--- [EMAIL PROTECTED] wrote: > Hi Joe, > > You want us to believe that PM is why we Pre-emp FM. > > Yup. It's a PM world, and you must make your FM > equipment work in that world. > > > That's simply not the case. This is not supported by > anything I have ever seen or read, only by you. > > These are conclusions we drew from old documents > from the 40s. We don't have > anything firmer than that, nor have we seen anything > that implies preemphasis > was intentionally added to a flat system by any > standards body. > > While I do agree that preemphasis improves > intelligibility, again there is no > evidence it was added with malice and forethought. > > I would request that if you, or anyone, has any > written material at all that > sheds light on how PM and FM came to be, share it > with the group. > > Frankly, we don't even know how 455 kHz became an IF > standard - - but > somebody, a long time ago, picked it and it stuck. > These things happen. > > 73, > Bob
This is an interesting article: http://www.repeater-builder.com/rbtip/fmtheorydiscussion.html He suggests that de-emph came first to get around the rising noise of an FM receiver. In any case, I think most people agree that it was done for noise control purposes, which it does do well. If not, it would have been abandoned. Most forms of FM use some sort of pre/de emp on the signal, whether it's broadcast FM, land mobile, FM TV, or analog FM microwave. In analog microwave, a lot of times you have the choice of pre-emp or not, and the pre-emp doesn't kick in until the last decade or so. In FM broadcast the pre-emp starts at about 2K, which is about a decade below the FM upper limit. On land mobile, the pre-emp is only specified from 300 to 3000, or about 1 decade. Pre and de-emphasis in any system doesn't run from DC to the upper limit- there is some upper and lower cut off frequency. Regarding links or repeaters with flat audio: I think that the only place the audio should be pre or de emped is in the end users radio, or in items that communicate as an end user (phone patches, voice synthesizers, etc.) There is no need to pre and de emp at a repeater, and especially no good reason for doing it on links. If you look how an FM stereo station gets it's audio to the TX site, through an STL, all of the processing, clipping, pre-emp, etc is at the studio. This pre-emped composite audio is then fed to a 900 MHz STL, containing FM and it's stereo and other subcarriers. At the STL RX site, raw baseband audio from the STL goes right to the FM mod- no de-emp or other processing, other than a LPF at 70KHz or so, depending on subcarriers. If the signal has to go through multiple hops, it goes as-is, without pre or de emping at each intermediate site. The best sounding analog microwave systems use IF repeating, they don't even break the signal down to baseband. If we wanted really true "simplex sounding" audio, that would be the way to do it, since we just take the users input frequency and "move" it to the output channel. This would be impractical for repeaters, but it could be implemented in point to point links that had multiple hops, and didn't need mod/demod at each site to accomodate users. Joe __________________________________ Do you Yahoo!? Yahoo! Small Business $15K Web Design Giveaway http://promotions.yahoo.com/design_giveaway/ Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
