PA8W Amateur Radio 

Wil, PA8W,  E-mail: PA8W@upcmail.nl           
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Microphones, microphone technique, tayloring your audio



Your audio frequency response is highly depending on your specific microphone.
Be aware of quality loss in old microphones; some microphones do age dramatically.
The pop filter in front of the membrane will surely get contaminated after a few years of use, and this will cost clarity.
Much more dramatic is the fact that some types of materials used for the membranes will change their mechanic properties pretty fast, resulting in a catastrophic degradation in sound quality.
Don't hide behind statements as: "My microphone was absolutely top notch when I bought my rig in 1977..."
That may be correct, but times change, and so do materials.

Handmikes:

Some handmikes really sound poor, but if your handmike uses an electret element, it should be quite useful. 
The electret has a nice flat frequency response, and is quite immune to RF fields.

However, my original Icom handmike -which has an electret element- was not quite what I wanted.
It had too much low end, and lacked clarity.
Additionally, I found that sometimes it would pick up RF. 
It turned out that there was a poor connection between the the aluminium housing of the little element and the earth track of the little PCB in the back.
So I replaced the electret element (by one that I saved from an old cassette recorder..) after I removed more than 80% of the felt filter in front of the Icom element. This thick, compact piece of felt obviously is meant to act as a pop-filter, but it does a poor job doing so. 
Instead, especially after contamination due to intensive use, it becomes a real barrier to the higher frequencies.

So I only left a very thin layer of it, and Eureka; my handmike audio suddenly entered the 21'st century...
More output, more clarity, nice low end, without sounding muffled.

But:

Take care to use a microphone in the right way; do not hold a handmike right in front of your mouth at only a few centimeters distance as you see in police movies; you will surely "pop" the mike with every letter P or F you say. Here's a bad example: sound/popping.mp3
You won't believe how many times I hear this. and even much, much worse.
It's better to hold a handmike in a 90 degree angle beside your mouth, so the top of the mike is towards you, in this way you will not blow into the membrane. Be sure to keep a constant distance, and train yourself to use a steady volume of speaking.


You may also consider a more fancy mike.

There´s lots of good models available, think of Heil microphones for example.
But also the transceiver brands themselves offer good alternatives to the standard mike they supply with the radio.

If you are thinking of a Pro Audio mike, try a cheap (Behringer?) studio condensor mike. 
But they need phantom feed:
 Both pin 2 and 3 of the XLR connector need to be connected each via a 6k8 ohm resistor to a +48V supply, however, the smaller types often work well down to +10V on each pin.
To overcome this inconveniency you could buy a small Behringer mixer to provide the necessary phantom feed,
 and it will additionally give you some means of equalization and level control!
The mixer output has to be connected to either a line input of your transmitter or to the standard Mic. input, using a -30dB or so attenuator, to prevent the rig's Mic. input to overload.
With this mixer, you also have a handy entry point for other audo sources, and also some kind of level indicator.

In case of HF coupling problems or ground loops, you may need a small audio coupling transformer to connect the mixer to the rig.
I use small and cheap "loop isolator" transformers out of the car-HiFi.
They do the trick most of the time.
They cost a few bucks only and for this little money they offer you two transformers.

I use one transformer for the line-input of my rig and the other one for the line-output, to connect them to my computers soundcard.
In this way, I can use the computer for recording, spectrum analysis, and as a signal source for measurements.


There are basically two main microphone pickup patterns:

Cardioid / Super-Cardioid and omnidirectional ones.

Pick a microphone with a cardioid or Super-Cardioid pickup pattern if you need the best suppression of background noise 
as from fans, room reverberation etc.
However, they do show proximity effect; getting near the mike will emphasize the low end, which is generally not what we want.
So investigate this carefully, and keep a constant distance of at least 10cm to minimize the proximity effect.

A omnidirectional mike is often easier in this respect, but it will pick up more background noises.


Additional tayloring equipment:

The last step is to hook up additional equipment to enhance your audio.
But be careful: By far the worst examples of bad audio I found at stations with additional equipment!

Additional equipment can give you extra possibilities to improve your signal, but it also gives you additional opportunities for poor adjustment...
My message is: Only add a device that has been proven necessary after proper testing and analyzing.
Note that additional equipment also means more entry points for HF radiation!
And every added device may add problems that need another added device to overcome these problems.
Before you know it, you may find yourself sticking bandage over bandage on a stinking wound... 

Note that considerable changes in frequency response will have effect on dynamic behaviour as well, and vice versa. 
So if you change one, check both!

If you want to add external tone control or limiter/compressors, you may check the Behringer range.
Although a budget brand in the Pro Audio world, (German name, but all made in China...) 
the performance will be absolutely sufficient for Ham's demands.
In Pro Audio, you will easily find devices that cost 20 times the price of a Behringer, 
but for this application they will not add anything useful.
So, save this money for a nice antenna, or take your XYL out for dinner a few times. (which may be the best investment anyway...)

 In case of an external tone-control, you could preferably pick a parametric one, 
that allows you to notch out peaks in your frequency response, or to excite certain frequencies for better intelligibility.
The lowes part and the highest part of the spectrum can be modified best by simple shelving tone controls.  

Don't be tempted to take a 31 band graphic EQ. 
There are lots of good reasons not to.
If you really need a 31 band EQ to get your frequency response right,  
there must be something deadly wrong in your mike or transmitter!


If your transmitter appears to be overloaded easily, you could decide to add a limiter-compressor to your audio chain, 
but take time for proper adjustment of this device as well.

Here's an example of what good compression does to your signal:



I recorded this graph using the internal compressor of my IC746pro.
The left half of the graph is without, the right half (after the beep) is with modest compression switched on.
The right portion has much better saturation, the average power will be at least 6dB higher than in the left portion.
This is 4 times more power!

If I had to choose between a 100w transmitter with optimized audio, and a 1kW transmitter with poor audio, I would immediately go for the first one!

Avoid unnecessary high settings of audio gain even if your set shows excellent dynamic behaviour; it will emphasize room reverberation and the noise of fans, and even your breathing noise will become audible.
Also, your microphone input will be more likely to pick up RF radiation.
So, just don't overdo it.

Here's an example of way to much compression, the S-meter showed S9 throughout the recording, during speech as well as in speech intervals. 
sound/compressed.mp3
And here is an example of far to much audio gain.
sound/compressed2.mp3

A very common misadjustment!


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