Xref: utzoo sci.electronics:2092 rec.music.synth:2491
Path: utzoo!utgpu!water!watmath!clyde!burl!codas!mtune!lzaz!mtunb!dmt
From: dmt@mtunb.ATT.COM (Dave Tutelman)
Newsgroups: sci.electronics,rec.music.synth
Subject: Re: guitar audio signals
Message-ID: <1165@mtunb.ATT.COM>
Date: 11 Feb 88 12:34:00 GMT
References: <502@m10ux.UUCP> <22670@yale-celray.yale.UUCP> <8948@ism780c.UUCP>
Reply-To: dmt@mtunb.UUCP (Dave Tutelman)
Organization: AT&T Information Systems - Lincroft, NJ
Lines: 59

In article <8948@ism780c.UUCP> gary@ism780c.UUCP (Gary Swift) writes:
>In article <22670@yale-celray.yale.UUCP> lisper@yale-celray.UUCP (Bjorn Lisper) writes:
>>In article <502@m10ux.UUCP> rgr@m10ux.UUCP (Duke Robillard) writes:
>>>
>>>Do turntables put out the same  audio signal as electric guitars?
>>
>>Plug your guitar into "AUX" at your amp. I've been doing this with various
>>keyboards when I've been playing with them at home and it works just fine,
>
>Were your keyboards (pre)amplified?  I've tried this with a
>(nonamplified) electric guitar and it didn't work, but it *did*
>work by going through turntable inputs.  I don't remember the
>sound quality though.  

	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

In terms of LEVEL alone, the turntable input should handle mikes
and guitar pickups, since they have the same output level as
turntables (to within an order of magnitude, which is all the
table above claims to represent).  Keyboards do well into the
AUX input, because it is designed for devices like tape decks
and CD players (typically around a volt).

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.

A couple of ways to deal with this:
   -	Pre-amplify the signal separately, and feed to AUX IN.
   -	Put the signal through a simple (passive, homemade) RIAA
	pre-emphasis approximation, then play back through
	you phono input.  My guess is the pre-emphasizer is two resistors
	and a capacitor, though I haven't worked the values.
   -	Live with it.  Some singers will sound more mellow with a
	low-pass filter.  (Most rock guitars will sound better with
	an extreme low-pass filter ;-)
	
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