Record Grooves

[Eyeball]

This is very interesting information-

Tom Appleby writes:
I noticed that an LP I just transcribed had only 13 minutes per side but the disc was seemingly as full as a 22 minute per side. Was grooves per inch varied from lp to lp? Also it had no print-thru (don't know the correct name) (editor's note : it is called "pre-echo') whereby on long classical LPs one can often hear the introduction of the piece by hearing a faint version of the first groove before the actual start.
tom
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Hi Tom,

Your observations are correct. When mastering an LP, the first step involves transferring the signal from audio tape to a lacquer master, from which metal-parts are later made. Various factors in the music can be changed during this process, including (but not limited to) adjustments in relative volume of the songs, adjustments in effects, EQ, etc.

The amount of volume coming from the tape machine is the determining factor in how wide a groove is cut by the lathe. Wider grooves naturally necessitate more margin between the grooves (to prevent pre-echo and/or outright skipping.) This amount of margin -- ie the distance between the grooves -- is indeed adjustable. Thus if you cut the music very loud, the disc could appear to be full, but might be quite short in duration. If a disc is cut very quietly, the groove will be narrower, enabling more rotations to fit on the disc -- thus more time, but all at a lower volume. That's why 45 EPs are usually quieter than standard single-song 45s.

Back in the day, the object in most disc mastering labs (aside from EQ considerations and matching the volume song-to-song) was generally to try and cut the disc as hot as possible without distortion, pre-echo, or skipping. Generally one would look with a microscope at the grooves to make sure that during a particularly loud section, they weren't coming close to touching one another. Oftentimes it took several lacquer attempts to get an optimal result.

hoping this is helpful,

Tom Ball

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Ted K. Hering writes-

You're exactly right, Tom: Different LPs had different number of "grooves
per inch."

Mercury Records introduced "Variable Pitch" in the 1950s, in which the
closeness of the grove was determined by the volume level of the
particular passage. So you'd have some grooves close together in the
quiet passages, and wider apart for the loud passages, all on the same
side of the record. The advantage of Variable Pitch was you could get
longer play time on one LP, and still keep the dynamic range (volume
variations).

[Eyeball]

(Getting back to 78s) I've noticed some widening of the pitch on late
'30s (c. 1937/1938) ACR cuts during or in anticipation of loud passages.
Possibly the technician made trial cuts during run-throughs, or gained
experience from earlier-recorded takes, or just listened. I think
these would be before the dubbing-to-78 business started!

Perhaps one chap who had an interest in such things! I suppose there
must be a knob or somesuch on lathes to adjust the pitch??

I haven't seen similar on HMV, Victor or Decca, say, although
sometimes the shorter sides in a classical set will have been cut with
a noticeably wider pitch so as to use more of the available (12")
side.

78 groove pitch varies quite a bit, allowing nearly 4 minutes on a 10"
side at maximum. Around 3'45, however, seems to be about the limit of
safety. I must someday count the tpi on some samples!

Pre-echo occurs fairly often on (lateral) 78s, especially on longer
and therefore more closely pitched sides. The inner wall of an already
cut turn is damaged by the immediately following turn when there's an
especially loud bit. Conversely, post-echo appears to be virtually
non-existent given that the following turn has yet to be cut, even
though you do seem to get it on microgrooves occasionally.

Best,
Antony Pepper