[hpsdr] FFT latency

[Jay Salsburg]

Dedicated Hardware like FPGAs and DSPs are what you get in these expensive
devices like HDSDR and Stand-alone Receivers (ICOM). There are many
techniques in hardware for reducing the computation burden, like Digital
Down Conversion (DDC).

There are dedicated chip sets that connect directly to the output of the
Analog to Digital Converters. This is absolutely necessary for real time
processing for extremely wide band detection of threats in RADAR and
surveillance, and frequency hopping interception of multiple clocks like in
GSM and LTE where multiple IFFTs provide multiple simultaneous receivers
from a single converter. However, for Short Wave receivers below 5 meters in
frequency, bandwidth is not an issue. 

The SDR can perform Zero IF conversion directly with a modest direct
conversion like a PC's Stereo Audio input and I/Q demodulation. The
sacrifice of agility and bandwidth using the PC's Audio input, gains ultra
deep dynamic range and low cost. Using the FFT on low bandwidth, high
dynamic range reception should not be a burden on a modern PC. In this case
the FFT is used just for display and not for filtering, demodulation, and
zooming (FFT/IFFT), like is necessary in wide bandwidth receivers that
operate through the microwave bands.

-----Original Message-----
From: Simon Brown [mailto:simon at sdr-radio.com] 
Sent: Tuesday, April 7, 2015 3:07 AM
To: G3XJP at RhodesG3XJP.plus.com; 'Jay Salsburg'
Cc: Hpsdr at lists.openhpsdr.org
Subject: RE: [hpsdr] FFT latency

Peter,

You don't have to perform FWD / INV FFT in the demodulation path at all.
Essentially there are two IQ paths in a generic design:

1) For the waterfall / spectrum display,
2) For demodulation.

Demodulation can get down to < 20ms on Windows if you're careful, using
WASAPI audio API helps a lot :) .

Simon Brown G4ELI
http://v2.sdr-radio.com

-----Original Message-----
From: Hpsdr [mailto:hpsdr-bounces at lists.openhpsdr.org] On Behalf Of G3XJP
Sent: 07 April 2015 08:21
To: Jay Salsburg
Cc: Hpsdr at lists.openhpsdr.org
Subject: Re: [hpsdr] FFT latency

***** High Performance Software Defined Radio Discussion List *****

Jay, this is the bit I don't get.  I think maybe this concern is what 
prompted me to ask the original question.   If the delay in an FFT is 
unavoidable and inherent, how does throwing more, better or dedicated 
hardware at it help in any way?   I can see how it would help release 
resources so you don't add to that inevitable delay.  cheers, Peter G3XJP

Jay Salsburg wrote:
> Yes, there is an unavoidable delay. What the SDR Software does is 
> perform the FFT on the Data stream coming from the Analog to Digital
Converters.
> Then, to hear the stream, an Inverse FFT is performed and sent to the 
> audio device. This delay may be reduced with better hardware and 
> dedicated SDR hardware to pre-process the data stream.
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