[AMRadio] AM Transmitter "quality"


Charles Ring w3nu at roadrunner.com
Tue Sep 15 15:05:19 EDT 2009


Grant Youngman wrote:
> On Sep 15, 2009, at 1:11 PM, Joe Hankins wrote:
>
>   
>> I
>> operation was effective radiotelephone service. Human speech falls  
>> into
>> the range between 300 and 3000 Hz. There are sounds outside this  
>> range,
>> but they do not contribute to intelligibility.
>>     
>
> That's actually a complete myth.  There are frequency components both  
> above and below this range which, if present, contribute in a major  
> way to speech intelligibility.  The fundamental frequency in  vowels  
> is closer to 100 Hz, and frequencies well above 3 Khz (actually above  
> 4Khz) are needed to reliably distinguish an "f" from an "s".  There  
> are also significant intelligibility cues in the 20-80Hz range for  
> some consonants.
>
> 300-3000 hz was convenient for the phone company and was proselytized  
> by Collins (which we all know is never incorrect in design philosophy)  
> as the gospel truth.  But it  contributed mostly to Uncle Bob and  
> Granny being so hard to understand and sounding so lousy on the  
> telephone.  And contributes still to "pinched" sounding radios.  Your  
> modern telephone (fortunately) exceeds this old accepted range in  
> bandwidth.
>   

While modern telephones can produce better audio (no carbon mics), 
modern phone exchanges don't. Th e old exchanges passed whatever audio 
was fed into them at least for local calls, but digital switching has a 
brick wall cutoff. I grew up a block away from the local Bell central 
office so I got spoiled with unusually good phone audio. Further out, it 
was limited by loading coils and line capacitance but now nothing 
outside the 300-3000 cps range gets through at all. There is also audio 
degradation from the digital chopping, not severe but I can hear it. A 
call from a digital cellphone, through a digital central office, to my 
digital answering machine (the worst in the chain) is often 
unintelligible to me.

The 500/2500 telephones, carbon mic and all, were the best sounding 
phones ever. It was the culmination of 75 years of phone evolution and 
was incrementally improved for decades after it was brought out in 1949. 
After those phones there was a break in continuity of knowledge and too 
many old lessons were forgotten. Electret mics can sound good but not 
when they are in a poor design meant to be as cheap as possible.

73 de W3NU, with a 2565HKM phone on my desk.


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