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<!-- USRP_Source.mediawiki -->
<!-- ALSAPulseAudio.mediawiki -->
The USRP Source Block is used to stream samples from a USRP device (i.e. act as the receiver). 
== Working with ALSA and Pulse Audio ==


There are two methods of setting parameters and adjusting them while running.
* 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.  


; Variables and GUI Widgets
== Talking to ALSA ==
: Parameters may be set by a [[Variable]] block, a GUI Widget such as [[QT GUI Range]], or directly in the block Properties. 


; Messages
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.
: The input message port called "command" can be used to change frequency, gain, and other parameters via [[Message Passing]]. A complete list of message commands can be found in [https://www.gnuradio.org/doc/doxygen/page_uhd.html#uhd_command_syntax_cmds Common command keys].


: A common arrangement is to use the [[QT GUI Sink]]'s output message port to connect to the USRP Source's input port, so that when a user double-clicks within the GUI to change frequency, the change gets propagated to the USRP device. The [[#Example_Flowgraph|Example Flowgraph]] section shows this and other message-based methods.
PulseAudio itself also provides a &quot;fake&quot; ALSA device, so 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 internally capable of resampling, but the results aren't always predictable. For GNU Radio applications, it's often desirable to know the raw device.


== Parameters ==
You can obtain a list the playback devices using the <code>aplay</code> program.


; Output Type
* from a terminal window enter:
: This parameter controls the data type of the output stream in GNU Radio.
<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).


; Wire Format
For audio input devices, use:
: This parameter controls the form of the data over the bus/network. Complex bytes may be used to trade off precision for bandwidth. Not all formats are supported on all devices.


; Stream Args
<pre>arecord -L</pre>
: Optional arguments to be passed in the UHD streamer object. Streamer args is a list of key/value pairs; usage is determined by the implementation.  [https://files.ettus.com/manual/structuhd_1_1stream__args__t.html#a4463f2eec2cc7ee70f84baacbb26e1ef Docs for stream args]
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;to obtain a similar list.
 
; Stream Channels
: Optionally used to specify which channels are used, e.g. [0,1]
 
; Devices Address
: The device address is a delimited string used to locate UHD devices on your system. If left blank, the first UHD device found will be used. Use the device address to specify a specific device or list of devices.
 
    Examples: serial=12345678
              addr=192.168.10.2
              addr0=192.168.10.2, addr1=192.168.10.3
 
; Device Arguments
: Other various arguments that can be passed to the USRP Source, see [http://files.ettus.com/manual/page_configuration.html USRP Device Configuration]
 
; Sync
: Can be used to get USRP to attempt to sync to either PC's clock, or PPS signal if it exists.
 
; Clock Rate [Hz]
: The clock rate shouldn't be confused with sample rate, but they are related.  The B2X0 and E31X USRPs use a flexible clock rate that will either be equal to the requested sample rate, or a multiple of it.  Best left to default unless specific behavior is needed.
 
; Num Mboards
: Selects the number of USRP motherboards (i.e. physical USRP devices) in this device configuration.
 
; Mbx Clock Source
: Where the motherboard should sync its clock references. External refers to the 10 MHz input on the USRP.  O/B GPSDO is the optional onboard GPSDO module, which provides its own 10 MHz (and PPS) signals.
 
; Mbx Time Source
: Where the motherboard should sync its time references. External refers to the PPS input on the USRP.  O/B GPSDO is the optional onboard GPSDO module, which provides its own PPS (and 10 MHz) signals.
 
; Mbx Subdev Spec
: Each motherboard should have its own subdevice specification and all subdevice specifications should be the same length. Select the subdevice or subdevices for each channel using a markup string. The markup string consists of a list of dboard_slot:subdev_name pairs (one pair per channel). If left blank, the UHD will try to select the first subdevice on your system. See the application notes for further details.Single channel example: ":AB"
 
; Num Channels
: Selects the total number of channels in this multi-USRP configuration.  Ex: 4 motherboards with 2 channels per board = 8 channels total.
 
; Sample Rate
: The number of samples per second, which is equal to the bandwidth in Hz we wish to observe. The UHD device driver will try its best to match the requested sample rate. If the requested rate is not possible, the UHD block will print an error at runtime.
 
; Chx Center Frequency
: The center frequency is the overall frequency of the RF chain. 
 
; Chx Gain Value
: Value used for gain, which is either between 0 and the max gain for the USRP (usually around 70 to 90), when using the default "Absolute" Gain Type.  When using "Normalized" Gain Type, it will always be 0.0 to 1.0, where 1.0 will be mapped to the max gain of the USRP being used.
 
; Chx Gain Type
: Absolute (in dB) or Normalized (0 to 1)
 
; Chx Antenna
: For subdevices with only one antenna, this may be left blank. Otherwise, the user should specify one of the possible antenna choices. See the daughter-board application notes for the possible antenna choices.
 
; Chx Bandwidth
: The bandwidth used by the USRP's anti-aliasing filter.  To use the default bandwidth filter setting, this should be zero. Only certain subdevices have configurable bandwidth filters. See the daughterboard application notes for possible configurations.
 
; Chx Enable DC Offset Correction
: Attempts to remove the DC offset, i.e. the average value of a signal, which is something that will be very visible in the Frequency domain.
 
; Chx Enable IQ Imbalance Correction
: Attempts to correct any IQ imbalance, which is when there is a mismatch between the I and Q signal paths, typically causing a stretching effect to a constellation.
 
== Example Flowgraph ==
 
The example flowgraph below is taken from [[Guided_Tutorial_Hardware_Considerations]]. It shows how the message ports can be connected so that changing the frequency from within the [[QT GUI Sink]] (by double clicking on the X-axis) is propagated to the USRP. Even though it is hidden by the GUI Sink block, there is a connection from the output message port (freq) on the left to the input message port (freq) on the right. Variables are used to set the sample rate, RF gain, and initial tuning.
 
[[File:HW_tutorial_fg.png]]
 
[[File:HW_tutorial_freq.png|648px]]
<br><br>
<hr>
A significant drawback to the method above is that once a frequency is selected by double clicking, the frequency displayed in the GUI Range is no longer correct. To remedy this situation, messages can be used to change all of the run-time functions. Two approaches are presented here. The first is to use a [[QT GUI Message Edit Box]]. Not only can it generate commands, but it can display commands from other sources such as a QT GUI Sink. That way, the parameter (such as frequency) is always displayed correctly. The following flowgraph using this method not only sets the frequency, but also the RF gain.
 
[[File:USRP_Source_fg.png]]
<br><br>
<hr>
A second message-based method can be found in [https://github.com/gnuradio/gnuradio/blob/master/gr-uhd/examples/grc/uhd_msg_tune.grc UHD Message Tuner]. Each of the parameters is set using a [[QT GUI Entry]] block. Every two seconds, the [[Message Strobe]] block sends all of the parameters to the USRP. Note that the initial frequency and gain values must be set using [[Variable]] blocks.
 
[[File:Uhd_msg_tune_fg.png]]
 
[[Category:Block Docs]]

Revision as of 14:17, 4 July 2020

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 itself also provides a "fake" ALSA device, so 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 internally capable of resampling, but the results aren't always predictable. For GNU Radio applications, it's often desirable to know the raw device.

You can obtain a list the playback devices using the aplay program.

  • from a terminal window enter:

aplay -L

  • a long list of options will be displayed, such as:
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
...
  • find the entry such as:
hw:CARD=Generic,DEV=0
    HD-Audio Generic, ALC662 rev3 Analog
    Direct hardware device without any conversions

      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: 

arecord -L

      to obtain a similar list.

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