This PR adds basic adaptive gain support, which adjusts SDR gain on the fly based on the noise & signal levels seen.
There are two control mechanisms:
Dynamic range control is enabled by the --adaptive-range option. This adjusts SDR gain to try to achieve a minimum dynamic range, regardless of the exact hardware in the RF path.
Burst (loud message) control is enabled by the --adaptive-burst option. This decreases SDR gain when undecodable loud messages are heard, allowing for better reception of nearby aircraft at the expense of range.
This is only the basic implementation - see the PR for remaining work to do.
This reinstates the fastpath in the 2.4MHz demodulator that
aborts message demodulation early if the DF bits don't match a message
type that we know how to decode, or stops early if the DF bits can
only correspond to a short 56-bit message.
Do this in a more general form where we test against a set of valid
DF values, rather than a switch/case. Then populate the sets based on
the current error-correction settings (e.g. if we are allowed to repair
DF damage with 1 bit error correction, then a DF17 message could appear
with any of DF=17, DF=16, DF=19, DF=21, DF=25, or DF=1 and still be
correctable to a valid DF17 message)
In the default case this produces a small CPU improvement as short
messages might be able to stop earlier and a few DF values can be ignored
early. With --no-fix-df it produces a larger improvement as relatively few
DF values are valid in that mode.
To further reduce CPU, the default behaviour has changed to _not_
decode DF24 messages (Comm-D ELM messages). These are rarely seen in
the wild and dump1090 can't do anything useful with them. Disabling
them allows us to further reduce the set of valid DF values as they
effectively use all DF values from 24..31, 25% of all possible values.
To re-enable DF24 decoding, use the new `--enable-df24` option.
Finally, avoid doing some CRC calculations twice. This was only happening
once per valid decoded message, not per demodulation attempt, so it's not
such a large win.
Major changes:
Try to error-correct all messages that potentially could be DF11/17/18
even if the original DF value is different. This allows messages with
correctable damage in the first 5 bits to be correctable.
Track recently-seen DF18 addresses separately to DF17 addresses.
DF18 does not imply Mode S support, so we don't want to treat it like
a known Mode S emitter, but once we've heard some DF18 for an aircraft
we can be more confident about future DF18 messages for the same
address.
Rework the scoring system so it's just a big enum that lists all the
possible outcomes in the order that we want. This makes the relative
ordering of different messages clearer, and makes it easier to move
messages above/below the accept thresholds as needed.
Don't accept 2-bit-error-corrected messages that are from aircraft we
have not previously seen. This greatly reduces the number of garbage
messages when using 2-bit error correction.
Overall results are:
* more CPU required for decoding (approx 30% increase in my tests)
as we're doing a lot more speculative CRC-checking work
* no significant change to message rates with error correction off
* about 5% more 1-bit-corrected DF17 decodes, with a
disproportionate increase in those messages contributing to
successful position decodes / unique aircraft counts
* _fewer_ decodes with 2-bit correction (versus old code with 2-bit
correction), but the message quality is substantially improved,
the rate of garbage decodes / phantom aircraft is greatly reduced
sample stats:
1-bit correction, old code:
141158 total usable messages
137852 accepted with correct CRC
3306 accepted with 1-bit error repaired
27446 DF17 messages
51 unique aircraft tracks
1-bit correction, new code:
141296 total usable messages
137854 accepted with correct CRC
3442 accepted with 1-bit error repaired
27528 DF17 messages
55 unique aircraft tracks
2-bit correction, old code:
142656 total usable messages
137809 accepted with correct CRC
3283 accepted with 1-bit error repaired
1564 accepted with 2-bit error repaired
28803 DF17 messages
349 unique aircraft tracks (<- note that most of these are garbage)
2-bit correction, new code:
142426 total usable messages
137822 accepted with correct CRC
3420 accepted with 1-bit error repaired
1184 accepted with 2-bit error repaired
28666 DF17 messages
55 unique aircraft tracks
Update all the SDR implementation to use it.
This was getting pretty ugly with code getting copy&pasted in all the SDR
implementations. Unify it all and give it a simpler API. Linked list works out
much simpler than the circular buffer. Also, simplify copying the overlap region
around by just using a separate buffer (it's only a few hundred bytes long, so
the double copy is not a big deal).
* use a global noise level rather than one computed from a few bits
* work out level vs power confusion in some thresholds
* fix the power calculation for working out the phase offset from
the framing bits
* require fewer quiet trailing bits
* relax the bit-threshold tests
Magnitude conversion now happens immediately when sample data is
received, so there is no risk of newly received data clobbering old
data under CPU overload.
the message being emitted immediately.
Fix computation of reception time so it's more sensible (the block timestamp
is some time after reception of the _end_ of the block, not the start) - this
means that message-emission times are always later than message-reception
times in SBS output, which is a bit more sensible.
Use clock_gettime in preference to ftime.
Switch signalLevel back to a power measurement, don't put SNR in there.
But make it a 0.0 - 1.0 double so we're not scaling everywhere.
Adjust for the amplitude offset when calculating power.
Adapt everything else to the new scheme.
Oliver Jowett
Oliver Jowett
rxseger