Trapezoidal modulation display (was "Re: [BC] Inovonics model 520")

[RRSounds@aol.com]

Date: Wed, 1 Feb 2006 07:16:33 EST, DHultsman5 at aol.com wrote:

<<
In a message dated 1/31/06 6:36:14 P.M. Central Standard Time,   
pirate at albanyis.com.au writes:

Ok guys,   thanks for the help.   Sounds like I need a bigger and brighter   
scope.   I have no problem setting mod on sine waves, but program is   much 
harder to see.


Thanks

Are you looking at RF carrier or modulated audio off carrier?

Dave
>>


Perhaps <pirate at albanyis.com.au> is not using a trapezoidal display, which 
makes things pretty clear.

For those who aren't familiar with this technique, you apply a sample of the 
modulated RF carrier to the vertical input of the 'scope, and the modulating 
signal to the horizontal input (don't use a sweep).

The resultant display resembles a trapezoid (hence the name) that becomes a 
triangle pointing to one side when modulation reaches 100% negative modulation 
(carrier pinch-off). (The side to which the trapezoid/triangle points is 
determined by the relative polarity of the audio with reference to the audio 
modulating the carrier).

By setting the dead carrier in the exact center of the screen and calibrating 
where the 100% neg. mod "point" occurs to, say, ten divisions from center, 
you can then use the horizontal scale on the scope screen to determine pretty 
accurately the positive modulation in the direction opposite from the negative 
modulation "point."

Like an X-Y display of stereo audio, this setup offers the user a plethora of 
information about the signal. With the trapezoid, you can see modulation 
level, density and amplitude and phase linearity.

But, if nothing else, it's fun to watch. :-)

Kind Regards, 
David


David P. Reaves, III
TransLanTech Sound, LLC
Creators of the Award-Winning "Ariane Stereo Audio Leveler"