In this month’s Shure Whiteboard Session, Shure Senior Applications Engineer, Tom Colman explains some of the basic differences between digital and analogue wireless systems. (Video at the bottom of this article).
For many years, analogue wireless systems ruled the airwaves, but in recent years we’ve seen increasing amounts of digital systems. On the surface, one might draw the conclusion that digital wireless is somehow better than analogue, but this isn’t necessarily fair — you first have to consider the price point. Generally speaking, an expensive, high-quality analogue system is likely to perform better than a cheaper digital system, and vice versa.
What’s the difference between analogue and digital wireless?
Analogue and digital wireless systems both have distinct advantages and disadvantages. The key fundamental differences lay in audio quality and RF quality, both of which are defined by how the audio is processed and then carried over RF.
Analogue Wireless Systems
Analogue wireless systems use frequency modulation to carry a signal. This process works by gently varying the frequency at which the system operates when a transmission is on air. There are some physical limitations to frequency modulation — notably frequency response and dynamic range. The frequency response of an FM radiowave is approximately 60Hz – 16 KHz, while the dynamic range is about 50dB.
So what happens if the dynamic range exceeds these limitations? How could we transmit say 100dB of audio through such a limited dynamic range? To achieve this, we use a process known as companding.
How Companding Works
A compander will first compress the signal on input ready to transmit over the airwaves within our limited dynamic range; the signal then expands at the receiver ready for amplification. This compression and expansion process is known collectively as companding.
More advanced companders use audio reference companding — meaning they only compress when they need to, resulting in a final signal less coloured by the companding process. The very best companders are so discreet, you’d be hard pressed to hear any effect on sound quality.
Digital Wireless Systems
Digital systems do not require a compander to adjust the signal ready for transmission. In this instance, the signal is digitized by the receiver ready for the carrier wave to transmit as a binary data stream of ones and zeros. A digital wireless system can transmit the full dynamic range and frequency response of the capsule as data, resulting in a more accurate representation of the original microphone signal.
The second key advantage of digital systems is their spectral efficiency, and this comes down to how frequency modulation works.
For example, if we’re operating an analogue system at say 610MHz, we would need to modulate (or change) the frequency. Therefore, as audio enters the system, the frequency will deviate away from 610Mhz. This deviation is quite unpredictable and takes up space within the RF spectrum landscape.
A digital system only needs to carry binary code, and thus the traditional frequency modulation carrier method will not work. For digital systems, we use different modulation systems that utilise ‘keying’ (moving stuff in discrete steps).
The methods are as follows: Frequency Shift Keying, Amplitude Shift Keying, and Phase Shift Keying where in turn you adjust the frequency, amplitude or phase is discrete steps. Carrying a digital signal using shift keying is far more predictable than frequency modulation — meaning we can often stack more frequencies closer together in a given portion of spectrum.
What are the downsides of digital?
Latency is the primary concern some professional RF engineers have about digital wireless systems. By latency, we mean the amount of time it takes for our audio signal to arrive at the output after entering the input of a digital device. Many early digital wireless microphones were plagued by audible high latency times, but this just simply isn’t a problem in modern digtial wireless setups.
The amount of acceptable latency depends on the application, and this is taken into consideration when designing a digital wireless system. In professional systems designed for musical performance (an application where accurate timing is imperative), the latency will often be as low as 2.9ms, which is far below the threshold of perception. In our opinion, this minor drawback isn’t enough to ignore the huge gains in spectral efficiency delivered by digital systems.