The first LP's were evenly black in appearance and had a fixed groove
pitch of 300 per inch, resulting in a maximum recording time of 23
minutes per side.

By the mid-1950s, however, virtually all classical-music recordings used
"variable-groove" recording, with the grooving closer together in quiet
passages and wider in loud ones, typically extending the playing time to
30 minutes or more.

The question I have is this: how was the groove pitch regulated?

Was some kind of automatic device used, and, if so, how did it work?

Or did the engineer simply familiarize himself with the performance and
manually adjust the lathe speed as needed (that is, effectively
contributing his own, skilled, "live" performance to the mastering
process)?

--
Daniel P. B. Smith, dpbsmith at world dot ess tee dee dot com
"Elinor Goulding Smith's Great Big Messy Book" is now back in print!
Sample chapter at http://world.std.com/~dpbsmith/messy.html
Buy it at http://www.amazon.com/exec/obidos/ASIN/1403314063/