Xref: utzoo sci.electronics:2240 rec.music.synth:2573
Path: utzoo!mnetor!uunet!husc6!mailrus!umix!umich!mibte!gamma!pyuxp!nvuxj!abeles
From: abeles@nvuxj.UUCP (J. H. Abeles)
Newsgroups: sci.electronics,rec.music.synth
Subject: Re: guitar audio signals
Message-ID: <306@nvuxj.UUCP>
Date: 16 Feb 88 06:13:46 GMT
References: <502@m10ux.UUCP> <22670@yale-celray.yale.UUCP> <8948@ism780c.UUCP> <1165@mtunb.ATT.COM>
Organization: Bell Communications Research
Lines: 43
Summary: Reason for equalization

In article <1165@mtunb.ATT.COM>, dmt@mtunb.ATT.COM (Dave Tutelman) writes:

> 	Probably deficient in highs.  See discussion below.
> 
> SOURCE			SIGNAL LEVEL		EQUALIZATION
> 
> Turntable		1 mv			RIAA
> Mike			1 mv			None
> Guitar pickup		1 mv			None
> Keyboard		100 mv			None
> Tape Deck		100 mv - 1 v		None
> 
> ... However, level alone doesn't tell the whole story.  When records
> are cut, they are PRE-EQUALIZED (high frequencies emphasized).  When
> played back, they must be POST-EQUALIZED (inverse operation, de-
> emphasize high frequencies).  The reason is to implement a primitive
> noise reduction technique.  The specific emphasis curve is specified
> by the Recording Industry Association, and is referred to as the RIAA
> equalization curve.  Now the catch:  ALL PHONO PREAMPLIFIERS CONTAIN
> RIAA POST-EQUALIZATION, since they're intended for playing back
> RIAA-equalized source material.  That means that if you play back
> your mike or guitar through the turntable input, you'll get exactly
> the RIAA de-emphasis curve cutting your high frequencies.

The reason for equalization is as follows, according to my belief:

When the grooves in a record contain equal amplitude fluctuations
for both lower and higher frequencies, the magnetic coil pickup
will emphasize higher frequencies.  The factor will be proportional
to frequency, because the faster the coil moves, the larger the
voltage that is generated.  Thus, some kind of equalization is needed
to reduce the high frequency response.  The RIAA curve is chosen
to approximate the opposite (i.e., a rolloff proportional to freqency).
Its several 3 dB break points are chosen to approximate this 1 over
omega rolloff.  A single pole filter is 1 over omega in voltage but
I think not in power, or something like that.

Thus, guitar pickups ought to require equalization as well since
they work according to the same physical principle.

Comments?  (Am I all wrong about this?)

--Joe Abeles