[Amps] Audio processing-Part 2

[Jim Thomson]

manfred sez...


Now if the radios you mean happen to use low quality second sideband 
filters, with slopes that aren't steep enough, and with poor stopband 
rejection, then of course there will be more splatter. But that's a 
problem of cheap implementation, not of the principle.

##  The DX engineering RF clippers used on my old drakes used
tiny filters, think they were 4 pole units.   Best version was the
rob sherwood unit, that used real  8 pole filters, one for each sideband.  





With current DSP technology pretty sophisticated and clean speech 
processing can be implemented, at very low cost. Good speech processors 
in DSP often shift the audio signal to some low RF (which is the same as 
passing it through a balanced modulator and sideband filter in the 
analog world) and then apply the compression and limiting, precisely to 
move most of the artifacts out of the passband and then filter them away.

###  Which xcvrs  do that ?    On paper it would work...provided you could
use a DSP filter  to filter out the DSP artifacts.  I havent see any version of a digital  DSP based
clipper, only limiters and compressors.. but that is not at low RF freqs... just 20-20k  audio. 
I have dsp  6 x band audio compressors, but they are for a different application.  


When I started writing this post, I intended to question that remark - 
but now, after having looked closely at all those ALC implementations, I 
see your point. Let's be kind and think that amplifier manufacturers 
provide ALC outputs tailored to specific radios. Obviously those 
manufacturers that make the amps as accessories for their own radios 
will tailor them to these radios, but I wonder what the amplifier-only 
manufacturers have in mind, when defining their ALC outputs... Maybe 
some specific, widely used radio? Or the radio the company's boss 
happens to use?

####  You just nailed it, there is no standard, hence the incompatibility 
issues.  



And broadcasting is very different from ham operation, too. In 
broadcasting, you set up the transmitter on one frequency, then usually 
run it there, 24/7, for years. Or at least for hours, in shortwave 
broadcasting. You have no frequency changes, and no thermal drifts from 
circuits heating up and cooling down. In ham radio instead you change 
frequency often, you also might change bands every now and then, and 
your transmitter stages are all the time changing temperature, due to 
the RX/TX cycling. So there is far more need for ALC than in broadcasting.

> VE7RF does extensive audio processing in his station.

###  Even if the xcvr in question has overshoots, its still not an issue.
Either develop the ALC voltage externally, or  limit the  audio...b4 it
gets into the  xcvr. 

Audio processing in ham stations, using external consoles with 
compressors, AGC, equalizers, etc, seems to be all the rage at present. 
Some hams indeed produce excellent transmission in that way. Others not 
so much. I keep hearing hams with lousy signal quality, bragging about 
their studio mikes and all the audio equipment they are running to 
process their audio signal.  Some even add cathedral-style echo effects, 
like those CB operators of 30-40 years ago! :-)

##  ESSB has been going on since late 90s.   Its a very tricky setup
depending on what gear is used, and how things are configured. 
If  I know the station on the other end is using narrow band RX, like
300-2700 hz,  I have to configure entirely differently  vs a  station that
has a wider RX BW... like say  100-3900 hz.   Wide band  TX ssb will never 
sound correct on a narrow band RX, it just doesnt work.   The EQ
setup on TX is such that it wont sound right unless the RX BW at the
other end is the same BW..or wider.   Even if you play with the shift control
on a narrow band width RX, it still wont sound right.   You get either the bottom,
middle, or top end, but you can never get all of it at once.   When a bunch of wider band
ops  all of  a sudden all get lousy audio reports from a fellow using narrow band RX..and 
also a  2 inch diam speaker built into the  top lid of his icom, we have a bw issue. 
For all intensive purposes we are transmitting a mode that the narrow  RX BW station
cant copy.   

 manfred sez...


The funny thing is that so many hams use this sophisticated audio 
processing on HF SSB, where HiFi audio makes little sense, due to all 
the usual QRM and QRN and the intrinsic limitations of SSB. It would be 
far more logical to strive for excellent audio quality on VHF and UHF in 
FM, but that's something no ham in my area does.

##  Not true.  It works superb on HF..it also works superb on VHF. 
On HF, I have not used phonetics since  2001, even on a real noisy 
80m band, during the summer months..with ssb signals  buried in the noise. 
Several of us ran thousands of exhaustive tests on  intelligibility vs BW.  
The results are jaw dropping to say the least.  On most tests the bottom 
end was cut off at 300 hz.   The top end was cut off initially at  2700 hz, then 
incremented slowly upwards in small steps to aprx 4000 hz.  Slight changes in
the digital tx eq had to be tweaked in as this is all going on.  
Once u hit aprx 3600 hz at the top end, the clarity  comes alive.   
3600-300 =  3300 hz BW.... which is not much wider
than ur typ ham xcvr.   With the top end at 3900+ hz,  its razor sharp.   You can easily
go from Q3  to Q5.  I recorded a fellow  doing the above tests on my sony rack mount
mini disc setup, and it still astounds me to this day.  Then the above tests  were done
again, but   TX po reduced such that signals were right at the noise level.  Same deal, but
even more pronounced effect.   Then a bunch of similar tests done with dx stations that
were also essb equipped.   We tried everything from  300-2700.....all the way up to
300-5800 hz.   Intelligibility  increases every time the top end was increased.  Turns out we just
re-invented the wheel, as the BW  vs  intelligibility is well  documented in  tech references, like 
SSB systems and circuits.... written by collins engineers.   They have already graphed it all out,
with supporting documentation.   They used standardized tests, which also included using
non sensical  random word groups. 

##   BTW, the audio eq setup used for say a 300-3000 hz tx BW is not even close to what is
used on say a 300-4100 hz setup.   Its not just the extra top end either. The  lower and middle portion
is tweaked quire a bit differently.  Hence when a RX station listens to wide band ssb.... on his narrow band
RX, it sounds bad. 

Its all fine as long as no QRM, and no contest is on.   Its not like there is 
loads of wide band ssb signals strewn across each band.  They typ congregate
on one  or two freqs on each HF band.   Typ what happens is even if I tx with a top end
cut off at say 3900 hz, I will RX  with a top end of 5800 hz....so I can listen to both
wide..and  wider ssb stations.   Not to harp on the subject, but some of us have experimented
with audio processing on RX.   Everything from gentle downward expansion on RX, to  extensive
digital eq-ing.   I also tried adding artificial harmonics to the RX audio, to widen the BW, then
eq-ing the added harmonics...with great success.    I 1st tried that stunt when listening to a KH6
calling cq in a contest..on 15M...just as the band was about to pack it in.   That was beyond an eye opener.

Jim   VE7RF



Manfred