The fundamental of the hum, at either 60 cycles or 50 cycles, is still one
of the most reliable "clues" to getting the pitch "right."  All of the
issues discussed mean that this issue will always be something of a
struggle, and certainty is not always guaranteed.

Pitch has been standardized, tho not always reliably, at A = 440, since the
1920's, but of course there are exceptions.  Several major orchestras
(Boston and Minneapolis, for example), deliberately tuned sharp in the
1940's to get a more brilliant sound.  But unless a reason for an exception
is known, A=440 is still the best choice in many instances.  For example,
studio-made pop records that include studio musicians, made during the
1950's will not likely be tuned oddly.  But "in the field" folk recordings
or gospel recordings could be anything (unless there is an electronic organ
being used, which might be reliable).  Church pianos are notoriously not
tuned for years, and over time a piano will often sink in pitch, so if the
piano is out of tune generally, look out.  But it is always good to check
the hum.   It really should not vary much from 50 or 60.

On vocal recordings, listen a lot to the accompaniment!

Despite all these problems, getting the pitch "right" is one of the most
critical things an audio restorer has to address.  Changes of even a
fraction of a half step can make quite a difference, particularly for human
voices.  In today's world, it is a real sin not to at least try to get the
pitch right, and I mean really right, not within a "range" of right.

I am restoring a mono RCA LP right now from the early 1950's, a compilation
of the same artists, obviously recorded at different times and places as
the tracks do not share the same "sound" for the space they were recorded
in.   The first two tracks on Side 1 required a drop of 4%--they were
playing way sharp, even tho I dubbed the record at 33.33 RPM.  I thought,
OK,since the first two match, the rest should be the same.  Nope.  The next
track required a correction of -1% to be dead on pitch, etc.

John Haley

On Fri, Nov 27, 2015 at 12:42 PM, Jolyon S Hudson <[log in to unmask]>

> Dear Andrew
> Sorry for going off at a tangent, easy done!
> It is possible that the power supply was not all that it could be. It may
> have
> been expedient to either drop the frequency or, more probable, the draw on
> the
> power stations was such that the generators lost momentum at certain times
> in the day.
> This is what happens even today when a blackout is tripped - too much
> energy
> being pulled from the grid and the network collapses as generators can't
> keep
> up with the demand. To alleviate this you can drop the frequency.
> I don't know enough about the power supply in Argentina I'm afraid but
> knowing that between the early 1930s and 1990s the Argentine economy
> progressively deteriorated I don't doubt that the power supply was not all
> that it
> should be.
> The power supply in the US and most other counties would be fine most of
> the
> time. However during wars, economic problems, lunch time and dinner time
> (nice combination of events) things can droop a bit.
> I don't think that the frequency of hum is a sure way of getting the pitch
> right in
> recordings but it is fairly accurate most of the time but not a silver
> bullet.
> This or a combination with other factors could be part of the answer. This
> is
> why a number of recording companies kept to weight driven recording
> equipment as it theoretically guaranteed a constant speed.
> Jols