On Tue, 7 Oct 1997 12:10:12 -0400 John Shreffler <johns@AVENUETECH.COM>
writes:
>------ =_NextPart_000_01BCD319.F7BB1F50
>Content-Type: text/plain; charset="us-ascii"
>Content-Transfer-Encoding: 7bit
>
>The old Magnavox (circa early 60s) remote created a supersonic
>whistling by pressing one of several bladders, like a dog whistle.
>It isn't a medium that could be converted to a digital encoding,
>I would guess.

The early Zenith remote sender had resonant metal rods inside.  When the
user pressed one of the buttons, a hammer would strike the corresponding
rod and ultrasonic sound came out the front.  The rods were differnt
length, thus each one produced a different frequency.  The frequencies
were not that far apart, due to the limited response of the receiver
transducer.  At the TV end, the signal from an ultrasonic microphone was
amplified and applied to a bank of filters.  Each filter had an envelope
detector, when the signal at that frequency was strong enough an action
would be taken.

The different frequency system is easily falsed by jingling keys,
clanking glasses, etc.  It was still an improvement over the
radio-controlled ones which would respond to your neighbor's remote as
well.  I think there were a few that used a digital code over ultrasonic,
but this was very short-lived, as IR soon replaced it.

If it only had 4 buttons its probably using different frequencies.  You
can generate the frequencies with a PIC but the transducer would like to
have tens of volts of drive, so a transformer maybe driven by a
transistor amplifier would be needed.  On the other hand, if you're
placing the converter close to the TV then only a very weak ultrasonic
signal would be necessary.  You might consider testing with a 555 timer
or signal generator circuit to see if the TV will respond to a simple CW
signal, and what frequencies are required.  The 40 KHz range was usually
standard for this.

>-----Original Message-----
>From:   Eric Thompson [SMTP:EricThompson@VERSALOGIC.COM]
>Sent:   Tuesday, October 07, 1997 11:57 AM
>To:     PICLIST@MITVMA.MIT.EDU
>Subject:        [OT] Old TV remotes that used ultrasonic?
>
>I was looking for some information on how the old TV remotes worked
>that
>used ultrasonics for the remote controls.
>
>I have an old TV that I lost the remote for (during a move) and would
>like to create an Infrared to Ultrasonic converter (using a PIC of
>course!) so that I can use my All for One IR remote with this TV.
>Does
>any one know how these remotes worked?  The remote only had 4
>functions:
>Chan. Up and down, and Volume up and down, so I'm assuming it uses a
>few
>different frequencies to trigger these four functions.
>
>As far as using a PIC for this, I have done the IR receiving part
>before
>so that's not a problem, it's the ultrasonic part that I don't know
>about.
>
>So my questions are:
>1) What sort of method was used to communicate from the remote to TV
>with ultrasonics?
>
>2) How would I create an ultrasonic sound using a PIC?
>
>
>Thanks,
>        Eric Thompson