I was just sitting here ruminating, and remembered something I'd read in some Ham magazine a millennium ago, about how phase modulation is easier to do than FM, but they wrote that it has a "tinny" sound.
Anybody wanna discuss that?
I was also thinking about the spectrum you can see on a 'scope when you use an RF sweep generator with the sweep fed into the X axis (horizontal, instead of the 'scope's own sweep), and the output goes to the Vertical. That's fun, too. :-)
Cheers! Rich
Rich Grise wrote: > I was just sitting here ruminating, and remembered something I'd read > in some Ham magazine a millennium ago, about how phase modulation is > easier to do than FM, but they wrote that it has a "tinny" sound. >
Worked at a TV station with an RCA TT10AL channel 3 back in '76 It used PM for the aural transmitter. The phase modulation was applied to a
152.2KHz signal that got multiplied x 432 for a final carier frequency of 65.75 MHz. The multiplier was needed to get the 25KHz deviation. The main difference was a 6dB/octave applied to the signal to avoid the 'tinny' sound. Needless to say, they needed a new exciter when they went stereo.GG
The aural cabinet in the RCA TTU25B did the same thing to create the modulated Aural carrier for their 25 KW UHF transmitter. Basically it was a standard 1 KW FM broadcast transmitter, where the final stage was replaced with another multiplier. The output was fed to the Aural power amplifier cabinet and amplified to 12.5 KW then on to the diplexer to combine the Aural and Visual signals to feed a single Jampro antenna. This transmitter was used by WACX on Ch 55, then I moved it to the Florida panhandle and built WRMX, Ch 58 around it. Who knows how many other stations it was used at before then. In fact, it was built from a TTU-1 and a TTU25B that had a fire in the exciter cabinets. Since the TTU25B was a TTU1 with power amplifiers, HV power supplies and control circuits added it was a simple matter of replacing the burnt panels with ones from the earlier transmitter.
-- Service to my country? Been there, Done that, and I\'ve got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
FM and PM are related. Phase is the integral of frequency. If you listen to PM on an FM receiver, the audio is differentiated (high-pass filtered) by the process. This is why you apply pre-emphasis (low-pass-filtering) before the phase modulator. This produces true FM; but it may be difficult to get the equalisation right throughout the audio range.
Pre-emphasis is high pass. The subsequent de-emphasis at the receiver reduces noise, since it is a PM effect.
Yes, broadcast FM transmitters use high-pass pre-emphasis to improve the signal-to-noise ratio; but the OP was asking about ham radio. Many amateur NBFM (narrow-band FM) transmitters use phase modulators or reactance modulators to generate "indirect FM" from what is essentially a PM transmitter, by low-pas-filtering (integrating) the audio input. Perhaps I was wrong to use the term "pre-emphasis" in the indirect-FM context.
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