Path: utzoo!attcan!uunet!dino!ux1.cso.uiuc.edu!ux1.cso.uiuc.edu!m.cs.uiuc.edu!irwin
From: irwin@m.cs.uiuc.edu
Newsgroups: sci.electronics
Subject: Re: Hissing from tape recorders ?
Message-ID: <21000070@m.cs.uiuc.edu>
Date: 11 Apr 90 17:13:18 GMT
References: <3190@moondance.cs.uq.oz.au>
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Nf-ID: #R:moondance.cs.uq.oz.au:3190:m.cs.uiuc.edu:21000070:000:2164
Nf-From: m.cs.uiuc.edu!irwin    Apr 10 10:10:00 1990


/* Written  9:57 pm  Apr  5, 1990 by anthony@batserver.cs.uq.oz.au in m.cs.uiuc.edu:sci.electronics */
>/* ---------- "Hissing from tape recorders ?" ---------- */
>Just want to ask a simple question.  I notice that my tape recorder when
>played without a tape hisses.  I just want to know why and how I can
>get rid of the hiss.  Is this because the head have been magnetised one
>way and need demagnetising ?

>Thanks in advance 
--
>Anthony Lee (Humble PhD student) (Alias Time Lord Doctor) 
>ACSnet:	anthony@batserver.cs.uq.oz	TEL:+(61)-7-371-2651
>Internet: anthony@batserver.cs.uq.oz.au	    +(61)-7-377-4139 (w)
>SNAIL: Dept Comp. Science, University of Qld, St Lucia, Qld 4072, Australia
/* End of text from m.cs.uiuc.edu:sci.electronics */

I have been following this thread for a time and note the responses
that have been received. Most were in jest in one form or another.

I would like to offer a serious response. I worked part time at a
service bench for a local TV and audio service shop for a few years.
I have serviced a lot of tape recorders. One thing that I noted was
that most recorders make use of an automatic volume control circuit.
This is to take up part of the differences in record level from tape
to tape.

If one purchases several music tapes and plays them, they can not
assume that all are recorded at the exact same level. If you set
the volume of the recorder on play back to what you like, then play
tapes, you note that most music is about the same level. This is
because of the automatic level in the pre-amp stages of the unit.
It will take care of minor differences in levels from tape to tape.

Now, if you put the recorder into play mode, with no tape, the pre
amp stages are running WIDE OPEN, as there is no signal to cause
them to throttle back, hence, lots of noise. The stages would not
normally run at such high gain, if a tape were actually in place
as the signal being sampled would cut down the gain.

I am not sure just why you want to put it in play mode without tape,
just to use the audio stages or what, you might offer an answer to
this.

Al Irwin
Univ of ILL
Dept of Comp Sci
Urbana, IL
irwin@m.cs.uiuc.edu