[hpsdr] FFT latency

[Jim Barber]

Oops... Make that "256-tap linear-phase FIR filter" in the below.

Jim N7CXI

On 4/5/2015 9:35 AM, Jim Barber wrote:
> 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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