Use 4 carriers wit QAM16 to achieve 16kbps.

common.py

@@ -7,32 +7,26 @@ log = logging.getLogger(__name__)
 Fs = 32e3
 Ts = 1.0 / Fs
 
-Fc = 9e3
+frequencies = (np.arange(4) + 8) * 1e3
+carrier_index = len(frequencies)/2
+Fc = frequencies[carrier_index]
 Tc = 1.0 / Fc
 
 Tsym = 1e-3
-Nsym = int(Tsym / Fs)
-
-F0 = Fc
-F1 = Fc + 1e3
 Nsym = int(Tsym / Ts)
 baud = int(1/Tsym)
 
-scaling = 10000.0
+scaling = 32000.0  # out of 2**15
+SATURATION_THRESHOLD = 1.0
 
 LENGTH_FORMAT = '<I'
 
 def pack(data):
     log.info('Sending {} bytes'.format(len(data)))
-    data = struct.pack(LENGTH_FORMAT, len(data)) + data
     return data
 
 def unpack(data):
-    length_size = struct.calcsize(LENGTH_FORMAT)
-    length, data = data[:length_size], data[length_size:]
-    length, = struct.unpack(LENGTH_FORMAT, length)
-    data = data[:length]
-    log.info('Decoded {} bytes, leftover: {}'.format(len(data), len(data)-length))
+    log.info('Received {} bytes'.format(len(data)))
     return data
 
 def to_bits(chars):