Even quite expensive cameras still only have a 3.5mm stereo socket to allow an external microphone to be connected. This has two main limitations:

1. The 3.5mm socket and plug are not very robust and are easily damaged. Unfortunately, damage is at least as likely to be done to the socket as the plug, so it pays to take extreme care to avoid the cable being pulled, perhaps by tying it to part of the camera. By contrast, the industry standard XLR connector is very solid and although like the 3.5mm jack it still only has 3 connections they are wired in a different way.

2. The input is Unbalanced. This makes it prone to mains hum pickup.

The XLR and 3.5mm Jack plugs. Each pin on the XLR is 2.3mm diameter and solid.
The outer body is metal so also screens the connections, whereas a typical Jack is plastic bodied so exposes short lengths of the cores to the open air (plastic doesn't
stop electro magnetic radiation)

In order to understand why a Balanced cable (line) is superior at rejecting unwanted interference it is necessary to have an appreciation of what is at each end of the line.

In this Diagram (Fig 1) the Microphone is represented as a coil, and wih no other connections made to it each terminal will produce a signal which is antiphase to the other end (Antiphase - a mirror reflection). These signals vary with the sound that the microphone is exposed to and they are represented above as a waveform which indicates the voltage appearing at the terminal with respect to time.

However, to make use of this signal in an unbalanced line we need to first establish a reference point, normally called the Earth (or ground, Ov). When we connect our reference point to one end of the coil this has the effect of fixing that end to ground, so the level (or Amplitude) at the other terminal is forced to double. Figure 2 shows this.

The ground and the signal in Figure 2 represent the two wires that are in a typical piece of single core screened cable. Any interfering signal such as mains hum will affect both wires (the ground and the signal wire) equally, but of course the ground is fixed to, err, ground, so it can't carry a signal. So the interference appears only on the signal wire. If the signal wire is carrying a high voltage (say 10 volts) and the hum is only 1mV (1 millivolt, 1/1000 volt) then the ratio between them is 10000:1. Ratiometric measurments in electronics are made with the Bel, and usually quoted in decibels (1/10 Bel), 10000:1 is 40dB. So the hum of 1mV would be 40dB down on the signal of 10 volts. Loudspeaker cables will pick up hum (as will any cable) but the voltages driving the speaker are far higher than the hum so it will not heard.

Unfortunately, microphones usually only produce a few tens of millivolts, so 1mV of hum would become a lot more audible...

This is a piece of high quality Neoprene covered LOC (Low oxygen content) twin screened audio cable. The screen is braided to give a more even coverage (some cables have twin screens), the red and white cables are stranded (more flexible) and it also has a core of string. This gives the cable a lot of strength and also makes it more flexible.

Now on to the right way to do it - the Balanced transmission line shown in Figure 3. Here the two signal wires are connected to each end of the transducer, and an equal and opposite signal (antiphase) appears on each core.

The Black wire in the diagram is connected to a clean (I.e. not noisy) earth, usually at the other end. It would usually also be connected to the microphone body if it is metal - but it carries no signal, it just acts as a screen. This can also have the effect of earthing anyone holding the microphone, as a human body can also reradiate hum. Touch a microphone input wire (This one, Simon...) and you'll hear the hum.

We haven't got to the other end yet, so let's look at that now:

Here the input is unbalanced, so there is little choice but to connect the white wire of an XLR wired cable (shown in Green) to ground leaving just the red wire to carry the signal. This effectively makes the balanced cable unbalanced. A single core screened cable would be adequate here, the screen also becoming the Earth.

The input of a theoretical Differential Amplifier (Figure 5) has a Positive and a Negative input. If a positive going signal is applied to the positive input, the output goes positive. If a negative going signal is applied to the negative input the output also goes positive.

So if a positive signal is applied to both inputs they will cancel each other out and the change at the output terminal will be zero. We're nearly there.

Now (Figure 6) we have a balanced cable connected to a balanced input. Because the signals on the two wires are in opposite phase (see Figure 1) they both contribute to the output.

Of course the two signal cores will still pick up mains hum and other interference, but because the hum will appear as two signals in phase, (I.e., they are both going positive at the same time - Hey Presto, they cancel each other out in the differential amplifier.

So you get the signal and reject the interference. There are always non-linearities in electronics, so perfect rejection of interference is not possible. The ability to reject an interfering signal which is common to both cores is known as the Common Mode Rejection and is quoted in dBs.

If you only have a choice of an unbalanced input, try to get a microphone giving a higher than average output level, this will increase the signal-to-noise ratio.

Let me know if you find this useful and interesting.

Copyright / Tony Wright / 2005-2010