Fast 12 bit to 16 bit sample conversion?

edmondo.giovannozzi at gmail.com edmondo.giovannozzi at gmail.com
Mon Jul 20 17:14:51 CEST 2015


Il giorno lunedì 20 luglio 2015 15:10:22 UTC+2, Peter Heitzer ha scritto:
> 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):
>     h0=struct.unpack_from('>h',diskBuffer,offset)
>     h1=struct.unpack_from('<h',diskBuffer,offset+1)
>     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.
> Can I map the '& 0xfff0' to the whole array?

I'll try to read the binary data with numpy.fromfile, reshape the array in [n,3] matrix, and then you can operate with the columns to get what you want.
:-)
 


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