Dennis Lee Bieber  wrote:
> On Sun, 4 Aug 2019 19:46:15 +0100, "NY"  declaimed the
> following:
>
>> Was the choice of ultrasonic rather than infra-red driven by the need to be
>> mechanical rather than electronic at the remote, or was there some reason
>> why ultrasonic was preferable to IR irrespective of whether it was generated
>> mechanically or electronically?
>>
>
>  In that day and age -- no IR LEDs, and no microcontrollers to
> encode/decode data. Two whistle tones (one per button) are easy to decode
> with simple RC bandpass filters (or even just a high-pass and low-pass),
> and detected output from filter just kicked a stepper to the next position.

A reliable ultrasonic remote requires fairly narrow bandpass filters to
reject the omnipresent ultrasonic noise. The transmitter also needs to
produce a quite robust tone to achieve good SNR.

Even then, it’s necessary to inhibit a response when more than one bandpass
filter indicates a signal in order to prevent a hearty “hiss” from
affecting the TV.

--
-michael - NadaNet 3.1 and AppleCrate II:  http://michaeljmahon.com

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