Many people equate stereo and two channel. In my post about mono, I went to some lengths to explain that mono does not mean one, and similarly stereo does not mean two. With our eyes we perceive visual depth because each eye sees the world from a different angle. With our two ears, each ear receives slightly different sounds because the source will likely be closer to one ear than the other.
I remember when I graduated college and got a gift of a “stereo system” – this was back in the 1980s, and I got a receiver, a turntable, and two speakers. So the notion that stereo means two is pretty ingrained, but it isn’t necessarily so.
A two channel recording simply means a separate left and right track. For example, I could create a recording with a singer and a guitar, and record each separately in mono, and place the singer in the left channel and the guitar in the right channel. Is that stereo? In fact, many recordings are done this way with the exception that the main interest, say the singer, is “mixed” equally in both channnels. So you might have the drums in the right, the guitar in the left, and the singer equally in the right and left. This is 2 channel, but it’s not stereo.
So what is stereo? Stereo is short for stereophonic, a word made up to sound like the visual term stereoscopic. The Greek word stereos means solid, not two, and stereoscopic is a technique to create the illusion of visual depth by having the left and right eyes see different images (like the 3-d movie glasses).
Some people feel adamantly that “true” stereo means two channels and two loudspeakers, for example see: http://www.tnt-audio.com/topics/realstereo_e.html. A true stereo recording, then, would suggest recording a live musical performance with the acoustics of the performing hall and the position of different voices and instruments captured by at least two microphones.
When a good stereo recording (two channel) is played back with two loudspeakers, and if the two speakers are placed properly in the room (at the same height, and equally distant from room boundaries), and if one is positioned exactly equidistant between the two speakers, one can get a good illusion of the spatial quality of the original performance. Notice three important caveats – good recording, the positioning of the loudspeakers, and where one is positioned when listening.
I will argue in the next post that since stereo means solid, the best way to create a “solid” aural image may be with more than two speakers.