[sdiy] VU meter algorithms

ASSI Stromeko at nexgo.de
Wed Mar 20 18:40:43 CET 2019

On Wednesday, March 20, 2019 7:51:17 AM CET rsdio at audiobanshee.com wrote:
> One pesky detail that always bothers me about the Nyquist frequency is
> related to the fact that no filter, analog or digital, has a step response
> in the frequency domain.

No realizable filter does if you also specify that you have a finite delay 
through the filter.

> Basically, your filter needs to output “nothing”
> above Nyquist to avoid all aliasing. Thus, you cannot have 0 dB response at
> Nyquist or even close by.

That is a common misconception.  In a nutshell, the Nyquist criterion is 
actually about the highest signal bandwidth you can reconstruct and the usual 
f_sample/2 answer applied to the highest frequency happens to coincide with 
using the first Nyquist zone of the signal and corresponds to needing a 
lowpass filter (as you noted, you'd need a brickwall filter if you want your 
passband go right up to that frequency).  If you use a bandpass filter 
instead, you can very well reconstruct higher frequencies as long as the 
signal bandwidth stays below f_sample/2.  So in a very practical sense you can 
ease your filter specifications at the top end considerably if you don't need 
some band of frequencies around DC (which for audio you'll usually not be able 
to use anyway).  You will still not get a flat frequency response across the 
whole band, but look up "maximally flat" filter designs to get a feeling for 
what is possible.  Keep in mind that for audio you'll usually also want a near 
constant group delay, which further restricts your design space.

> In my estimation, digital signal processing that never gets turned back into
> an analog signal at the end is kinda pointless. At least I can’t think of
> an example.

The speech recognition folks would disagree, for starters.  There is in fact a 
lot of digital signal processing that never leaves the digital domain again.

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