If Digital Terrestrial Television Broadcasting (DTTB) is to be introduced in Australia it will need to co-exist for some time with existing PAL broadcasting. Under some possible introduction scenarios this coexistence might require operation on channels immediately adjacent to existing PAL services. One scenario for the introduction of DTTB in Sydney, and perhaps other capital cities, is to locate DTTB services on VHF band III channels between existing channel 7, 9 and 10 PAL services.
It is noted however that the existing channel 9A only provides 6 MHz of spectrum. This note investigates ways that a near 7 MHz wide DTTB signal might be accommodated in the nominally 6 MHz wide channel 9A.
The factor that limits transmission of a DTTB signal in the channel 9A gap is the adjacent channel interference rejection performance of domestic PAL television receivers. The results of tests performed in September 1994 on a range of domestic receivers using a simulated 7 MHz DTTB signal interfering with a 7 MHz PAL service have been analysed in this report.
Those results show that by operating the DTTB services at a power levels well below the adjacent PAL services it is feasible to fit a 6.4 to 6.6 MHz DTTB signal into channel 9A.
Further, if the existing PAL channel 10 service could be shifted up 400 kHz in frequency it would increase the possible width of the DTTB service to between 6.8 and 7.0 MHz. It is believed this frequency offset could be achieved without large numbers of viewers needing to retune their receivers, but this assertion may need to be confirmed by measurements.
In some areas (around Melbourne particularly) it might also be necessary to study the effect such a 400 kHz offset of PAL channel 10 would have on PAL channel 11 services in surrounding regions.
Regardless of whether or not an offset is applied, the protection of a channel 9A DTTB service from higher powered channel 10 PAL services could be improved by truncation of some of the vestigial sideband of the channel 10 PAL transmission.
Although this report has focussed on increasing the capacity of channel 9A, the techniques discussed could equally be applied to increase the capacity of nominally 7 MHz wide channels.
Finally it should be emphasised that this report describes a theoretical study. If there is serious interest in using channel 9A in the way described in the report, experimental work should be performed to verify the assumptions made in the report. These areas for further investigation are identified at the end of this report.