Radio Microphones and Field Recording

April 28, 2011

I have been wanting to test the use of radio microphones for my field recording work for a while now. I finally got round to it.

Radio mics or wireless microphones are two terms commonly used for a number of different types of devices. In the broad sense the terms refer to any wireless system used to transmit audio signals over a wireless connection. In a narrow sense the terms refer to a wireless enabled microphone such as the Shure SM58.

Wireless systems are common in concert, drama and film applications both for PA and monitoring. Singers performing on stage use dynamic or condenser hand held microphones with a wireless transmitter built in. In drama/theatre or film it is mostly lavalier microphones connected to a wireless transmitter body pack that are being used. All of these systems usually use the UHF 470 MHz to 870 MHz bands for transmission. Which frequencies you will be able to use is dependent on country specific regulations. Often you will need a license to be allowed use of the equipment.

Wireless microphone systems operate in the same frequency ranges as TV channels and wireless internet (wireless DSL). As of 2012 the new LTE (4G) mobile phone networks will impact the usage of wireless mics and wireless personal monitors quite heavily. Strict usage regulations to be expected.

Manufacturers like AKG, Sennheiser and Shure have wireless systems in their product offering but these are geared toward stage performance. For field recording one needs to look at wireless systems for ENG/ EFP applications. To me only two manufacturers offer systems potentially of interest to the field recordist: Lectrosonics and Micron. Their hardware fits in an audio bag, the build quality is rock solid and the audio quality is excellent. I am ruling out Zaxcom because their systems low pass everything at 16kHz.

The obvious advantage of wireless is mobility. No cable clutter and less weight to carry around. But what about audio quality?

I tested the Micron TX700B transmitter SDR550 receiver combination with a DPA 4060 miniature microphone.

Frequency response of this system is 80Hz to 20kHz. S/N ratio is better then 100dB. The receiver uses a diversity design meaning two antennas are used to prevent audio from dropping out. This works really well. I have experienced no audio drop-outs even in the RF hostile city where I live.

Much of the discussion on audio quality in wireless systems evolves around the companding that is used on the radio connection. The dynamic range of the audio is compressed on the transmitter side, usually in a 2:1 ratio. An expander restores the audio to the original values at the receiver side. Depending on the technology used side effects referred to as “breathing” can sometimes be heard as a result of this signal processing. The “breathing” can occur by a sudden rise in the noise floor on quiet sections of the audio.

Here again the Micron system performed extremely well without any side effects.

In all I am quite happy using this system for field recording use. I can position the DPA microphones in locations I would be unable to reach using cables. The audio quality is more then acceptable. I have had no trouble with signal strength with distances as far as 80 meters.

I will continue the testing with phantom powered condenser microphones, probably the Sennheiser MKH 8020.

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