Demonstrations of Sound Buzz
TVMS Model 4200 In-Service Test Processor
Gated CTB tests necessarily cause sound buzz. The magnitude of the buzz is dependent on the duration of the gate (how long the signal is gated off during the VBI), the frequency of the gate (one field or both fields), and the characteristics of the television receiver or set-top box.
In most cases, the buzz is low level and unlikely to cause concern if the test is kept short - typically it takes approximately 20 seconds to complete the test. In many cases, the buzz would not be noticed. However, the design of some receivers makes the buzz very apparent.
Click on the links below to hear the buzz from a Mitsubishi 41 inch TV or for a Philips 5 inch portable set. The Mitsubishi is an example of the typical response - perhaps a little better than most, but representative. The Philips is the "worst case" example we have found to date - with the possible of exception of a Tektronix precision demod. The small speaker in the portable set also aggravates the buzz. None-the-less, keep in mind that some of your subscribers may hear something similar to the Philips. In general, the program sound material is understandable even for the worst case buzz.
Although not demonstrated, there is no impact on the picture!
Receiver
Gate settings
Mitsubishi 41 inch
Typical?/low levels of sound buzz
- Both fields, 83 uS width (typical settings for 8591C)
Note: the buzz is contiuous during this sample
- One field, 83 uS width (typical settings for 2715)
- One field, 53 uS width (typical settings for AT2000R)
Note: the buzz is essentially inaudible in this case
Philips 5 inch portable
"Worst case" buzz.
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