|Many electronic goods, from DVD players to
stereo systems are packaged with an infrared remote control, allowing you
to control your equipment from a distance. Long gone are the days of an
electronic cable, leading from your TV to a cumbersome box in your hand.
Nowadays, most audio, video and even lighting systems are operated using
infrared remote control technology.
DVD player manufacturers use different region free unlock codes and their
handsets transmit on differing data-rates. That’s why a Panasonic remote
won’t work on a Sony system. Well, you wouldn’t want your DVD remote
control to start playing with your stereo would you? To stop one system
interfering with another, many data-rate coding systems are used.
Typical IR signals flow on a carrier frequency of around 36 kHz. Control
codes, including those used to unlock DVD players are beamed along a
modulated 36 kHz carrier frequency by turning the carrier on and off,
usually in serial format.
If you didn’t know already ‘IR’ stands for infrared. As its frequency is
below that of visible red, infrared light is invisible, to the naked eye
at least. This makes it perfect for home entertainment use, making sure
you don’t get a laser show every time you switch channels.
Although invisible, infrared waves function in exactly the same way as any
other light source, such as the simple light bulb. However, it has the
practical capacity to carry date along its wave form. Generally, an IR
signal is generated by a Light Emitting Diode source, more commonly known
Your average TV or DVD remote control can only send commands one way, up
to 30 feet in a low-speed packet. The infrared waves sent via LEDs have a
‘moderate cone angle’ allowing you to use your controller without having
to aim directly at the receiver. You can even bounce the signal off walls.
Again, the IR modulation signal sent by home entertainment devices is
generally set to around 38 kHz, using ‘amplitude shift keying’ (carrier
on, or off). If this sounds like binary, you’re very close the mark.
Data-rate use for appliances in the home are generally within the 100-2000
bps range, enough to tell your DVD player or TV what you want it to do –
change channel, alter the sound level, activate a region code, and so on.
Although reliable and inexpensive, infrared devices can be confused by
other IR sources, but in short, this is a minor cause for complaint. Types
of interference may come from other IR remote controls, IR audio systems
that transmit infrared signals constantly, and ‘harsh’ light sources such
as fluorescents. Light interference is due to ‘ballast’ that either
dilutes the signal or refracts it, rendering the IR signal wave front
ineffective. In fact, even an electronic powered light can cause problems
depending on the frequencies involved.
To avoid interference, using the same operating frequencies of ‘ballast’
have been avoided. This means that the LED 36 kHz frequency is generally
focused on your TV, DVD or stereo. Likelihood of ballast interference
around the 40 kHz frequency is more common, so one way to dampen
interference is to use an elevated IR carrier frequency. You can actually
find infrared systems using carrier frequencies well into the megahertz
range, stopping most ‘ballast’ altogether. Think of the carrier as a laser
given a little more zap to reach its target.
Read on and we’ll get a little technical – learning more about the
dynamics of infrared. IR handsets communicate on a 32-40 kHz modulated
‘square wave’. This wave is sent to the infrared LED transmitter where the
frequency is modulated by data, usually full on/off type modulation. You
could say that this is where the signal is programmed before being beamed
across your room.
Depending on the IR system in use, the data-rate will fall into the range
of 50-1000 bits. The transmitter is designed so that an ‘oscillator’ (the
hardware that drives the infrared LED) can be modulated on or off by the
application of a TTL voltage (transistor-transistor-logic). The signal is
made up of serial data generated from a remote control keyboard decoding
IC, or Integrated Circuit.
So, we know how remote control commands are sent, but what takes up the
signal on the receiver side, say, one on a DVD player? Well, this is the
work of a photodiode. The Integrated Circuit inside an average receiving
chip picks up infrared signals around the 32-40 kHz range. Once picked up,
this is where demodulation occurs.
The demodulated output of the receiving chip will then ‘command’ the DVD
player’s actions using exactly the same wave used to drive the
transmitter. When an infrared carrier is present, the output is high and
when no carrier is detected, the output low. A circuit like this can
usually transmit a 1-3 kHz digital signal through infra light.
An infrared remote control, as well as most other light barrier systems
and optoelectronic sensors work within a wavelength of 870nm and 950nm (nano-meters).
Of course, the methodology outlined is not the only infrared system in
use. However, for home use, it’s certainly the most common.