SDRangel Developer's notes

General flow

A consistent flow of samples is grouped into a "device set" that combines one device plugin and many channel plugins sharing the same baseband. The following figure depicts the flow of samples taking place in one device set.

The critical parts of plugins run on their own thread. FIFOs at their boundaries ensure the independence of threads and thus effective multi-threading. The "DSP device engine" also runs on its own thread.

The DSP Device Engine is specialized for each category of device sets:

Source device sets (Rx): the DSPDeviceSourceEngine dispatches the baseband samples to the attached channels
Sink device sets (Tx): DSPDeviceSinkEngine combines the samples extracted from the attached channels into the baseband
MIMO device sets (v5 only): DSPDeviceMIMOEngine dispatches and combines samples coming from attached channels into a multi-stream baseband

The GUI and management modules of the plugins run on the main thread. Communication between threads is done via message queues.

Code structure and flow

Legend of figures:

Black arrows follow (loosely) the UML standard
- Hollow arrows represent inheritance oriented towards the parent class
- Filled diamond arrows represent composition oriented towards the composed class
- Hollow diamond arrows represent aggregation (reference) oriented towards the referenced class
Green arrows represent the direction of the Rx stream
Red arrows represent the direction of the Tx stream
Blue arrows represent the direction of the MIMO stream. The MIMO stream is a simultaneous (synchronous or asynchronous) stream composed of any number of input or output streams

Rx path

Since v4.12.2

The I/Q samples are collected from a physical device or UDP flow with a device plugin that derives from the DeviceSampleSource class.
These I/Q samples are downsampled in a bank of half-band decimators to reach the baseband sample rate and are fed into a BasebandSampleSink by the DSPDeviceSOourceEngine
The Xxx class delegates the processing of samples at baseband sample rate to the XxxBaseband class
The XxxBaseband class realizes the baseband processor described in the device set general flow. It has a DownChannelizerclass to decimate the samples down to the channel sample rate and send them to the XxxSink class. The DownChannelizer class contains a bank of half-band decimators that cascade the downsampling at the center, left or right half of the input band at each stage in order to fit the Rx sink bandwidth.
The xxxSink class realizes the Sink processor described in the device set general flow. It contains a NCO to adjust to the Rx plugin center frequency and a rational downsampler to adjust the sample rate at the output of the DownChannelizer (channel sample rate) to fit the exact rate required (sink sample rate).
The FileRecord class records the baseband stream.
The baseband stream is also sent to the spectrum display.

Tx path

Since v4.12.0

The I/Q samples are sent to a physical device or UDP flow with a device plugin that derives from the DeviceSampleSink class.
These I/Q samples have been upsampled in a bank of half-band interpolators from the baseband sample rate and have been taken from a BasebandSampleSource using the DSPDeviceSinkEngine
The Xxx class delegates the generation of samples at baseband sample rate to the XxxBaseband class
The XxxBaseband class realizes the baseband processor described in the device set general flow. It has an UpChannelizerclass to interpolate the samples generated by the XxxSource class up to the baseband sample rate. The UpChannelizer class contains a bank of half-band interpolators that cascade the upsampling to the center, left or right half of the output band at each stage in order to fit the Tx source bandwidth.
The XxxSource class realizes the Source processor described in the device set general flow. It contains a NCO to adjust to the Tx plugin center frequency and a rational upsampler to adjust the sample rate at the input of the UpChannelizer (channel sample rate) to fit the exact rate required (source sample rate).
The generated baseband stream is also sent to the spectrum display.

MIMO path (since v6)

Work in progress...

Device plugins development order

Generally one would start from an existing plugin and adapt it. In any case developing from bottom to top will allow you to validate the code compilation progressively. Thus this is the recommended order for the main source files:

start with the "...settings"
do the "..thread(s)"
do the Swagger .yaml file(s) and generate REST API code (you may as well do it after the "settings")
do the static "web...adapter"
do the "...input" or "...output" or "...mimo"
do the "...gui" with the ".ui" XML form file from QtCreator
end with the "...plugin"

Device sample source plugins

At present the following plugins are available:

