Why DTT Ratings Differ from Analogue Ratings

Traditional amplifier power ratings were developed around analogue television signals. Analogue TV signals concentrated much of their energy around the vision carrier, and unwanted intermodulation products tended to appear at discrete frequencies.

DTT signals behave differently. Their energy is spread more evenly across an 8 MHz channel, and any intermodulation products tend to be spread more widely and appear more noise-like.

• Analogue TV signals were particularly sensitive to interfering tones, usually producing visible picture patterning.
• DTT signals distribute energy across the channel rather than concentrating it at a single carrier.
• Intermodulation products from DTT signals tend to be spread and noise-like.
• DTT signals are generally less sensitive to co-channel interference than analogue signals.

MER and Multiple DTT Multiplexes

Work undertaken by Mandercom for the CAI, also involving the DTG, established an important principle for DTT amplifier performance. If a single DTT channel is carried through an amplifier at a power level that produces a given MER value, then several equal-level DTT channels can achieve the same MER provided their combined total power does not exceed the power of that single-channel signal.

This means it is possible to measure the output power of an amplifier using a single multiplex at a given MER value, then calculate the available power per multiplex when carrying multiple DTT multiplexes.

Power Per Multiplex Formula

The power available for each multiplex can be calculated using the following formula:

In this formula, N is the number of equal-level DTT multiplexes being carried by the amplifier.

Example Using a proMHD14R Masthead Amplifier

The output power versus MER characteristic of a proMHD14R masthead amplifier was measured using a DTT signal from a modulator.

The red line connects the measured data points, while the black line shows the trend.

The CAI Code of Practice for systems suggests a value of 30 dB for carrier-to-noise ratio, equivalent to MER, for a DTT signal at the aerial.

Using this figure, the amplifier degrades the test signal to 30 dB MER at an output power of 102.7 dBµV for one DTT multiplex.

Calculated Power Per Multiplex

Using the formula above, the following table shows the calculated power per multiplex for different numbers of equal-level DTT multiplexes at MER = 30 dB.

From this table, it should be possible to achieve 93.7 dBµV per multiplex for eight equal-power DTT multiplexes.

Important Considerations

The calculated values are useful for understanding amplifier performance, but real installations rarely behave as perfectly as a controlled test condition.

• The calculation assumes that all multiplex powers are equal. In real installations, this is seldom the case.
• The slope of the MER versus power characteristic is steep, meaning a small change in output power can cause a large change in MER.
• An amplifier operated close to its output limit is more vulnerable to overload from interference, including LTE signals.
• Additional signals may arrive during periods of enhanced propagation, increasing the risk of overload.