Microphones for Radios and Mic Powering
This article gives an outline of the Microphones used in radio systems.
Radio Mic Inputs
Radios usually have an automatic gain control (AGC) and limiters on their Mic input . This prevents over-modulating the transmitted signal and guarantees transmitted clear speech. It also means the radio Mic input is quite forgiving of the huge dynamic range of audio levels, from speaking in a quiet environment to speaking in very noisy environments.
The majority of radios expect an Electret Mic to be connected. Aviation radios are an exception.
Electret Mics
The majority of radios and radio earpieces have an electret Mic. Electret Mics are small and have a small internal amplifier so need power feeding up the earpiece cable to power them up.This is a small current of a couple of milliamps and with a voltage of around 3 to 8v at the Microphone terminals.
The level of the speech signal from the electret mic is dependent on how loud you speak but a nominal value might be around 30mV RMS for normal speech when the mic is around 50mm from your mouth.
Dynamic Mics
Some headsets that are designed for high noise sometimes have a dynamic mic although this is not common.. Dynamic mics are passive and require no power to use them. Applying power to them can damage them.
The level of the speech signal from a Dynamic mic is dependent on how loud you speak but a nominal value might be around 5mV RMS for normal speech when the mic is around 50mm from your mouth. This level is much lower than radios normally expect however for an ear defender microphone it works well because speech levels are naturally much higher in high noise environments.
Aviation Carbon Replacement Mics.
In the radio world carbon replacement Mics are associated with aviation radios. They are a historical hangover from microphone technology common up until the 70’s. They were quite large up to 50mm diameter and filled with carbon granules and generated a very high level of audio but were quite noisy and frequently unreliable.
Because aircraft radios were very much at the forefront of new radio communications technology and aircraft have a very long service life the carbon mic audio level seems to have remained the de-facto standard for small aircraft radios, intercoms and headsets.
The level of the speech signal from a Carbon mic is dependent on how loud you speak but a nominal value might be around 200mV RMS for normal speech when the mic is around 50mm from your mouth.
Because the carbon mic is large and noisy the Carbon Replacement Mic is usually a combination of the smaller and more reliable electret Mic and an amplifier to bring the Electret mic level up to the carbon level. The amplifier usually needs around 6 to 8v and a few milliamps to be fed up the Mic cable to power the Mic amplifier. Many aviation radios provide this power via the Mic port.
Connecting an Electret mic directly to an aviation radio without an amplifier sometimes works but often won't give a good depth of modulation.
Plug in Power (PIP)
Plug in power is a defacto standard outside the radio world for powering electret microphones devices that use short cables (less than three metres) between Mic and electronics device.
Mobile phones and Handheld radios are all examples of the PIP. PIP feeds around 3 to 8v up the cable to power the Mic.
PIP won't work well over longer cables as it uses unbalanced cables that are prone to picking up interference.
Balanced Microphone cables
Where Microphones are used on longer cables (up to tens of metres) and balanced microphone cable is used. This technology is what has made the original landline telephone feasible. This is a technology not commonly used in the radio world.
PIP or Electret Mics can be connected to balanced line cables using a device like the Rode VXLR+.
Phantom Mic Power
Associated with Balanced Microphone cables Phantom power is a term used in Professional Audio equipment to power distant microphones over long cables. It provides 12 to 48v over Balanced line cables to power distant Microphone amplifiers. Phantom power is not normally used in the radio world.
