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From George Brock-Nannestad: "...From interviewing the resident cutting
engineer at EMI Denmark I know that he corrected many things on the master
tapes he was presented with, in order to get a passable mechanical signal
for the singles and LPs he mastered.

We also know that the mixes available from mono and stereo, and single, EP
and LP of the same selections were entirely different around 1960, and I
suspect that in many cases the master tapes were basically the same. However
I have not evidence for that..."

There are several deterioration issues to consider.

First off, international releases were generally sourced from tapes that are
two or more analog generations down from the master tape. I understand this
was done because of enormous duties on metal parts in most countries.

Around 1960 most major label masters were mixed live to mono tape. Stereo
was derived from a 3 track "safety" master which was typically divided into
left, right and a vocal or soloist track. A notable exception was RCA Victor
and later Monument Records engineer Bill Porter here in Nashville. He
recorded a live stereo mix and folded that down to mono because he didn't
care for the sound of the three track machines. A great disappointment is
that the mono reissues of Roy Orbison's classic Monument recording "Oh
Pretty Woman" sound way better than the stereo which suggests the original
master has not been used for stereo.

Another exception was Robert Fine who recorded to a three track machine and
mixed it down to stereo in the mastering room. Maybe Tom knows if a mono
master was generally recorded at the same time.

Bob Olhsson Audio Mastery, Nashville TN
Mastering, Audio for Picture, Mix Evaluation and Quality Control
Over 40 years making people sound better than they ever imagined!