Shure Whiteboard – Analogue vs. Digital Wireless

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.

shure-whiteboard-image

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.

companding

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.

Spectral Efficiency

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.

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Marc Henshall

Marc forms part of our Pro Audio team at Shure UK and specialises in Digital Marketing. He also holds a BSc First Class Hons Degree in Music Technology. When not at work he enjoys playing the guitar, producing music, and pretentiously dabbling in the odd craft beer or two from his own personal glass at the local pub — like a total nerd.

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  • Chris Jones says:

    I have not had the opportunity to work with Shure digital mics, but I have used some others and although I have found them to be very good as far as sound quality is concerned, I have encountered serious problems with muting and loss of handshake where the transmitter is taken out of range of the receiver, as happens when an actor goes off stage. The system we were using didn’t just mute, but would completely lose handshake and switch off! Unless a stagehand remembered to check that the body pack was still linked when the actor came back on stage, we would have no communication. Does anyone else have experience with this, and have any manufacturers got around this problem?

    • shureUK says:

      Hey Chris, thanks for your comment. According to our tech department, it would be incredibly unusual for this to happen with a Shure system. If there’s no TX in range then the RX output will mute. (By “handshake,” we’re assuming you mean the transmission between Rx and Tx). I hope this helps, feel free to contact us if you need further help. We’ll happily advise where possible. social@shure.co.uk

      Thanks for watching/reading!

      • Chris Jones says:

        Thank you for responding to my comments. I am very interested in your reply. To give you a bit more detail,
        the units I have used have been AT System 10 and we were hopeful that, being licence exempt, this would be the way to go but, as I said, our big problem was that the Tx packs actually switched themselves off completely when out of range of the receiver so, even if the user realised that the unit had done this, it would take several seconds to get the Tx switched back on and re-linked with its receiver once it was within range again before the mic could be used. As you can imagine this proved something of a nightmare with actors coming and going from the stage, particularly where body packs were under costumes! We had both handheld and body pack units, and they all exhibited the same problem so it wasn’t just one faulty unit. The other thing we noticed was that the Tx were quite heavy on batteries, presumably because they are constantly checking and modifying their frequency, and as soon as the two AA cells dropped below about 1.2 – 1.3 volts per cell they would shut down. We had hoped we could use NiMH cells, but had to revert back to alkalines, and we were lucky if we got six hours out of them. The analogue Ch 38 units we hired in the past happily gave us eight to ten hours on a set of AA alkalines, and I have just used a Toa Ch 38 system which only uses one AA cell per Tx and that runs for ten hours!
        So I guess my two main questions are i) do the Shure digital mics remain in a standby mode when out of range, and re-link automatically when they come back into range, and ii) what sort of battery life could I expect, and would they be happy running on NiMH cells, or do yours use rechargeable lithium packs?
        With the threat of further reductions in the analogue channels available for radio mics and IEMs I am still looking at the possibility of going digital, but my experience so far has not been encouraging.
        Many thanks
        Kind regards
        Chris Jones

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