[78-L] EQ ideas for transferring acoustic 78s?

[Ken Matheson]

Date: Sat, 23 Apr 2011 18:14:44 -0700
From: "J. E. Knox" <rojoknox at metroeast.org>
Subject: Re: [78-L] EQ ideas for transferring acoustic 78s?
To: 78-L Mail List <78-l at klickitat.78online.com>
Message-ID: <E1074A20-334F-4F63-816A-8EDEB13E49F3 at metroeast.org>
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Greetings from FixitLand!

DanKj wrote:

>  That is correct - we hear 120 because the "60" refers to how many  
> times the
> electrical current is reversed each second ...

Power frequency in the US is 60 cycles per second (or, nowadays, 60  
Hz). Each cycle is typically measured from one positive-going zero  
crossing to the next positive-going zero crossing, thus one cycle has  
a positive peak and a negative peak. If the current reversal occurred  
60 times each second, the frequency would be only 30 Hz since each  
cycle encompasses two polarity reversals -- one going positive that  
defines the beginning of the cycle, and the other at mid-cycle as the  
polarity goes negative.

We hear 120 Hz in many cases because full-wave rectification is  
frequently used in power supplies (because it's more efficient); by  
"flipping" the negative-going halves of each cycle to positive-going,  
a 120-Hz pulsating DC is output from the rectifier. Insufficient or  
failing power-supply filtering causes 120-Hz ripple to be heard in  
audio. Harmonics of that frequency are prevalent, because the ripple  
component is usually not sinusoidal.

> ... - it doesn't refer to a sound frequency.


I don't understand this statement. 60 Hz is in fact quite audible (a  
"sound frequency"). Hook a speaker to the secondary of a 115V:6.3V  
filament transformer, and plug in the primary, and you'll  
*definitely* hear it!

Thanks for the comments on various power frequencies; interesting.

Take care,


Joe


The above is well stated. 
 
But also other power line frequencies will creep in to a recording.
180 cycles (Hz.), for 3 phase power, or 150 in the case Western Japan.
I have never run across 400 or 1200 cycles in a recording but I know
it is used in some portable or mobile power equipment.
 
Some electrical recordings will have this problem also.
 
Ken