OP25j

= Signal Scope =

== Installation ==

Before running the scope app, check that the following standard pre-requisite packages are also installed:

 * GNU Radio

 * The op25 blocks

 * Frank's gr-fsk4 block

 * The op25 repeater block

The Python Numeric package is now also required.  If you're running Debian or Ubuntu:

{{{

apt-get install python-numeric

}}}

Other than these pre-reqs, no special setup or installation is needed.

== Targets ==

 * Choice of USRP, discriminator-tap, or "audio-IF" hardware modes

 * Supports either GL or non-GL display mode

 * Both FSK4 and CQPSK demodulation modes supported

 * Lower and middle range PC hardware, not just the latest

== Running the Program ==

There are three overall options or modes depending on your hardware; these are

 * USRP

 * External receiver with discriminator tap connected to sound card

 * External receiver with IF in the audio sound card range (e.g., 24 KHz), referred to as "audio IF"

=== Running with the disc-tap option ===

The signal scope does not require the USRP.  If you have a discriminator-tapped receiver, use the "-a" option:

{{{

./scope.py -a -v 10 -g 50

}}}

=== Running with the USRP ===

It's helpful to find out the current calibration error beforehand (I use kalibrate), for example, +1234 hertz:

{{{

./scope.py -f 412.34e6 -RA -g 65 -c 1234 -v 10

}}}

=== Running in the Audio IF mode ===

Receivers equipped with an IF output in the sound card range can be used.  This is known as "audio IF" mode.

A soundcard sampling rate of 96K is used and the IF frequency (typically 24 KHz) is given using the {{{--calibration}}} parameter:

{{{

./scope.py -A -c 24e3 -g 50 -v 10

}}}

== Feature overview ==

 * Spectrum plot

 * Baseband oscilloscope

 * Eye Pattern Diagram (Datascope) display supporting several standard symbol rates

 * Constellation Diagrams

 * Demodulated Symbol Output

 * Correlation (including Fast Auto-Correlation)

 * Direct-frequency entry, signal gain and fine-tuning controls

 * User-selectable demodulator (FSK4 or QPSK)

In the baseband AF (disc. tap) mode, several program functions are disabled (spectrum FFT, constellation diagram, PSK demod, FAC and iDEN correlation)

because these functions require direct access to the signal with no demodulation.

In all modes, the {{{--wireshark}}} option is used to write received P25 packet data to Wireshark.

== Program Options ==

Here is a full list of program options:

{{{

Usage: scope.py [options]

Options:

  -h, --help            show this help message and exit

  -a, --audio           use direct audio input

  -A, --audio-if        soundcard IF mode (use --calibration to set IF freq)

  -I AUDIO_INPUT, --audio-input=AUDIO_INPUT

                        pcm input device name.  E.g., hw:0,0 or /dev/dsp

  -i INPUT, --input=INPUT

                        input file name

  -b Hz, --excess-bw=Hz

                        for RRC filter

  -c Hz, --calibration=Hz

                        USRP offset or audio IF frequency

  -C Hz, --costas-alpha=Hz

                        value of alpha for Costas loop

  -f Hz, --frequency=Hz

                        USRP center frequency

  -d DECIM, --decim=DECIM

                        source decimation factor

  -v VERBOSITY, --verbosity=VERBOSITY

                        message debug level

  -p, --pause           block on startup

  -w, --wireshark       output data to Wireshark

  -W WIRESHARK_HOST, --wireshark-host=WIRESHARK_HOST

                        Wireshark host

  -R RX_SUBDEV_SPEC, --rx-subdev-spec=RX_SUBDEV_SPEC

                        select USRP Rx side A or B (default=A)

  -g GAIN, --gain=GAIN  set USRP gain in dB (default is midpoint) or set audio

                        gain

  -G GAIN_MU, --gain-mu=GAIN_MU

                        gardner gain

}}}

== Spectrum Display ==

[[Image(a.png)]]

The controls arranged along the bottom of the page are:

 * Frequency: to retune, type the new frequency here and press ENTER

 * Signal Gain: adjusts the baseband (demodulated) signal level

 * Fine Tune: adjusts tuning frequency over +/- 3000 Hz range

 * Demod: Selects demodulator (currently used in Demodulated Symbols only)

Except for the signal gain control, these controls are only available in USRP RX mode.

