[time-nuts] Misuse of word "decimate" (was Re: Short term 10MHz source)

[Bob kb8tq]

Hi

For a quick dive into the basics of what has to happen in order to get more bits
from an ADC when decimating:

http://www.ti.com.cn/cn/lit/an/snoa232/snoa232.pdf <http://www.ti.com.cn/cn/lit/an/snoa232/snoa232.pdf>

Is a free on the internet source. There are somewhat better (and more complete)
descriptions in various books. “Multirate Signal Processing” by Harris is one, “Digital
Signal Processing in Communications Systems” by Frerking (a TImeNut …) is 
another one. There are *many* others. 

While this is normally applied to voltage to digital conversion (ADC), it also very 
much applies to time to digital conversion as well. 

Bob


> On Jan 10, 2019, at 2:29 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> 
> 
> petervince1952 at gmail.com said:
>> Could you please clarify then exactly what "decimate" means in this context.
> 
> Perhaps an example will help.
> 
> Consider an audio input system.  You want 0-20 kHz, so you need 40 k 
> samples/second.  You also need a sharp cutoff filter at 20 kHz.
> 
> An alternative approach is to sample at 160 k samples/second and implement the 
> filter in software.  You still need an external anti-aliasing filter, but this 
> one doesn't need to be sharp cutoff - just good enough to eliminate everything 
> over 80 kHz.
> 
> But now you have 160 k samples/second when you only want 40.  The answer is to 
> throw away 3 out of 4 samples.  That's decimation or downsampling.
> 
> You can downsample at non-integer rates by interpolating.
> 
> You can gain bits/sample with this sort of setup.  I'm not sure how to explain 
> it.
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
> 
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