Maybe it is not a bad idea to talk about audio quality in general. I have been listening on HF, VHF, UHF to the audio quality of different stations in QSO's and also relaying weekly bulletins. I have come to the conclusion that in many cases the lack of good audio quality deter me from listening to such "broadcasts". The same must apply to listeners if my audio is bad on any frequency or VOIP.
In some instances the bulletin's audio were so low that it was difficult to hear what the station was talking about. No I am not complaining but way back in the olden days in amateur radio, audio test transmissions were carried out on a daily basis.
Who can still remember the local ZS AM transmissions and how the stations were experimenting with different microphones and pre-amps. Yes I know that today's equipment is far more sophisticated than in the olden days. If you agree to this fact why do we experience bad audio quality nowadays. One reason could be the lack of knowledge how to setup modern amateur radio transceivers. Another could be not reading the owners and setup manual etc. etc.
No I am not going to teach you how to achieve good signal and audio quality as this is quite a complex issue. The whole intent of this post is to maximize the readability and audio quality of
your transmissions. However, nothing you do on your end, will compensate for a
lousy setup on the other end! I would however like to provide you with a few hints and kinks in this regard.
Generally radio amateurs
need to speak in a normal tone, at a normal pace and hold the microphone 5-10cm
from their mouth. In high noise environments, the radio can be held closer
(i.e. 2-5cm).
Stay away from speech processing if you are not familiar of how to use this feature. If you do use speech processing it is important to remember, that no
amount of speech processing, regardless of how it's done, will improve
upon your natural speech pathology!
Every single type of modulation, AM, FM (both phase and true FM), SSB (Single Side-Band), etc., all have a specific set of operating parameters.
It will become very evident that correct
microphone gain adjustment is the most important factor in achieving
good readability on the other end of your contact. Unfortunately, most
owners' manuals, and on-line articles seldom cover this important
attribute, focusing instead on adjustments for the internal and/or
external DSP and/or equalizers. While those adjustments are important,
excessive microphone gain will effectively negate their viability.
FM transceivers usually don't have (external) microphone gains, and never have speech compression in the usual sense. They do have pre-emphasis, but that's a whole new subject that we can go into in a future posting!
Modern solid state transceivers (almost universally) use a low impedance (nominally 500 ohm) microphone. The elements are usually electret condenser types, but may be dynamic. Some do have preamps built in, but unlike a power microphone, their gain and impedance matching is fixed. Gain and DSP adjustments aside, the way you use your microphone can have a major affect on your audio quality.
Talking too far away from the
microphone! Remember, the output level of electret and/or noise
canceling microphones drops off rather quickly as the distance
between your lips and the microphone increases. Therefore, you should
speak directly and closely into the microphone and not across it as is often suggested. In some cases, 50 - 100mm is
too far! This is especially important when using (background) noise
canceling microphones, which the majority of mobile transceivers come
equipped with.
As mentioned above, most modern transceivers, SSB and FM alike, are supplied with electret condenser microphones. There are several reasons why this is so, not the least of which is their diminutive size, and power requirements. Their frequency response (as shown in the chart at right) is almost flat across the audio spectrum. No other commonly used microphone type can even come close. This fact makes designing the requisite audio stages easier.
If they have a drawback, it is improper use. As noted above, you should speak directly into the microphone, not across it. Follow the rules, and you won't need to replace the microphone, or perform any mods to get good audio reports. If your radio is equipped with a microphone gain all you have to do is use a moderate amount of microphone gain. If the truth be known, having to set the microphone gain higher than about 10% to 15% is an indication you're not using your microphone correctly.
If you use an amplifier, any microphone gain adjustment should be made with the amplifier turned off, and the power output setting at its maximum. Once the microphone gain is properly set, the power output control should be dialed back to that required by the amplifier. This is almost never full power out, and may be as little as 25 watts PEP.
Noise canceling microphones come in a variety of configurations. How they work varies with the make. Unfortunately, there are some misdirected radio amateurs modifying stock microphones with the supposition of improved audio quality, and output level. They do this by defeating the noise canceling feature. If you've been paying attention, you know what the ramifications are. For example the IC-7000, and its companion HM-151 hand microphone can be "problematic". The microphone gain is set at 5% (you read that right!), not 60% as one modification expert suggests.
As alluded to above, there is no cheap-and-dirty way to measure peak to average speech levels. It takes a decent storage scope, astute knowledge of how to interpret the resulting readout, and a handful of other laboratory-grade hardware. Nor is there a cheap-and-dirty way to measure the frequency response of any given microphone. You certainly can't do it with your ear alone, as some mod-pundits would have you believe.
The use of speech processing (aka speech compression or speech clipping) has become the major bane of amateur radio, especially mobile operation. It allows every little nuance of engine noise, AC fans, the kids in the back seat, and that squeak in the left quarter panel to be plainly heard. It is important to remember, the average vehicle traveling at 60 mph, is at least 25 dB louder than the average living room, and some are over 40 dB louder. Adding insult, most amateurs who use it do not know how to properly set their microphone and/or processing controls, resulting in some really lousy-sounding, on-air signals. They might sound passable when properly tuned in, but a few kHz away the distortion products (IMD) can be clearly heard.
It should be noted at this point, that any form of speech processing does increase the average power level. This fact does cause the signal to appear louder on the receiving end. However, if used excessively, it also removes most of the nuances our brain uses to comprehend what's being said. And, no matter what the advertising hype says to the contrary, that means decreased readability!
Speech compression in its simplest form, is nothing more than an automatic level control. The softer nuances of speech are amplified more than the louder ones. In some cases, a different (narrower) band-pass filter is used and/or different DSP settings, all in an effort to minimize IMD.
Speech processing, however it is done, not only increases the average output power, is also increases the average current draw! Depending on the configuration, it could in fact double! This can easily tax the heartiest of electrical systems, especially when running high power. As the voltage sags, the IMD goes up, and readability decreases even more.
Here is the very best advice you'll ever get with respect to using speech processing and/or excessive microphone gain—don't!
I have only touched the tip of the iceberg. As stated before adjusting for good audio quality can be a complex task. However "kiss" is the answer here. Before transmitting use a Echo Repeater to listen to your audio quality or ask a fellow radio amateur for a audio report on a frequency where you cannot disturb others.
If you relay or broadcast a bulletin ensure that your audio quality is good enough to listen to. If you transmit a bulletin via a pre-recorded audio file then make sure that the audio recording is of good quality and suitable codec. Many pre-recorded bulletins is of low audio quality and it can even vary with different contributors to the bulletin. Listen to the recorded bulletin before transmitting and adjust the level and audio with a suitable audio editor. I use Audacity for this purpose.
Finally: We should be proud of our radio- and bulletin audio. The quality must always be good and we must remember lousy audio nowadays is unacceptable and will deter listeners from listening to you or your bulletin. Do not be afraid to tell the bulletin broadcaster that his audio is bad. Do it in a nice way and offer to assist with tests to correct issues.
Source: Own content and various Internet clips.
