[BC] HDTV rollout wrinkle

[RichardBJohnson at comcast.net]

8VSB requires exactly 6dB (voltage) of headroom, i.e., a peak power of 4 times the average power, just like 100% AM modulation has 4 times the peak power as the carrier. Unlike AM, 8VSB continuously transmits, sending a pseudo-random scrambled message, even if there are no scene changes. Therefore, its power input and output, as well as the bandwidth occupied is constant, unlike NTSC television where a white scene might be at 10 percent power and a black would be at about 70 percent. Because it is continuously transmitting, with all of the power contributing to information, the only way to measure is its RMS value. Think of it as a bunch of band-limited amplified noise because that’s what it is.

The horrible results we get with HDTV reception is because we have horrible HDTV receivers, designed to work with 100 mV of signal input, while NTSC receivers were designed to work with 100 uV of signal input. It really has nothing to do with the transmitters. It’s the absolutely awful receivers.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "harold stanton" <k5gvr at hotmail.com>

 As most of you do radio, and are familiar with those form of power measurements, the DTV powers that you see are not what you surmise.
 The methods of measurements is done on an RMS detecting meter such as an HP4418.
 The REAL peak powers are 6 to 7db higher that what is measured.
 So 1MW at UHF is really equal to 4 to 5MW peak power.