For the sake of optimum clarity and intelligibility of radio transmission, the radio modulation depth must be controlled by ensuring that the microphone input level is maintained well within the automatic Level control (ALC) correction range of the modulator. Failure to do so would result in the introduction of audio distortion. Overdriving the modulator will invariably result in the creation of unwanted AF harmonics and spurious products. Consequently, the intelligibility of the demodulated audio on the receiver at the DX end will also suffer proportionately.

Several ill-effects of overmodulation will begin to surface. In extreme cases, the overmodulating radio station will begin to splatter across the band. However, with the modern radio transmitters, sideband splatter is usually not the problem. Nonetheless, this could have been a huge problem in earlier transceivers because it would have produced spurious RF components across a significant portion of the band. At times the splatter could have extended across the entire band. This kind of slatter from just one operator might have rendered the band more-or-less useless for a large number of other operators… Thankfully, as I mentioned before, this is usually not a problem with modern transceivers. All radio rigs that are manufactured to conform to the minimum stipulated OEM standards have adequate post-modulation bandpass filter arrangements with sharp skirt-selectivity to be able to eliminate the out-of-band spurious components that might be produced on account of inadvertent overdriving of the modulator.

Despite the measures taken in the transceiver design to prevent sideband splatter, we could still have the other problem caused due to the overdriving of the modulator. It is the loss of intelligibility that would be caused due to the in-channel generation of AF harmonics and other associated spurious. If this were to occur, it would have to be addressed by the radio operator by taking appropriate corrective measures. It is this adverse aspect of overmodulation that we intend to examine in this post.

At times, people feel that some voices have a texture that is better suited for radio communication and that’s why they appear to produce better intelligibility. Although it is true to an extent, that’s not the full story. Besides the obvious fact that one must speak clearly and distinctively articulate each syllable, word, etc, the most important factor is to ensure a clean radio signal modulation by maintaining the AF modulating baseband voltage levels within the boundaries of the ALC circuit negative feedback error correction range.






Loss of Intelligibility occurance in Radio Communication
Instances of occurrence of the degradation or partial loss of intelligibility in radio communication happen from time-to-time and it is a pretty common phenomenon especially on the HF radio bands. In the regular course of events, such issues are caused by natural phenomena that determine propagation conditions. Some of the typical reasons are multi-path ionospheric skips, phase distortion, group delay, selective fading, scintillation, etc. The consequence of these effects is more pronounced in a weak signal (low SNR) communication environment under unstable propagation conditions. Long-range HF communication is often a typical candidate that could be classified as susceptible to channel degradation, distortion, and compromised intelligibility. These difficult conditions do not persist at all times but are intermittent in nature.

However, it is not these natural factors that we are discussing at the moment. We are currently concerned about the completely avoidable yet often ignored aspect of additional man-made distortions that result in compromising communication intelligibility.

These avoidable issues invariably occur due to either operator malfunction or due to the inability of the operator to make the right choice while selecting and interfacing a non-standard custom microphone with the transceiver. Unless the operator applies due diligence and makes sure that that the AF electrical output from the microphone into the transceiver falls within a voltage range that is compatible with transmitter’s AF pre-processing circuits that drive the modulator system, problems are likely o occur. An overdriven modulator circuit will create a mess. That is the reason why all radio transceivers feature a modulator ALC circuit to address the microphone AF level variations, however, the ALC system also has its limits.


How does Overmodulation cause loss of Intelligibility?
In terms of waveforms, as might be visible on an oscilloscope, the physical manifestation of overmodulation might appear different depending on whether it is an AM, SSB, or another form of modulation. The factor that would be common to all cases would be either of the following two conditions. Without dwelling on technical intricacies, let us take a brief intuitive look into it. To brush up the fundamentals concepts of radio modulation, please check out the article on Radio Signal Modulation Principles.

