A tropospheric interference phenomenon exists between television stations GTV-9 and TNT-9 on VHF channel 9 either side of Bass Strait. Viewers near the coast in Victoria experience interference from TNT-9 over the summer months due to VHF ducting. Precision offset transmission techniques are being used to minimise interference however some interference still exists.
ITU-R Recommendation BT.655-4 provides information which indicates that a 20 dB reduction in interference levels may be achieved by using a -1.25 MHz precision frequency offset. This report investigates this information to check if such an improvement is obtainable. The Communications Laboratory has simulated the precision offset conditions and evaluated both small and large negative precision frequency offsets for the unwanted interfering television signal (TNT-9).
The investigation has shown that no improvement will be achieved by a large offset compared with the current small offset. The ITU figures for -1.25 MHz offset do not clearly or unambiguously show the effect of soundcarriers of the unwanted television signal on the required protection ratio. In comparison with the visible effect of interference at 2/3 (5/3) offset (luminance line-pairing) the interference seen at -1.25 MHz offset is quite different and is primarily noise in the chrominance channel.