Welcome to the eleventh installment of All About Wireless. In this issue, we will discuss the bandwidth characteristics of analog and digital wireless microphone systems.
FM Wireless Microphone Systems
In an FM wireless microphone system, the amplitude of the modulated carrier remains constant, but the bandwidth varies. The frequency of the carrier is modulated by an amount proportional to the instantaneous amplitude of the input audio signal at a rate equals to the frequency of the input audio signal. When plotted as a function of time, the envelope of the carrier frequency modulation represents the input audio signal.
Specifications for FM systems typically include a value for Frequency Deviation. This is a measure of the maximum possible carrier modulation, expressed in kilohertz. However, the bandwidth required for a frequency modulated wireless microphone is typically far greater than the Frequency Deviation specification suggests. The reason for this is that the Frequency Modulation process results in the generation of multiple sideband components. These are spaced sequentially, both above and below the modulating carrier frequency, at intervals equal to the frequency of the input audio signal. Fortunately, the power contained in each sideband component decreases significantly as they move further away from the carrier frequency.
One challenge with coordinating an analog FM wireless microphone system is that the varying frequencies and power levels of the sideband components can generate unexpected Intermodulation products if they interact with the sideband components of adjacent carriers. This is a key factor limiting maximum channel count within a given range, as adequate bandwidth needs to be reserved for each carrier to prevent sideband interaction. Current best in class analog FM wireless microphone systems may be capable of supporting 15 or so active transmitters within a 6MHz slice of spectrum.
Digital Wireless Microphone Systems
In contrast to a Frequency Modulated carrier, a digitally modulated carrier occupies a fixed bandwidth regardless of the fidelity and dynamic range of the input audio signal. As the bandwidth consumed per channel is constant, the risk of unexpected IMD caused by sideband interaction is effectively eliminated. This is a key advantage of digital wireless microphone systems. The predictable and consistent nature of the digital modulation scheme enables the precise characterization of all possible Intermodulation products, allowing for a denser coordination of active microphones with a greater degree of operational stability.
Well-designed digital wireless microphone systems may exhibit a wider frequency response and increased dynamic range compared to analog FM systems. However, it is not possible to directly transmit data representing a full fidelity audio signal with 120dB dynamic range within a single channel’s allowable bandwidth. Not all digital wireless microphone systems are created equal. Different systems often employ different modulation schemes, many of which are proprietary, and some are more spectrally efficient than others.
The spectrum scans above compare the bandwidth consumed by three systems transmitting high-fidelity audio via three different modulation schemes: 2-bit Frequency Shift Keying (FSK), 8-bit FSK, and a proprietary modulation scheme implemented in the Shure ULX-D system. Total span in each scan is 1MHz, which equates to 100kHz per division.
On reviewing these scans, 8FSK measures several times more spectrally efficient than 2FSK. The ULX-D scan is actually showing two active transmitters, so is twice as efficient as 8FSK! It’s clear from these scans that the modulation scheme employed has a great impact on spectral efficiency, the importance of which is compounded as system channel counts increase.
If planning a high channel count system, it is important to review the bandwidth specification of the systems you intend to use. Shure Axient Digital and ULX-D are industry-leading in terms of spectrum efficiency. These systems support up to 47 active transmitters within a 6MHz slice of spectrum, more than three times the channel density of leading analog FM systems.
In addition to specialized modulation schemes, data compression algorithms are employed to further increase spectral efficiency. Most practical audio compression schemes are lossy in nature. A lossy data compression scheme discards a certain amount of the input data stream deemed to be least significant. Quality lossy compression schemes typically divide the full fidelity data stream into a number of sub-bands. Each sub-band is then processed separately to deliver the desired ratio of data compression. The sub-band streams are then reassembled into a single full fidelity data stream that is used to modulate the carrier.
Time Division Multiple Access
Digital systems also allow for the implementation of multiplexing techniques to further enhance spectral efficiency. Multiplexing is a general term for the process of combining several signals or data streams into one for transmission over a shared medium. While many multiplexing techniques exist, the one most commonly implemented in digital wireless microphone systems is Time Division Multiple Access (TDMA).
TDMA is a multiplexing scheme whereby multiple transmitters use the same carrier frequency to transmit bursts of data in individual time slots. This technique increases spectral efficiency by enabling multiple microphones to use the same carrier without interfering with each other. Every device in the system is afforded the opportunity to use the carrier for a fixed amount of time. At any point in time, only one device will be transmitting on that shared carrier. The compromise is a significant increase in latency, so multiplexing techniques are usually only implemented in wireless microphone systems designed for conferencing or discussion applications rather than live performance.
There is a trade-off between the complexity of the modulation scheme, the degree of data compression, and the resulting improvement in terms of spectral efficiency, demodulated audio quality, and system latency. Ideally, the result is a robust and spectrally efficient system with low latency and decompressed audio that is practically indistinguishable from the source audio.
However, the quality of this process varies greatly between systems from different manufacturers. High-tier digital wireless microphone systems from industry leading manufacturers typically sound considerably better than older analog systems. However, older high-tier analog systems from these leading manufacturers remain competitive, and in some cases, may still outperform new digital systems from less established manufacturers.
Next month, we will continue comparing the performance analog and digital wireless microphone systems, focusing specifically on the ingress of noise. We will examine several common noise mitigation techniques implemented in digital systems to improve audio quality, extend operating range, and increase operational stability.
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