I can provide the following, and since remotes are so ubiquitous I'd be interested
to know if there is some sort of an up-to-date "one-stop repository" somewhere
on the web for the technology behind them.
http://repairfaq.ece.drexel.edu/sam/irrfaq.htm
I searched the repairfaq site but didn't find the 10 year old file I have archived. So
I've included most of it below, with credit to originator Juergen Putzger.
6. Decoding IR Remote Controls (From Juergen Putzger)
The origin of this posting was the question what to do with an old TV. I
suggested to use the infrared remote control as an input keyboard for a
microcontroller board and mentioned a piece of code I had written for
the 8052 microcontroller. I was asked by some people to share my
information about remote controls, so here it is:
There are at least two international standards which are used by remote
controls to encode the commands, the RC5 and RECS 80 code. The RECS 80
code uses pulse length modulation. Each bit to be transmitted is encoded
by a high level of the duration T followed by a low level of duration 2T
representing a logical '0' or 3T representing a logical '1'.
Notice that a '1' takes more time to be transmitted than a '0'. The RC 5
code instead has a uniform duration of all bits. A transition in the
middle of the time interval assigned to each bit encodes the logical
value. A '0' is encoded by a high to low transition and a '1' by a low
to high transition. Therefore we need additional transitions at the
beginning of each bit to set the proper start level if a series of equal
bits is sent. We don't need this additional transition if the next bit
has a different value. This is also called a 'biphase' code.
Instead of being fed direct into the IR emitter, most remote controls
modulate a 20-30 kHz carrier with this signal. A logic one is
represented by a burst of oscillations.
The reason is, that you can use a filter tuned to the carrier frequency
to distinguish the signal from noise in the ambient light. Fluorescent
lamps are the main source of such noise. Photodiodes behind an optical
filter which transmits infrared light but blocks visible light are used
as detectors. The signal from the photodiode is fed through a filter
tuned to the carrier fequency and then amplified. The amplified signal
is demodulated just like the carrier is demodulated in any AM radio
receiver.
It can be a lot of pain to design a sensitive receiver that does'nt
start to oscillate. It is also necessary to have some automatic gain
control to avoid overload of the amplifier at close distance to the
emitter. It is easier to use some integrated circuit that does all of
the job. The best i have ever seen (and used) is the SFH505A
manufactured by SIEMENS (no, I don't work for this company). It looks
like one of this three legged voltage regulators and uses a single 5V
supply. It incorporates an optical filter, the photodiode, a filter
tuned to about 30 kHz , the amplifier with automatic gain control and
the demodulator.
If you don't know which code your remote control is transmitting you can
identify it by viewing the output of your receiver with an oscilloscope.
The RECS 80 code uses high pulses of uniform length while the low pulses
differ in length. If there are high and low pulses of two different
lengths it might be RC5 code. Note that your receiver may invert the
levels.
How are commands like volume control or channel selction encoded? In the
case of the RC5 code there is an international standard. Every command
is encoded by 14 bits. The first two bits S are startbits to allow the
receiver to adjust the automatic gain control and to synchronize. Next a
bit T follows, that toggles with every new keystroke. Next is the
address A of the device which shall respond to the command. At last the
command itself follows.
| S | S | T | A4 | A3 | A2 | A1 | A0 | C5 | C4 | C3 | C2 | C1 | C0 |
Some important addresses and commands:
Address: Device: Command:
0 TV1 0...9 Numbers 0...9 (channel select)
1 TV2 12 Standby
5 VCR1 16 Master Volume +
6 VCR2 17 Master Volume -
17 Tuner 18 Brightness +
18 Audio Tape 19 Brightness -
20 CD Player 50 Fast rewind
52 Fast run forward
53 Play
54 Stop
55 Recording
There are integrated decoder circuits which have inputs to select the
device address and parallel outputs activated by the commands. Since
this is comp. robotics the devices you wish to control will have a
microcontroller on board which can do all the decoding. Here is an input
routine I have written for the 8052 microcontroller family to receive
RC5 codes. My cousin has written a similar routine for the RECS80 code
which i will try to make available also. Perhaps we can start a
collection of such routines and archive them somewhere.
