This document summarizes the digital signal processing which takes place in
the sameold crate. The top-level receiver is in receiver.rs.
A DC blocker (dcblock.rs) removes DC bias that may result from analog audio
connections. An automatic gain control (agc.rs) algorithm then normalizes the
input power to a fixed amplitude range. These pre-processors improve the
performance of the symbol synchronizer, which makes certain assumptions about
the signals it is receiving.
The demodulator (demod.rs) is responsible for turning waveforms into symbols.
In binary constellations like 2FSK, the symbols are -1 and +1. The demodulator
uses a matched filter for each of the two frequencies and compares their output
powers non-coherently.
The demodulator also downsamples to a low-speed sampling rate. The low-speed rate is approximately twice the SAME baud rate (2× 520.83 Hz). Everything else in the system happens at this low rate. The demodulator is cleverly designed to avoid calculating output samples that will just be discarded later.
For optimum performance, the demodulator must sample the signal at exactly the
center of the transmitted symbol. On its own, the demodulator does not know when
this occurs. The symbol synchronizer (symsync.rs) is responsible for
estimating the correct sampling time. The demodulator is tightly integrated with
the symbol synchronizer.
The synchronizer uses a feedback algorithm, known as a zero crossing detector, to estimate when the transition between symbols occurs. The output of the symbol synchronizer is used to adjust the receiver's downsampling clock. The low-speed signal is also output.
The individual bits which make up a SAME message must be assembled together into
characters—in this case, ASCII bytes. The code squelch (codesquelch.rs) takes
synchronized symbols and attempts to turn them into synchronized bytes. It
listens for a preamble sequence which precedes every SAME transmission. The
preamble sequence is crafted specifically to aid in acquiring the bit and byte
timing.
The code squelch also suppresses (i.e., squelches) its output when no SAME signal is present. It contains a power detector to prevent garbage symbols from "noise" (i.e., the normal program audio a station transmits) from tripping the detector. The code squelch also helps detect the end of each transmission.
Byte-synchronized symbols are then provided to the adaptive equalizer. Up until this point, the "bits" of a SAME transmission are represented as floating-point samples. Some parts of the system may make their own internal decisions about whether the symbol is -1 or +1, but these decisions are not propagated. The adaptive equalizer is responsible for making the final decision for each symbol. The adaptive equalizer is a decision-feedback equalizer that contains an adaptive filter. The adaptive filter can compensate for inter-sample interference (ISI) and non-flat channel frequency response. The output of the adaptive filter is a byte stream.
The bytes are then provided to a framer (framing.rs). Like all framers, this
framer is responsible for detecting when messages begin or end.
SAME messages are repeated three times for redundancy. The assemble
(assembler.rs) is responsible for combining up to three bursts together into
a single Message estimate. The assembler performs 2-of-3 bitwise parity
correction, where applicable, and produces fully-formed Message estimates
for the client.