Digital Wireless Latency Explained

Shure UK Project Engineer, Stuart Stephens explores digital wireless latency and what causes  latency in a digital wireless system.

Latency is the amount of time it takes for our  signal to journey from input (microphone capsule) to audio output (analogue audio output on our receiver). In the case of analogue wireless systems, the microphone capsule is converting the acoustical  energy of the sound source into an electrical signal, which is then  transmitted over radio frequency waves. Both the electrical and RF signal travel at the speed of light, and therefore, the latency of  analogue wireless systems is negligible.

In the case of digital wireless systems, the acoustical to electrical  transformation remains the same, however, this time we're converting  the electrical audio signal into a digital bit stream. This conversion from analogue audio to digital binary code takes time, and thus,  introduces some latency within digital wireless systems.

The amount of latency in a digital wireless system will depend on the  amount of signal processing involved, and also the RF mechanisms  employed. A Shure ULX-D system, for example, produces around 2.9ms of latency from audio input to the analogue output on our receiver.

How Much Latency is Acceptable?

When considering how much latency is acceptable in a live performance  environment, it's important to consider the application. For example,  if you're using stage monitors, 5 - 10 milliseconds of latency is  generally acceptable for most performers. Once you get beyond 10 milliseconds, the signal delay can start to become noticeable, which can  have a detrimental effect on the performers timing and overall  delivery.

In live applications that utilise in-ear monitor systems, latency  figures become even more critical, especially for vocalists. The reason  is, vocal performers will hear their performance both from the  monitoring system and through vibrations in their bones that lead to the  ear. This additional dimension makes the delay issue far more acute.  For in-ear applications, 5 milliseconds and below is recommended to  avoid compromising performance.

Very low latency can also cause different issues for in-ear monitors.  Once you get under 5 milliseconds, you can start to introduce comb  filtering, which happens when the direct sound and the monitoring system  sound interacts, causing dips in the frequency response. These dips can  change the tonal characteristics, but most performers can adapt and  work with minor comb filtering as a compromise.

Total System Latency

When considering latency, it's important not to focus on a single  device. As mentioned above, a Shure ULX-D wireless system has a latency  of 2.9ms. However, the chances are, you'll have more than one digital  device in your entire audio system. For example, once your vocal leaves  the analogue output of the receiver, your signal might enter an audio  network, which could then progress to a digital mixer — you might even  want to process this signal using outboard digital processors. All of  these stages and additional analogue to digital conversions contribute  to the total system latency, and this is the key figure you want to  consider.

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Shure UK Project Engineer, Stuart Stephens explores digital wireless  latency and what causes latency in a digital wireless system.