Digital Wireless Latency Explained
Whiteboard Session
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.
