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<p>
The receiver part for this driver is described <a href="http://people.inf.ethz.ch/mringwal/lirc/">here</a>.
</p>
<p>
</p>
<a name="audio_transmitter"></a><hr>
<p>
</p>
<h1>Using the audio driver for transmitting.</h1>
<p>
</p>
<p>
Pros:
</p>
<p>
</p>
<ul>
<li>Very simple circuit.</li>
<li>No need for a kernel module.</li>
</ul>
<p>
</p>
<p>
Cons:
</p>
<p>
</p>
<ul>
<li>Doesn't transmit very far without an amplifier (about 3 meters
when directly aiming).</li>
<li>A reasonably good sound-card is required (cheap cards might not
provide enough voltage, or might not be able to output a correct 19
kHz sine).</li>
<li>It takes some time to set up (50 ms or so) so when no clients are
connected to lircd the first transmission will have some higher
latency.
<p>
A workaround for this is to keep irw running with a bash
script like this:
</p>
<code>
#!/bin/sh<br>
while [ true ]; do<br>
irw || true<br>
sleep 1<br>
done<br>
</code>
</li>
</ul>
<p>
</p>
<h2>How it works:</h2>
<p>
</p>
<br><p>
</p>
<p>
The audio driver can send IR signals using a (reasonably good)
sound-card and a very simple circuit. It does this by modulating
the data onto a sine which is half the carrier frequency (38 kHz is
the default if the remote doesn't have a frequency specified).
After rectification the sine becomes a full frequency carrier.
The wave is inverted on the right channel, so the left and right
channels can be used to double the voltage.
</p>
<p>
</p>
<center><img></center>
<p>
</p>
<p>
The top wave is how the wave looks when it comes out the sound-card,
the bottom wave is how it looks after rectification, as you can see
the frequency is doubled.
</p>
<p>
</p>
<p>
The rectification is done using the following circuit:
</p>
<p>
</p>
<center><img></center>
<p>
</p>
<p>
LED1 and LED2 are 950 nm infra-red LEDs, R1 is a 0.25 watt resistor.
</p>
<p>
</p>
<p>
Because LEDs are diodes, they only conduct one way. Since the
sound-card outputs a wave that goes both positive and negative, two
LEDs are placed anti-parallel, that way infrared is emitted on both the
positive and negative cycles.
</p>
<p>
</p>
<p>
R1 is used to limit the current, this presents a load to the sound-card
that is roughly the same as a pair of 32 ohms headphones. To make the
transmission more powerful, you can try lowering the value of R1 (or
just short it out), but this might damage your sound-card, the LEDs, or
both. So try at your own risk!
</p>
<p>
</p>
<p>
Another way to make the transmission more powerful is to use a small
speaker amplifier (5 watts or less), in this case a 5 watt resistor
should be used for safety. The volume should be adjusted so that the
amplifier outputs its full voltage without clipping.
</p>
<p>
</p>
<h2>Setting up:</h2>
<p>
</p>
<p>
Compile LIRC with the audio driver (not the IR diode or alsa ones) and
install it as usual. Connect the circuit to the sound-card and set the
volume to the maximum level. Start lircd, the -d flag can be used to
select the audio device and/or sample-rate, the syntax is
<code>api:device[@samplerate[:latency]]</code> or
<code>@samplerate[:latency]</code>.
</p>
<p>
</p>
<p>
Examples:
</p>
<p>
</p>
<code>
lircd -d ALSA:default<br>
lircd -d ALSA:default@48000<br>
lircd -d ALSA:default@48000:0.02<br>
lircd -d @48000<br>
lircd -d @48000:0.05<br>
</code><p>
</p>
<p>
Use irsend to test if it works.
</p>
<p>
</p>
<p>
A samplerate of 48 kHz is about the bare minimum needed to make a
modulated 19 khz sine, if your soundcard supports 96 kHz, you should
use that.
</p>
<p>
</p>
<h2>Choosing an audio device:</h2>
<p>
</p>
<p>
If you want to use a different soundcard, check the lircd log for lines
like these:
</p>
<p>
</p>
<p>
<code>
Found input device 0 OSS:/dev/dsp<br>
Found input device 1 OSS:/dev/dsp1<br>
Found input device 2 ALSA:SB Audigy 1
[SB0092]: ADC Capture/Standard PCM Playback (hw:0,0)<br>
Found input device 3 ALSA:SB Audigy 1
[SB0092]: Mic Capture (hw:0,1)<br>
Found input device 4 ALSA:SB Audigy 1
[SB0092]: Multichannel Capture/PT Playback (hw:0,2)<br>
Found input device 6 ALSA:HDA ATI SB: HDA Generic (hw:1,0)<br>
Found input device 13 ALSA:spdif<br>
Found input device 14 ALSA:default<br>
Found output device 0 OSS:/dev/dsp<br>
Found output device 1 OSS:/dev/dsp1<br>
Found output device 2 ALSA:SB Audigy 1
[SB0092]: ADC Capture/Standard PCM Playback (hw:0,0)<br>
Found output device 5 ALSA:SB Audigy 1
[SB0092]: Multichannel Playback (hw:0,3)<br>
Found output device 6 ALSA:HDA ATI SB: HDA Generic (hw:1,0)<br>
Found output device 7 ALSA:front<br>
Found output device 8 ALSA:rear<br>
Found output device 9 ALSA:center_lfe<br>
Found output device 10 ALSA:surround41<br>
Found output device 11 ALSA:surround50<br>
Found output device 12 ALSA:iec958<br>
Found output device 13 ALSA:spdif<br>
Found output device 14 ALSA:default<br>
Found output device 15 ALSA:dmix<br>
</code>
</p>
<p>
</p>
<p>
Now use the string for the device you want to use after the device number
as argument for the -d flag like this:<br>
<code>lircd -d "ALSA:HDA ATI SB: HDA Generic (hw:1,0)"</code><br>
</p>
<p>
</p>
<h2>Known issues:</h2>
<p>
</p>
<p>
The audio driver uses PortAudio to interface with the sound-card, there
seems to be a bug in some later versions that makes portaudio hang
completely, lircd becomes unresponsive and you have to kill it with
<code>killall -9 lircd</code>. To get around this use the PortAudio
stable release from December 7, 2007.<br>
</p>
<p>
</p>
<p>
Low latency values will cause dropped audio, when this happens lines like
this will be printed in the lircd log:<br>
</p>
<p>
</p>
<p>
<code>
WARNING: Input overflow ALSA:HDA ATI SB: HDA Generic (hw:1,0)@96000:0.02<br>
WARNING: Output underflow ALSA:HDA ATI SB: HDA Generic (hw:1,0)@96000:0.02<br>
</code>
</p>
<p>
</p>
<p>
To prevent this you can set the niceness of lircd to -20 or increase the latency.
</p>
<p>
</p>
<p class="footer">
[<A HREF="http://www.lirc.org/">LIRC homepage</A>]
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