SV: [Speex-dev] About "Convert 8->16 bits"

[Jonas Tärnström]

I’d say this is out of scope for this mailing list. Any how, you can’t
just shift a signed number by 8 (multiplying with 256) without handling
the signedness-bit.
PCM 16-bit samples are stored as signed values.
 
Try something simpler which works first and then optimize your code
afterwards.
 
unsigned char inSamp;
short s1, outSamp;
 
s1 = (short) inSamp;
s1 -= 128;
s1 *= 256;
outSamp = s1;
 
It’s probably possible to reduce the complexity of this using bitshifts,
unless you know what you are doing and are in a lot of need for speed I
don’t see the reason.
//JT
 
 
 
//JT
 
-----Ursprungligt meddelande-----
Från: speex-dev-bounces at xiph.org [mailto:speex-dev-bounces at xiph.org] För
yaseminyilmaz
Skickat: den 15 december 2006 17:46
Till: speex-dev at xiph.org
Ämne: [Speex-dev] About "Convert 8->16 bits"
 
 
Hi everybody,
 
I have been using speex-1.2beta1 and my input data is 8-bit unsigned so
I know that I should convert from 8 to 16 bits. I’ve done the convertion
as following.
 
for(int i=0; i<160; ++i)
{
            input_frame[i] = (short)( (data[2*i] << 8) | data[2*i+1]);
      }
 
When my codec application didn’t work right, I think that the reason is
an error at this convertion. For this aim, I looked at read_samples
function in speexenc project and I can’t understand why the convertion
is done in this way:
 
      for(i=frame_size*channels-1;i>=0;i--)
      {
         s[i]=(in[i]<<8)^0x8000;
      }
 
(in my application, channels is 1 and frame_size=160)
 
I can understand why shift left 8, but can’t why we should do XOR
operation with 1000 0000 0000 0000. Should I do the convertion operation
so and why? Please can you clarify? Thanks a lot for your helps.
 
Sincerely
Yasemin
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