IR Remote Control Primer - Phidgets Support
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`6/6/14 3:12 PM
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`Products for USB Sensing and Control
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`IR Remote Control Primer
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`From Phidgets Support
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`Contents
`1 Introduction
`2 Data Encoding Properties
`2.1 Header
`2.2 Trail
`2.3 Repeat Code
`2.4 Bit Length
`2.5 Code Length
`2.6 Gap
`2.7 Minimum Repeat
`2.8 Toggle Mask
`2.9 Carrier Frequency
`2.10 Duty Cycle
`3 Data Encodings
`3.1 1. Pulse Distance
`Modulation (PDM) or Space
`Encoding
`3.2 2. Pulse Width Modulation
`(PWM) or Pulse Encoding
`3.3 3. Bi-Phase or Manchester
`Encoding
`4 Receiving Data
`5 Transmitting Data
`6 Resources
`
`Introduction
`IR Remote Controls can send and receive data encoded in various fashions as pulses of infrared light. The various encoding methods that most IR
`remotes support are grouped under the general term ‘Consumer IR’ or CIR.
`CIR is generally used to control consumer products such as TVs, DVD players or game consoles with a wireless remote control, but in general can be
`used for any application that needs to transmit low speed data wirelessly. CIR is a low speed protocol. This means its commands generally contain no
`more then 32-bits of data with a maximum bit rate of 4000 bits/second (but usually much less). There is no concession for anti-collision, so only one
`code can be transmitting at any time. Transmission distance depends on the power of the transmitter, and the receiver typically needs to be within line of
`sight. However, the signal can still reach the receiver without line of sight by bouncing off of the walls and ceilings if the transmitter is strong enough.
`CIR data is transmitted using a modulated bit stream. Data is encoded in the length of the IR light pulses and the spaces between pulses. The pulses of IR
`light are themselves modulated at a much higher frequency (usually ~38kHz) in order for the receiver to distinguish CIR data from ambient room light.
`Data Encoding Properties
`CIR codes encode data using a specific encoding scheme. Apart from the actual data part of the bit stream, there are a number of other features to
`describe. These features can all be specified when transmitting a code using our API, and they are filled in when learning a code.
`Header
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`A CIR code will often start with a header. This is a pulse and space that immediately precedes the data. Usually the pulse is quite long (several ms), and
`is used by the receiver to adjust its gain control for the strength of the signal.
`Trail
`The trail is a trailing pulse that follows the data stream but is not part of the data. This is only used in some encodings.
`Repeat Code
`The repeat code is a series of pulses and spaces that are sent at a specific amount of time after the data packet. This allows the receiver to the know that
`the button is being held down. Not all encodings have a specific repeatpacket, but all encodings support repetition (either with a repeat code, or simply by
`repeating the data over and over).
`Bit Length
`This is the number of bits in the data. Most codes are between 8 and 32 bits long.
`Code Length
`Codes can have either a constant length or a variable length. Constant length codes will have a specific amount of time within which the code can be
`sent, which includes the code itself followed by a gap time. The gap time plus the data time adds up to the constant code time, so that if the data takes a
`bit longer because there are lots of 1’s then the gap will be shorter, and if the data time is short because a repeat code is being sent, then the gap time will
`be much longer. Variable length codes will have a varying data encoding time followed by a constant gap time.
`Gap
`Gap refers to the time between codes. This is a long (>20ms) space time between codes that helps the receiver see the beginning of the next code. If the
`code length is constant, this gap time refers to the entire code length - the data time plus the gap time. If the code length is variable, this gap time refers to
`the constant gap time only.
`Minimum Repeat
`Some encodings require a code to be repeated a number of times before it is recognized. This usually cannot be automatically detected.
`Toggle Mask
`CIR data is sometimes toggled for two reasons. In one case some of the data is toggled every time it is sent. This can help the receiver to recognize the
`data as valid, and is often combined with the Minimum Repeat feature. This can sometimes be detected automatically.
`In a more common variant, one bit of data is toggled every time the button is released - in this way when a button is pressed and held down, you get the
`same code repeated over and over - once the button is released and pressed again, you get the same thing, but with one bit toggled. This helps the
`receiver to know when a code is being repeated. This ‘toggle on repeat’ cannot be detected automatically.
`Carrier Frequency
`The carrier frequency is the frequency used to modulate the IR pulses. IR receivers are tuned to a specific frequency so it’s best to transmit on the
`frequency that they are tuned to receive. Most consumer IR devices use 38kHz, and this is usually set as the default. Others that are sometimes used
`include 56kHz, 40kHz, 36kHz or 39.2kHz. If you are unsure what frequency your receiver is tuned to, we recommend that you check the datasheet or
`documentation online.
`Duty Cycle
`Duty cycle is the period of the carrier frequency. This can be set from 1 to 50, but useful values are in the range 25-50. The default is 50. The duty cycle
`of an incoming data stream cannot be automatically determined.
