The secrets of spill: What's the problem?
Most of the undesired artifacts of equipment from a bygone era of audio are now available as simulations in digital rackmounts or plugs-ins. We can have all the distortion, noise and even vinyl scratches that we want, when we want them. But there remains one commonplace problem that exists even today that, to my knowledge, there is no simulation available, and that is spill.
Spill, otherwise known as leakage and probably by half a dozen different names, is where the signal from one channel leaks through to another and is generally considered to be undesirable. In fact spill is thought to have no practical benefits whatsoever and therefore is undeserving of the digital simulation designer's attention. Spill can occur acoustically, electrically and magnetically (in the latter two cases it is more commonly known as crosstalk).
Fortunately spill does not occur digitally and we do receive some respite from it. Spill exists in recording, public address, and also in broadcasting where one radio frequency signal might interfere with another. It has to be said that in PA and broadcasting spill has no practical value and no effort should be spared to eliminate it. But spill in recording, studio or live, has been made out to be a worse problem that it actually is and there are strong grounds for saying that spill, when present in moderation and under control, is the recording engineer's friend, not enemy
Spill has been around ever since people starting using more than a single microphone to record, but it wasn't seen as a major problem until multitrack recording came along. Multitrack recording was initially seen simply as providing a useful extra degree of versatility, but it soon became apparent that there were great advantages in recording each instrument on its own track, to be 'remixed' later - the sense in which that word was originally used.
Once people had that idea fixed in their minds, then spill suddenly became very important since instead of each instrument having its own track, there might be several other instruments and perhaps vocals bleeding through in the background. Separation became the key word and no effort was spared to keep instruments as isolated as possible. Acoustic screens were ordered, drum tents were constructed, studio walls were covered in acoustically absorbent material (topped by a trendy layer of hessian), all in an effort to make whatever spill might linger absolutely as quiet as possible.
This led to two problems: firstly the acoustics of many studios were simply too dead, and also the lack of understanding of the importance of providing absorption at all frequencies led to them being bass heavy and dull, and being acoustically dull is probably even worse than being acoustically dead. Secondly, the isolation imposed by acoustic screens between musicians led to a lack of communication which tends to make performance more difficult. Obviously, thinking engineers were prepared to compromise, and eventually there came a move towards live rooms and an acceptance of spill to a certain extent, and the wider availability of equipment such as noise gates led to better control.