The disadvantage of the whole stereo LP scheme is that although the mastering engineer can adjust the groove pitch for loud or soft passages, the maximum and minimum depth of the groove are constant. Cut too much low frequency information with a wide stereo spread, and you get a lot of deep peaks and valleys in the groove and styli tend to pop out of the groove. Turn that down, and your stereo image collapses.
So the amount of stereo information has a lot to do with the level that can be cut to disc. No matter what you do beforehand, out-of-phase low frequency content will lift the stylus from the groove or drive it into the substrate. On the other hand, in-phase low frequency information causes lateral excursions wide enough to cut into the previously cut groove and into the area where a groove would be cut in the next revolution.
Mastering today has become a catch-all term for any kind of post-mixing audio processing, but LP mastering is the process of making an acetate out of the original tape. The processing is secondary. However, the processing is almost essential to get the most out of the limited channel.
There is a lot of poking and prodding that is often done to get the stereo signal to fit into place, because the LP has less information on it than the original master tape does. Often, you’ll see mastering engineers roll off a lot of the very low bass and add a false bass peak around 200 Hz or so, just to compensate for the mechanical limitations of the equipment. The other alternative is to reduce the running time per side radically.
The one thing that saves us from bass being a big problem is the RIAA pre-emphasis curve. Most of the noise in the recording process is at higher frequencies. So on record, we pre-emphasize the signal by pumping up the highs, and then on playback the phono amplifier has a roll-off curve that is the exact inverse of the curve in the record chain, which rolls them off. This means that the music has the same frequency response, but the noise is reduced, primarily on the high end.
Even with this, though, the mastering engineer is constantly juggling signal processing versus recording time versus groove pitch. Most systems today automatically control the groove pitch, although an expert engineer can override them to some extent and make constant tweaks to get that last bit of performance out.
Traditionally, the way this is done is with a “margin control” system. The tape is played back on a machine with a special “pre-hear” or “pre-listen” head that picks up the signal about half a second before the playback head (i.e., for 30 ips tape, the head is about 15 inches away from the playback head), and feeds that signal into some control electronics.
This means that that control system has information about what the signal level is going to be like on the next rotation of the disk. It can constantly ride the groove pitch up and back so that the grooves don’t get too close that the walls between them get deformed. But they are still as close as possible so that the maximum running time on the disk occurs.
This, incidentally, is why 12" singles are invariably cut much, much hotter than conventional LPs. There is plenty of space for very wide groove spacing, so they are cut as hot as possible and therefore play back much louder. Some of the 12" singles are even cut constant pitch, without any margin control, because there is just too much safety margin available for the mastering engineer.
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