I had a question this week from a reader who was wondering why there was excessive noise in the audio channels of a recording. He'd positioned an external microphone close to the presenter and thought this would eliminate any potential problems. And yet there was still noise in the end result.

There are three primary causes of noise. The first cause, as we've discussed before, is ambient background noise. We film, generally, in non-ideal acoustic environments, and the working world has to go on around us. By positioning the microphone as close to the source as possible, we get a much better ratio of signal to background noise.

The second is electrical interference, which is definitely a potential problem on long cable runs from the microphone to the camera or recorder, but can even be a problem on relatively short cable runs when the environment is electrically noisy. This can be mitigated by using balanced audio, when noise on the positive signal line is duplicated on the negative signal line, and cancelled out when one is subtracted from the other within the camera's pre-amp.

Balanced audio outputs can be transformer balanced, actively balanced or impedance balanced. Transformer and actively balanced outputs transmit the signal on one line and an inverse of the signal on the other. Any noise generated will be identical on both lines. The balanced input in the camera or recorder subtracts one from the other, so not only is the noise eliminated but the signal is boosted. This means the pre-amp gain can be lower, so you get less problems with the pre-amp's self noise.

Impedance balanced lines don't provide the inverse signal, with only have the noise running on both lines. You can therefore eliminate the noise, but you don't get the benefit of the signal boost.

Most balanced microphone outputs will be transformer balanced, although I can think of at least one prominent lavalier mic manufacturer that prefers the impedance balanced configuration. It says that although you don't get the boosted signal at the balanced input, this is more than offset by the improved linearity of frequency response and elimination of phase problems that you can get with transformers.

If your camera or recorder only has a 3.5mm jack input rather than a 3-pin XLR, then you don't have the facility for balanced audio. You can solve this by buying a microphone with a balanced output, and using it with a balanced XLR input adapter.

The third source of noise is a 50Hz mains hum that can begin to float around earth loops. Easiest way to get rid of this is to have an earth lift switch in the signal path.

With all this in mind, the reader with the question wondered whether a wireless solution might have been better. Well, in many instances it will indeed be the most appropriate solution, but it's as well to be aware of potential problems too. First off, you're going to have a slightly curtailed frequency response and a reduced dynamic range. Would I tend to worry about either when it comes to speech? Honestly, I don't think I'd lose sleep over it. More problematic, though, is that when you go wireless, you open yourself up to interference from outside sources, and you introduce new elements of complexity into the audio chain: there's more to go wrong.

No single solution will ever be ideal in every environment. I like to go into every job with as much flexibility as possible: always test everything before you go, always have options, always have a plan B.

With that in mind, I've recently added to my arsenal of microphones with a Sony ECM-55B, modified so that it will work with my wireless transmitter or in a balanced wired configuration with the option of a ground lift. I can see it being a real problem solver.

I found a great video that compares the ECM-55B with a sub-£30 clip mic of the type we've discussed before, and with the camera's built-in mic. The 55 is really good, but the low cost mic is still decent. Hopefully what the video will reinforce again is that you should never rely on the camera's built-in microphone. That will always deliver the worst audio quality.