AirspyXxx classes in plugins/samplesource/airspy: Interface with Airspy devices
AirspyHFXxx classes in plugins/samplesource/airspyhf: Interface with Airspy HF devices
BladeRF1InputXxx classes in plugins/samplesource/bladerf1input: Interface with BladeRF v1 devices
BladeRF2InputXxx classes in plugins/samplesource/bladerf2input: Interface with BladeRF v2 (or micro) devices
FCDProXxx classes in plugins/samplesource/fcdpro: Interface with Funcube Pro devices
FCDProPlusXxx classes in plugins/samplesource/fcdproplus: Interface with Funcube Pro+ devices
HackRFInputXxx classes in plugins/samplesource/hackrfinput: Interface with HackRF devices
KiwiSDRxxx classes in plugins/samplesource/kiwisdr: Interface with remote Kiwi SDR devices
LimeSDRInputxxx classes in plugins/samplesource/limesdrinput: Interface with Lime SDR devices
Perseusxxx classes in plugins/samplesource/perseus: Interface with Perseus devices
PlutoSDRInputxxx classes in plugins/samplesource/plutosdr: Interface with Pluto SDR devices
RTLSDRXxx classes in plugins/samplesource/rtlsdr: Interface with RTL-SDR devices
SDRPlayXxx classes in plugins/samplesource/sdrplay: Interface with SDRplay RSP1 devices
XTRXInputXxx classes in plugins/samplesource/xtrxinput: Interface with XTRX devices
RemoteInput class in plugins/samplesource/remoteinput: Special interface collecting I/Q samples from an UDP flow sent by a remote instance of SDRangel using Remote sink channel.
LocalInput class in plugins/samplesource/localinput: Special interface collecting I/Q samples from another device set having the LocalInput as its device.
FileInput classes in plugins/samplesource/fileinput: Special interface reading I/Q samples from a file directly into the baseband skipping the downsampling block
SoapySDRInput classes in plugins/samplesource/soapysdrinput: Special interface working with devices interfaced with SoapySDR library
TestSource classes in plugins/samplesource/testsource: Special device to generate samples internally for mocking a device source

Device sample sink plugins

At present the following plugins are available:

BladeRF1OutputXxx classes in plugins/samplesource/bladerf1output: Interface with BladeRF v1 devices
BladeRF2OutputXxx classes in plugins/samplesource/bladerf2output: Interface with BladeRF v2 (or micro) devices
FileSink classes in plugins/samplesink/filesink: Special interface writing baseband I/Q samples to a file skipping the final upsampling block
HackRFOutputXxx classes in plugins/samplesource/hackrfoutput: Interface with HackRF devices
LimeSDROutputxxx classes in plugins/samplesource/limesdroutput: Interface with Lime SDR devices
LocalOutput class in plugins/samplesource/localoutput: Special interface getting I/Q samples from another device set having the LocalOutput as its device.
PlutoSDROutputxxx classes in plugins/samplesource/plutosdroutput: Interface with Pluto SDR devices
RemoteOutput class in plugins/samplesource/remoteoutput: Special interface getting I/Q samples from an UDP flow sent by a remote instance of SDRangel using Remote source channel.
SoapySDROutput classes in plugins/samplesource/soapysdroutput: Special interface working with devices interfaced with SoapySDR library
TestSink classes in plugins/samplesource/testsink: Special device to direct generated samples to a spectrum display
XTRXOutputXxx classes in plugins/samplesource/xtrxoutput: Interface with XTRX devices

Channel receiver (Rx) plugins

At present the following plugins are available:

ChannelAnalyzerXxx classes in plugins/channelrx/chanalyzer: Signal analysis tool pretty much like a DSA/DSO signal analyzer like the venerable HP 4406A (although still far from it!)
AMDemodXxx classes in plugins/channelrx/demodam: AM demodulator with audio output
ATVDemodXxx classes in plugins/channelrx/demodatv: Demodulator and visualization of analog TV signals mostly those used in Amateur TeleVision (ATV)
BFMDemodXxx classes in plugins/channelrx/demodbfm: Broadcast FM demodulator with audio mono/stereo output and RDS
DATVDemodXxx classes in plugins/channelrx/demoddatv: Demodulator and visualization of digital TV signals mostly those used in Digital Amateur TeleVision (DATV)
DSDDemodXxx classes in plugins/channelrx/demoddsd: Digital Speech demodulator/decoder built on top of the DSDcc library. Produces audio output and some communication data from various digital voice standards: DMR, dPMR, D-Star, Yaesu System Fusion (YSF).
FreeDVDemodXxx classes in plugins/channelrx/demodfreedv: Demodulator and decoder of signals using the Free DV digital modes
LoraDemodXxx classes in plugins/channelrx/demodlora: Decodes LoRa transmissions. This is legacy code that is not maintained so it does not work most probably.
NFMDemodXxx classes in plugins/channelrx/demodnfm: Narrowband FM demodulator with audio output.
SSBDemodXxx classes in plugins/channelrx/demodssb: SSB/DSB/CW demodulator with audio output.
WFMDemodXxx classes in plugins/channelrx/demodwfm: Wideband FM demodulator with audio output. This is a basic demodulator not for broadcast reception.
FreqTrackerXxx classes in plugins/channelrx/freqtracker: Plugin that follows the frequency of the input signal using a Frequency Locked Loop or a Phase Locked Loop for signals with phase modulation (BPSK, QPSK, ...). To be used in cooperation with an external script to control the frequency of other plugins via REST API.
LocalSinkXxx classes in plugins/channelrx/localsink: Sends channel I/Q to another source device set that has a LocalInput source for its device.
RemoteSinkXxx classes in plugins/channelrx/remotesink: Sends channel I/Q samples to another instance of SDRangel via UDP. The RemoteInput plugin is used to receive the samples.
UDPSinkXxx classes in plugins/channelrx/udpsink: Sends channel I/Q or FM demodulated samples via UDP

