It is essential for the beginning sound engineer to understand the differences between mono and stereo. In essence, mono means one channel, stereo means two channels. But there is more...
In the early days of recording, there would be one microphone in the recording studio and one loudspeaker in the reproducing equipment in the home. In between there was one 'channel', into which all the sound components of the recording were mixed, and mixed in such a way that they could never be separated again.
As recording equipment became more sophisticated (and in broadcast too) then the outputs of several microphones could be combined into one channel.
Both of the above examples are mono (from the Greek for 'one') because there is just one channel in which the sound signal is recorded, or through which it is transmitted.
Even if there is more than one microphone, or more than one loudspeaker, because there is a single channel then the result is mono.
But we two ears. The brain is capable of receiving sound through two channels. So even as far back as the 1930's it was thought that it would be a good idea to use two channels to record or transmit sound.
So if two microphones are used to record sound, and are connected through two completely independent channels to two loudspeakers, it is likely that the result will be more convincing than mono. And it usually is. We call it 'stereo' (from the Greek for 'solid'). Stereo, unless something has gone disastrously wrong in the recording process, sounds better than mono because it satisfies the brain's requirement for two channels of sound.
It is important to note that for stereo, there must be at least two microphones, two channels and two loudspeakers. If at any point in the chain the channels are combined into one, or only one is used, then the result is mono.
There are two principal ways in which stereo can be used...
Of course, there will be a whole spectrum of applications in between. It is important to note that in the first case, the illusion of a 'sound stage' where instruments appear to come from points all the way across the gap between the loudspeakers is perfectly achievable, given correct recording technique, good equipment and correct loudspeaker placement.
One interesting point comes from the 1970's, which was when ordinary people, other than hi-fi enthusiasts, started to buy stereo systems for their homes.
At first, people didn't know what stereo was and had to be shown very clearly and audibly exactly how different it was to mono, and therefore how superior.
So there was a trend in the early part of this decade for stereo demonstration records to be made (vinyl of course, because CD had not been invented yet). There had been such demonstration records since the 1950's, but the early 1970's seems to be their heyday.
These records emphasized the spatial differences between instruments. So some instruments would be completely on the left, some completely on the right, with little or nothing in the middle. Sometimes this was known as 'ping pong' stereo.
These days it is possible to choose how natural or otherwise the stereo effect should be, without going to such extremes.
One more point. In the 1950's, when stereo was just becoming practical rather than just a theory, it wasn't necessarily so that there were just two channels. Three-channel stereo was tried, and in the cinema even more channels were used to reproduce stereo sound.
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