[hpsdr] FFT latency

[Jim Barber]

The actual latency introduced by an FFT will depend on the overlap used 
between frames.
For very long FFT's with substantial overlaps it can be burdensome, yes.

The reasons the FFT is used are for the most part performance, 
convenience and sometimes
because that's the "way that the other guys did it". There are also 
algorithms that are inconvenient
or difficult to execute in the "time domain" - spectral subtraction 
noise reduction being one.

You can, of course perform filtering, demodulation and processing 
routines strictly in the time
domain. If you use linear-phase FIR filters, the additional latency or 
delay for
each filter added to the chain is very close to N/2 samples. So for 
example, a 256-tap linear-phase
FFT filter in the audio section of the DSP @ Fs=48 kHz will add ((256/2) 
/ 48000) = 2.7ms delay.
The equivalent delay using an FFT to execute the filter will be longer. 
Specifying it exactly will
start arguments, but it will indeed be substantially longer.

So... The first question that comes up is can you afford the additional 
processing overhead?
That calculation isn't simple, and is subject to implementation tweaks.

Another thing you can do is use IIR filters where appropriate - audio 
EQ's for speaker output
channels and the like. They are computationally efficient and add almost 
no delay - at the
expense of non-linear phase response, which you may or may not care 
about at a given
stage.

My .02,

Jim N7CXI


On 4/5/2015 3:54 AM, G3XJP wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> I need your collective wisdom!  Is it not an inescapable fact that any 
> serious FFT in the real-time path between your antenna and your ears 
> must introduce an inherent and inevitable delay that will preclude a 
> conversational VOX/QSK QSO?  I can see that it would not preclude 
> listening to a monologue - or to watching spectral pictures on a 
> display.  But anything more than about 1/5 sec is doomed to feel like 
> Skype on a bad day - where you often end up having to say "over".  As 
> a generality, am I missing something fundamental here?  Peter G3XJP
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