Topband: [Bulk] Re: ADC Overload from MW transmitters

[Grant Saviers]

Not having any particular axe to grind, I'll pile on a bit with some 
comments.

1.  The superhet/SDR vs direct sampling radio manufacturer and 
technology competition is and will continue to be very good for the ham 
community.

2.  The early days of CD audio yielded, "I can't stand the sound" for 
which there were sound engineering reasons. Today with 24bit 196KHz 
sampling and playback, I think it is impossible to discern that analog 
signals via LP vinyl are technically superior.  However, that won't sway 
a number of folks who "like the sound of analog". Sounding "different" 
will always appeal to some segment Direct Sampling vs superhet/IF DSP vs 
pure superhet.  btw I suspect that the inherent amplitude/phase 
distortions at the audio level of different amateur radio filtering 
techniques affects intelligibility more than is commonly recognized.

3. Unfortunately, the ham radio market isn't currently large enough for 
a major Si firm to design for our needs.  The requirements are driven by 
the cell base station market and perhaps a bit of military needs.  So 
sample rates are going up beyond the current 300 MSPS rather than more 
bits of resolution which would solve the overload concerns.  However, as 
the technology of fast + wide A to D's disperses, there may be a 
boutique firm that takes up the challenge as has happened in the audio 
market where the best A/D's are not made by Analog Devices, TI, or 
Linear Tech.

4. It seems that the overload issue is now confined by consensus to 
duplex operations on the same band since high pass and bandpass filters 
and stubs take care of most BCB/MW/multi multi overload situations.  I 
agree that overload otherwise is a minor concern.

5. As always there will be a range of good and not as good 
implementations of technologies.  Also, the terminology of "SDR" is a 
mess since it is applied to several generations - 1. outboard audio DSP, 
2. integrated audio DSP, 3. integrated IF DSP, and 4. direct RF 
sampling.  Many posts seem to me to confuse generations 3 and 4 and 
perhaps different capability radios.

6. Moore's Law continues and more MIPS and FPGA gates will become 
cheaper and better.  It seems to me that the direct sampling technology 
offers a number of opportunities for better signal processing than IF 
SDR's.  Maybe not, time will tell.  FLEX has a teaser with "wide band 
noise reduction" in their latest 6000 release.  What is that?

7. One thing unlikely in affordable superhets in the opportunity to 
decode many signals on many bands simultaneously.  The contest rule 
writers have/are going to have increasing difficulty with what that 
capability means to contesting.  e.g. a 5 band skimmer for SSB? (since 
my Samsung S5 has nearly perfect speech recognition in a noisy car, is 
that too far out?).

8. The content (syntax and semantics) of contest and DX messages are 
mostly proscribed, so our brains ability to discern (or make a good 
guess) of content in very poor signal to noise and QRM/QRN situations is 
pretty impressive and varies hugely among operators. (rant: why do 
DXpedition callers insist on telling me their state or they are QRP or 
are running 50w to a dipole in the basement?)

9. This thread has been quite interesting and informative.

73,
Grant KZ1W
ex TX5D, TX7G, E51MKW


On 10/19/2015 13:06 PM, Tom W8JI wrote:
>> I'm still of the opinion --right or wrong-- that there will *always* be
>> hams using analog technology that will be able to out-hear anyone 
>> using an
>> SDR (even DDS) to copy very weak CW signals at the low end of 160. 
>> *But I
>> have an open mind.* I think it was Barry N1EU that disagreed with me on
>> that (I think he has an Anan DDS SDR). But we need people like him that
>> drive us to investigate SDR further. :-)
>
> I think it depends on the individual. If an individual has the mental 
> ability to "process" noise out of the signal, external filtering and 
> "noise reduction" won't mean nearly as much. Some people I've operated 
> with are better than I am, some the same, and many others just cannot 
> hear the signals unless they are crystal clear.
>
> I'm poor at SSB, but good at tone.
>
> My first experience with this was when a group of people came over to 
> pick me up to go to the Cincinnati hamfest. I was working VK's on 160 
> (using a modified SX101) through heavy noise, copying the callsigns 
> fairly easy, but no one else could even tell there were signals.
>
> Another case was at Dayton, when MFJ was demonstrating a DSP. I could 
> hear the signals the same with or without the DSP, and people walking 
> up were marveling. Others walking up couldn't hear the difference.
>
> When a human is part of the decoding system, results will vary.
>
> A similar thing is true for results at different stations, when we 
> talk about overload. One size does not fit all applications. I see now 
> where the one station's comments about a bunch of modest signals 
> overloading an SDR kicked off the "popular folklore" rebuttal, but 
> 1500 watt transmitters into antennas less than 2 wavelengths from an 
> RX antenna are not the same as something far out of band one or more 
> miles away.
>
> We have to read carefully, and not mix cases.   :)
>
> 73 Tom
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>