Foundations of Amateur Radio
One of the most misunderstood settings on your radio is the microphone gain. You'll often hear people talking about adjusting it up or down depending on what they hear and the results are often displeasing to the ear.
The very first thing to know is that the microphone gain is likely the single most audible setting on your radio, right after the tuning frequency. It's pretty much the first variable between your voice and your transmitter. Set it too low and you'll hear nothing, set it too high and you'll hear gibberish.
I said it's pretty much the first thing, but it's not the very first. That's your voice, unique in all its glory, loud, soft, happy, sad, funny or not, it's the thing that your microphone captures to transmit. Closely coupled to your voice is the distance between your mouth and your mike. The closer you are, the louder, the further, the softer and the more background noise creeps in.
As an aside, speaking of noise, there's background noise at play, but there's also the noise that comes from the audio circuitry itself, which can for example change depending on the temperature of your radio. I'm going to refer to both as noise here, even though they're slightly different.
So, starting with the ideal model where you always speak in the same way, at the same volume, at the same distance from the microphone, with a constant temperature in your radio, at all times, the next thing is the microphone gain, or gain.
Gain is an imperfect attempt at corralling your utterings into electrical signals without causing the audio circuit to distort or drown in noise. Distortion comes as a result of overloading of the audio circuit when the gain is too high, causing clipping, which essentially changes the audio waveform into something that no longer resembles your voice. At the low end of the gain range there is no difference between audio and noise which results in your voice being buried inside a hissing noise.
You might wonder why we don't just build transmitters that cannot clip and increase your volume. Well, we do. We use things like AGC, or Automatic Gain Control to attempt to prevent such things from happening, but this isn't perfect.
All this results in the microphone gain being a setting that you need to tune to your voice and adjust as things change. Overall, the best outcome is when you set the gain so the AGC just engages when you talk normally.
This gain setting also applies to computer generated signals, often fed into your radio via an audio or microphone input. If you set the gain too low, noise is the problem, set it too high and the Automatic Gain Control will distort the signal to the point where it no longer works and causes interference for everyone else including the station that you're trying to contact.
On older radios the output power was fixed. This is also true for Software Defined Radios. To reduce output power, you can change the microphone gain down and reduce the power. Change it to halfway and your output power is essentially reduced to half power. This works for a range of settings, but get too low and we're back to noise and audio fighting each other.
The opposite isn't true.
You cannot increase the microphone gain to increase power. The moment you exceed the audio circuit range your signal is distorted. On an SDR this means that you're exceeding the ability of the Analogue to Digital converter to represent your audio. In digital terms, zero means no sound and all on means 100%. If your audio is so loud as to only be 100% on, that's like sending a tone out the transmitter, resembling anything but your voice.
All of what I've talked about is related to SSB signals and to some extent AM. FM is a different animal entirely. For starters, output power on FM is fixed. The next difference is the signal or channel width. Without going into full detail, FM comes in different widths, WFM or Wideband FM, NFM, or Narrowband FM, and between the two, "normal" FM. To make things more fun, not everyone agrees on what each one means at any given time. Also, channel width and channel spacing are not the same thing, but that's for another day.
Gain aside for a moment, consider two matched FM radios using the same channel width. Your voice volume is determined by how much of the channel you use. Louder means wider, softer means narrower. Adjust the gain up, the signal gets wider, but the limit of the channel width remains, get too high and it clips at the channel width and distorts. At the other end, changing the gain down, you'll use less of the channel width and eventually the noise and your voice will be at the same level and you won't be heard.
Let's look at what happens when you use a normal FM signal to transmit to a narrowband FM receiver. Essentially your signal is too wide and the result is that your voice will be clipped unless you speak really softly or if you've set the gain really low, either way comes with more noise.
Similarly, if you transmit a narrowband FM signal to a normal FM receiver, then your voice will be very low, regardless of the microphone gain setting and turning it up will only distort it due to clipping inside your transmitter.
So, for FM, before fiddling with the gain, make sure that you're using the same FM mode as the other station. One thing to remember is when you use a repeater, if the audio is always too loud for everyone, your mode is probably too narrow. Similarly, if the audio is always too soft and you always need to turn up the volume on your radio, your mode is likely too wide. Check your radio specifications to determine what each mode means.
In broadcast audio this whole thing is managed by calibration using standard tones, but as amateurs we tend to rely on other people reporting their feelings on the quality of your voice with the often heard admonishment to adjust the microphone gain.
I'm Onno VK6FLAB