On 2015-07-20 14:10, Peter Heitzer wrote:
I am currently writing a python script to extract samples from old Roland 12
bit sample
disks and save them as 16 bit wav files.
The samples are layouted as follows
0 [S0 bit 11..4] [S0 bit 3..0|S1 bit 3..0] [S1 bit 11..4]
3 [S2 bit 11..4] [S2 bit 3..0|S3 bit 3..0] [S3 bit 11..4]
In other words
sample0=(data[0]<<4)|(data[1]>>4)
sample1=(data[2]<<4)|(data[1] & 0x0f)
I use this code for the conversion (using the struct module)
import struct
from array import array
def getWaveData(diskBuffer):
offset=0
words=array('H')
for i in range(len(diskBuffer)/3):
If the 2 12-bit values are [0xABC, 0xDEF], the bytes will be [0xAB,
0xCF, 0xDE].
h0=struct.unpack_from('>h',diskBuffer,offset)
This gives 0xABCF, which is ANDed to give 0xABC0. Good.
h1=struct.unpack_from('<h',diskBuffer,offset+1)
This gives 0xDECF, which is ANDed to give 0xDEC0. Not what you want.
words.append(h0[0] & 0xfff0)
words.append(h1[0] & 0xfff0)
offset+=3
return words
I unpack the samples in an array of unsigned shorts for I later can use the
byteswap() method
if the code is running on a big endian machine.
What options using pure python do I have to make the conversion faster?
I thought of unpacking more bytes at once e.g. using a format '>hxhxhxhx' for 4
even samples
and '<xhxhxhxh' for 4 odd samples vice versa.
You could try using lookup tables to decode even-numbered and
odd-numbered pairs of bytes.
Can I map the '& 0xfff0' to the whole array?
That's something the numpy could do.
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