Friday, September 14, 2018

About Microphones



A Communicator Reprise: September 2012

A Primer On Mics For Ham Radio

All Hams use them as a basic piece of operating equipment but most never give it a second thought—the microphone.

A microphone colloquially called a mic or mike is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal.


Both Thomas Alva Edison and Emile Berliner filed patent applications for the carbon microphone, in March and June 1877 respectively. After a long legal battle, Edison emerged the victor, and the Berliner patent was ruled invalid by both American and British courts. 

There are basically two kinds of microphone technology, dynamic and condenser.
Dynamic mics are actually backwards speakers and generate a small amount of electricity when the diaphragm of the mic moves back and forth under the pressure of the sound waves hitting it.

Condenser mics are powered or biased by electricity and so are more sensitive; they use a more lightweight diaphragm and are better at picking up nuances of sound. "Large diaphragm" condenser mics are more sensitive and more expensive than "small diaphragm" types. 

Pickup Patterns isn't what you get when you drive your truck in circles in the snow, but refers to the relative sensitivity of a microphone to sounds coming from the side. A pickup pattern can be… 

Omnidirectional picks up equally well in all directions
Unidirectional picks up mostly from one direction
Cardioid picks up in a heart-shaped pattern (hey, you think your dad was kidding when he said studying Latin would come in handy sometime?) 

Exotica ribbon mics, tube mics, and most other technologies are probably way out of your budget anyway… except for the PZM (Pressure Zone Microphone), which is patented by Crown and was used by Radio Shack for many years. The current Radio Shack mic is not considered to be much good, but if you can find one of the older ones, you can modify it for serious use.

Plugs everyone is used to teeny little 1/8" plugs found on consumer mics, 1/4" plugs found on guitar cables or the little square plastic ones on your transceiver mic  that look like an over-sized telephone plug. Forget all that. Professional mics have XLR plugs and balanced cables, which have the following characteristics.

The plugs lock in and don't rip out easily when someone trips over something.
the cables have three conductors, which not only make them thicker and more resistant to rough handling, but also means that they're less likely to pick up buzz, hum, etc.

Balanced vs Unbalanced

An unbalanced audio path has two conductors. One carries the audio signal and the other is the shield/ground. There is nothing at all wrong with an unbalanced signal but at times can be susceptible to picking up interference from radio frequencies or electro magnetic fields causing noise and buzz and picking up the occasional unwanted radio station! In fact, a lot of gear is unbalanced on the inside even though it has a balanced input and output. Including some high end consoles. 

A balanced signal has three conductors. It relies on a sum and difference principal. 
Sum and difference is the combining (summing) of two signals that are out of phase from each other. Whatever doesn't cancel out is what you're left with (difference).
When two identical signals of identical amplitude (volume) are combined and one is 180 degrees out of phase from the other you have complete cancellation of that audio. However, if one of those signals is a different amplitude, you don't get complete cancellation. And it's this principal that makes a balanced audio path work.
The output from a balanced piece of gear will have the audio signal on XLR pin 2 (hot). That same signal will be present on pin 3 (cold) however that signal is at the opposite phase from the signal on pin 2. The shield/ground will be on pin 1. 

When the signal reaches a balanced input, the signal on pins 2 and 3 are combined with either pin 2 or pin 3 (usually pin 3) out of phase. If that cable happens to pick up interference along the way, it will be on all pins, in phase together and at the same amplitude. When it gets to the input, pins 2 and 3 are combined out of phase and any signal exhibiting the same amplitude (the noise) will cancel out completely. Since the audio is at different amplitudes, it doesn't cancel out and you're left with the difference: clean audio!

Heil, Shure and Sennheiser are manufacturers of premium microphones. They generally come with XLR plugs but Heil in particular provides a vast array of adapters to interface their mics to almost any make of transceiver. Bob Heil [K9EID] is a ham himself and has devoted a great deal of time to perfecting mics that sound good despite the poor conditions often encountered with HF Amateur Radio.

All amateur radio transmitters (except the new Yaesu FTdx9000) unfortunately use an unbalanced microphone input. It's sad, but true. In connecting a balanced microphone, equalizer, or audio device that uses balanced signals, care has to be taken in how the balanced signal is UNBALANCED in order to feed that unbalanced input.

So you can't generally just plug a mic with an XLR cable into a radio jack, even with a properly wired adapter. That's because the mic will almost certainly have a lower impedance than the input of what you're plugging it into, and that means that unless you correct things with a matching device, it will sound like junk. 

One option for HF home use is to invest about $45 in a small mixer such as a Behringer Xenyx 802. This unit comes with 1/4” and XLR jacks but will adapt to almost any mic.  It will even provide ‘phantom’ power to condenser mics when the radio does not provide it. The output of the mixer goes to your radio and you have full control over what your mic sounds like, particularly of you want to adjust the equalizer to punch through HF interference. I purchased mine at a music store here in Surrey and the difference is noticeable.



VHF and UHF Transceivers generally come with low cost and sometimes low quality microphones. The mic on your handy talkie or a typical handheld mic is a condenser microphone. Manufacturers use different plugs and pin configurations  though most amateur supply houses stock adaptors. 

If you are participating in an event where you may be on the radio as Net Control for an extended period, a headset and boom or desk mic and a foot pedal are a must. Not only does it leave your hands free but the audio quality and lessened background noise will provide much better communication. My Heil desk mic and Heil headset will interface with all my transceivers (and even my computer) with adapters.

Hints on using your mic effectively

If using a handi talkie, invest in a hand-held microphone. It will he healthier not having that antenna radiating right beside your brain and also more comfortable in use.
Talk across the front of the handheld mic rather than directly into it. This will provide a less harsh and therefore clearer sound.

Push the ‘Talk’ button and pause a second. This will permit the repeater and any interface equipment with an opportunity to fully power up and avoids a portion of your transmission being cut off.

In closing, be careful about switching mics from one transceiver to another. Not only are different makes using mics with different pin connections but some manufacturers use different mics with the same plug on different models within their own brand. Transceivers generally carry a small voltage on one of the mic pins to power the condenser element. If this voltage is shorted because a pin is connected to another point inside the mic, say to ground, damage to the transceiver may result.

There is an excellent site with pin-outs for different mics at https://www.scribd.com/doc/54320681/Microphone-Pinouts




1 comment:

  1. "The output from a balanced piece of gear will have the audio signal on XLR pin 2 (hot). That same signal will be present on pin 3 (cold) however that signal is at a lower amplitude than the signal on pin 2. "

    That is not correct. The "cold" has the same amplitude, but opposite phase. See https://en.wikipedia.org/wiki/Differential_signaling

    Good article. You might add a follow-up and talk about mic bias and phantom power.

    ReplyDelete

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