OP25j

Hardware

There are a number of alternatives for hardware to capture a P25 baseband signal. These include:

• USRP (using TVRX daughterboard and appropriate antenna).
• SSRP with an appropriate radio front-end and antenna.
• Custom interface to existing scanner using soundcard for quadrature sampling.
• Custom PSK to serial interface hardware.

For those without any hardware it is possible to investigate using signal samples which are available on the DemodulatorPage.

=== USRP ===
Initial work is proceeding using the USRP which is available from [[http://ettus.com Ettus Research LLC]]. This is a relatively low-cost software radio (< 1000USD) that works with the GNURadio framework. The example USRP shown below shows a fully populated USRP with TX and RX capabilities as well as identifying the major components:

Image(usrp.jpg)

Figure: the USRP (used with permission, source: [http://kd7lmo.net KD7LMO])

The USRP is capable of wideband reception and requires no special-purpose hardware as everything is done in software. A USRP with the TVRX daughterboard (not illustrated) and an appropriate antenna can be used out-of-the-box or the BasicRX card combined with an appropriate RF interface.

=== SSRP ===
The SSRP is a promising alternative to the USRP from [[http://oscar.dcarr.org/ssrp/index.php David Carr]] and is significantly cheaper. There are presently no suitable radio front ends, the sampling rate is reduced in comparison to the USRP and it is not nearly so mature. Nevertheless it may be a good bet in future for mobile use or where the expense of the USRP cannot be justified.

=== Sound Card Sampling ===
The ADC built into the soundcard of many home computers is capable of sampling at rates of up to 200KHz. Using the soundcard to quadrature sample the discriminator output of a receiver is a cheap and cheerful solution. Not all scanners provide a discriminator output but there are scanner modifications that that can be made when this is not provided as standard. This approach may require some custom interface circuitry and the filtering within the receiver will probably limit reception to a single channel.

=== Custom hardware ===
Another approach for interfacing to existing scanners is to feed the discriminator output to a custom hardware circuit. Some are available built and tested from a variety of sources such as [[http://www.discriminator.nl/index-en.html discriminator.nl]]. This approach is low cost and pretty straightforward although limited to reception of a single-channel at a time.

==== Discriminator Tap with Sound Card ====
The scanner's discriminator tap output is fed directly into the sound card's line input, instead of into a custom external hardware circuit. It should be possible to run two radios simultaneously (one each on the left and right audio channels), although this configuration has not been tested. On my radio I found that adding a 10K-ohm resistor in series with the audio signal line was desirable. It has been rumored that adding a series capacitor in place of or in addition to the resistor is a bad idea. Some writers also suggest adding a small audio buffer amplifier stage, although I have found that unnecessary. A simple audio cable with standard stereo phone plugs on each end was used to complete the connection.

The author's system uses a standard sound card with the sampling rate set to 48K. Most sound cards should support at least this rate.

In terms of GNU Radio software blocks used to demodulate and decode P25 signals, input discriminator tap (baseband) audio is sampled and digitized and read using gnuradio audio.source(). The FM signal has already been demodulated (by hardware inside the radio), so the quadrature_demod_cf() (and its predecessor) signal processing blocks aren't needed; the input audio signal is fed directly to the symbol filter preceding the P25 FSK4 demodulator.

The "voltage level" of baseband audio signals input to the symbol filter block is critical. Amplification or attenuation (i.e., multiplication by a constant value greater than or less than unity, respectively) must be applied to bring the signal into the proper "voltage" range such that the symbol decision points appear at the standard (-3, -1, +1, +3) levels.

Unfortunately, the proper multiplication factor may vary. One method to get correct reception is to use your audio mixer application to set the proper signal level while viewing the symbol scope output. Tune the radio to a strong P25 signal. Starting with the volume control all the way down, the scope should display a single horizontal line. The scope should be set to use the plus "+" format. Slowly raise the audio level until the display breaks up into a band, which may weave up and down. Continue to increase the audio level until four distinct bands begin to appear, and the weaving should cease. The signal will overload eventually if you increase it too far. Adjust for best reception.

Another potential problem occurs due to inaccuracies in sound card sampling rates, and less severely due to variations in the symbol rates among different transmitter stations. This slop should be fully handled by the decoder used in this project, which applies phase and frequency correction; but some other decoders require a manual adjustment to ensure exact timing.