Channel transmitter (Tx) plugins

At present the following plugins are available:

FileSourceXxx classes in plugins/channeltx/filesource: takes I/Q samples from a file saved in .sdriq format
LocalSourceXxx classes in plugins/channeltx/localource: takes I/Q samples from another sink device set that has a LocalOutput sink for its device
AMMmodXxx classes in plugins/channeltx/modam: AM modulator
ATVModXxx classes in plugins/channeltx/modatv: Modulator for analog TV signals mostly those used in Amateur TeleVision (ATV)
FreeDVModXxx classes in plugins/channeltx/modfreedv: Coder and modulator for signals using the Free DV digital modes
NFMModXxx classes in plugins/channeltx/modnfm: Narrowband FM modulator
SSBModXxx classes in plugins/channeltx/modssb: Single Side Band modulator. Can be used for CW.
WFMModXxx classes in plugins/channeltx/modwfm: Wideband FM modulator (not broadcast quality)
RemoteSourceXxx classes in plugins/channeltx/remotesource: takes I/Q samples from another SDRangel instance using the RemoteOutput plugin to send samples over UDP.
UDPSourceXxx classes in plugins/channeltx/udpsoutce: takes channel I/Q or FM demodulated samples sent via UDP as raw samples (not SDRangel UDP format)

Source tree structure

This describes only the main components

httpserver

The HTTP server used for REST API communication and internal web server

logging

Classes used to log messages to console or file

qrtplib

Rewrite of the jrtplib library for the Qt environment. It handles RTP over UDP communication.

rescuesdriq

Script written in Go language to process or restore .sdriq files so that their header is compatible with the present version of SDRangel

scriptsapi

Collection of Python scripts that use the REST API of SDRangel to perform common tasks

sdrbase

The sdrbase subdirectory contain the common core components. It is further broken down in subdirectories corresponding to a specific area:

ambe contains classes to interface with AMBE digital voice processors
audio contains the interface with the audio device(s)
channel contains the channel API interface and related classes
commands classes to deal with the commands
device contains the device API interface and related classes
dsp contains the common blocks for Digital Signal Processing like filters, scope and spectrum analyzer internals
plugin contains the common blocks for managing plugins
resources contains Qt resources including web server static files
settings contains components to manage presets and preferences
util contains common utilities such as the message queue
webapi contains common classes to handle web REST API

sdrgui

The sdrbase subdirectory contain the common GUI components. It is further broken down in subdirectories corresponding to a specific area:

device contains the UI parts of the Device Sets
dsp contains DSP related GUI specific classes including the scope and spectrum interfaces
gui contains the GUI core components
resources contains Qt resources with GUI artifacts (icons...)
soapygui contains GUI components specific to the SoapySDR plugins
webapi contains GUI related classes related to the web REST API

This is also where the MainWindow class resides

sdrsrv

The sdrsrv subdirectory contains classes specific to the server variant of SDRangel

swagger

The swagger sudirectory contains classes and resources related to the REST API interface of SDRangel. As the name suggests the REST API is designed using Swagger.

plugins

The plugins subdirectory contains the associated plugins used to manage devices and channel components. Naming convention of various items depend on the usage and Rx (reception side) or Tx (transmission side) affinity. MIMO is a work in progress and not described here yet.

