The myriad sources of clicks and pops in a digital television plant are sometimes confusing and sometimes completely counter-intuitive. Normally, clocking is the correct answer but occasionally it is deeper than this. An often-overlooked source of on-air clicking, popping, and even buffer under- and over-flow is the incorrect configuration of the external Dolby Digital (AC-3) encoder.

First, it is not well known that some HDTV encoder manufacturers require the clock source for external audio encoders such as the Dolby DP569 to be supplied from the video encoder back to the audio encoder. In some cases, it is not enough to simply lock the video encode to video reference and the audio encoder to a DARS reference locked to that same video reference! The video encoder may develop an internal reference based on an internal clock used to produce the DVB-ASI transport stream output. This clock is used to generate a “local” DARS reference that is output and needs to be fed to the Dolby Digital (AC-3) encoder to lock it to the transport stream. Note this locally generated DARS reference may have no relation to any references applied to the DTV encoder and should be used only to lock this local encoder and not for anything else in the plant.

To correctly set up a system, feed audio and video to the audio and video encoders, then feed the DARS reference that the video encoder creates to the reference input of the audio encoder. Make sure to set the audio encoder to use this external reference, and in some cases you may need to check the video encoder to make sure that it outputs the proper DARS signal. Check with your DTV encoder manufacturer to determine if this special clocking is required.