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SCA Injection/Modulation Levels

B

Brian Bowers

Hi guys. I don't know why I have such a hard time understanding the FCC rule on this, but maybe someone can clarify this for me, so I can understand it better. I am already aware that for every 2% SCA injection, you are allowed 1% modulation. I'm correct so far, right? However, can we for a moment forget about the injection part of this, and just look at it from a modulation viewpoint? Because that is really what I am looking at on the modulation meter for measurement Here is how I understand it. I could be wrong. Please correct me if I am.

Let's talk only in stereo operation mode on FM. Am I correct in thinking I have 100% total modulation for the main carrier (let's say 9% Pilot and 91% main modulation)? Now before I go any further, when adding SCA's, does any of that 100% main modulation ever go away? To put it another way, do I ever have to dial that back, or turn it down when adding SCAs?

As I understand Part 73.319 you have a maximum of 110% total modulation when any amount of subcarriers are used on an FM signal. Am I correct in thinking that 10% modulation, after you already have 100% modulation for the main, is devoted entirely to SCA injection?
 
Simplest thing to do is set your total mod to 100%, turn on the SCA and adjust the SCA injection to where the total mod is 110%, then reduce the output of the main channel composite to 105%. That assume you desire 10% SCA injection. So to answer the question, yes, you do need to reduce the composite output once the SCA is introduced into the mix.
 
I could have sworn years ago, it was pointed out in the rules, that if you had one SCA, you were allowed no more than 105% total modulation. I can no longer find that mentioned anymore. Here is part 2 of section 73.319 - During stereophonic program transmissions, modulation of the carrier by the arithmetic sum of all subcarriers may not exceed 20% referenced to 75 kHz modulation deviation. However, the modulation of the carrier by the arithmetic sum of all subcarriers above 75 kHz may not modulate the carrier by more than 10%
 
One way I've seen this interpreted is a 67 KHz only subcarrier could be injected 20% with 110% total mod. But a 92 KHz only subcarrier could only be injected 10% with a total of 105%.
?
 
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