sdrangel/plugins/channelrx/demodpacket

Packet Radio Demodulator Plugin

Introduction

This plugin can be used to demodulate packet radio (APRS/AX.25) data packets. Received packets can be sent to the APRS Feature for decoding and display.

Interface

The top and bottom bars of the channel window are described here

1: Frequency shift from center frequency of reception

Use the wheels to adjust the frequency shift in Hz from the center frequency of reception. Left click on a digit sets the cursor position at this digit. Right click on a digit sets all digits on the right to zero. This effectively floors value at the digit position. Wheels are moved with the mousewheel while pointing at the wheel or by selecting the wheel with the left mouse click and using the keyboard arrows. Pressing shift simultaneously moves digit by 5 and pressing control moves it by 2.

2: Channel power

Average total power in dB relative to a +/- 1.0 amplitude signal received in the pass band.

3: Level meter in dB

• top bar (green): average value
• bottom bar (blue green): instantaneous peak value
• tip vertical bar (bright green): peak hold value

4: Modulation

This specifies the baud rate and modulation that is used for the packet transmission. Currently 1200 baud AFSK is supported.

5: RF Bandwidth

This specifies the bandwidth of a LPF that is applied to the input signal to limit the RF bandwidth.

6: Frequency deviation

Adjusts the expected frequency deviation in 0.1 kHz steps from 1 to 6 kHz. Typical values are 2.5 kHz and 5 kHz.

7: Filter Packets From

Entering a regular expression in the From field displays only packets where the source address, displayed in the From column, matches the regular expression.

8: Filter Packets To

Entering a regular expression in the To field displays only packets where the destination address, displayed in the To column, matches the regular expression.

9: Filter PID No L3

Checking this option displays only packets where the PID (Protocol ID) field is 0xf0 (no L3). This value is used by APRS and BBS data packets, and helps to filter out control packets.

10: Clear Packets from table

Pressing this button clears all packets from the table.

11: UDP

When checked, received packets are forwarded to the specified UDP address (12) and port (13).

12: UDP address

IP address of the host to forward received packets to via UDP.

13: UDP port

UDP port number to forward received packets to.

14: Use Date and Time from File

When checked, if the source device is a File Input device, the date and time used for packet reception time is taken from the file playback time. Otherwise, the current system clock time is used.

15: Start/stop Logging Packets to .csv File

When checked, writes all received packets to a .csv file.

16: .csv Log Filename

Click to specify the name of the .csv file which received packets are logged to.

17: Read Data from .csv File

Click to specify a previously written .csv log file, which is read and used to update the table.

Received Packets Table

The received packets table displays the contents of the packets that have been received. Only packets with valid CRCs are displayed.

• Date - The date the packet was received.
• Time - The time the packet was received.
• From - The source address / callsign of the sender of the packet.
• To - The destination address.
• Via - List of addresses of repeaters the packet has passed through or directed via.
• Type - The AX.25 frame type.
• PID - Protocol Identifier.
• Data - The AX.25 information field displayed as UTF-8 character string.
• Data (Hex) - The AX.25 information field displayed as hexadecimal.