== Eye Pattern Diagrams ==

The scope input source can be connected either before or after the symbol filter using the Viewpoint toggle.

Also the proper speed must be selected from the available options.

[[Image(b.png)]]

== Constellation Diagram ==

[[Image(f.png)]]

The signal scope also features an angular population graph (shown above) in addition to the traditional constellation display.

In this mode the symbol magnitude (distance from center) is discarded.  Instead the circle is sliced into segments and a count

of symbols found in each segment is plotted.  This is similar to a histogram except that a straight line is drawn between each

result, and that the results are arranged in polar form instead of rectangular form.

With this display, the zone at the exact center of the plot can be used precisely to measure the degree of separation or margin

between the symbol decision points.  The plots below illustrate the difference, with poorer convergence showing on the left image:

[[Image(mhp7a.png)]][[Image(c.png)]]

The two-color mode is used in these images, providing natural relief to highlight the distinctive feature of π/4 DQPSK in which 

successive symbols are chosen from two distinct constellations (each containing four possible symbol values) separated by 45°

== Demodulated Symbols ==

[[Image(d.png)]]

== Correlation ==

[[Image(e.png)]]

Cross correlation allows rapid identification of signals with known characteristics.  Frame Sync (FS) signatures of several commonly

used radio systems are included.

By convention correlation results are usually displayed using positive correlation peaks only.  In this system

however it is possible (and legal) for negative correlation products to be produced.  This can occur for two wholly separate reasons:

 * If the hardware polarity is inverted 

 * When the FS symbols are purposely inverted as an integral part of protocol processing (commonly used in certain protocols but not used in P25)

The first case commonly happens when using the disc-tap method of hardware connection, because the actual polarity of the signal seems to vary 

randomly among different sound cards and receivers.  In one actual case, two PC's of the same PC brand bought from the same store had opposite polarity.

The P25 software framer automatically detects the proper polarity and issues a message if negative polarity data is received:

{{{

Reversed FS polarity detected - autocorrecting

}}}

The automatic correction applies only to software framing and doesn't help with correlation.  For correct results for both software framing and

correlation, you should correct the polarity reversal problem at its source; this is done using negative values for the {{{--gain}}} parameter at 

program start time:

{{{

./scope.py -a -v 10 -g -50

}}}

The second cause of negative correlation peaks is that some protocols (although not P25) make use of both normal- and inverted-polarity FS 

sequences as a standard part of their processing.   Instead of clogging the GUI menu with several choices that are merely inverses of others,

just for the sake of always having positive-peaked correlations, we allow the correlation graph to reflect the natural polarity of the data.

Thus both + and - peaks are shown, allowing quick diagnosis of incorrect hardware polarity (see above), and allowing identification of the

particular family and sub-protocol in use.

== Auto Correlation ==

Also included is Frank's Fast Auto Correlation (fac):

[[Image(g.png)]]

For further details about Fast Auto Correlation refer to Frank's page at http://sites.google.com/site/radiorausch/

== BUGS ==

Possibly bugs exist, here are a few of the known ones as of this writing

 * Symbol filters totally brain damaged (need separate filters for each speed)

 * When switching modes using the notebook tabs, leftover data from before may appear momentarily

 * Highest and lowest speeds are not well tuned resulting either in sluggish updates or CPU exhaustion

== CREDITS ==

I ripped off the "tabbed notebook" theme (and code) from Stevie; it already had the spectrum, baseband, and decoded symbol displays. I added the data scope, constellation scope, and cross correlation features.

The Fast Auto Correlation (fac) code came from Radiorausch.

Special thanks to Mossmann for leaving one commented-out "print" statement in c4fm-decode.py. Of course it was tempting to wonder "what would happen" if that statement were un-commented. The final result? The cross-correlation feature.

Special thanks also to GPL(v3), for encouraging these mashups.