Good processing, is of course, the key ingredient in good sounding audio, but that's not the entire issue. Processing can sound good on one station and not as good on another despite identical settings. The quality of the source material comes into play here. The old saying of Garbage in = Garbage out is as true today as ever. Starting with clean, uncompressed audio is a good start. Then make sure that the audio path is good and that no hum is being introduced on the path to the transmitter.
Last, the choice of transmitters can have an effect on audio quality. The heart of an FM transmitter is its exciter. This small FM transmitter is responsible for generating the initial signal that is then amplified through various stages to the needed power level. Nothing affects the sound quality of a transmitter more than the exciter's modulated oscillator. The modulated oscillator is where the signal is generated. Up until the past 10 years or so, transmitters usually used a device called a "voltage controlled oscillator" to generate modulated FM signal. The most common method to modulate an oscillator is to use something called a varactor diode. These are sometimes also called "varicaps". All VCO/varactor diode based systems have certain limitations that have plagued the broadcast industry for decades. The chief complains are that the oscillators are microphonic, suffer from AFC problems and add distortion.
If you bump into one hard enough, it will make a sound that will be heard on the air. If the exciter is used in a noisy environment, rumble and other sounds may be faintly heard on the radio. This is called being microphonic. Those of us in broadcasting for a long time have probably seen some VCOs potted in silicone or even put into sealed boxes on rubberized standoffs in some transmitters in an attempt to mitigate this problem.
Another serious issue occurs with the Automatic Frequency Control (AFC). AFC is required to make sure that the transmitter is operating on the correct frequency. To do this, there is a feedback circuit, called an AFC loop, that produces a correction voltage if the VCO tries to drift off of the assigned channel. This works well enough, but the AFC can be fooled by high modulation levels. When high modulation is applied, especially in low frequencies, this can simulate an AFC voltage change and cause the exciter to momentarily drift. This is sometimes known as AFC tearing because it can manifest itself on the air as a tearing sound in the demodulated audio. It also manifests as added multipath. The folks at Harris made a big stride towards reducing or eliminating this problem when they introduced the MS-15 exciter in the late 1970s. The MS-15 used a dual AFC loop operating at different frequencies. There are some variations of this technology and they're far more robust than the simple single speed AFC loop. Today, almost all commercially available transmitters using VCOs have multispeed AFCs. Still, there's an unforeseen drawback. The dual AFC loop prevents or distorts certain low frequency audio energy, preventing it from properly modulating the exciter. This is really just the old AFC unlock problem, reversed. Instead of the audio energy unlocking the AFC, the newly stabilized AFC is so stable that it suppresses some parts of the modulation. Oops! There's no real solution to this.
Last, the varactor diode always introduces distortion to the modulated carrier. Great VCO design and proper Varactor selection can minimize this a lot and produce some very good results, but some distortion always remains.
A number of years ago, a new technology known as Direct Digital Synthesis (DDS) was introduced. It was off to a slow start because it's more expensive owing to the fact that only one manufacturer in the world produces the best chipset for this method.
The DDS method doesn't use an AFC or a varactor diode. Instead, the audio is sampled by a high speed A/D chip and sent digitally to a dedicated DDS encoder chip. The DDS encoder digitally creates a modulated FM carrier that is sonically as pure as the original CD. There's no tearing, multipath or distortion when the signal is received. This creates a noticeably cleaner sound than standard VCO FM transmitters. The point is to always look for a transmitter that uses DDS for the best sonic performance.
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