[ARC5] Lopsided modulation

[Richard Knoppow]

     Most voices are asymmetrical toward higher pressure. While 
there are recent standards about the polarity of microphones many 
older ones are not wired according to the standard. Also this 
applies to pressure microphones, which includes non-directional 
types but not cardioid or bi-directional ones.
     One can check the polarity of a pressure mic with the aid of 
a squeegee, just apply a puff of air to the diaphragm and see 
which way the scope trace moves. Note that this is the reverse of 
checking a loudspeaker for polarity, there a positive voltage on 
one terminal should make the diaphragm move outward, i.e., the 
speaker is compressing the air.
     There is a great deal in the literature about upward vs 
downward modulation. There is a definite limit to downward 
modulation, if exceeded it cuts the carrier off and produces 
something akin to a key click. However, if extended upward more 
than 100% what one gets is essentially reduced carrier AM, also a 
cause for distortion although it may not cause QRM if the 
modulator is capable of it. There have been all sorts of devices 
to get large upward modulation, none work although some were 
popular for a while. Some very fancy speech processors, like the 
famous (and infamous) CBS Volumax-Audimax, turned the polarity of 
the input signal over continuously in order to make its asymmetry 
always toward upward modulation. I could always tell stations 
using this combination because they generally sounded awful. 
Nonetheless they were popular for a time in the broadcast industry.
     As far as power supply regulation, I suspect a supply with 
poor enough regulation to cause chirping is also capable of 
causing FM on AM since the two are related.
    Someone asked about the BC-375, my first transmitter was a 
BC-375 modified to run with an external exciter. I still have it. 
I don't know if it had symmetrical sidebands because I didn't 
have the instruments to measure it, and never thought of it. The 
external exciter would have eliminated pulling due to the power 
supply anyway. It is a very well built and very simple 
transmitter and I may dig it out and put it on the air again. I 
think I had a bunch of spare 211s for it.

On 2/26/2018 2:27 PM, J Mcvey via ARC5 wrote:
> My TX dynamotor only sags about  35V between standby and 
> transmit. Probably contributes to the "chirp" on CW, but I don't 
> think AM modulation would drag it down that much.. I'm using a 
> 25V 65 amp SMPS to drive it , so once it starts, there's lots of 
> current available for steady operation. I get about 50W cw, 22 
> watts on AM carrier.  My SCR274 is all unmodified circuitry, 
> though, just like they did it on the B-17.
> 
> Someone mentioned that it may be that voice has more negative 
> spikes . This may indeed be the case. I know mine does! Take a 
> look at it on a scope while holding  a tone. Try different 
> octaves.  Not very symmetrical at all. The only way to test 
> reproduction symmetry, is to use a sine wave iat say, 1 Khz and 
> see what that looks like.
> jim

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL