7727b7c902
Also changed the reference level printout to show dB instead of sample value. This seems more useful. 0dB is full scale.
73 lines
2.1 KiB
Python
73 lines
2.1 KiB
Python
#!/usr/bin/env python
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#from string import split, join
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#betcha this would be faster if you used a table for mode C
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#you could strip out D1 since it's never used, that leaves 11 bits (table is 2048 entries)
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#on the other hand doing it this way is educational for others
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def decode_alt(alt, bit13):
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if alt & 0x40 and bit13 is True:
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return "METRIC ERROR"
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if alt & 0x10: #a mode S-style reply
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if bit13 is True:
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tmp1 = (alt & 0x1F80) >> 2 #first 6 bits get shifted 2 down
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tmp2 = (alt & 0x20) >> 1 #that bit gets shifted 1 down
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else:
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tmp1 = (alt & 0x0FE0) >> 1 #first 7 bits get shifted 1 down but there are only 12 bits in the representation
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tmp2 = 0
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decoded_alt = ((alt & 0x0F) | tmp1 | tmp2) * 25 - 1000
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else: #a mode C-style reply
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#okay, the order they come in is:
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#C1 A1 C2 A2 C4 A4 X B1 D1 B2 D2 B4 D4
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#the order we want them in is:
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#D2 D4 A1 A2 A4 B1 B2 B4
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#so we'll reassemble into a Gray-coded representation
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if bit13 is False:
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alt = (alt & 0x003F) | (alt & 0x0FC0 << 1)
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C1 = 0x1000
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A1 = 0x0800
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C2 = 0x0400
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A2 = 0x0200 #this represents the order in which the bits come
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C4 = 0x0100
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A4 = 0x0080
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B1 = 0x0020
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D1 = 0x0010
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B2 = 0x0008
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D2 = 0x0004
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B4 = 0x0002
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D4 = 0x0001
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bigpart = ((alt & B4) >> 1) + ((alt & B2) >> 2) + ((alt & B1) >> 3) + ((alt & A4) >> 4) + ((alt & A2) >> 5) + ((alt & A1) >> 6) + ((alt & D4) << 6) + ((alt & D2) << 5)
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#bigpart is now the 500-foot-resolution Gray-coded binary part
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decoded_alt = gray2bin(bigpart)
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#real_alt is now the 500-foot-per-tick altitude
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cbits = ((alt & C4) >> 8) + ((alt & C2) >> 9) + ((alt & C1) >> 10)
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cval = gray2bin(cbits) #turn them into a real number
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if cval == 7:
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cval = 5 #not a real gray code after all
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if decoded_alt % 2:
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cval = 6 - cval #since the code is symmetric this unwraps it to see whether to subtract the C bits or add them
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decoded_alt *= 500 #take care of the A,B,D data
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decoded_alt += cval * 100 #factor in the C data
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decoded_alt -= 1300 #subtract the offset
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return decoded_alt
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def gray2bin(gray):
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i = gray >> 1
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while i != 0:
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gray ^= i
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i >>= 1
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return gray
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