While we all can agree that -100% modulation is a hard limit to be avoided, there are a surprising number of amateur radio ops who strive for positive modulation far above 100%. Why? What are the benefits of soaring positive peaks?
What makes this question even more meaningful is that the broadcast audio industry has perfected the reduction of peaks by increasing average modulation energy, so there are very stark contrasts in how peak energy is viewed between professional and amateur radio.
Peaks may be of high amplitude but are always of short duration and therefore do not contain useful energy. Conversely, average power is sustained energy and contributes significantly to perceived loudness.
A simple example of peak energy is within a static discharge between your finger and a wallplate. These sparks can be of 20-40kV but have a duration of just 1-100nS. It would make a significant difference if this energy had a longer duration of several seconds, right?
So what effect does the peak component of modulation have if it doesn't contribute to loudness?
In AM, negative peaks are the first part of the audio waveform to reach -100%, leaving average modulation reduced. Fortunately, negative peak clippers address this nicely and are widely used. Positive peaks are the first to reach 1500 Watts PEP even with modest carrier power. A 375 Watt carrier reaches 1500 Watts when modulated to +100%. 1500 Watts is also reached with a 240 Watt carrier modulated at +150% and just 167 Watts of carrier reaches legal limit at +200%. So why no positive peak clippers?
An equation for PEP of an AM signal: PEP = ((M+1)^2)C, where C is the carrier (Watts) and M is the modulation factor. (
https://www.qsl.net/wa5bxo/amplitude-modulation-and%20pep.htm)
100% modulation would be M = 1, 200% would be M = 2, etc. So with 100% modulation, M = 1, M + 1 = 2, 2^2 = 4 (2^2 means 2 squared), so PEP = 4C (Carrier * 4).
With 200% modulation, M = 2, M + 1 = 2 + 1 = 3, 3^2 = 9, so PEP= 9C (Carrier * 9).
There would be a lot more flexibility if communicating were just about transmitting, but it's important to include the effects of propagation and signal detection in receivers. A highly asymmetric waveform is more susceptible to the effects of QSB than a symmetric waveform, resulting in increased distortion. Further, many detectors do not handle asymmetry well and will clip off peaks extending beyond a symmetric waveform, resulting in increased distortion. So much for that FB audio.
Even though SSB is properly measured in PEP, peaks are the first part of the audio waveform to trigger ALC action, leaving overall modulation reduced.
In any mode, excessive peaks increase the risk of arc-overs in RF and modulator components which can be particularly damaging to expensive mod iron.
This illustration shows the relationship between the various components of a signal:
This display shows received audio with peaks left as-is and clipped off:
The latter contains significantly higher average audio energy and is dramatically louder even though the modulation level remained the same. Despite the extreme difference, there was no noticeable increase in distortion because only short duration peak energy was removed.
The goal of this post is to shed light on the difference between average and peak energy. As mentioned earlier, the broadcast audio industry has put a significant investment in competitive audio over the past half century and the lessons are there for the taking.