jgorman01 wrote: > I may be wrong but I beleive your theory doesn't assume that the RF > energy at your reciever's antenna is not additive. In other words, > the signal from the transmitter you want to hear and the interfering > signal do not add together. You can only discern the strongest > signal. An example is, that if you put a carrier on the air and I > receive it at S9 and then someone else puts a carrier on the exact > same frequency but it only arrives at S8, I'll never know it is there. >
The only kind of receiver that does this by design is one in which a limiter is in the front end (or other undesireable nonlinearity) and exhibits the capture effect (such as FM receivers). So you statement is false. They do add to the extent that the front end and following stages continue to operate linearly. If BPL is very close to you, and the other signal is weak, then it is possible (easy) for the audio difference coming from a typical amateur radio transceiver to exceed the dynamic range OF THE AUDIO CIRCUITS (not the front end or the IF's). For some of the SDR transceivers, where the audio dynamic range is presented digitally to the processing programs (the audio never leaves through a D/A or speaker or wire of any type) through a virtual audio hookup (done with software wires rather than hardware wires). I feel this is one of the major assets of these SDR approaches (SDR-1000, GnuRadio, HPSDR). The other major asset of each of these SDR systems is that they can, or soon will be able to, tune a signal that is almost 200 kHz wide. > Therefore, when you remove the interfering signal, you also remove any > possibility of retreiving information from the signal you want to > hear. Consequently, you will never have a coherent signal to decode. > It will always have missing information. > There is information degradation but your "nonlinearity" reason is incorrect insofar as it went. > Any other assumption means noise, especially random noise, would not > be a problem, and that you could always subtract a signal from it. > Every mode I know of, digital or analog, has a minimum signal to noise > ratio that is required to decode it. > This is correct. BPL is not very noise like in comparison to a very well designed digital system where randomization of the data, forward error correction, source coding (compression) all make the data look flat random. So a system to defeat BPL must be designed to overcome the nonrandom statistics of the BPL excitation. There will be a signal / (noise+interference) ratio beneath which we cannot go further. This is well understood what it is even if it is difficult to compute (with interference being decidely nongaussian and the channel having memory). If you demand 1000 bps from the channel and the capacity is 999 bps, you have exceeded the capacity and information will be lost. Any system that will communicate through BPL interference must degrade gracefully as the channel information capacity decreases. It is the demand for fixed rate where that rate exceeds capacity that leads to a graceless collapse in our HF systems. One of the things the illegal encrypted communications Pactor III does, is decrease the information rate to be below the capacity (any digital system where the complete specifications allowing duplication are not published is a violation of Part 97) to allow for continued communications at the reduced rate. There is no "one size fits all" with these horrid HF channels. Any system designed to replace what we have will ultimately have to accept this and build for it. > Jim > WA0LYK > > > > Bob N4HY -- AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman "You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat." - Einstein Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/digitalradio/join (Yahoo! ID required) <*> To change settings via email: mailto:[EMAIL PROTECTED] mailto:[EMAIL PROTECTED] <*> 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/
