Add RS ECC

common.py

@@ -1,4 +1,6 @@
 import numpy as np
+import reedsolo as rs
+
 import hashlib
 import struct
 import logging
@@ -21,17 +23,8 @@ SATURATION_THRESHOLD = 1.0
 
 LENGTH_FORMAT = '<I'
 
-def pack(data):
-    log.info('Sending {} bytes'.format(len(data)))
-    return data
-
-def unpack(data):
-    log.info('Received {} bytes'.format(len(data)))
-    return data
-
-def to_bits(chars):
-    for c in chars:
-        val = ord(c)
+def to_bits(bytes_list):
+    for val in bytes_list:
         for i in range(8):
             mask = 1 << i
             yield (1 if (val & mask) else 0)

ecc.py

@@ -0,0 +1,52 @@
+''' Reed-Solomon CODEC. '''
+from reedsolo import rs_encode_msg, rs_correct_msg, ReedSolomonError
+
+import struct
+import logging
+log = logging.getLogger(__name__)
+
+DEFAULT_NSYM = 25
+BLOCK_SIZE = 255
+
+LEN_FMT = '<I'
+
+def encode(data, nsym=DEFAULT_NSYM):
+    log.info('Encoded {} bytes'.format(len(data)))
+    data = bytearray(struct.pack(LEN_FMT, len(data)) + data)
+    chunk_size = BLOCK_SIZE - nsym
+    enc = bytearray()
+    for i in range(0, len(data), chunk_size):
+        chunk = data[i:i+chunk_size]
+        if len(chunk) < chunk_size:
+            padding = b'\x00' * (chunk_size - len(chunk))
+            chunk.extend(padding)
+        enc.extend(rs_encode_msg(chunk, nsym))
+
+    return enc
+
+def decode(data, nsym=DEFAULT_NSYM):
+    data = bytearray(data)
+    dec = bytearray()
+    for i in range(0, len(data), BLOCK_SIZE):
+        chunk = data[i:i+BLOCK_SIZE]
+        try:
+            dec.extend(rs_correct_msg(chunk, nsym))
+        except ReedSolomonError:
+            break
+
+    n = struct.calcsize(LEN_FMT)
+    payload, length = dec[n:], dec[:n]
+    length, = struct.unpack(LEN_FMT, length)
+    assert length <= len(payload)
+    log.info('Decoded {} bytes'.format(length))
+    return payload[:length]
+
+
+def test_codec():
+    import os
+    x = bytearray(os.urandom(1024))
+    y = encode(x)
+    assert len(y) % BLOCK_SIZE == 0
+    x_ = decode(y)
+    assert x_[:len(x)] == x
+    assert all(v == 0 for v in x_[len(x):])

errors.py

@@ -2,16 +2,15 @@ import common
 import sys
 
 tx, rx = sys.argv[1:]
-tx = open(tx).read()
-rx = open(rx).read()
+tx = bytearray(open(tx).read())
+rx = bytearray(open(rx).read())
 
 L = min(len(tx), len(rx))
 rx = list(common.to_bits(rx[:L]))
 tx = list(common.to_bits(tx[:L]))
 indices = [index for index, (r, t) in enumerate(zip(rx, tx)) if r != t]
 
-if indices:
-    total = L*8
-    errors = len(indices)
-    print('{}/{} bit error rate: {:.3f}%'.format(errors, total, (100.0 * errors) / total))
-    sys.exit(1)
+total = L*8
+errors = len(indices)
+print('{}/{} bit error rate: {:.3f}%'.format(errors, total, (100.0 * errors) / total))
+sys.exit(int(errors > 0))

recv.py

@@ -9,6 +9,7 @@ logging.basicConfig(level=0, format='%(message)s')
 log = logging.getLogger(__name__)
 
 import sigproc
+import ecc
 import show
 from common import *
 
@@ -173,8 +174,7 @@ def main(t, x):
     else:
         data = iterate(data_bits, bufsize=8, advance=8, func=to_bytes)
         data = ''.join(c for _, c in data)
-        log.info( 'Demodulated {} payload bytes'.format(len(data)) )
-        data = unpack(data)
+        data = ecc.decode(data)
         with file('data.recv', 'wb') as f:
             f.write(data)
 

send.py

@@ -12,6 +12,7 @@ import itertools
 logging.basicConfig(level=0, format='%(message)s')
 log = logging.getLogger(__name__)
 
+import ecc
 import sigproc
 from common import *
 
@@ -70,7 +71,7 @@ if __name__ == '__main__':
         for c in sym.carrier:
             train(sig, c)
 
-        bits = to_bits(pack(data))
+        bits = to_bits(ecc.encode(data))
         modulate(sig, bits)