how does a PULSE code MODULATION WORKS?


When I press a button in my IR transmitter it will send binary codes example.
1 1 1 0 with a 38 KHZ carrier.

The IR receiver device will detect the 38 KHZ ( with the filtered code, 1 1 1 0) and will let it pass.
BUT… how does the device know where to put the 1 1 1 0 code? does it have some sort of low pass filter or something ?


http://www.tpub.com/neets/book12/0217.GIF

 

Synchronising the IR signal can be done a few ways:

* By a known preamble (eg.. the remote sends 1100110011001100 before sending 1110), thus the RX knows to sync to the '1100' and then capture what ever doesn't match that pattern as the real command.

* By activation (eg.. the transmitter turns on the IR LED and begins encoding the TX data immediately, therefore the start of transmitting is the start of the command).

There are dozens more methods to synchronise an RF/IR signal that escalate in bit length requirements and complexity, but for bottom-line IR - either of the above would be used.

 

The NEC protocol uses a "START" sequence of 9 msec of carrier on followed by 4.5 msec of silence. The individual data bits that follow all have a period of carrier followed by a period of silence.

 

will this work?

1. I add in the receiver 4 D type flip flops, with clock input and the Data input
2. I send the code from the IR transmitter example 1 1 1 0
3. the 4 D type flip flops will “detect” the 1 1 1 0 signal (Wave length on/off time) in its INPUT pin, and its OUTPUT will be “exactly” the same code as the IR transmitter.


dunno, will this somehow also work?

 

No not quite. In the NEC protocol a '1' bit is 1.125 milliseconds long and a '0' bit is 2.25 milliseconds long. Each bit starts with 560 microseconds of 'carrier on' at 38 kHz. The remainder of each bit is silence. It is common for a byte of data to be followed by the complement of that data to equalize the total number of '1's and '0's in a message.

http://wiki.altium.com/display/ADOH/NEC+Infrared+Transmission+Protocol
http://www.sbprojects.com/knowledge/ir/nec.php

I think it is highly unlikely that a simple collection of logic circuits will do the job. IR receivers usually use custom decoder chips or a microprocessor.