Receiver Summary



The program was recently redesigned to support more than one VCOs or channel per receiver. Most receiver models can tune to one channel at a time so the terms VCO and receiver are nearly interchangable.

Discriminator Audio

This is unfiltered audio produced by the narrow FM demodulation circuitry (the so-called FM discriminator) inside a traditional superhererodyne radio receiver. The audio is filtered before being amplified and delivered to headphones or speaker. Filtering alters the shape of the waveform. This is bad for decoding. You want a radio that is tapped to provide access to the unfiltered audio.

Signal Receiver

A signal receiver is just a manually tuned receiver that is either manufactured with a discriminator tap or the tap was added as a modification. Signal receivers are useful for signal decoding only. They don't need to support computer control (more on that later) or trunking. Prior to the Realtek USB devices (see below), these were the cheapest and probably most common type of radio used for decoding. Program the control channels of your favorite trunked systems into the radio. Select the system you want to monitor by dialing in an active control channel on the radio's front panel. The decoding software won't know what system or frequency you've selected so, in some instances, the software may prompt you for additional information (like control channel frequency).

Tip: Trunked systems often rotate control channels. After extended monitoring, your decoding session suddenly goes dead. Program the site's control channels into a separate bank (no other channels). Enable only that bank and set the radio to scan. It will lock onto the active control channel providing nearly uninterrupted signal to the decoding application.

The signal receiver connects to the computer through a shielded audio cable plugged into the computer's line input or microphone input. Configuring the receiver in Unitrunker means selecting the correct audio recording device. The sample rate is the number of measurements taken in one second of recording. Set the sample rate to 44100, 48000 or 96000. The sample size is the number of bits uses to represent a waveform at a given point in time. 8, 16, 24, and 32 are supported. 16 bit samples are recommended. Unless you are using a stereo splitter for left/right channels, use mono. However, if needed (eg. two signal receivers on the same stereo input), left & right channel inputs are accepted.

Before configuring a signal receiver in Unitrunker, fire up UniScope and follow these instructions to validate your discriminator tap, cable, and recording device selection. You're dealing with discriminator audio which means resistors, capacitors, adjusting recording levels, and recording device "enhancements" that disrupt decoding (like microphone AGC). UniScope gives a clear visual presentation of the signal so you can confidently move on to decoding.

You will be able to hear the control channel noise through your computer's speakers. Use the Mute option.

When reporting problems to the community, you may be asked for a decode log.

Control Receiver

This class of receivers accept some level of control from a computer - usually through a serial or USB port. By control, I mean the radio's operation (such as frequency and mode of demodulation) can be directed by a computer. Some models can be programmed but not controlled by a computer. Control receivers are useful for voice following. The computer can steer the radio to follow voice calls as they move from one channel to the next on a trunked system. Audio comes from the radio's speaker or headphone.

Port configuration for a control receiver means selecting a serial port (either real or virtual) and setting baud rate, data bits, stop bits and parity. Most later model Uniden, GRE, Radio Shack and Whistler models like 115200 baud, 8 bits, 1 stop bit and no parity. Some of the CI-V compatible radios like using 2 stop bits. Consult your model's user guide.

When reporting problems to the community, you may be asked for a tuner log.

Lots of radios accept computer control. Manufacturers announce new models yearly so this is an ongoing area of expansion for the program.

Both Receiver

This is a control receiver that also has a discriminator tap. A few models are produced with discriminator audio. Most must be modified to bring the unfiltered audio outside the radio. "Both" receivers can be used as a signal source for decoding or as a control receiver for voice following. For this reason, when configuring your receiver in Unitrunker, you must specify the role - either voice or signal. You can of course change the role later. Recall that trunked systems often rotate control channels. The program can direct the receiver to chase the control channel. For this to work, the monitored site must have all control channel frequencies entered. The receiver must have the "Chase" option selected. Audio may come from the radio's speaker or headphone. It can also be routed through the host PC's speaker. When operating in a voice role, the audio may be split into two paths: one path for analog voice calls and a second path for digital calls. The digital path is unfiltered discriminator audio meant to be piped to a third party decoder.

