On 2/26/2011 12:52 PM, Rsoifer@aol.com wrote:
> Jerry, Lee, and others,
>
> It may be useful to draw a distinction between digging weak signals out of
> the noise and improving the SNR on stronger signals so they sound better.
> As we know, most of the intelligence in (male) human speech is below about
> 2400 Hz. The human ear is very good at disregarding higher frequencies, so
> passing the signal through a low-pass filter will make it sound better
> but, for most good operators, won't make much of a difference in their
> ability to dig it out of the noise.
When the operator is fresh, yes. After several hours the operator can
become fatigued and then needs all the help the hardware can give. But
the brain extraction of weak signals from noise is sort of a correlation
process and if the noise bandwidth is too narrow, just like correlation
noise reduction in a DSP it works less well with narrow band noise.
Same for CW. One year at FD at our club station the CW rig was a TS-430
owned by a ships sparks bought overseas with a factory narrow CW filter.
That radio didn't seem to have the option of selecting the filter or not
for CW and so it was always in the circuit. So I couldn't switch to a
wider filter and that filter rang enough on noise and was narrow enough
the noise had a pitch to it, so copying CW I had a constant tone to
discriminate against which wore me out in less than 4 hours of operating.
In my FT-857D, I've found the audio DSP CW filters do nothing to improve
S/N of a CW signal below the noise level or to make it easier for my
brain processing to do it, but the Collins mechanical CW filter does
improve the S/N of a CW signal below the noise. The difference between
copying and not copying on long VHF paths.
73, Jerry, K0CQ
>
> 73 Ray W2RS
>
>
> In a message dated 2/26/2011 6:16:09 P.M. GMT Standard Time,
> geraldj@weather.net writes:
>
> That's where I find my passive speaker filter shines. It passes no audio
> section noise and no IF noise, an few DSP HF artifacts.
>
> A fundamental of receiver design is that selectivity works best as close
> to the antenna as possible. Unfortunately that ignores the noise
> contributions of all the stages after that. The typical product detector
> is double sideband so the IF noise of the image is there along with the
> signal and the RF noise that passed through the filter plus the same
> sideband noise much wider than the filter that was up front. Receivers
> would benefit from having a SSB filter at the product detector, but I
> know of only one design that way, called the Hohentweil, a 2m
> transverter kit. Then they would benefit from making the audio output
> stage, often essentially a power op amp into an active low pass filter.
>
> In tube receivers a simple capacitor from audio output tube plate to
> ground combined with the tube and the audio output transformer to make a
> rudimentary low pass filter. In the 75S-3B, it was effective enough to
> make using 2125/2975 tones for 850 shift RTTY (and for all recorded
> history, the standard tones for 850 shift RTTY due to an AT&T standard)
> difficult until the capacitor was removed from the circuit.
>
> 73, Jerry, K0CQ
>
> On 2/26/2011 11:57 AM, kc9cdt@aol.com wrote:
>> I think one of the reasons the Drake R-4B, Hallicrafters SX-117 and
>> many others are beter in a noisy condition is simply they do not have
>> all the high frequency respnse in the audio, or maybe it is the tube
>> amp??. I wish there was a HF cutoff on the OII, full EQ like Bob Heil
>> recommended day one to TT way back may have helped.
>>
>> Interestng...last nght, on 40 I was working a really nice guy in St
>> Kitt. There was quite a lot of QRN, He was just above the noise floor
>> I found that if I used the old Hallicrafers SX-117 to receive
>> him...copy was more clear!!!!
>>
>> OMG, Maybe we need to go back to the older stuff (I have both) Unless
>> of course it is contesting at a high level...where you need lot of speed
>>
>> I use the Collins S line& KWM-2
>> Drake C line (all Sherwood mods)
>> Halli SX-117/HT-44
>> Halli SX-115/HT-32B
>>
>> Along with the OII of course.....
>> 73,
>> Lee
>>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|