Infrared protocol analysis with PC soundcard

Published on December 20, 2014 . Updated on: -
Infrared communication is widespread among electronic devices that use remote controls. Because of this, there are more protocols in use and the data (bytes) sent to the device depend on manufacturer. This post will show you how to view the waveform of an infrared signal emitted by a remote control, how to analyze and decode it. Then it will be possible to reproduce it. In this way you can program your universal remote control or your mobile device (smartphone with IR transmitter) with the right code for the best results. Instead of using an oscilloscope for signal analysis, due to the low frequency of the IR carrier an ordinary soundcard will be used.

Infrared protocol analysis with PC soundcard

The IR receiver

Modify a receiver

The simplest receiver you can build will connect straight to the microphone plug. This supplies the IC with 5 V too so no external power supply is needed. If you already have the IR receiver from a TV tuner card or a SCART receiver don't rush to plug it in the sound card because pinouts are different and you may destroy the IR receiver IC.

IR adapter
IR receiver of a SCART DVB-T tuner.
Note that it has four pins instead of only two needed by a microphone.
The above receiver contains an IR IC and a bicolor LED. Nowadays, in all IR controlled devices you'll find an integrated circuit that receives the IR, amplifies and filters it. Here are such ICs: TFMS5300, TSOP1738, SFH506.

All these ICs and devices have only three pins: power supply, ground and signal output. Notice the similarities with electret microphones. The only difference is that a microphone's signal output is capacitor coupled to power supply, thus the microphone has only two pins. That's what our adapter will do. It will connect the signal output to power supply via a capacitor and it will use a current limiting resistor to power the IC.

Here is the schematic and a photo of the simplest IR receiver with microphone plug. You can modify something similar to the above receiver (for a SCART DVB-T tuner) according to this schematic. If you want to play with an old module similar to the above keep in mind that supply voltage is different - you'll need separate power because microphone jack power cannot be used anymore.

Soundcard IR adapter schematic
Soundcard IR adapter (based on Matthias Ringwald's schematic)

Soundcard IR adapter picture
Built IR receiver. There's no need for a PCB.
R1 and R2 form a voltage divider that lowers output signal maximum level to less than 1 V. C1 couples the AC waveform to the VCC line (replacing it with a 4.7 - 10 uF electrolytic with positive pole to R1/R2 and negative to GND would produce better results). Inside the soundcard another capacitor will feed this signal into the ADC. Plug this adapter into the microphone input and go to the next section.

View the waveform

The simplest to use is Audacity. It is a cross-platform software that can record and display the signal.

Audacity IR remote recording - screenshot
Audacity main window.
First you must select the right device (rear microphone in my case) and choose mono input (1). Do a few test records (4 and 5) and if the level is too low adjust it (2) - if there's no signal at all check input and adapter. Set the project sample rate to the maximum value (3) - note that after the record is done, the real sample rate will be displayed in the bottom right corner. As you can see the level is a bit low in my example. But if you click on the drop arrow next to Audio Track (7) and select Waveform (dB) things get a lot better. Now start zooming in (6) to see the actual protocol.

Looking at my example, this remote control sends the actual command bytes once and then if I keep pressing the button it sends a repeat pulse. So I will zoom in the blueish part of the signal - you can use Ctrl++. This is similar to what you'll get:

IR waveform in Audacity - screenshot
IR waveform in Audacity
Of course I didn't have any idea how to decode this protocol. But, using San Bergmans' IR Remote Control page I was able to tell that this is some kind of NEC protocol. Now, if I analyze this it has the following meaning:

IR analysis and decoding - Audacity screenshot
IR analysis and decoding (the last byte is 0x07, not 0x0B)
The waveform starts with a long burst needed by the receiver IC to calibrate its gain. Then the remote sends four bytes in total but only two are useful. First is the address byte. Then it should be the inverted address byte for error control. With some manufacturers, no matter what button you press on the same remote control, the address byte stays the same. I don't know why this remote control sends the address wrong (0x40 + 0x9F is not 0xFF). The third byte is the actual command and the fourth is the inverted command byte (0xF8 + 0x07 = 0xFF). Whether the bytes are sent with MSB or LSB first I'm not sure.

Here are some other software solutions for viewing the signal waveform. Note that with all you must first choose the audio input device before using. Always choose the highest sample rate possible and mono input. Note that some Windows versions disable audio devices when nothing is plugged in.

Soundcard Oscilloscope - Windows only

It's quite difficult to "catch" the signal on this software. Go to Settings and choose correct Input device.
IR waveform displayed on Zeitnitz Scope
The waveform displayed on Scope

Protocol Analyzer

This is a Java based cross-platform software that can record and analyze various protocols. The analysis doesn't always work as it should so it's best to sample the raw signal. Go to View - Settings - Audio Settings tab and select the correct Source. Go to Action - Sample x seconds and quickly press a button on the remote control. A new line should appear in the main window. If not tweak the input volume from sound settings. Double click the line with raw recorded signal and you should see a graph. Click and drag to select a rectangle that will be zoomed in. Also right-click and zoom each axis as you like. With the right zooming my protocol looked like this:

IR waveform in Nethome Protocol Analyzer
The waveform in Protocol Analyzer


WinLIRC contains the IRGraph utility which displays a windows with a processed square wave. It starts recording after the first rising edge which is not displayed. Continue reading to see how to set up LIRC.

