Welcome to the twelfth installment of All About Wireless. In this issue, we will compare the ingress of noise in analog and digital wireless microphone systems. We will then examine some noise mitigation techniques implemented in digital systems that improve audio quality, extend operating range, and increase operational stability.
Carrier-to-Noise Ratio in Analog Systems
A key difference between analog and digital wireless systems is the way in which the demodulated audio is impacted by the RF carrier-to-noise ratio (CNR). Analog systems exhibit a fairly direct relationship between the RF CNR and the audio signal-to-noise ratio (SNR). There will always be some amount of noise present in the demodulated audio of an analog wireless microphone or IEM system.
Analog receivers are designed to minimize the impact of a poor RF CNR on the demodulated audio signal, but they cannot eradicate noise completely.
Whilst the noise present in the demodulated audio signal may be almost inaudible under optimal operating conditions, the audio SNR degrades as the RF CNR degrades so the signal will become noisier as operating conditions worsen. The ingress of noise will eventually render system performance unacceptable and this is where the squelch point is typically set.
Carrier-to-Noise Ratio in Digital Systems
In digital systems, the baseband audio signal is processed by a manufacturer specific codec, the output of which is a series of binary symbols. This digital audio bitstream is then used to modulate the carrier by means of a manufacturer specific modulation scheme. Each receiver model is designed specifically for the modulation scheme and codec used by the corresponding transmitter model so that the carrier waveform can be demodulated to reproduce the original digital audio bitstream.
The carrier in a digital system is subject to the same noise ingress as an analog carrier. The key strength of a digital wireless microphone system is that there is an innate immunity to a reasonable level of carrier noise. The original digital audio bitstream can be accurately recovered in poor CNR conditions as long as the noise is not so severe as to cause the receiver to demodulate a symbol value incorrectly.
Unless some form of error detection is incorporated into the digitally transmitted bitstream, the receiver won’t know when an error has occurred. The error detection scheme must be designed for high accuracy, minimum latency and maximum spectral efficiency.
Shure digital systems employ a Cyclic Redundancy Check (CRC) process that meets these requirements and enable the receiver to detect 99.95% of transmission errors once the CNR falls below a pre-determined threshold.
How CRC Works
The CRC process begins in the transmitter. Repetitive binary divisions are performed on each block of data and the remaining values are transmitted along with the digital audio signal. The process is repeated at the receiver and the remainders calculated are divided by those values sent by the transmitter.
If the result of the CRC division is zero, no errors will be present in that block of demodulated data. The decoded digital audio signal will, therefore, be an exact noise-free replica of the original audio signal.
If carrier noise becomes severe enough that errors are detected, a Shure proprietary algorithm will control a receiver muting process according to the severity of the noise. No clicks, pops, or other audible artifacts are ever perceived – the system will simply mute.
This process, combined with highly efficient and robust modulation schemes, enables Shure digital wireless microphone systems to deliver clean audio in RF environments where most analog systems would be unusable due excessive noise in the demodulated audio signal.
The risk of strong interference severely compromising the carrier cannot be completely mitigated in fixed carrier digital wireless microphone systems.
For enhanced operational stability in challenging RF environments, Shure manufacture frequency agile wireless microphone systems like Axient Digital. In these frequency agile systems, the error detection process is so effective that the system can hop to a clean carrier frequency to avoid interference before any audible artifacts are perceived.
Frequency Diversity is a Shure technology implemented in ULX-D and Axient Digital Wireless Systems whereby the same signal can be transmitted simultaneously on two separate frequencies to a dual or quad channel receiver.
ULX-D and Axient Digital receivers set to Frequency Diversity mode continuously analyze the incoming signal quality, using both signals to provide an optimized audio signal on a single channel output. If RF interference or dropout occurs on one frequency, the receiver automatically uses the other frequency. DSP controlled routing between channels when interference occurs is seamless and inaudible.
Frequency Redundancy is implemented in the Shure GLX-D and GLX-D Advanced Wireless Systems. The GLX-D and GLX-D Advanced microphone transmit on three unique frequencies, repeating the most important data such that one frequency can be compromised by interference without causing errors in the demodulated audio bitstream. The GLX-D and GLX-D Advanced systems can then replace the compromised frequency by automatically hopping to a clean backup frequency.
Many Shure digital wireless systems also feature AES256 encryption. It may not be immediately obvious how or why encryption may be regarded as a noise mitigation technique in digital systems. In addition to the clear security benefits, encryption also ensures that a receiver will only demodulate audio from a linked transmitter.
Similar to the pilot tone in analog systems, encryption prevents unwanted audio, artifacts, and noise from being demodulated when the linked digital transmitter is powered off or is out of range. To optimize the process for low latency, counter-mode key stream ciphering is the method used to encrypt the transmitted bitstream bit-by-bit.
The combination of Shure’s advanced audio codecs, spectrally efficient modulation schemes, and robust noise mitigation techniques result in digital wireless microphone systems that exhibit far greater audio quality, operating range, and operational stability compared to older analog systems.
In the next post, we cover some recommended practices regarding site evaluation and frequency coordination planning. We will look closely at the Shure Wireless Workbench software to highlight ways in which the application can assist with RF spectrum planning and the monitoring of wireless microphone and IEM systems.
To stay updated about this and other educational content, subscribe to our email list at shure.com/subscribe.