ir_nec: optionally accept input signals that include the carrier

Introduce optional detection of a carrier signal. Immediately "go active"
when edges are seen. "Go inactive" again in the absence of edges in a
specified period of time. Cope with input signals that already had the
carrier removed.

By default carrier detection is disabled, to remain pixel compatible to
the previous implementation. When a carrier frequency is specified and
thus detection is enabled, edges of already filtered input are shifted
by one carrier period, and thus changes the output of the decoder. For
unfiltered inputs that still contain the carrier, detection of activity
is reliable and immediate, but the active phase is extended by one
period of the carrier frequency (which is considered acceptable).

1 files changed, 30 insertions, 2 deletions

diff --git a/decoders/ir_nec/pd.py b/decoders/ir_nec/pd.py
index 0362609..02d70a9 100644
--- a/decoders/ir_nec/pd.py
+++ b/decoders/ir_nec/pd.py
@@ -38,6 +38,7 @@ class Decoder(srd.Decoder):
     options = (
         {'id': 'polarity', 'desc': 'Polarity', 'default': 'active-low',
             'values': ('active-low', 'active-high')},
+        {'id': 'cd_freq', 'desc': 'Carrier Frequency', 'default': 0},
     )
     annotations = (
         ('bit', 'Bit'),
@@ -158,9 +159,36 @@ class Decoder(srd.Decoder):
     def decode(self):
         if not self.samplerate:
             raise SamplerateError('Cannot decode without samplerate.')
+
+        cd_count = None
+        if self.options['cd_freq']:
+            cd_count = int(self.samplerate / self.options['cd_freq']) + 1
+            prev_ir = None
+
         while True:
-            # Wait for any edge (rising or falling).
-            (self.ir,) = self.wait({0: 'e'})
+            # Detect changes in the presence of an active input signal.
+            # The decoder can either be fed an already filtered RX signal
+            # or optionally can detect the presence of a carrier. Periods
+            # of inactivity (signal changes slower than the carrier freq,
+            # if specified) pass on the most recently sampled level. This
+            # approach works for filtered and unfiltered input alike, and
+            # only slightly extends the active phase of input signals with
+            # carriers included by one period of the carrier frequency.
+            # IR based communication protocols can cope with this slight
+            # inaccuracy just fine by design. Enabling carrier detection
+            # on already filtered signals will keep the length of their
+            # active period, but will shift their signal changes by one
+            # carrier period before they get passed to decoding logic.
+            if cd_count:
+                (cur_ir,) = self.wait([{0: 'e'}, {'skip': cd_count}])
+                if self.matched[0]:
+                    cur_ir = self.active
+                if cur_ir == prev_ir:
+                    continue
+                prev_ir = cur_ir
+                self.ir = cur_ir
+            else:
+                (self.ir,) = self.wait({0: 'e'})
 
             if self.ir != self.active:
                 # Save the non-active edge, then wait for the next edge.