--- [EMAIL PROTECTED] wrote: > Hi Kevin, > > When we (Scott and I) go to build a repeater for > someone or are talking about > a radio set that has or needs an FM modulator, it > gets referred to as true > FM. This helps us to eliminate confusion about what > type of modulator needs to > be installed into a radio set that ultimately > transmits FM in any event. > > Yup, I know what you're saying. The Handbook people > mention 'direct FM' and > 'indirect FM' as alternate terms, but I don't like > those any better. > > But if I may critique your last sentence from a > strict definition standpoint > (and here's another place where Bob goes over the > edge): Yes, you can install > either an FM or a PM modulator. If you choose the > FM, it must be accompanied > by a preemphasis circuit that covers the entire > voice audio spectrum.
I think this is where you go astray. FM doesn't require pre-emp, it will work fine without it. Pre-emp has been added to make the FM sound better by redistributing the voice energy. You can have a "true FM" exciter that runs or doesn't run pre-emp. In digital paging, we don't run pre-emp, but we still use true FM exciters. We end up with FSK, which is basically the data input modulated onto the carrier frequency. A "true FM" modulator or exciter is denoted by it's ability to shift a certain amount of deviation for each volt of signal applied to the modulator, and that doesn't change with frequency. A PM exciter doesn't do this. If you > choose the PM, you won't need a preemphasis circuit. > Such a system, by strict > definition, is not an FM system, it's a PM system. > If we had an FM system, we'd > all be transmitting flat audio. Since we don't > transmit flat audio and we do > transmit preemphasized audio, an engineer outside of > our circle would say we > have a PM system. Or an FM system with pre-emphasis. This would be more correct, since the PE only covers a certain portion of the bandwidth. You can't run a "true PM" system and have it sound right, since the clipping has to be done "post pre-emp". If you run it without clipping, you will exceed your allocated bandwidth. Here is how a commercial PM transmitter works: Audio comes into the exciter and is pre-emped. PE audio is then clipped at some value, then applied to a LPF. Filtered audio is then applied to a "de-emp" stage, then on to the PM. We are not using the "inherent" pre-emp built into PM as such; conversely, we have to engineer solutions around it! > > I can hear it coming: "Whoa! This here's a True FM > transmitter, and no > lilly-livered engineer is gonna tell me that this > thing transmits anything but True > FM!" Yes, but there's nothing wrong with a PM > system. In fact, there doesn't > have to be a single PM modulator in the whole system > - - it can consist of 100% > FM modulators and still be a PM system. The whole > definition rests on whether > or not the audio is transmitted preemphasized. See > where I'm coming from? > So, if I build a repeater using "flat audio" techniques, is that a PM or FM repeater? If someone talking through it has bypassed their pre-emp, would it be FM for that user, and the next guy has got regular pre-emp, so would it be PM for him? I think when we talk on this board, and FM or PM is mentioned, we're usually talking about specific hardware that has these specs, not talking about a "system". A Micor mobile may be PM and a Micor base may be FM, not because they will sound significantly different, but due to the way their exciters were built. > I will readily admit that available PM exciters can > have all sorts of > shortcomings. They distort when you ask for a lot of > deviation at very low > frequencies like CTCSS. There are inherent > nonlinearities in the varactor tank circuit > that create a little distortion. Tuning the > multipliers correctly is important > for low distortion. And we've seen some sloppy > engineering in the design of > the audio and limiter circuits. But if a guy wanted > to, he could design one heck > of a great PM exciter using, say, DSP, and it would > work exactly as well as > any FM modulator. > This is probably true, but no-one I know of has done this. Reason why? It's much easier to build/buy a "true FM" exciter that has the characteristics people want and need, without going through the trouble of re-inventing PM to do the same thing. > Has this topic has been bantered about for a long > time? Yes! Will I change > anyone's mind? No! Is it time I finished this and > did some real work? Yes! Am I > starting to sound like Donald Rumsfeldt? Heavens, > yes! > > Have a great day! > > 73, > Bob > I don't dislike PM modulators, I have used them on a number of repeaters (and they sound good). But as you have ponted out, there are certain limitations to them, especially at low modulation freqs. There are certain people out there (KC comes to mind) that like their repeaters to faithfully reproduce sound down to 40 or 50 Hz. This is very difficult for currently used PM exciters (ala Micor or MII) to do. I prefer filtered audio on my repeaters, everything below 300 hz is thrown out. Does this make it sound different (ref "simplex")? Yes, it would have to. Does this make it sound worse? That depends on the individual. I like the filtered HPF sound. And in so filtering, I can make use of the PM equipment, since there are no really low freqs to worry about. In this instance, it is nearly impossible to tell the difference between a PM and an FM exciter, since over the area being used (300-3000 Hz) the exciters will have virtually identical freq responses. 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/
