Wireless microphones – a digital minefield22 November 2010
Wireless microphone technology is a minefield. As if the complications of analogue wireless systems were not enough, there is now the difficulty of understanding the relative merits of digital and digital hybrid systems. Analogue wireless mics typically use frequency modulation (FM) to transmit high quality audio on a radio frequency (RF) carrier at some hundreds of megahertz. In order to obtain a sufficiently high dynamic range, such systems often use some form of audio compression and expansion (compansion), which can give rise to ‘pumping’ side effects. One of the claims of those promoting the various forms of digital wireless is that these nasty side effects can be avoided. Transmitting high quality digital audio over a narrow RF channel, however, is not a trivial task. A typical analogue FM channel might occupy about 200kHz of RF bandwidth, and a number of recent systems aim to squeeze a high-quality digital audio signal over a similar channel. This can mean only one thing – data reduction. Although it’s not obvious in the descriptions of commercial systems, many of them use it. That’s because otherwise a full bandwidth channel of digital audio would take up far too much RF spectrum space. (When they claim something like ‘24-bit audio’, it almost certainly refers to the A-D converter.) There are both ‘digital’ and ‘digital hybrid’ wireless systems out there. The fully digital systems mostly use proprietary encoding methods to squeeze a high-quality digital audio channel with wide dynamic range down the relatively small RF ‘pipe’ available. Manufacturers are remarkably coy about the encoding methods used, but one mic from Lectrosonics (its UT700) uses sub-band ADPCM encoding to deliver one channel of audio at around 220 kbit/s. AKG’s DMS700 uses ‘AKG Premium Audio Compression Technology’, for example. Forms of DQPSK modulation are used, which vary the phase of the carrier according to the state of the digital data. Latency (delay) is typically low, in the order of 1–4 ms. Potential advantages of digital transmission include greater robustness to interfering signals in the same band and the potential for more efficient packing of the channels into the available space. Sony, for example, claims it can squeeze up to 16 channels into one 8MHz TV channel, whereas only 10 analogue wireless channels would have been possible. Encryption is another major ‘plus’ of digital wireless, enabling secure reception. Zaxcom’s systems can have built-in digital recording, so a copy of each microphone’s output can be stored. Audio-Technica has done something novel with its SpectraPulse system, which operates in the band above 6 GHz, using Ultra Wide Band (UWB) transmission. In this case an audio sampling frequency of only 24kHz is used, along with 16-bit quantisation, to deliver 14 channels of uncompressed digital audio over a link with a data rate of 8Mbit/s. The possible downside here is an upper audio frequency limit of 12kHz, but that may be a price worth paying for uncompressed encoding. The company claims a greater freedom from the interference problems that plague other RF bands. Lectrosonics’ trademarked ‘Digital Hybrid Wireless’ approach claims to encode the audio information digitally yet transmit it over an FM link as an ‘analogue data signal’. US Patent 7225135, by David Thomas, assigned to Lectrosonics, appears to discuss the technology behind this system. It describes a form of signal prediction whereby a compressed error signal is transmitted to the receiver using FM. The error signal represents amplitude-compressed deviations between the input signal and a computed extrapolation of that signal. In the receiver a similar predictor is used to reconstruct the signal, with help from the transmitted error signal. The signal is said to degrade gracefully under weak RF conditions, rather more like analogue than digital.