[AMRadio] Audio response and Long distant QSO's (was: Suggestions Please)

[Donald Chester]

>From: "Jeff Edmonson" <w5omr at w5omr.shacknet.nu>


>...the 'rack audio' SSB guys who are adding pre-emphasis, tone-tailored 
>audio
>into their SSB rig, AFTER the Balanced modulator...

Jeff,

I'm not exactly sure what you mean.  You put the audio INTO the balanced 
modualtor and rf (in the form of a DSB signal) comes out.  Immediately 
following the balanced modulator is a bandpass filter to suppress one of the 
sidebands.  No matter how wide the response of the audio fed into the 
balance modulator, the filter will limit the bandwidth of the SSB signal, 
except for distortion products generated in the amplifiers that follow.  
Maybe the "hi-fi SSB" ops are replacing their bandpass filters with wider 
ones to achieve a wider audio  frequency response in the SSB signal.  The 
problem with that is that the skirt slope of a wider filter  is not as 
steep, so at lower audio frequencies, they lose sideband suppression.  
However, I have seen some phasing type SSB circuits using digital techniques 
in the audio phase shift nework, that can result in real hi-fi SSB with good 
sideband suppression down to 50 cps or so.

We need to make a distinction between "wide" signals resulting from wide 
audio frequency response, versus spurious sideband products resulting from 
distortion.  Whether AM or SSB, most "broad" signals result from distortion 
products (splatter), not the frequency response of the audio itself.  I 
doubt if a clean hi-fi AM or SSB signal would get much attention from Riley. 
  It's the guys who don't know what they are doing, and generate garbage way 
beyond the normal passband of the signal that are causing the problem.

At the Dayton FCC forum, this topic came up, and both Riley and Bill Cross 
seemed to indicate that the FCC was not contemplating specific bandwidth 
limits, because that would hamper experimentation.  They said the rules are 
intentionally vague in order to allow the maximum flexibility for 
experimentation.  But the rules do call for "good engineering practice" and 
they could use that to go after someone who repeatedly causes harmful 
interference with splatter from a distorted signal.

If activity on a band is light, for example during the daytime on 75 or 160, 
or on 10m when there is no skip propagation, I see no reason why a ham 
shouldn't run hi-fi double-sideband AM with audio response 20-20,000 cps if 
he so wishes.  But it wouldn't be good amateur practice to run the same 
signal when the band is heavily occupied.  It's a matter of common sense and 
consideration, not more restrictive FCC regulations.

On my signal, with the 3400 cps cutoff, the pre-emphasis curve with the 
rising response compensates for the loss of highs.  Normally, with flat 
response, if the  highs are cut off at 4000 cps or less, you need to cut the 
bass somewhere around 200 cps, or else the audio will sound bassy.  There 
has to be a balance in frequency response.  I have found that with the 
pre-emphasis, many report my signal as "broadcast quality", completely 
unaware that I am cutting off the treble at such a low frequency.  The upper 
midrange boost balances out the flat low frequency response.

With a "bassy" signal, there is a difference  between too much bass and not 
enough highs.  "Tinny" audio is usually the rusult of not enough bass, not 
too much treble.  Many times I have heard ham signals that lacked any high 
frequency response above 2000 cps, and they would get "bassy audio" reports, 
so what they would do was cut the bass by reducing the values of some 
coupling capacitors, and the rusult was extremely restricted audio, perhaps 
600-2000 cps, and it sounded weak, unintelligible, like a tin-can telephone. 
  The secret is to strike a  balance  between the highs and lows.  For good 
intelligibility, the lows need to be flat down to 200 cps or below, and the 
highs up to at least 3000 cps, with a proper response curve to strike a 
tonal balance.

A SSB signal should be approximately 1/2 the bandwidth of an AM signal with 
the same audio.  The pro-SSB advocates who claim a SSB signal is 1/3 the 
bandwidth of AM need to review their arithmetic:  one sideband is, by 
definition, one-half as wide as two sidebands.  The "1/3 bandwidth" signals 
can be achieved only by pinching the frequncy response of the SSB audio to 
the point of compromising intelligibility.  You hear plenty of SSB signals 
like that.  Next time, listen closely, and try to note how much of the audio 
you actually hear, and how much is missing while your brain subconsciously 
fills in what is left out.  Of course, most ham QSO's are trivial enough 
that it isn't hard to do, and in the case of DX contacts "you're 59 in ..." 
the vocabulary of conversation is so limited that it is easy to guess the 
missing parts, especially if phonetics are used.

We are not aware of how much information we miss even in normal 
conversation.  When I lived in France many years ago, I knew a girl who was 
a real George Harrison fan.  She had a record of one of his songs, and had 
difficulty understanding the words.  She asked me to write them down in 
English for her.  I thought it would be easy, since I had heard that song 
many times and thought I understood every word. So I listened to the record 
and  tried to write out the lyrics word by word.  I was amazed at how often 
I hadn't a clue exactly what the word was, due to slurred speech or the 
loudness of the instruments in the background.  Casually listening to the 
song, my brain subconsciously filled in the holes in the text, and left me 
with the impression that I understood perfectly.  We all do that every time 
we listen to anyone speak.  Just try to transcribe word for word what is 
said in a recorded conversation or speech.

That's the only reason anyone can understand so-called "communications 
quality" voice restricted to 600-2700 cps or so with a 2.1 kc/s SSB filter.

Don K4KYV

_________________________________________________________________
Send and receive Hotmail on your mobile device: http://mobile.msn.com