`Data Encodings
`CIR device manufacturers use many different protocols to encode data using pulses of IR light. Few of these protocols have been standardized, and many
`are very similar but have different bit lengths, or different headers, etc. There are three major ways to encode data that are supported automatically by our
`devices, and these cover the encoding used by almost all CIR devices:
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`1. Pulse Distance Modulation (PDM) or Space Encoding
`This is the most common encoding scheme. Here, data is encoded by modulating the duration of the space between pulses, while the pulse time remains
`constant. For example, a ‘1’ could be encoded by a 500us pulse followed by a 1ms space, and a ‘0’ encoded by a 500us pulse followed by a 500us space.
`The data stream will be terminated by a trailing pulse which lets us determine the length of the last space.
`2. Pulse Width Modulation (PWM) or Pulse Encoding
`This is like Space encoding, except that the data is encoded by modulation of the pulse width. For example, a pulse of 1.2ms followed by a space of
`600us would encode a ‘1’, while a pulse of 600us followed by a space of 600us would encode a ‘0’. There is no need for a trailing pulse because the
`space length is fixed, and the trailing space can be inferred. This protocol is used by Sony.
`3. Bi-Phase or Manchester Encoding
`This encoding breaks each bit in the data stream into a fixed length of time, where each time slice is half pulse andhalf space. For example, if the bit time
`is 800us, a ‘1’ could be encoded by a 400us pulse followed by a 400us space, while a ‘0’ would be encoded by a 400us space followed by a 400us pulse.
`The PhidgetIR supports two specific variations of Bi-Phase coding, called RC5 and RC6. There are protocols that have been somewhat standardized -
`with specific bit times and headers, and they can be automatically recognized and used for transmission. There are many other ways to encode data, but
`they are not covered here, and not supported automatically by our devices. They can be reproduced by capturing / retransmitting a RAW bit stream. One
`such protocol would be RCMM.
`Also note that, with the exception of RC5 and RC6, there is no way of knowing how ‘1’s and ‘0’s were originally encoded by the manufacturer, and the
`automatic decoding logic used by our IR Remote Controls will simply guess. Therefore, a code returned by the device may be the complement of the
`same code published elsewhere. That being said, it will always consistently return the same code for a specific bit stream.
`Receiving Data
`An IR transceiver continuously receives IR data when it is not transmitting. There are three ways to intercept this data.
`1. Code data: Use this method if you are wanting to act on specific codes (button presses) but don’t care about reproducing them. The transceiver does
`its best to decode every code that comes in and displays it as a hexadecimal string (array of bytes). This string should be unique at least on one remote.
`Access this data using the Code event or the LastCode property.
`2. Learn Code data: Use this method if you want to learn or retransmit a code, the transceiver will determine as much about a code as possible and pass
`the CodeInfo structure/object which contains the code semantics, along with the code string (byte array) itself. This can be used to retransmit the code as
`it was received. In order to learn a code the button must be held down longer (~1 second). Access this data using the Learn event or the LastLearnedCode
`property.
`3. Raw data: Use this method if the transceiver cannot decode the data automatically, you can access the raw data stream directly. This is an integer
`array of data in micro-seconds. Each access to the Raw Data array will return an even number of array elements, with the first element always being a
`space and the last element always being a pulse. Access this data using the RawData event or the getRawData (readRaw) function.
`Transmitting Data
`Data can be transmitted in two ways. When data is being transmitted, reception is momentarily paused.
`1. Code data: Use this method if you want to transmit a code that can be encoded automatically by the PhidgetIR. This will be almost any code. In order
`to send a code like this, you will need to fill in a CodeInfo structure. The easiest way to get all of these values is to learn the code from the original
`remote.
`Note that the remote will probably use the same CodeInfo settings for every button, with only the actually code (data) changing. You can also fill in the
`CodeInfo structure manually using known data (for example, in the LIRC remote control database). Just be aware that their codes may not exactly match
`the codes you need to use because of a most/least significant bit first convention mismatch or the flipped bits methods mentioned above.
`2. Raw data: If you can't represent your bit stream using a CodeInfo structure, you can send Raw data directly. This is just an array of pulses and spaces
`in microseconds. The array must start and end with a pulse. An optional gap can also be specified which serves to guarantee spacing before any more
`data is transmitted.
`Resources
`Database of CIR codes (http://www.lirc.org/)
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`IR Remote Control Primer - Phidgets Support
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`6/6/14 3:12 PM
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`General CIR reference with links (http://en.wikipedia.org/wiki/Consumer_IR)
`CIR protocols (http://www.sbprojects.com/projects/ircontrol/ircontrol.htm)
`Retrieved from "http://www.phidgets.com/wiki/index.php?title=IR_Remote_Control_Primer&oldid=21664"
`
`This page was last modified on 2 August 2012, at 07:32.
`This page has been accessed 3,915 times.
`This work by Phidgets Inc., except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0
`Unported License (Click the image on the right for more).
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