The FCC permits commercial AM broadcast stations' to transmit a signal that has up to 125% positive modulation but the negative modulation cannot exceed -100%. The best description I have seen for why exceeding -100% at the modulation troughs is not allowed is this quote:
"[The] effect, commonly described as "pinch off", produces at the "corners" at the start and finish points in time of the straight zero line, a spectrum of spurious emissions that typically `spatter` beyond the band limits and potentially interfere with adjacent channels in violation of broadcast regulations."
Follow that statement up with this quote from the same material:
"In recognition that the interference problem originates at "pinch-off" at the envelope troughs and not at the envelope peaks, regulations have been relaxed to allow some amount of "upward" modulation, e.g. to a level corresponding to 125% modulation as long as "pinch-off" and resultant out-of-band "splatter" interference are avoided."
So my first question is, if the problem with splatter is not caused by the positive modulation, why is it regulated to be cut off at 125%? Are there some ill effects by exceeding that percentage that the FCC (and other agencies) are wanting to avoid?
On a related note, I'm reading that because the human voice (and other things) that are broadcast are assymetrical, the phase of the source (normal or inverted) becomes a factor in maintaining at least 100% modulation. Because of this, stations use sophisticated equipment to monitor and detect whether or not the input signal has inverted or normal phase and then automatically switch the output phase to maintain modulation percentage. A write-up on the issue and (of course) a device to handle this issue is described here: Posi-Phase. My question is, does this issue apply to the typical AM ham station? I mean, since we're typically using our same mic, transmitter, amp, etc., do we need to concern ourselves with phase shift?
I think I have more questions...
"[The] effect, commonly described as "pinch off", produces at the "corners" at the start and finish points in time of the straight zero line, a spectrum of spurious emissions that typically `spatter` beyond the band limits and potentially interfere with adjacent channels in violation of broadcast regulations."
Follow that statement up with this quote from the same material:
"In recognition that the interference problem originates at "pinch-off" at the envelope troughs and not at the envelope peaks, regulations have been relaxed to allow some amount of "upward" modulation, e.g. to a level corresponding to 125% modulation as long as "pinch-off" and resultant out-of-band "splatter" interference are avoided."
So my first question is, if the problem with splatter is not caused by the positive modulation, why is it regulated to be cut off at 125%? Are there some ill effects by exceeding that percentage that the FCC (and other agencies) are wanting to avoid?
On a related note, I'm reading that because the human voice (and other things) that are broadcast are assymetrical, the phase of the source (normal or inverted) becomes a factor in maintaining at least 100% modulation. Because of this, stations use sophisticated equipment to monitor and detect whether or not the input signal has inverted or normal phase and then automatically switch the output phase to maintain modulation percentage. A write-up on the issue and (of course) a device to handle this issue is described here: Posi-Phase. My question is, does this issue apply to the typical AM ham station? I mean, since we're typically using our same mic, transmitter, amp, etc., do we need to concern ourselves with phase shift?
I think I have more questions...