There are actually a number of subtle physical processes going onīut by far the most significant effect (other than volume, which panning models) is the phase change. You also pick up echos and reverberation or ambience from the walls of the room in which a recording is made. This can cause frequency filtering effects or echoes. Some of them have to do with the way sound goes around and/or through your head as it gets to the "far ear" from the sound source. There are actually a number of subtle physical processes going on. If you listen to these via earphones and close your eyes, I think you'll be impressed by how complete the auditory scenes appear in your head - you're able to localize the sounds much better.
In this way, you can take a bunch of independent mono recordings (of individual instruments, for example) and spread them out across the left-right spectrum. It's also very easy to do in a mixing console or in a mixing application like audacity - you just change the relative left and right amplitudes of the waveforms for different elements you are going to record. This is very popular, and works quite well. Simple stereo sound produces a limited 3D illusion by changing the loudness of the signal coming from each speaker - imitating the simplest aspect of binaural hearing That's called "panning", we say that the sound signal has been "panned" to the left speaker. I think most people realize that the main way that "stereo" sound works is by having sound come at different volumes in each ear - if it's louder in your left ear (or coming out of the left speaker), then it seems to be coming from that direction.
In fact, there's a wealth of information in the sounds that reach our ears, and our brains do some very sophisticated processing of that information. We can distinguish the direction and, to some degree, distance of a sound source. We can distinguish the direction and, to some degree, distance of a sound source This was fine for many purposes, but it was a little weak when it came to things like music. The first sound recordings were "mono" - that is to say, they just recorded a single waveform, representing frequency and volume information, but no directionality. An intriguing step along the way is true "binaural" sound, and how it differs from ordinary "stereo" sound.
In order to process surround sound with free software tools, it's first important to understand what surround sound is, and why we might want to use it.
This column may seem a little abstract for Free Software Magazine, but it provides a launching off point (and a technical introduction) for a number of topics I hope to cover in upcoming.
This article is part of an on-going series on the challenges I've faced in producing two free-licensed movies, Marya Morevna, through the Morevna Project and Lunatics, which we are working on as Anansi Spaceworks.