IR square wave signal in LIRC IRGraph
The square wave signal in IRGraph

Record the signal

There are multiple ways of recording that waveform but the point is to be able to reproduce it. The waveform as it comes out of that IR receiver IC is a square wave that is highly distorted especially by that coupling capacitor. You can try to improve the hardware by using a separate power supply (can be derived from USB) and feed the output of the IC straight into sound card.

Audio recording

This is almost pointless. You'll have the waveform recorded as-is but even if you build a simple amplifier with an IR transmitter LED and plug it into the output of the sound card, then play the record through it you'll get nothing. That's because the signal is more complex than that. The remote sends a 38 kHz pulse that is modulated by the waveform you recorded. The high pulses you see are actually 38 kHz signals that:

  • are interpreted by the soundcard as high. If you can set up the sample rate to a higher value, multiple of 38 kHz you could see the carrier of 38 kHz.
  • are converted by the receiver IC to logic 1 (high). Yes, most modern IC receivers remove carrier from the output signal, thus making it easier to interface with a microcontroller. In this case, no matter how high is the sound card sample rate, you will not see the carrier!

This is how a not filtered IR waveform looks when sampled at 192 kHz. The signal is distorted a lot by the adapter but it can be seen clearly the carrier with a 38 kHz frequency.

IR Waveform sampled at 192 kHz - Audacity screenshot
Waveform sampled at 192 kHz. The high peaks after each 38 kHz carrier shouldn't be there.
If I plot the spectrum of this waveform I can see that the carrier really is at 38 kHz. Totally irrelevant results are displayed at low sample rate (44 kHz).

IR waveform spectrum analysis - Audacity
The spectrum shows a high level of the 38 kHz carrier. Generated in Audacity (Analyze - Plot Spectrum)

Probably the waveform looks much better with another adapter but Audacity can only display audio waveforms.

Note that 38 kHz is the most used carrier frequency but there are remotes which use 36 kHz or 40 kHz.

Data recording

LIRC (and its Windows equivalent WinLIRC) will be used here.

On Linux it's easy to install LIRC. Just run sudo apt-get install lirc in a Terminal on Ubuntu distros. You'll be prompted to choose your hardware. I couldn't get the receiver working in Ubuntu with the ALSA audio input source so I built a serial port transceiver like this. To record the signals open a Terminal and run (the configuration file will be created in the directory you are running irrecord from):
irrecord -f -d /dev/lircd test.cfg
On Windows just extract the downloaded archive and start winlirc.exe. Click OK at the error message and when the Setup dialog appears choose AudioCapture.dll plugin. Configure it with the right input and the maximum sample rate. Click the Create Config button and close the CMD windows that appears. Click OK at the error message and when prompted to open the Setup dialog again select Cancel. Now go to plugins folder and open WinLIRC.ini. You should see the audio configuration in this file. If it doesn't exist try to configure WinLIRC again by launching it. Here's my audio configuration:
AudioDeviceName=Microphone (Realtek High Defini
Now open a command prompt in the folder where winlirc.exe and irrecord.exe are and execute the command:
irrecord -f -d AudioCapture.dll ../test.cfg
All OSes
Follow on-screen instructions (press Enter a few times, then press and hold remote buttons and name them). Note that I always used the -f argument which disables any processing of the recorded waveform, saving it as raw timeframes. After you finish open test.cfg with a text editor. It will look like similar to this (just the codes for a button showed):
name OK  
   9395  4072   781   343   770  1458
   770    354   760   354   760   354
   760    364   760   354   750   364
   750   1489   750   364   750   375
   739   1500   10    10    708  1500
   739   1500   10    10    708  1510
    10     10   708   1510   10    10
   708   1500   10    10    729  1489
    10     10   718   1500   10    10
   718   1489   10    10    718  1500
    10     10   708   385   739   375
   729    395   718   395   718   395
   718    406   708   406   708   416
   708   1531   10    10    687  1520
    10     20   10    10    666  1520
    10     10   687
What are those numbers? It's very simple. They represent time in microseconds. So 9.395 ms is high followed by 4.072 ms low (doesn't it look like the AGC burst and the space after it?) and so on. Let's rewrite it by grouping data bits timeframes:
( 781, 343)( 770,1458)( 770, 354)( 760, 354)( 760, 354)( 760, 364)( 760 ,354)( 750, 364)=01000000
( 750,1489)( 750, 364)( 750, 375)( 739,1500)( 708,1500)( 739,1500)( 708,1510)( 708,1510)=10011111
( 708,1500)( 729,1489)( 718,1500)( 718,1489)( 718,1500)( 708, 385)( 739, 375)( 729, 395)=11111000
( 718, 395)( 718, 395)( 718, 406)( 708, 406)( 708, 416)( 708,1531)( 687,1520)( 666,1520)=00000111
Note that I completely ignored 10 and 20 us frames as they can't be anything but noise. To be able to modulate a 38 kHz signal, each pulse must be at least 1/38000 * 1000000 = 26 us.

Reproduce the signal


IRplus by Binarymode is probably the most versatile application of this type. Besides having a large database that you can contribute too, it can import LIRC files you recorded earlier. You can also edit straight from the app every button (command, text label and color). The default configuration files used by irplus are based on XML.

irplus main screen
irplus main screen showing an example remote control
To import the LIRC configuration file, go to Options (the top left icon), then tap on Import.

Import LIRC file in irplus
Import LIRC file in irplus
More than this, irplus can be used with devices that don't have an IR blaster. You need a simple to make adapter plugged into the audio jack.

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