From: mwandel@bnr.ca (Markus Wandel)
Subject: Re: Television remote control units
Organization: bnr
Date: Tue, 10 Nov 1992 17:51:19 GMT

In article <721012191snx@nlbbs.UUCP> paula@nlbbs.UUCP writes:
>
>In article <1d9ka1INNmta@usenet.INS.CWRU.Edu> ah335@cleveland.Freenet.Edu writes:
> > 
> > Can someone give a technical explanation on how TV remote control
> > units operate?  I'm sure I'm not the only one who doesn't have
> > a clear technical understanding of this and would like to know.
>                                -------
> >
>                               M A G I C !
> > 

Well, if THAT was good enough to post, maybe the following is too.

TV remote controls send information to the TV by infrared light.  The
light is generated by one or more LEDs in the remote control and picked
up by a photodiode or phototransistor in the TV.  The wavelength is one
which you can't see, and the filters in front of the LEDs and detectors
are nearly opaque at the wavelengths at which you can see, but pass the
IR easily, which is why you can't see the devices behind them.

A normal room environment contains plenty of IR 'noise', from sunlight
and incandescent lamps, for example.  So that its signal may be
distinguished from the noise, the TV remote control emits a carrier
frequency of approximately 40KHz.  The LED is simply flashed at that
frequency.  The receiver contains a bandpass filter which picks this
frequency out of all the IR noise.  It's very much like a radio
transmission but using light pulses instead of electromagnetic waves.

Information is encoded on the carrier frequency by turning it on and off,
so that the remote control actually emits bursts of 40KHz flickering.
The length of the bursts and the spaces between them contain the
information.

Inside the remote control is a keyboard matrix, an IC, a battery, a
clock source (a 455KHz ceramic resonator typically), the IR LEDs, and
the driver transistors for the LEDs.  The IC energizes the keypad "row"
lines with DC and watches the "column" lines (for example) so it can
detect any key being pressed without scanning the keypad continually.
When it detects a key press, it fires up the clock oscillator,
determines which key is down, and generates a serial data stream which
is used to modulate the IR carrier.  On the remote controls I have,
a preamble is sent (long pulse), then 32 bits of data.  Each bit is
a burst of constant length.  The pauses between the bursts can be
"long" or "short" to encode 1 and 0.  Part of the code is constant for
all keys of a given remote control, the other part determines the key
which is down.  The length of the code ensures that IR bursts from
other (incompatible) remote controls don't accidentally spoof a key
code and the constant part of the code allows multiple remotes to use
the same protocol.

The remote control achieves its range in part by careful tuning of the
receiver, and in part by pumping TONS of current through the IR LEDs.
1/2 AMPERE of current in each of two LEDs is quite reasonable.  The 
LEDs and the battery survive because the duty cycle is small -- even
if you hold a key down the LEDs are off most of the time.

I've found that (on my VCR and remote control) the carrier frequency
and data rate of the signal don't have to be all that accurate: 10% off
in either direction produced a decrease in range but it still worked.

Markus Wandel
markus@pinetree.org <-- NOT the source of this posting
Newsgroups: sci.electronics