radio/util/codecs.py

   1 #!/bin/env python
   2
   3 from __future__ import print_function
   4
   5 SIGN_BIT = (0x80)   # Sign bit for a A-law byte.
   6 QUANT_MASK = (0xf)  # Quantization field mask.
   7 SEG_SHIFT = (4)     # Left shift for segment number.
   8 SEG_MASK = (0x70)   # Segment field mask.
   9 BIAS = (0x84)       # Bias for linear code.
  10
  11
  12 def alaw2linear(a_val):
  13     a_val ^= 0x55
  14
  15     t = a_val & QUANT_MASK
  16     seg = (a_val & SEG_MASK) >> SEG_SHIFT
  17     if (seg):
  18         t = (t + t + 1 + 32) << (seg + 2)
  19     else:
  20         t = (t + t + 1) << 3
  21
  22     if a_val & SIGN_BIT:
  23         return t
  24     else:
  25         return -t
  26
  27
  28 def ulaw2linear(u_val):
  29     # Complement to obtain normal u-law value.
  30     u_val = ~u_val
  31
  32     # Extract and bias the quantization bits. Then
  33     # shift up by the segment number and subtract out the bias.
  34     t = ((u_val & QUANT_MASK) << 3) + BIAS
  35     t <<= (u_val & SEG_MASK) >> SEG_SHIFT
  36
  37     if u_val & SIGN_BIT:
  38         return (BIAS - t)
  39     else:
  40         return (t - BIAS)
  41
  42
  43 def pcmTable(fn):
  44     result = []
  45     for i in range(256):
  46         result.append(fn(i))
  47     return result
  48
  49
  50 def tableToString(name, table):
  51     result = 'const int16_t ' + name + '[256] = { '
  52     result += ', '.join(str(i) for i in table)
  53     result += ' };'
  54     return result
  55
  56
  57 print(tableToString('alawTable', pcmTable(alaw2linear)))
  58 print(tableToString('ulawTable', pcmTable(ulaw2linear)))