Receiver functions (Rx):
- samplesource: Device managers:
  - xxx : Device manager (e.g. xxx = airspy)
    - xxxinput.h/cpp : Device interface
    - xxxgui.h/cpp : GUI
    - xxxplugin.h/cpp : Plugin interface
    - xxxsettings.h/cpp : Configuration manager
    - xxxthread.h/cpp : Reading samples
    - xxxwebapiadapter.h/cpp: Detached web API functions to handle settings via REST API
- channelrx: Channel handlers:
  - demodxxx : Demodulator internal handler (e.g xxx = demodam)
    - xxxdemod.h/cpp : Demodulator core
    - xxxdemodgui.h/cpp : Demodulator GUI
    - xxxdemodplugin.h/cpp : Plugin interface
    - xxxdemodwebapiadapter.h/cpp: Detached web API functions to handle settings via REST API
  - xxxanalyzer : Analyzer internal handler (e.g xxx = channel)
    - xxxanalyzer.h/cpp : Analyzer core
    - xxxanalyzergui.h/cpp : Analyzer GUI
    - xxxanalyzerplugin.h/cpp : Analyzer plugin manager
    - xxxanalyzerwebapiadapter.h/cpp: Detached web API functions to handle settings via REST API
  - xxxsink : Interface to the outside (e.g xxx = udp):
    - xxxsink.h/cpp : Interface core
    - xxxsinkgui.h/cpp : Interface GUI
    - xxxsinkplugin/h/cpp : Interface plugin manager
    - xxxwebapiadapter.h/cpp: Detached web API functions to handle settings via REST API
  - xxx : Not conventinally named module (ex: freqtracker):
    - xxx.h/cpp : Module manager
    - xxxgui.h/cpp : Module GUI
    - xxxplugin/h/cpp : Module plugin manager
    - xxxwebapiadapter.h/cpp: Detached web API functions to handle settings via REST API
Transmitter functions (Tx):
- samplesink: Device managers:
  - xxx : Device manager (e.g. xxx = bladerf1)
    - xxxsinkoutput.h/cpp : Device interface
    - xxxsinkgui.h/cpp : GUI
    - xxxsinkplugin.h/cpp : Plugin interface
    - xxxsinksettings.h/cpp : Configuration manager
    - xxxsinkthread.h/cpp : Writing samples
    - xxxsinkwebapiadapter.h/cpp: Detached web API functions to handle settings via REST API
- channeltx: Channel handlers:
  - modxxx : Modulator internal handler (e.g xxx = modam)
    - xxxmod.h/cpp : Modulator manager
    - xxxmodbaseband.h/cpp: Baseband generator
    - xxxmodsource.h/cpp: Source generator
    - xxxmodgui.h/cpp : Modulator GUI
    - xxxmodplugin.h/cpp : Plugin interface
    - xxxmodsettings.h/cpp : Configuration manager
    - xxxmodwebapiadapter.h/cpp: Detached web API functions to handle settings via REST API
  - xxxsource : Interface to the outside (e.g xxx = udp):
    - xxxsource.h/cpp : Interface manager
    - xxxsourcebaseband.h/cpp: Baseband generator
    - xxxsourcesource.h/cpp: Source generator
    - xxxsourcegui.h/cpp : Interface GUI
    - xxxsourceplugin/h/cpp : Interface plugin manager
    - xxxsourcesettings.h/cpp : Configuration manager
    - xxxsourcewebapiadapter.h/cpp: Detached web API functions to handle settings via REST API

Device interface and GUI lifecycle

To be reviewed ...

Overview

Since version 3.4.0 the opening and closing of the physical device has been split off the start and stop methods of the device interface. The opening and closing of the physical device is handled in private methods called from the constructor and destructor of the device interface respectively.

The device interface is itself created in the constructor of the GUI and is deleted at the destruction of the GUI.

The lifecycle of the GUI is controlled from the "Sampling Device Control" device selector in the main window using the plugin manager. When there is a change in the hardware device selection (you can also re-cycle the same device by clicking again on this button) validated by the confirmation button (check sign icon). the following steps are executed in sequence:

Stop streaming
Delete the current GUI this will in turn delete the device interface and always close the physical device unless the physical device has a SISO or MIMO architecture (more on that later)
Remove the current device API from the relevant buddies lists. Buddies list are effective only for physical devices with SISO or MIMO architecture (more on that later)
Create the new device API
Add the new device API to the relevant devices APIs buddies list
Creates the new GUI and hence new device interface. This will always open the physical device unless the physical device has a SISO or MIMO architecture

Here is the relevant part of the code (source side) in the MainWindow::on_sampleSource_confirmClicked method:

deviceUI->m_deviceSourceAPI->stopAcquisition();
deviceUI->m_deviceSourceAPI->setSampleSourcePluginInstanceUI(0); // deletes old UI and input object
deviceUI->m_deviceSourceAPI->clearBuddiesLists(); // clear old API buddies lists
m_pluginManager->selectSampleSourceByDevice(devicePtr, deviceUI->m_deviceSourceAPI); // sets the new API
// add to buddies list
... here it looks for open tabs with the same physical device and adds the deviceUI->m_deviceSourceAPI to the corresponding m_deviceSourceAPI buddies list ...
// constructs new GUI and input object
QWidget *gui;
PluginManager::SamplingDevice *sampleSourceDevice = (PluginManager::SamplingDevice *) devicePtr; // lightweight representation of the device
PluginInstanceUI *pluginUI = sampleSourceDevice->m_plugin->createSampleSourcePluginInstanceUI(sampleSourceDevice->m_deviceId, &gui, deviceUI->m_deviceSourceAPI);
deviceUI->m_deviceSourceAPI->setSampleSourcePluginInstanceUI(pluginUI);
deviceUI->m_deviceSourceAPI->setInputGUI(gui, sampleSourceDevice->m_displayName);

