Following the Independent Television Commission’s (ITC) decision to award the BBC, BSkyB and Crown Castle the Digital Terrestrial Television licence back in July, important changes to the DTT broadcast parameters have been made. Tom Carnie, Product Manager, explains what these changes mean for test meter users.

It was a landmark decision for the Digital Terrestrial Television platform and the television trade, paving the way for low cost entry to digital viewing for the masses, and of course paving the way for analogue switch off. To coincide with the launch of the new digital terrestrial services in October, the BBC changed the DTT broadcast parameter from 64QAM to 16QAM.

The reason for the change is simple. When OnDigital launched, many industry experts believed the transmission system adopted was not as robust as it could have been. (In fact the NTL report on coverage predictions stated this fact in their data-rate to coverage tables). This resulted in many viewers being disappointed with performance and the coverage map was somewhat restricted.

The change to 16QAM makes the signal more robust and less susceptible to impulse noise, improving the performance of the Set Top Box. This change has been seamless for the one million or so DTT viewers, but there are implications for test equipment users.

Most test equipment manufacturers allow the instrument to be pre-programmed with the broadcast parameters. You are probably already using the instrument for the current 64QAM parameters, but it is crucial that you change the parameters to 16QAM for the relevant broadcasts.

An additional point to consider is that broadcasters continue on the 64QAM parameter. You will need to be very careful at the installation site to make sure that you have the correct QAM setting for the multiplex that you are attempting to measure. The reason that the Freeview consortium are keen to make this change is a result of recent trials carried out from Crystal Palace on the unused ITV Digital multiplexes. The 16QAM gave an improvement of around 5dB Digital Carrier Power.

Therefore an improvement in Carrier-to-Noise and subsequent improvements in the susceptibility to impulse noise will be apparent. This in turn will mean that fewer STB's will be returned for poor reception problems. It does not retract any of the best working practices that we have learned for successful DTT installation. It does not relax the input requirements of the set top box, it merely improves its performance.

Further information on future changes can be found at www.dtg.org.uk. For technical queries, call the Solutions Group Technical Team on 08456 444 000.

Here is a list of steps which should be followed to change the parameters of the Unaohm DaTuM 10 and EP314. It is necessary to verify that the Digital Parameters are correct for the transmission that you are to measure. This information will be available from the relevant broadcaster.

Unaohm DaTuM 10 Digital Parameters
The DaTuM 10 has been pre-programmed for 2K Carrier, Modulation 64QAM, Code rate 2/3 and Guard interval 1/32. To change the QAM parameters proceed as follows;

Modulation:
1) From the SPECTrum mode, press the yellow BER key
2) Press the Soft Key and rotate the knob until the cursor
highlights “Mod” in the top right of the display.
3) Press SPECT key to save changes and exit.
4) Press the ENTER key
5) Rotate the knob until the cursor highlights the desired mode
(16Q, 64Q or QPS).

Unaohm EP314 Digital Parameters
1) From the Spectrum mode, press the Digital key
2) Press the soft key for the BER function
3) Pressing the soft key for PRMT1, this gives a menu to select
Modulation or Spectrum modes.
4) Press the soft key for MOD function. The cursor will now
highlight the current setting.
5) Rotate the shaft encoder knob to display 16QAM.
6) Press the soft key for the BACK function to save and exit.
7) Pressing the soft key for PRMT2 gives a menu to select Carrier, Code and Guard modes.