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From: morris@jade.jpl.nasa.gov (Mike Morris)
Newsgroups: sci.electronics
Subject: Re: Speaker phone needed (circuit?)
Message-ID: <1913@jato.Jpl.Nasa.Gov>
Date: 15 Oct 89 06:56:13 GMT
References: <3268@puff.cs.wisc.edu> <6584@viscous.sco.COM>
Sender: news@jato.Jpl.Nasa.Gov
Reply-To: morris@jade.Jpl.Nasa.Gov (Mike Morris)
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In article <6584@viscous.sco.COM> rosso@sco.com (Ross Oliver) writes:
%
%In article <3268@puff.cs.wisc.edu> aber@gumby.cs.wisc.edu (David Aber) writes:
%>I am looking for a GOOD speaker phone.  Most of the ones I have seen are only
%>half duplex.  The one full duplex one I found has very low volume.  I don't 
%>know if this it to prevent the echos from being picked up or what.  
%
%Since the speaker and microphone are usually right next to each other
%on the phone, most speaker phones are half-duplex to prevent feedback,
%and the ear-splitting whine that results.
%
%>What I would really like is a speaker phone with:
%>  
%>         Lots of volume;  both ways (Its not fair to make the other person 
%>                                     listen to you from the end of a cave)
%
%You're fighting the inverse-square law here.  The sound energy of your
%voice receieved by the microphone falls off in proportion to the square
%(or is it the cube?) of your distance from it.  However, the background
%noise and echoes remain at the same level.  A studio-quality directional
%microphone might help.  Getting the microphone away from the speaker
%would also reduce feedback and allow you to use full duplex.

Years ago a ham radio acquaintance built a wireless mike in a small box
that would fit in a shirt pocket.  The device ran a few mw on a FM frequency
(i.e. license free) and worked very well as a "microphone" for a speakerphone
since the "mike" was kept away from the speaker.  He later added a jack
for a "plantronics" style headset - he had picked one up with a dead
earpiece for free.

Another cute trick that can be done with a "PC-4" style speakerphone from
Ma Bell (the kind with a rocker-style on-off switch) is to disassemble the
mike unit, and install a 1/8" closed-circuit jack in the mike element
leads.  Plug in a good quality electret desk mike.  This trick will 
improve the speakerphone audio quality 100% (and it's pretty good to
start with...   Well, pretty good as far as speakerphones go...).
ZZ



Mike Morris                      UUCP: Morris@Jade.JPL.NASA.gov
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