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In a message dated 4/8/2004 11:11:35 AM Mountain Daylight Time, [EMAIL PROTECTED] writes:
FM was forced upon the land mobile mfg as users wanted to modulate digital signals directly onto the RF carrier and not go through the modem/demod process. FM and PM are equally linear, its the additional low pass & high pass filters, audio limiters, audio amplifiers etc that introduce the distortion products into the system. Land Mobile was designed to sent understandable information between two points, not Hi-Fi audio. Look at what we are now accepting in cell phones.
Yes!
RF Links: The Land Mobile radio equipment was never designed to be used for RF point to point links, but as there are people that want to get by with as little expense as possible most anything will be put to use. If you are fortunate enough, you should use 900 mhz and higher microwave equipment with baseband multiplexing. Then you won't have to worry about the links audio quality.
Yes! Yes!
For those that still want to use mobile radio equipment for linking I would offer the following practical suggestion from my experience. One should look at the manufactures specifications of their pre/de-emphasis, you should find something like +3/-8 db of a standard 6 db per octave curve. This presents you with a considerable problem. The audio levels and sounds can vary considerably over the radios audio pass band, even from radios of the same manufacture.
I think it's real important to point out here something to those who want to bypass the preemphasis and deemphasis in their repeaters. It is NOT the slope of the 6 dB/octave curve that varies +3/-8 dB! That curve will always be 6.000 dB/octave - - that's first-order filter theory. Likewise, the inherent preemphasis in a phase modulator will always be 6.000 dB/octave. There is no such thing as a 5.999 dB/octave curve or a 6.001 dB/octave curve. As far as I know, it is impossible to even construct such filters without using some kind of high-powered digital technique.
The +3/-8 dB specification tells the manufacturers that any filter or other circuitry they design into their radio cannot result in a response that is outside the specification.
73,
Bob
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