In the case of AM, where the RF carrier is retained, the first of the two factors often play up. If the AF modulating signal amplitude is increased to become greater than the RF carrier amplitude, then overmodulation would occur by virtue of carrier blocking on each negative peak of the AF. This will result in the generation of harmonics of the AF modulating frequency.



The second condition that leads to overmodulation is perhaps more common and often more sinister. This condition is not determined by the relative level of the RF carrier but is determined by the limitations of the electrical circuit design. If the modulating AF voltage amplitude exceeds the voltage handling ability of the circuits, then AF will clip at the top and bottom of the waveform by virtue of what is known as swing limitation of the circuit. As a consequence of AF clipping due to swing limitation, the AF signals produces several harmonics, all of which are undesirable. These AF harmonics will also modulate the RF along with the desired speech frequencies. Of course, these modulated harmonics will not cause out-of-channel splatter but they will make the speech fuzzy and reduce its intelligibility. The degree of distortion on account of this effect will be proportionate to the magnitude of the overdrive that would be causing the swing limited clipping of the AF.

While working on the HF radio bands, so often we find that there might be a fairly weak station with not-so-good SNR, yet that station would sound very clear. Despite all other propagation-related factors like QSB, etc, we can understand every word or syllable uttered by the operator. Copying the station might be effortless, like a breeze. On the other hand, we also work many stations that might be much stronger with good SNR, yet quite often, without even realizing the cause, we find that we need to concentrate more while listening to the operator...

Even though the extra effort required to copy the station in our second example might not be at a conscious level, it produces a slight strain on the mind... why is it so?

Some might dismiss it by concluding that the first weak-signal operator perhaps had a voice texture that was better suited for radiotelephony in comparison to the other operator. As I mentioned before, to a small extent that might be true but the actual reason would lie in better quality and a lower-distortion modulation. Spurious-free modulation may only be achieved by controlling the modulation depth and consciously ensuring that the amplitude of the modulating AF from the microphone is not exceeding the boundary limits of the modulator's ALC system. This is what forms the primary means of optimizing intelligibility and retaining clarity of messaging in radio communication.

These overdrive-related overmodulation issues generally do not occur when an original microphone that is supplied with the transceiver is used. The requisite AF voltage level compatibility is taken care of by the manufacturer. However, when another generic microphone is used or a computer-based hybrid SDR implementation is used, then extra care needs to be taken to ensure correct microphone levels.

Thi principle of modulation level control not only applies to radiotelephony but also to all other modulation modes like SSTV, or digital modes that utilize the AF baseband.




Does unduly high TX power cause Overmodulation Distortion?
Last but not least, let me take this opportunity to address this question. I have come across several operators who have unfortunately tried to control overmodulation by reducing their transmitter power. This is certainly not a solution.

Overmodulation cannot be corrected by transmitter power reduction. However, perhaps some of the bad effects might be perceived to get slightly masked by reducing the power, the fact is that varying the RF power level would only vary the distorting components proportionately. This is akin to a photograph that is printed into a large poster. If the person in the picture were to have skin blemishes, they are likely to show up. Now, rip off the poster-sized blowup and put up a postcard size copy in its place. Anyone watching the smaller picture is less likely to notice the skin blemishes. The magnitude of RF power is similar to picture sizes. Remember, the core blemishes (distortion) still remain intact, no matter how little or how much we magnify the picture (power).

The way to correct overmodulation is to reduce the microphone output voltage level. If your mic level is set for a clean modulation, then no amount of power can cause distortion. If power were to ever cause TX distortion, then high power radio broadcast stations like the BBC would have been out of business long ago...

Having said that, let me put a caveat. Although cleanly modulated high power TX is OK, it might, under certain circumstances, result in a very strong signal level at the receiver, thereby, it may overload the receiver front-end resulting in an apparent perception of distortion. This is due to limited signal handling capability of any receiver. This is a radio receiver limitation determined by virtue of its design. Under these circumstances, reducing the receiver RF gain, or engaging a receiver input attenuator comes in very handy to resolve all distortion and unintelligibility related issues.