Whether you need a discriminator tap or not depends solely on the signal and the app you are trying to use. If it's a high speed control channel, such as those found on P25 systems, then yes, a discriminator tap is needed for reliable decoding. MPT 1327 signals (there are a few systems in the US, not many...), on the other hand, being low speed, doesn't. That is something that needs to be modified inside the scanner, not in a PC.
Each laptop seems to have a somewhat flaky standard for audio inputs; some would need a level-reducing cable that would go to either a mic input or headphone jack, others don't seem to need it. Such cables are easy to find at places like RS, etc. Again depending on the application, some need a level converter to convert a digital signal to audio. Looks like Spectran just needs an appropriate cable.
Decoding ACARS signals (found in the VHF aircraft band) would not use a discriminator, since AM detection happens differently than in the FM-based bands. Same thing with CW signals (nominally on HF, but trunk systems- using FM - use CW for the IDer).
I should think that neither a discriminator tap nor a headphone/speaker connection would give you the bandwidth you would want to see a 'chunk' of the spectrum. That takes sampling that would happen before the detection/audio stages of a scanner or receiver. When you see a display like Spectran's, you are only seeing a couple of khz bandwidth in either direction (plus/minus)
Software defined radios are a different animal altogether. They take a signal sent to a PC, sometimes via the USB port (such as the new Pegasus SDR that was demo'ed at the recent Winterfest), sample the output at various rates, and then display the output in a graphical or other format on a screen. In true SDR (and this is a matter of some controversy) all the processing is done in software, the 'receiver' is essentially a black box that generates a USB (serial, too) signal. That's part of the reason why it's so expensive. There are, however, software packages that allows one to view a spectrum of frequencies (say a Mhz wide or so) and click/tune the receiver to a selected signal. The RX320 (which is HF only, and although it does use DSP for generating RF, the audio stages are completly analog, and thus not true DSP) can do this, for example.
Where this is going is a very expensive solution. There are what are known as spectrum display units, or SDUs. These are not hooked into a discriminator tap, but to a very different point in the receiver. No scanner I know of has this capability. It shows a whole portion of a spectrum in a graphical manner. Very expensive receivers, such as the AOR AR5000, can drive these SDUs. There have been whole books and treatises written on the subject of DSP, sampling and so forth - so there's no way to cover such topics here.
Bottom line - what you want to do is possible, but costs mucho bucks
73 Mike
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Last edited by ka3jjz; 03-18-2008 at 08:16 AM..
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