[Elecraft] Comparison: K3 and SDR 5000A

[Lyle Johnson]

> As I understand it, latency is no longer an issue.  Latency issues with 
> the early versions of PowerSDR and the SDR-1000 have been resolved.  
> That being said, signal processing latency is a fact of life for all 
> digital radios. We are talking microseconds,

I beg to differ.

Latency (delay) in a DSP-based radio (or SDR, if you prefer) is caused 
by several things.

The one we can't get rid of is the filter delay.  By making "shorter" 
filters (fewer taps), we can reduce the delay through the filter, but at 
the expense of filter performance.  We get wider skirts, less ultimate 
rejection, more passband ripple, or some combination of these three factors.

However, DSP can be applied in many ways.  One sure way to increase 
latency is by processing the incoming signal in blocks.  This means you 
collect a certain number of samples, then process them all at once while 
collecting the next block, etc.  PC implementations of DSP typically use 
this method.

Another method is to process the signal after each sample.  This 
eliminates the block delay.  This is how the K3 processes signals.

As an example, let's consider an SDR using 2048-sample blocks (common in 
the SDR world) and 96 kHz sampling.  It will take (2048/96,000 =) 22 
milliseconds to acquire this block.  This is 22 ms more delay, or 
latency, than a K3 will have, assuming similar delays in each radio for 
the filter(s).

For a real world example, I connected an SDR-14 receiver (with 
associated dual core 3 GHz PC) to the IF output of my K3.  I then tuned 
in an SSB signal and listened to it through the K3's speaker as well as 
demodulating it and listening through the PC's audio system.

The audio coming from the PC was very noticeably delayed versus the 
audio from the K3.

This delay or latency may not be an issue in all cases.  But in come 
common operational scenarios, like QSK CW or SSB using "syllabic" VOX, 
it can be critical.  By paying close attention to such latency issues in 
the architecture of the K3, we are able to provide QSK CW operation at 
speeds well over 30 WPM without "reducing the taps" in the filters or 
otherwise compromising the performance of the radio.  The 22 ms 
additional delay cited in the example above would kill QSK performance.

Latency is just one of many considerations that arise when comparing 
radios, architectures, and one's own needs and preferences.

73,

Lyle KK7P