Having discussed in detail the microphony issue in the last two blogs, it’s time to look at Radio Frequency Interference and Electro Magnetic Interference ((RFI and EMI). But let’s start at a simple level, just to check we’re all on the same page – with some science!
Forgetting the terms RFI and EMI for a moment, let’s just consider basic signal theory. If you have two sine waves and add them the result will be quite literally at any point in time, the sum of the amplitude of both signals. If the sine waves are the same frequency and in phase the result will be a signal of double the amplitude. If the signals are out of phase the result will be zero. And if one signal is a large amplitude audio signal and the other is a small noise signal at a much higher frequency then you can see that the shape of the audio signal at any time will be polluted by the presence of the small signal. It has superimposed noise – it’s less pure. Keep adding noise a little at a time as the audio signal passes through your system and you can end up with quite a clouded, unrealistic sound. The expressions you might hear for this are a high noise floor or a lack of transparency.
But there’s another problem. When two frequencies are mixed you also get what are called intermodulation products. This process is best described by talking about frequency and when you add a frequency of say 400 Hz and 100 Hz you will produce additional signals at 300Hz and 500Hz. So if your hifi is allowing an unwanted signal in of 100 hz, and that’s not uncommon because its the frequency of rectified mains (120hz in the US), then you are going to get unwanted signals at plus and minus 100Hz for any musical tone and not just a 100 Hz hum as most people imagine. So our ‘noise floor’ is a dynamically changing multiple set of unwanted signals that grows as the level and complexity of the musical programme grows. Ever wondered why a system might sound good on gentle music but goes bright and hashy with complex stuff?
Next it’s important to understand that these unwanted high frequency signals come from so many internal processes within hifi equipment. Rectifiers, motors, LEDs, any form of digital processing, displays, digital controls and of course two of the worse culprits, switch mode power supplies and switching amplification. The old-school thinking that all you need is a simple inline filter in your mains is not correct – most of the noise that damages performance is generated within your equipment. And whereas we were talking before about a hypothetical 100Hz noise signal from the mains, all these other components actually generate noise at much higher frequencies, and some of that comes from another scientific process called harmonics. It’s a complicated subject but in this context we are talking about how anything that switches state rapidly, like chips going from ‘zeros’ to ‘ones’, then the leading edge of the rapidly changing signal will produce multiple spikes of harmonic noise up and down the frequency spectrum.
And of course, if the frequency of this noise is up in the order of frequencies that could be transmitted over the airwaves, it’s termed Radio Frequency Interference. And this brings us to the next important point - once these noise signals are up at those frequencies, by definition they can be transmitted from one wire within a hifi component and received by another wire in that component. So what this means is that something that is generating a lot of RFI, like a switch mode power supply, cannot be so easily isolated from sensitive analogue circuits because it’s RFI will be radiated out and picked up by the other circuits that are close by. We say that the analogue circuits are being affected by Electro Magnetic Interference (EMI). And of course the process now works in reverse – the EMI being picked up by the analogue circuits gets turned back into an RFI current within that circuit.
So there it is! I hope that helps a little. In my next blog I’ll start to talk about systematic ways to tackle these problems.