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RE: SDR-1000 (was FT-847 discontinued?)



Hi Dave

> The faster the CPU and the better the sound card the less the
> delay...

To a very large extent this is correct. Windows' preemptive multitasking is
not very friendly to applications requiring rapid fire low latency access to
peripherals.

You've probably seen it yourself: there you are, typing away in Word, and
suddenly what you type doesn't appear on the screen for a second or two,
during which there is a flurry of disk, CD and/or CPU activity.

To circumvent the possibility of missing a time slot, and therfore getting
short silent gaps,  applications typically use larger buffers for the
incoming and outgoing signals, hence the delay.

There is also some dependency on the API and drivers used for sound cards:
the best performance is typically demonstrated by using ASIO.

On the DSP side, if FFT's are used for filtering, there is also some
noticeable delay proportional to the size of the FFT and inverselecy
proportional to the sampling rate. For example, say you have a sampling rate
of 48kHz and an FFT length of 1024. You'll have a latency of at least 21ms
(FFTLen/Fs). FFTs by their nature deal in batches of data. There are
non-batch DSP methods that can deal with operations such as filtering and
demodulation, one sample at a time, thus giving you a software latency of up
to one sample time (20us at 48kHz).

Embedded DSP real time operating systems are typically light weight and make
Windows look like batch processing (remember that?). The only way you're
going to achieve full break in CW is to dump Windows I'm afraid.

73, Howard G6LVB
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