Simple Python struct issue

MRAB python at mrabarnett.plus.com
Fri Oct 2 18:49:56 CEST 2009


Carlo DiCelico wrote:
> I saw an article on O'Reilly about using NumPy and Dislin to analyze
> and visualize WAV files. It's a really fantastic article but was a
> little out of date. I updated the script to work with the newer
> modules &etc but am still having trouble getting it working.
> 
> The line
> 
> temp[i,:] = array(struct.unpack("%dB"%(fft_length), tempb),Float) -
> 128.0
> 
> always returns the same error: "Traceback (most recent call last):
>   File "pysono.py", line 31, in <module>
>    temp[i,:] = array(struct.unpack("%dB"%(fft_length),tempb),float) -
> 128.0
> struct.error: unpack requires a string argument of length 256" when I
> do python pysono.py test.wav 256
> 
> I'm sure it's probably something simple but I just can't see what it
> is!
> 
> Here's the original code: http://onlamp.com/python/2001/01/31/graphics/pysono.py
> 
.readframes(n) returns (at most) n _frames_ as a bytestring (str), not n
bytes. I tried reading 256 frames from a .wav file containing stereo at
16 bits per channel and got 1024 bytes (4 bytes per frame, not
surprisingly!).



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