In theory, the computer should be able to determine the frequency the receiver is monitoring to avoid prompting the user. This generally expedites the process of matching an incoming signal to previously logged site and system data.

"Both" receivers support two types of logging.

All of the above discussion on signal receivers applies to "both" receivers when operating in the signal decoding mode. Likewise, all of the above discussion on control receivers applies to "both" receivers operating in the voice or signal role.

The iCOM PCR-1500 and PCR-2500 can provide audio over the USB cable. The audio appears on a virtual audio port. Select this virtual port as your audio input if you want speech audio. For discriminator audio, you'll need to patch the "packet out" audio from the radio into a microphone or line input and select that for your input.

Inline Receiver

This class of receiver streams decoded information through the same communication channel (serial or USB) used for computer programming or control. Audio must come from the radio's speaker or headphones.

Decoding takes place inside the radio so you are limited to protocols the radio understands. This simplifies the computer / radio interconnect to a single cable and eliminates fussing with recording levels, impedance matching resistors or DC blocking capacitors. Unitrunker does not decode LTR from discriminator audio so this is currently the only way to monitor LTR in Unitrunker. An inline receiver is functionally a "both" receiver, so it also has a role selection. You can use it as a voice receiver or as a signal source for decoding.

Inline receivers support two types of logging.

Beware the Uniden inline models don't provide streaming data while under computer control. Likewise, the GRE / Radio Shack / Whistler models don't provide streaming data for LTR while under computer control.

Tip: Follow this sheet to get your Uniden model working.

Tip: For LTR on GRE / Radio Shack / Whistler models, program the system into the radio and scan as LTR.

The Uniden HP-1E is unique in passing discriminator audio over the same USB cable used for control and programming. Unlike the other inline models, you get a 'scope display and the option to route and mute the audio.

Inline receivers can operate as Signal Role receivers. Some models can also be used for voice following.

Realtek RTL2832U

The Realtek is a very low cost SDR. This receiver type can operate as a voice or signal receiver. Audio can be heard from the host computer's speaker or headphones. Since this receiver has no front panel, you can control it through fields in the receiver window. Typing a frequency in the Park field, for example, will tune the RTL to that frequency. While operating in Signal Role, you'll want to mute the audio to suppress the control channel noise. This class can also be used for voice following.


The AirSpy is a relatively low cost SDR that can serve as a voice or signal receiver. The program provides up to five VCOs in your choice of voice or signal role.

Remote API

This receiver is a skeleton receiver that extends the signal receiver above to support voice role or signal role through a third party DLL interface. The Remote API has been available since 2011. The remote API provides three commands:

All three commands accept a channel description that includes frequency and bandwidth. The control command indicates the receiver is being used for control channel signal decoding. The Listen command directs the receiver to a voice channel for an active call. The Park command tunes the receiver to a voice channel when there is no call activity.


This receiver is intended for debugging Unitrunker functionality but the community has found other uses. An SDR# plugin allows the voice-role debug receiver to control a running instance of SDR#. The Debug receiver has been largely replaced by the Remote API receiver above.


All of the receiver classes that directly handle discriminator audio can pass that audio - filtered or not - to any of your host PC's audio output devices. This can be PC speaker, headphone, line output, and a virtual audio device for routing audio to another application (for recording or online streaming).

Here's a quick self-test of your understanding of the different classes of receivers.

  1. To decode a trunking control channel, which of the six receiver classes listed below can do so? Check all that apply.
  2. To follow voice calls, which of the six receiver classes listed below can do so? Check all that apply.
  3. To follow voice calls, what is the required minimum number of configured receivers / VCOs?

Time's up! Put down your pencils. Hand the paper to the person on your right. Here are the correct answers:

  1. Answer: everything BUT control.
  2. Answer: everything BUT signal.
  3. Answer: two (one signal role receiver for decoding and one voice role receiver for voice following).

© Copyright 2008-2015 Rick Parrish