Wireless microphone signal path
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All the wireless systems used at the OptEQ workshop were the Lectrosonics Digital Hybrid type.
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Testing the Lectrosonics Digital Hybrid wireless against a microphone cable has with most test signals given me less than a 1 dB difference in the measured response. The end result is not a perfect transmission, but is amazingly close to the original signal. In the receiver the digital signal is used to generate the predicted audio signal, and then this is corrected by the use of the analog transmitted difference signal. The digital output of the predictive encoder is transmitted above the 20 kHz top of the analog difference signal. Since the predictive encoder is not perfect, they then take the difference between the incoming audio and what was predicted and transmit that error signal as an analog audio signal. The output of the predictive encoder has a very low bit rate so it is easy to transmit. They use a predictive encoder which attempts to guess what the next digital sample of the audio will be based on what came before. Lectrosonics came up with an interesting scheme that tries to blend the best of analog and digital transmission. Sending digital audio across a wireless link is difficult because with most encoding schemes the transmission requires too much RF bandwidth to allow it to be used in our common frequency bands used for wireless mics. With the advent of digital signal processors other approaches were tried to improve the S/N of wireless mics. In that case poor S/N was considered less of a problem than the compander artifacts. Many years ago there was a custom made analog wireless made for measurement that did not use companding, and you had to live with the resulting poor S/N. Trying to use a conventional analog wireless mic for measurements does not work well since what fools the ear will generally not fool the test software. There was one analog wireless mic that used a two band compander, but none I am aware of that have used a 4 band compander. The vast majority of analog wireless mics use some single band companding scheme. For many years Dolby A was the best companding scheme available, but later Telefunken developed an improved compander that was also 4 band. Even still with some sorts of program material it was possible to hear audible artifacts in the playback. The big difference between Dolby A and other schemes is that it divided the audio into 4 separate frequency bands, and applied independent compression and expansion to each frequency band. The first commercially successful companding scheme for tape recording was invented by Dolby and became known as the “Dolby A” companding scheme. In other words they applied the same compression and expansion to the entire frequency range.
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All those early companding schemes were “single band”. Most companders used in wireless mics have been optimized for voice and may not work as well with certain instruments or other program material including the sweeps and noise we use for acoustic measurements.Ĭompanding was first applied to tape recording, but the early companding schemes were not widely accepted because the artifacts were often quite audible. How well companding does at fooling the ear depends very much on the program material, how complex a companding scheme is used, and how well matched the companding parameters are to the characteristics of the program material. This does not actually improve the S/N, but it fools the ear into thinking the S/N is better. Analog wireless mics use “companding” to improve the apparent S/N by compressing the signal in the transmitter to reduce its dynamic range, and then doing a precisely matched expansion of the signal in the receiver.
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Wireless mics operate at very low RF transmit power levels (typically 5 to 50 mW) and as a result the audio signal to noise ratio is not very high. So what makes wireless performance not as good as that of a cable? Photo 1 – The best microphone/mixer interface. That is true, and we should always consider if a wireless link is really needed, because not only is it expensive, but the performance will never match that of the $25 microphone cable. Someone once said that a wireless microphone system was an expensive replacement for a $25 microphone cable. Recently someone who had attended the OptEQ Workshop emailed to ask about using an analog wireless mic he had available for test and measurement, such as the links he had seen in use during the workshop. Should a wireless microphone system be use in the test and measurement process? By Ray Rayburn This article addresses this question.