This decoder is loosely based on this article and adds some extra functionality:
- Address Decoding
- Continuous Resynchronization
- Verbose Data Display
- Sampling Rate Detection
I wrote this primarily to make sure that the POGSAG encoder in radiolib was doing what it should and also get get a better understanding of exactly how pagers work.
Pager Transmission in GQRX
- Use GQRX or other SDR software to record a pager transmission as a WAV file.
- Apply a low pass filter in Audacity and trim the result to start just before the first positive zero crossing and end after the bit period after the last zero crossing.
- Run the python script.
Example Low Pass Filter in Audacity
As many references state, the CAP-CODE address is split in two parts: The first 19 bits come from the address field of the address codeword and the last three bits come from the position of the address codeword in the frame. What I could not find was how to translate the position of the address codeword into bits — there are 16 possible positions within each batch, but three bits distinguish only eight possibilities. Naively, I expected these bits to be encoded such that an address codeword transmitted immediately after a sync word would have LSB 000 and an address codeword transmitted with seven idle codewords between the sync word and address word would have an LSB of 111. I expected this to be the case since I expected this to minimize air time (at most, seven idle words would be transmitted before the address is transmitted). This is not how it works!
What actually happens is that the address word is offset from the sync word by 2x the idle words as the LSB of the address. As such, an address with an LSB of 001 has two idle codewords between it and the sync word and an address with an LSB of 111 has fourteen idle codewords between it and the sync word. This is clearly shown in the source code for radiolib's pager library, but I don't see it in other references that come up on Google.
The example I started with here samples the waveform based on assuming its bit period synchronized only to the first zero crossing. I had enough offset in my data compared to the ideal rate of 1200 bps that I was no longer sampling at the right point on the waveform after about 3/4 of a transmission. To solve this, I added some code to resynchronize the sampling points to positive zero crossings.
Input Waveform Along with Bit Sample Points and Resynchronization Events
Message Received by Pager
Output of Python Script:
rruark@rruark-MS-7C56:~/Website/website/projects/RF/Pager/python/POCSAG-Decoder$ python3 pager.py Sampling Rate: 48000 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Preamble: 10101010101010101010101010101010 Sync: 01111100110100100001010111011000 Idle: 01111010100010011100000110010111 Idle: 01111010100010011100000110010111 Idle: 01111010100010011100000110010111 Idle: 01111010100010011100000110010111 Idle: 01111010100010011100000110010111 Idle: 01111010100010011100000110010111 Idle: 01111010100010011100000110010111 Idle: 01111010100010011100000110010111 Addr: 0 100001100100011100 11 10000000011 Msg: 1 00101010001011100101 01001010010 Msg: 1 11100111000001010010 11101001010 Msg: 1 11110011100000101000 10101110111 Msg: 1 01100000100010111101 00000001010 Msg: 1 00111100111001011100 10101001000 Msg: 1 00010111101101100110 01110000100 Msg: 1 00001000101110001011 01010010000 Sync: 01111100110100100001010111011000 Msg: 1 10100110000010000011 00100100000 Msg: 1 11000011111001110100 00101000110 Msg: 1 11010011101110100000 11111111010 Msg: 1 01010010010010111000 11111100010 Msg: 1 00101100111000101111 01000110100 Msg: 1 11011101011100110110 11100010011 Msg: 1 01001100000101100111 01110011111 Msg: 1 00001111000011011101 00101010000 Msg: 1 10000010100001100000 00110101101 Msg: 1 10110001111110111010 11110000000 Msg: 1 11100001110011011101 01011100111 Msg: 1 00110000010011001101 00100111010 Msg: 1 00111100001110110111 11101000111 Msg: 1 01001111001110111010 11111110101 Idle: 01111010100010011100000110010111 Idle: 01111010100010011100000110010111 This Message was addressed to 1100004 Message consists of 7-bit words: ['0010101', '0001011', '1001011', '1100111', '0000010', '1001011', '1100111', '0000010', '1000011', '0000010', '0010111', '1010011', '1100111', '0010111', '0000010', '1111011', '0110011', '0000010', '0010111', '0001011', '1010011', '0000010', '0000111', '1000011', '1110011', '1010011', '0100111', '0111010', '0000010', '1001001', '0010111', '0000010', '1100111', '0001011', '1111011', '1010111', '0011011', '0010011', '0000010', '1100111', '0000111', '1000011', '0111011', '0000010', '1000011', '0000010', '1100011', '1111011', '1010111', '0000111', '0011011', '1010011', '0000010', '0110011', '0100111', '1000011', '1011011', '1010011', '1100111', '0111010'] Message decodes in ASCII as: This is a test of the pager. It should span a couple frames. rruark@rruark-MS-7C56:~/Website/website/projects/RF/Pager/python/POCSAG-Decoder$