P.E.P of a System A television transmission

[Panrock]

Hi all,

Back again with another question, which I am sure you can answer more clearly than the relevant Wikipedia article, though the lack of understanding could be down to me...

This concerns the term Peak Envelope Power, abbreviated to P.E.P... here applied to a positively modulated, System A, RF television signal.

As an example, take such a signal with a RMS carrier power at peak white of 1 kilowatt.

What will be the P.E.P ? Also 1 kilowatt? Or perhaps √2 times this figure (1.414kW) because it may relate to the peak power of the carrier sine wave? Or something else?

If you can clear this up, I'd be most grateful. Cheers.

Steve

[Argus25]

Steve,

One problem here is that the video signal as a modulating source should or would never completely modulate the carrier completely to zero.

With a sine wave modulation, that is enough to take the carrier just to zero, the peak amplitude is twice the unmodulated amplitude and since the power is proportional to V^2, then the peak envelope power is 4 times the unmodulated power.

But for a video signal as a modulation source since the sync is there all the time at least, then that would have to represent the rms power situation and peak white the peak power situation, so you would have to calculate the average and peak powers from that.

I have always regarded peak powers as an urban myth for purposes of marketing & not science;

We all know that the essence of a power value contains a time domain quality, Joules per Second, it is an energy delivery rate.

A peak power value (always sounding more impressive due to the square law relationship) is simply an instantaneous value or "number" that does not represent progressive energy delivery over time (unlike rms) and is just the peak voltage value^2/R or the peak VA, and being a peak value has lost the time domain quality.

You can convince yourself this is the case by wondering, how long is the peak ? is it 1nS, 1uS, 1mS etc. The modulation peaks are rarely there all the time, especially with audio or video modulation, but only perhaps for a test waveform.

Peak powers were exploited in the marketing of a lot of audio amps because the peak power numbers sound so much more impressive, but the rms values are much more meaningful.

[Panrock]

Thanks Hugo. I've noticed filters etc. are often rated by "P.E.P" and wanted to see how this would stack up against a System A signal, which is normally rated by its RMS carrier power at peak white (positive) video modulation. At the other end of things - the sync bottom - there is zero power.

I'm sending you a PM.

Steve

[Synchrodyne]

As far as I know, PEP does have a specific meaning and utility in the RF domain, and is often associated with SSB applications.

My understanding is that it means the instantaneous power provided by a transmitter at the peak of the modulation envelope.

For a normal DSB AM transmitter, PEP at 100% modulation is four times the unmodulated carrier power. At 100% modulation, each sideband has half the amplitude of the carrier, and at the peak of the envelope, they are all in phase and their vector sum is twice the amplitude of the carrier alone, meaning four times the power (at the same impedance).

An SSB full-carrier transmission loses one sideband, so at 100% modulation, and when the remaining sideband is in phase with the carrier, their vector sum is 1.5 times the carrier amplitude, meaning 2¼ times the unmodulated carrier power.

For a VSB TV vision transmitter, the PEP would thus occur in the DSB region. TV transmitters are usually specified on the basis of peak white power for positive modulation, and peak sync power for negative modulation. That seems to imply that the peak power for a vision transmitter is in fact its PEP…

Cheers,

[Panrock]

Originally Posted by Synchrodyne

That seems to imply that the peak power for a vision transmitter is in fact its PEP…

Yes, this does seem to be the verdict emerging here. Thanks for adding your knowledgeable voice.

Steve