Hi Zach, This idea is feasible (as Tamas points out) and is done, but it is very difficult to get the error rate down to as low as you can get with binary modulation. That's why it tends to be used in analog comms rather than digital. This idea reminds me of one I had when I was about 13 years old. I thought that we could send so much more data in a given bandwidth if we would simply use analog voltage to indicate value. I also thought that we could include "reference pulses" which were a known voltage to compensate for changes in gain. All of these ideas have the same problem, and that is that in a real-world comms channel with noise, they become just as limited if not more than the typical methods. You should Google for Shannon-Hartley Theorem. This is a theorem in information theory which tells you the maximum overall bitrate you can transfer (after errors are corrected) through a communications channel of a given bandwidth and signal to noise ratio, assuming additive Gaussian noise. No matter what modulation scheme/error control coding you use, you cannot do better than this limit. There are several modern digital comms methods which get very close to this limit. Most of the challenge in digital comms comes in dealing with distortion (both in terms of a non-flat frequency response, delay which varies with frequency and time, multipath, and nonlinear distortion) and non-Gaussian noise (like spikes from lightning strikes or man-made devices). In general, bandwidth efficiency and noise rejection are at odds with each other. In other words, complex schemes like multi-level phase shift keying, quadrature amplitude modulation, etc. are good at cramming as many bits per second into a small bandwidth, but require a high signal to noise ratio to do so. Simple bandwidth-hogging schemes like FSK (frequency shift keying) take up at least several Hertz of BW per bit per second, but are more tolerant of lower signal to noise ratio. Keeping total power fixed, and assuming that the noise is white so that there is equal power per Hertz of bandwidth, then there comes a point where increasing bandwidth doesn't buy you any more bits per second. Spread spectrum modulation methods operate near this limit so that they can achieve the greatest bit rate at the lowest signal power. More complex digital modulation schemes are usually employed to deal with the "challenging" items mentioned above, like multipath. Multipath is when you have several paths which the signal can have from TX to RX (like the direct path plus several reflections) and they arrive out of phase with each other, distorting the signal. The lower the symbol rate (the fewer changes per time in the signal), the less effect multipath will have, so schemes which work well in multipath often use parallel channels, like several different relatively slow FSK channels close together which, combined, give a respectable bit rate. It is interesting to note that all modulation schemes which approach the Shannon-Hartley limit for bitrate per signal strength sound pretty much like white noise (QAM, OFDM, and CDMA/DSSS are three prominent examples). Sean On Tue, Jun 24, 2008 at 7:36 PM, Zachary Noyes wrote: > Hi, > > I have a really neat idea that could save a lot of time and money on an RF > project (not to mention have a greater baud rate), but I don't know if it > will work. > > Lots of pics like the 16F877 have a feature that when a signal on a pin goes > high, an interrupt can be triggered. > Pulse duration modulation is an RF modulation technique like morse code > except the info is incoded in how long the pulse is high or low. > > > Why don't we have an RF signal coming into an antenna, which is fed into a > rectifier, which is then fed into a low pass filter, which is fed into an > non-inverting amplifier so we get a 0-5 v DC signal. This signal would go > into a pin with an interrupt enabled, say RB0 for the PIC16F877. > The pulse is off most of the time and on for a short period of time. When > the pulse turns on, a timer in the pic starts counting up. The timer stops > and the value is recorded on the next pulse. > > In this way all that you need to encode a number in a radio signal is two > pulses, rather than log2(n) number of bits (encoded in a modulation which > requires a DSP to decompose), where "n" is the maximum value of the > information. The info can be directly read by the PIC. > > Is this feasable, impossible, already done, missing a piece. I think its > awesome. Thanks. > > --Zach > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist