As broadcasters rapidly implement digital TV, it is important that they remember to cover the operational bases and insure that their digital signal is managed as professionally as their current NTSC one. In a market with more than one DTV station, the first thing viewers will notice with their new DTV receiver are the differences between stations' output.
Audio and video levels vary widely both between DTV stations and DTV and NTSC counterparts. While receivers could be the problem, the operating parameters for the digital chips are similar and most receivers will exhibit the same problem on the same station. Most variations occur in the station where the audio and video feeds to the DTV encoder are often 1) not monitored, 2) not fed from a point in the program chain that has been certified as correct, and 3) not part of normal operations (patch cords and temporary hookups). The result will be viewer discontent and frustrated retail store sales staff trying to demonstrate DTV receivers to potential viewers.
What to do? Obtain at least four or five DTV receivers (set-top boxes are OK) of different makes and models. Set up the receivers in the maintenance shop and feed each with a signal from an antenna. Connect the outputs to a VGA monitor and simple stereo audio monitor and VU meters. If there is an NTSC output, feed it to a small NTSC monitor and WFM. Tune each receiver to your own DTV signal and observe and listen to the results. Check the electronic program guide (EPG) and PSIP for consistency and correctness. When switching between network and local sources, look for discontinuities, lip-sync, and level changes. Also look at the other DTV stations in the area. Use the same equipment to look and listen to your NTSC signal and others in the area. If a transport stream monitor is available, look at all DTV stations in the area to determine what they are transmitting.
The experiences obtained from the exercise above will provide useful information to help viewers tune in your DTV station. As more viewers begin using your DTV signal as their normal signal, the need to insure that the DTV facility is properly configured, operated, and maintained grows.
In general, your DTV signal will look and sound better than your NTSC signal. When viewers and cable companies discover this fact they will begin to use the DTV signal as the main service and the NTSC as the backup. Some exceptions to this better quality will be the lack of audio and video processing in the DTV program path that will result in serious variations, especially in the audio level. Monitoring the DTV signal in master control will be uncomfortable because of the delay caused by the video and audio compression encoders. But the DTV signal must be monitored because viewers are beginning to depend on it.
My own experiences with the Washington, DC area DTV stations have demonstrated significant audio level variations (10 dB or more) and video variations between stations and programs. An audio gain rider would solve the problem but would render moot the DNYRNG feature in the AC-3 decoder in the receiver. Consider the gain rider as an interim fix until the control room has been fully converted to digital. At that time, network audio with metadata can be passed on to the DTV encoders and local metadata will be created for local programs.
Video quality on DTV receivers varies according to each station's infrastructure. Digital video appears cleaner and has no composite artifacts. Analog video when converted to DTV often appears soft with washed-out color unless an effort is made to improve it.
Conversion from SD to HD in the station looks better than SD to HD conversion in the receiver. SD appears softer on an HD monitor than on a 480i monitor because the scan lines on the SD monitor give the impression of "sharpness." One possible solution to this effect might be to apply some degree of sharpness to the SD video prior to encoding to DTV. Many viewers will use low-cost DTV set-top boxes to feed NTSC receivers.
You can determine all these variations and more through tests with multiple DTV receivers in your own plant. Conduct the experiments now while there are fewer viewers. Doing it this time next year may be too late.
Stay tuned.
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