check for saturation during demodulation.

amodem/common.py

@@ -6,7 +6,7 @@ import logging
 log = logging.getLogger(__name__)
 
 scaling = 32000.0  # out of 2**15
-SATURATION_THRESHOLD = 1.0
+SATURATION_THRESHOLD = (2**15 - 1) / scaling
 
 LENGTH_FORMAT = '<I'
 
@@ -24,7 +24,7 @@ class SaturationError(ValueError):
 
 def check_saturation(x):
     peak = np.max(np.abs(x))
-    if peak > SATURATION_THRESHOLD:
+    if peak >= SATURATION_THRESHOLD:
         raise SaturationError(peak)
 
 

amodem/recv.py

@@ -237,19 +237,22 @@ def decode(bits_iterator):
     return ecc.decode(blocks())
 
 
-def iread(fd):
-    check = common.check_saturation
-    reader = stream.Reader(fd, data_type=common.loads, check=check)
-    return itertools.chain.from_iterable(reader)
+def iread(fd, skip):
+    reader = stream.Reader(fd, data_type=common.loads)
+    signal = itertools.chain.from_iterable(reader)
+
+    skipped = common.take(signal, skip)
+    log.debug('Skipping %.3f seconds', len(skipped) / float(modem.baud))
+
+    reader.check = common.check_saturation
+    return signal
 
 
 def main(args):
     try:
         log.info('Running MODEM @ {:.1f} kbps'.format(modem.modem_bps / 1e3))
 
-        signal = iread(args.input)
-        skipped = common.take(signal, args.skip)
-        log.debug('Skipping %.3f seconds', len(skipped) / float(modem.baud))
+        signal = iread(args.input, args.skip)
 
         size = 0
         signal, amplitude = detect(signal, config.Fc)

tests/test_common.py

@@ -54,9 +54,9 @@ def test_dumps_loads():
 
 
 def test_saturation():
-    x = np.array([1, -1, 1, -1]) * 1.01
+    x = np.array([1, -1, 1, -1]) * 1e10
     try:
         common.check_saturation(x)
         assert False
     except common.SaturationError as e:
-        assert e.args == (1.01,)
+        assert e.args == (max(x),)