at an output power of 102.7 dBμV for one DTT multiplex.
Using the formula stated above to calculate the power per multiplex that could be achieved for different numbers of multiplexes at MER = 30 dB, we get the following table:
|No. of muxes||
Back-off per mux (dB)
|Power per mux (dBuV)|
From this table we see that it should be possible to achieve 93.7 dBμV per multiplex for eight equal power multiplexes.
It should be noted that:
- This figure assumes that the powers of the multiplexes are equal. In reality they seldom are.
- The slope of the MER vs power characteristic is quite steep; in other words, a small change in power can cause a large change in MER.
- The more closely an amplifier is operated to its power limit with the wanted signals, the more vulnerable it is to be overloaded by interferers such as LTE signals, or additional signals arriving during periods of enhanced propagation.