Monday, February 17, 2014

The Wireless Microphone

The Microphone Book by John Eargle gets this wrong. He dates the development of the wireless microphone to the 1960s. It is easy to find references to wireless microphones that predate this date. For example: in a 1958 Issue of Electronics, F.G. Montgomery authored a story titled "Wireless Microphone uses FM Modulation."  In 1953 Tru-Sonic was marketing a miniature wireless mic meant for use on stage.  It was the size of a pack of cigarettes.  The truth is that while Commercial grade wireless mikes hit the market before 1960 the basic components of the technology existed decades earlier. So it's no surprise they existed before that date. Rules for wireless devices in the AM broadcast band have been in existence since 1938 and were revised and updated in 1955.  Despite that, it's actually quite difficult to name a sole inventor.
DeForest's Training by mail was advertising classes with wireless microphones as early as 1943. But in 1955 Electronic Industries attributed the first wireless microphone to Stephen's Tru-Sonic industries. Below is a dated list of known references to brands of wireless microphones. These are not kits... these are branded shrink-wrapped products. 
  • 1946 - Mike Jr.(aka Real Mike)
  • 1947 - Ultra Mike
  • 1952 - Micro-Vox
  • 1953 - Tru-Sonic
  • 1953 - Shure Vagabond
  • 1957 - Sennheiser /Telefunken Mikroport
  • 1961 - Comrex  Wireless Stereophone
  • 1961 - Sony CR4
  • 1964 - Vega-Mike wireless microphone
  • 1969 - Soundette Wireless Microphone PA System
The first wireless microphoen patent was issued in 1964 to Peter K. Onnigian the founder of the Jampro Antenna company. It's patent number 3105938 A and it was first filed in 1960.  He later pioneered circular polarization in FM radio. He is a very under-appreciated figure in radio. More here.

The second recorded patent (no. 3134074 ) for a wireless microphone was filed by Raymond A. Litke in 1964. The FCC  granted Litke 12 frequencies for the use of wireless microphones. The Vega Electronics Corporation began manufacturing models based on his design after 1960. It was fist used at the Democratic and Republican National Conventions that year.

Years later, there were 4 bands for wireless microphones. There' s an obvious problem with that first band. As a secondary application, it takes a back seat to FM radio but is allocated exactly on top of it. This was very stupid. Operating below 50 miliwatts, it's clear which device is going to lose that fight in a real world scenario. See list below:
  • 49 - 108 Mhz
  • 169 - 216Mhz
  • 450 - 806 Mhz
  • 900 - 952 Mhz
Modern models operate with "diversity reception." This means they have two separate receiver modules with two separate antennas usually operating in the 900 MHz, 2.4 GHz or 6 GHz bands. This diminishes dead spots caused by phase cancellation. Today, broadcast auxiliary service devices are no longer to operate in the 698–806 MHz band.