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README


(C) Copyright 2010, KA1RBI

This package contains implementations for a number of loosely related
GR blocks and related utilities including:
- improved ctcss squelch block
- general pipe block (gr seems to fight this)
- p25 framer / assembler block
- fsk4 slicer with settable decision points
- Combined Gardner/Costas loop block
- RS and BCH routines

-------------------------------------------------------------------------

Two key internal interfaces are used throughout the system:

1. 88-bit IMBE voice codewords (input to Pavel's decoder and output by the
encoder; also referred to as the u[] vectors) are represented as printable
"hex" strings of the format
"%03x %03x %03x %03x %03x %03x %03x %03x\n" (each string is 32 bytes)
When sent via UDP, the nine voice codewords (from the LDU1/2) result in a
packet payload size of 288 (9 * 32).

2. The "wireshark" packet interface: P25 frames are sent as packed bytes,
four symbols (8 bits) per byte, one UDP packet per P25 frame. The P25
frame sync (FS) is aligned starting at data offset zero of each UDP packet,
occupying the first six bytes. Example: total data length of voice packets
= 216 bytes (864 symbols * 0.25 bytes/symbol). All decoded symbol dibits
(including status symbols) are included. When unpacked into symbols (dibits)
this format is the 4800 baud stream that would be applied to a FM/PSK
modulator stage or would be as received out of a FSK4 decoder/slicer.