SIRC Part I

BASICS
  
Wireless infrared communications refers to the use of free-space propagation of light waves in the near infrared band as a transmission medium for communication. Signals sent are encoded using a an encoding algorithm in the transmitter circuitry and is decoded and the essential information is obtained at the receiver part. IR communication is a basic form of wireless communication. It's frequency range is 32-60 KHz. There are several protocols for IR communication. Some are Sony's SIRC, Phillips RC5 protocols.

 SIRC (Sony Infra Red Communication Protocol), is an IR communication protocol for controlling SONY equipment. It consists of three versions based on bit length (12, 15, 20 bit). For convenience, we here discuss about 12 bit version of SIRC. Here Pulse Width Modulation (PWM) is made use of.

  The 13 bits are divided into 12 data bits and 1 start bit. The start bit indicates the start of the information and the following 12 bits indicate the information itself. The data bits are further classified into address and command. The seven bits followed by the start bit represent the command (from LSB to MSB) (ie) in reversed manner. The following 5 bits represent the address in reversed manner. 

What is a command and address?

Command:

    It is more of a self defining term. It consists of a digital word that will server as a command to the receiving system. The following table gives the commands and their digital values in a Sony IR remote.

Command	Function
0	Digit key 1
1	Digit key 2
2	Digit key 3
3	Digit key 4
4	Digit key 5
5	Digit key 6
6	Digit key 7
7	Digit key 8
8	Digit key 9
9	Digit key 0
16	Channel +
17	Channel -
18	Volume +
19	Volume -
20	Mute
21	Power
22	Reset
23	Audio Mode
24	Contrast +
25	Contrast -
26	Colour +
27	Colour -
30	Brightness +
31	Brightness -
38	Balance Left
39	Balance Right
47	Standby

Address:

   It is a digital word that represents the address of the receiving system. Sony has unique address for its products. The following table gives the digital address for various Sony equipment.

Address	Device
1	TV
2	VCR 1
3	VCR 2
6	Laser Disc Unit
12	Surround Sound
16	Cassette deck / Tuner
17	CD Player
18	Equalizer

How does a signal to be transmitted look like?

   The following figure gives a rough idea about how the signal looks like:

  The data bits to be transmitted consist of marks and spaces. The marks are the high regions in terms of voltage while spaces are the low regions. Digital high (1) is represented by a mark of 1.2 ms (milli seconds) and digital low (0) is represented by a mark of 0.6 ms ( PWM). The time period of a start bit is 2.4 ms. A space is always represented by 0.6 ms low with respect to voltage.


What is PWM?

       PWM (pulse width modulation) is an encoding process where variation of width of each pulse is used to show the variation in the actual data.

Basic materials for transmission:
      We use an IR LED to transmit PWM signal and an IR receiver to receive the transmitted signal. The receiver always inverts the signal after which the decoding process is done.

 Coming Up :

    * More parameters

    * A platform to move on

    * Encoding and Decoding codes

    * Practical model

    * Building a project ( An Universal Remote)

Posted in: IR,Remote,Seeker1.0,SIRC,TSOP