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Barry Duggan is a graduate of Georgia Tech in Electrical Engineering and is a career computer programmer specializing in real-time control, data communication systems, and email security. He has been an amateur radio operator since 1953, and now devotes most of his time to the GNU Radio project. He is a member of the GNU Radio General Assembly, the GRCon21 Organizing Committee, and is currently the Documentation Lead for GNURadio.
== Working with ALSA and Pulse Audio ==
 
 
 
* Sound cards don't support arbitrary sampling rates. If your audio is choppy, check the rate of your audio sink or source: 44100 Hz works under all audio cards known, 48000 Hz on most, others may not.
 
* Don't clip: The maximum amplitude in your signal '''must not''' exceed 1.0.
 
* Set "OK to Block" to "No" when the flowgraph is throttled by another hardware device.
 
 
 
== Talking to ALSA ==
 
 
 
The GNU Radio Audio Sink and Source blocks use ALSA (unless ALSA support was disabled during build time). ALSA has been the standard sound API under Linux for a decade or more, so basically all programs that produce Audio know how to deal with it. Alternately one can use a ''sound server'' like PulseAudio that takes care of dealing with all the low-level details for the application.
 
 
 
PulseAudio also provides a &quot;fake&quot; ALSA device to make ALSA applications talk to PulseAudio instead of directly with the hardware driver, allowing one central volume control, etc. However, PulseAudio's device isn't always perfect. PulseAudio is capable of resampling internally, but the results aren't always predictable. For GNU Radio applications, it's often desirable to use the raw device.
 
 
 
You can obtain a list the playback devices (for an Audio Sink) using the <code>aplay</code> program.
 
 
 
* from a terminal window enter:
 
<code>aplay -L</code>
 
* a long list of options will be displayed, such as:
 
<pre>
 
default
 
    Playback/recording through the PulseAudio sound server
 
null
 
    Discard all samples (playback) or generate zero samples (capture)
 
pulse
 
    PulseAudio Sound Server
 
hdmi:CARD=HDMI,DEV=0
 
    HDA ATI HDMI, HDMI 0
 
    HDMI Audio Output
 
hw:CARD=Generic,DEV=0
 
    HD-Audio Generic, ALC662 rev3 Analog
 
    Direct hardware device without any conversions
 
plughw:CARD=Generic,DEV=0
 
    HD-Audio Generic, ALC662 rev3 Analog
 
    Hardware device with all software conversions
 
...
 
</pre>
 
* find the entry such as:
 
<pre>
 
hw:CARD=Generic,DEV=0
 
    HD-Audio Generic, ALC662 rev3 Analog
 
    Direct hardware device without any conversions
 
</pre>
 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;in the list which matches your desired device.
 
* use the first line of that entry (e.g. "hw:CARD=Generic,DEV=0") as the device name (without the quotes).
 
 
 
For audio input devices, use:
 
 
 
<pre>arecord -L</pre>
 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;to obtain a similar list.
 
 
 
== Monitoring the audio input of your system with PulseAudio  ==
 
 
 
<p><b>IMPORTANT: this procedure only applies to an Audio Source block!</b></p>
 
 
 
PulseAudio has its own monitor &quot;ports&quot;. You can list all PulseAudio monitor sources by running:
 
 
 
<pre>pactl list|grep &quot;Monitor Source&quot;|sed 's/^[[:space:]]*Monitor Source: //g'</pre>
 
 
 
This will give you one or more lines containing something like
 
 
 
<pre>alsa_output.pci-0000_00_03.0.hdmi-stereo.monitor
 
alsa_output.pci-0000_00_1b.0.analog-stereo.monitor
 
alsa_output.pci-0000_06_00.1.hdmi-stereo.monitor</pre>
 
 
 
Select the right name; assuming we use the analog input, the second line would be the right.
 
 
 
=== Add ALSA Pseudodevice for monitor ===
 
 
 
Now, we need to edit (or create, if it doesn't already exist) <code>~/.asoundrc</code>.<br />
 
Add
 
<pre>
 
pcm.pulse_monitor {
 
    type pulse
 
    device alsa_output.pci-0000_00_1b.0.analog-stereo.monitor
 
}
 
 
 
ctl.pulse_monitor {
 
    type pulse
 
    device alsa_output.pci-0000_00_1b.0.analog-stereo.monitor
 
}
 
</pre>
 
of course, replacing the <code>device</code> name with the correct one from the previous step.
 
 
 
=== Using the newly created device ===
 
 
 
In the Audio Source block, use <code>pulse_monitor</code> as the device name:<br />
 
 
 
[[File:PulseAudio-ALSA-Monitoring.png|PulseAudio-ALSA-Monitoring.png]]
 

Latest revision as of 19:10, 22 July 2021

Barry Duggan is a graduate of Georgia Tech in Electrical Engineering and is a career computer programmer specializing in real-time control, data communication systems, and email security. He has been an amateur radio operator since 1953, and now devotes most of his time to the GNU Radio project. He is a member of the GNU Radio General Assembly, the GRCon21 Organizing Committee, and is currently the Documentation Lead for GNURadio.