We have recently added a few digital channels to our system, using the DDC806 digital down converter and the DUC550 and DUC860 up converters. We noticed some interference, when we try to select an output channel on the DUC that is adjacent to an existing analog channel.

When an analog NTSC signal is present on the lower adjacent channel to the selected digital input channel of the DDC806, there will be some amount of the audio carrier (250 kHz below the edge of the selected channel) from this lower adjacent NTSC signal present in the IF output of the DDC806. This is then translated to the new output channel with the DUC upconverter, even though it will also be attenuated some in the upconverter. This will not cause a problem if the translated output channel has no channel or another digital channel located in the lower adjacent channel position in the output channel lineup. However, if there is a NTSC analog channel located adjacent to the lower edge of the translated digital channel, there will be a potential beat formed between the audio carriers of this lower adjacent analog and the translated lower adjacent analog. To avoid this potential problem, avoid having a lower adjacent analog at both the input and output of the translated digital channel(s). It is OK if there is a lower adjacent NTSC signal at either the input or output sides – just not both.