SC546 Project
An analytical study of IR signals used by a SONY remote control

Objective

To study the signals emitted from a SONY remote control and evaluate the possibility of interfacing it with the PC as an input device for applications.

Research on remote control IR communication

IR remote control units communicate using light waves in the Infra Red spectrum. The physical communication layer is formed by the emmiter (a photo diode) that emits the signal and a receiver (another photo diode) that receives the signal.

The light waves are modulated by the emitter at a frequency of 40 KHz. This is done in order to cut out other sources of IR such as electric lamps, etc.

The data link layer is implemented by using binary pulses. Some of the common encoding standards are..

Pulse coded

The length of the pulse is varied to represent data.
Used by SONY remote controls.


Space coded

The length of the space between the pulses is varied to represent data.
Also known as REC-80
Used by Panasonic remote controls.


Shift coded

The direction of transitions represent the data and all the bits have a constant timeperiod.
Also known as RC-5
Used by Philips remote controls.

In addition to this headers are used to initiate a transmission. Headers usually consist of a long pulse sent before the train of data pulses.

The network layer is implemented by dividing the code into 2 parts.

  • Address : specifies the target device
  • Command : specifies the command to be sent to the device.
Most of the above research data was collected from Notes on IR Remote Controls

Analysis of data signals used by SONY remote controls

Equipment used
  • SONY RM-DM5 (Remote control used to control portable CD players)
  • SONY RM-DM5K (Infra red receiver that plugs into the portable CD players)

The receiver with the pin connection details

Circuit setup and demo

Please move your mouse over the different buttons of the remote control to see the waveform of the signal and it's respective encoding.

The first 7 bits are the commands and the last 5 bits are the address bits.


Data codes and other facts

To view a list of all the codes, please click here...

Some other interesting facts..

It makes more sense to have the first bits as the address with the command following it. However, in this case, we found that the address was coded in the last trailing bits. On a similar tone, when analysing the encoding of the numerical buttons, the data bits seem to form mirrored binary numbers.

For example,

Button 0 = 000000010001
Button 1 = 100000010001
Button 2 = 010000010001
Button 3 = 110000010001
Button 4 = 001000010001

If you look at the first 7 bits, when mirrored, they denote corresponding binary numbers. This leads me to assume that the decoding logic decodes the bits the other way around. That also seems to explain the reason for the header being sent in the end.

There seems to be 1 error in my analysis. The plot of the data signals obtained for the button 6 seem to be wrong. It only has 11 bits and a faulty address. I haven't had the opportunity to go back to the lab and confirm this. However, the error seems to be in the remote control itself since all the data was measured and recorded using reliable electronic and digital equipment.

Computer Interfacing Solutions

The 3 possible input ports that could be used to connect the receiver are
  • IRDA port
  • Serial Port
  • Parallel Port

IRDA port

The IRDA port on the computer can be interfaced directly to the receiver, thus minimizing requirements for intermediate hardware. However, on further research I found out that the protocols and standards used by IRDA were totally different from the physical and data link layers used by the SONY remote control.

In brief, the IRDA standard is based on a 3 layer protocol..

  • Physical Layer / Data link : SIR
  • Data link / Network Layer : IrLAP
  • Transport Layer : IrLMP
The SIR specifications use 2 schemes depending on the transmission speeds. For lower speeds (between 2.4 kbps and 1.152 Mbps), it uses RZI (Return to zero-Inverted) and for higher speeds (around 4 Mbps), it uses PPM (Pulse Positon Modulation).

Since all the readily available IrDA software are only tailored to these standards, it is not possible to connect the SONY IR receiver to IrDA port unless we have specific drivers written for reading in the data signals as per the SONY specifications.

In any case, I tried connecting the receiver to the port to see if anything could be achieved. As expected, I had no luck.


Serial Port

The Serial Port on the computer receives asynchronous data at suitable speeds for our data. This, however, requires an extra hardware/logic interface between the IR receiver and the port. Some of the main incompatible features between the data links were..

  • The serial port expects asynchrounous data, while the remote control sends self-timed data.
  • The serial port can only accept words of length 5 or 7 bits. It also expects start and stop bits. The remote control has 12 bit words with a header.
To overcome these incompatibilities, we need hardware such as a demodulator and logic that encodes the IR signal as 5 bit / 7 bit words perhaps sending 1 data signal from the remote control at a time. Thus, a word received on the serial port would represent the data signal equivalent to a time period of T in the remote control pulse code.

After this, specific software needs to be written in order to decode / record the pulse code and interpret it as application commands. This software can be also part of the application.


Parallel Port

The Parallel Port on the computer is more flexible. Here is a solution borrowed from the Infra Red (IR) Interface with Windows CD Software page. The only extra hardware used is a high speed (109 kHz) timer, which is connected to Pin 11 as a clock to sample Pin 12, which is connected to the IR receiver output. This ensures that the data is sampled constantly through the parallel port.

However, we do need to write some low level software, that would collect these samples from the pararllel port and store them for further use. After this, the application software has to take the responsibility of decoding the pulse code and appropriately interfacing the commands.

Conclusion

The greatest learning experience in this project comes from the remote control signal analysis which answers a lot of questions regarding the communication technology and standards used by remote controls. After analysing the signals that are emitted by a remote control and the options available with regards to interfacing an IR receiver to one of the computer ports, I have come to the conclusion that it is possible to implement a communication line between your remote control and software applications on the computer without much difficulty. Depending on the convenience, various methods can be implemented using different layers to convert the pulse coded signals received from the remote control into a software application command.

Bibliography and Links

Initially, when I started this project, I kept running into several dead ends. However, as time progressed, my search on the web for information proved more successful and by the end, I found my project to be more redundant that a discovery. I am surprised at the amount of information readily available out there..

Last but not the least, a very special thanks to my roommate Brandon Itkowitz for getting me started with HP Vee, the sample JAVA script code (which I modified for the oscilloscope demo on this page), letting me use his scanner to scan in all the plots, help with photoshop and finally for just being there..

Related interesting links..