Until recently, in a wireless microphone system the signal between the transmitter and the receiver was analog, consisting of a very high frequency radio wave carrier that was continually varied slightly in frequency by the audio signal from the microphone (or other transducer). The electronics in the receiver removed the carrier frequency and left the audio signal – the same principle that is used in FM radio broadcasts. The new PGX Digital system from Shure (and those from a handful of other manufacturers) changes that paradigm.
Within the microphone transmitter, the audio signal from the voice or other source is digitally sampled, and the sample is converted into a digital “word” consisting of the electrical equivalent of a string of 1's and 0's. As before, a very high frequency carrier wave is modulated, but in this case with the digital “stream” of samples so that the carrier frequency only has two distinct states that represent the signal in the same manner that the flat areas and pits on a CD represent the music. The receiver retrieves this information from the carrier and decodes it via a D/A converter and outputs an audio signal that is the replica of what was encoded at the mic.
Why Digital?
Analog wireless transmissions are susceptible to a variety of noise and interference conditions, related to signal strength and/or interference from external electronic devices and other wireless signals. These can ride along with the carrier frequency and its audio signal as added noise, affect the receiver directly because the antennas that pick up the transmitter signal are also wide open to pick up other radio signal in the same general RF band, or interact with the carrier frequency to create additional harmonic frequencies.
While the same physics applies to a digital signal riding on a carrier wave, the digital signal with just two states is more difficult to damage. If the receiver finds that something has come in that is not equivalent to a digital word of 1's and 0's, that information will be ignored. If noise is riding on those digital words, it is still decoded as one of two states – rather than something in-between, if it were analog. As long as the digitally modulated carrier arrives at the receiver's antenna with sufficient level, it will be accurately decoded. And as with CD players and other digital audio devices, error correction algorithms may be added to fill in the gaps where there is missing information. Typically with a digital wireless system, the signal will retain its quality until the signal level is too low, and then it's gone.
