This article is fairly long and somewhat technical, so you should feel free to skip it if you care to take my word that FM does *not* transmit a change in frequency based on a difference in source from moment to moment. I'm posting this because I don't like to see completely incorrect information flying about. In article <[log in to unmask]> 82 Malcolm Dean 213-5-5676 <[log in to unmask]> writes: > > ...FM does transmit a signal directly, rather than > > any difference. Both FM and AM radio modulate a carrier to transmit > > their signal, an AM transmitter changes the amplitude of the carrier > > in proportion to the input signal, an FM transmitter changes the > > frequency of the carrier in proportion to the input signal. > > This is not a forum in radio theory, but your explanation > confirms what I said, which is that FM transmits a change in > frequency reflecting the difference in the source from moment to > moment. Nope. The following explanation is pretty simple. I'll just go over some basics first to make sure that nobody gets lost. When we convert sound to an electrical voltage using a microphone we come out with a signal that varies in amplitude (level) with time. This amplitude difference is measured in dB. (A dB is a measure just like a centimeter or a gram, but it measures the voltage or power of an electrical or sound signal based on an agreed-upon reference level.) This changing electrical signal can be sent to an FM transmitter directly or stored on a tape or a record or a CD or whatever and sent to a transmitter at a later time. Now, the FM transmitter always transmits a signal at the same level. The way it describes these level differences in the input, which is the information that we want to transmit, is though a corresponding change in frequency of the signal. The receiver looks at the frequency of the signal coming in, changes it back to an amplitude, and sends it out to an amplifier, which puts it out a speaker. (Incidentally, this need to convert a frequency to an amplitude is why an FM receiver is more complex than an AM receiver.) Now, our current FM system transmits a frequency that is *directly proportional* to the *absolute* value of the source from moment to moment. Here is a brief example of what the difference would be between linear FM modulations and delta FM modulation: Original Linear Delta Change in Signal Frequency Frequency Signal Amplitude Modulation Modulation Amplitude 0 dB 10.0 MHz 10.0 MHz 0 dB 5 dB 10.1 MHz 10.1 MHz 5 dB 10 dB 10.2 MHz 10.1 MHz 5 dB 10 dB 10.2 MHz 10.0 MHz 0 dB 15 dB 10.3 MHz 10.1 MHz 5 dB 20 dB 10.5 MHz 10.2 MHz 5 dB -10 dB 9.0 MHz 8.5 MHz -30 dB What the two systems have in common is that you designate a frequency, 10.0 MHz in this case, to be your carrier. Now here is where the difference is. An absolute level of 0 dB corresponds to the carrier frequency in linear FM. That means that every frequency has one, and only one, possible signal level associated with it. 0 dB is 10.0 MHz, 5 dB is 10.1 MHz, and so on. The formula, in this case, would be Frequency = Carrier + (Amplitude * a constant) Frequency = 10.0 MHz + (Amplitude * 0.1 MHz) One of the implications of this is that linear FM is not context sensitive. You can take any small randomly selected portion of the signal and deduce the original waveform from it. Delta FM, on the other hand, expresses a *change* in level, rather than an absolute level. Thus, the carrier frequency, 10.0 MHz in this case, corresponds to a change in level of 0 dB. If the previous level is the same as the current level, no matter what it is, the transmitter will send a frequency of 10.0 MHz. Thus you can get a 10.0 MHz signal for *any* level, so long as it's the same as the previous one. For example, if the previous signal level were 0 dB and the current one is 5 dB the transmitter would transmit a frequency of 10.1 MHz because 5 dB - 0 dB = 5 dB. If the previous signal were 10 dB and the new one were 15 dB, the transmitter would also transmit a frequency of 10.1 MHz because 15 dB - 10 dB = 5 dB. The formula here is: Freq. = Carrier + ((Current Amplitude - Previous Amp.) * Constant) Freq. = 10.0 MHz + ((Current Amplitude - Previous Amp.) * 0.1 Mhz) Here, any particular frequency could correspond to any particular amplitude level, depending on what the amplitude level before that was. You can't decode the original signal from any small random portion of the transmitter signal because you have to find out what the current frequency at the the beginning of your segment was. In short, our current system of modulation is linear FM, not delta FM. I've explained why, for the curious. I hope this clears up this mess, and that the explanation wasn't overly technical. cjs -- | "It is actually a feature of UUCP that the map of [log in to unmask] | all systems in the network is not known anywhere." [log in to unmask] | --Berkeley Mail Reference Manual (Kurt Schoens)