SISO and MIMO devices support

Some SDR hardware have Rx/Tx capabilities in a SISO or MIMO configuration. That is a physical device can handle more than one I/Q sample flow in either direction. Such devices supported by SDRangel are the following:

SISO full duplex: BladeRF
SISO half duplex: HackRF
MIMO: LimeSDR

Note that the following would also work for multiple sample channels Rx or Tx only devices but none exists or is supported at this moment.

In SDRangel there is a complete receiver or transmitter per I/Q sample flow. These transmitters and receivers are visually represented by the Rn and Tn tabs in the main window. They are created and disposed in the way explained in the previous paragraph using the source or sink selection confirmation button. In fact it acts as if each receiver or transmitter was controlled independently. In single input or single output (none at the moment) devices this is a true independence but with SISO or MIMO devices this cannot be the case and although each receiver or transmitter looks like it was handled independently there are things in common that have to be taken into account. For example in all cases the device handle should be unique and opening and closing the device has to be done only once per physical device usage cycle.

This is where the "buddies list" come into play. Devices exhibit a generic interface in the form of the DeviceAPI class. Through this API some information and control can flow between receivers and transmitters. The point is that all receivers and/or transmitters pertaining to the same physical device must "know" each other in order to be able to exchange information or control each other. For this purpose the DeviceAPI maintain a list of DeviceAPI siblings called "buddies". Thus any multi flow Rx/Tx configuration can be handled.

The exact behaviour of these coupled receivers and/or transmitters is dependent on the hardware therefore a generic pointer attached to the API can be used to convey any kind of class or structure tailored for the exact hardware use case. Through this structure the state of the receiver or transmitter can be exposed therefore there is one structure per receiver and transmitter in the device interface. This structure may contain pointers to common areas (dynamically allocated) related to the physical device. One such "area" is the device handle which is present in all cases.

Normally the first buddy would create the common areas (through new) and the last would delete them (through delete) and the individual structure (superstructure) would be on the stack of each buddy. Thus by copying this superstructure a buddy would gain access to common areas from another (already present) buddy along with static information from the other buddy (such as which hardware Rx or Tx channel it uses in a MIMO architecture). Exchange of dynamic information between buddies is done using message passing.

The degree of entanglement between the different coupled flows in a single hardware can be very different:

BladeRF: a single Rx and Tx in SISO configuration loosely coupled:
- independent Rx and Tx sample rates
- independent Rx and Tx center frequencies
- independent Gain, bandwidth, ...
- only the device handle and indication of the presence of the XB200 accessory board is common
HackRF: this is a half duplex device. Rx and Tx might appear as tightly coupled but since you can use only one or the other then in fact you can control them differently as this is done in sequence. In fact only the common device handle has to be taken care of
LimeSDR: dual Rx and dual Tx in MIMO configuration. This implies tightly coupling between receivers on one part and transmitter on the other. But in the case of the LimeSDR (LMS7002M chip) there is even a tight coupling between Rx and Tx parts since the ADC/DAC clock is in common which means both have the same base sample rate that can only be varied by the mean of the hardware decimators and interpolators. This means:
- Rx share the same sample rate, hardware decimation factor and center frequency
- Tx share the same sample rate, hardware interpolation factor and center frequency
- Rx and Tx share the same base sample rate (decimation/interpolation apart)
- Independent Rx and Tx center frequencies
- Independent gains, bandwidths, etc... per Rx or Tx channel

The openDevice and closeDevice methods of the device interface are designed specifically for each physical device type in order to manage the common resources appropriately at receiver or transmitter construction and destruction. For example opening and closing the device and the related device handle is always done this way:

openDevice:
- check if there is any "buddy" present in one of the lists
- if there is a buddy then grab the device handle and use it for next operations
- if there is no buddy open the device and store the handle in the common area so it will be visible for future buddies
closeDevice:
- if there are no buddies then effectively close the device else just zero out the own copy of the device handle

Exchange of dynamic information when necessary such as sample rate or center frequency is done by message passing between buddies.

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