Mail Archives: geda-user/2014/03/29/15:41:14
I don't know why I didn't think of this earlier (too much of a hurry)
but after reading John's response and realizing that I had forgotten
about the sign bit it became clear that you cannot just toss the high
bits - you need to toss the low bits (essentially an integer divide by
16). However, now I think that the best technique is to formally rescale
the data to 12 bits. This is really quite simple. Convert each 16 bit
sample into a float, divide by 16.0 and then convert the float back into
an integer, which should now fit into your target 12 bit magnitude. It
takes care of the signed nature of the data as well and should provide a
better result than an integer divide.
Steve Besch
On 03/29/2014 11:02 AM, Stephen R. Besch wrote:
> On 03/29/2014 10:36 AM, Rob Butts wrote:
>> I have a short 8kHz 16 bit wav pcm file that I converted to an 8kHz
>> 16 bit
>> unsigned C numerical array. I'm using a 12 bit dac so I need to convert
>> the 16 bit numerical array to a 12 bit numerical array.
>>
>> Before I get started is this just a matter of shifting out the four most
>> significant bits of the binary 16 bit value?
>>
> That depends on the nature of the signal. Usually the noise is mostly
> in the low order bits so you may want to dump at least one or 2 of
> those. But first, you may also want to examine the high order bits.
> Depending upon the initial gain settings when the data was sampled,
> one or 2 of the high order bits may be all the same anyway. In either
> case, I think I would do this in 2 stages. First, shift up to get rid
> of unused dynamic range at the top end. Then throw away the rest at
> the low end where the noise is. You may want to signal average the low
> bits, say 3 bits into 1 or 4 bits into 2
>
> Steve Besch
>
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