[plug] Interference

[Tony Clark]

While this has little to do with linux, personally I find it an interesting
subject.  To keep it on topic, has anyone got an HP48G emulator to run on
linux  :)

>
>	This may or may not work. Assuming that you are trying to
>achieve 56k, this 
>	would require a signal of 56+ kHz while your friendly local
>transmitter will be 
>	pumping out 700-740 kHz (720[6WF]+/-20kHz). Some calcs will be
>required to get 
>	the parameters just right. Any filtering WILL degrade the signal
>performance.

This not actually correct.  The upper limit of a standard PSTN line in
terms of frequency is 3.4KHz.  The exhange won't pass anything higher than
that.  This is why filtering should and if done correctly will work.  You
should be able to get around 24dB of attenuation at the interfering
frequency using a simple second order filter.

>
>I think that you'll find that the underlying signalling rate for a
>56kb/s is 9600 baud

The underlying symbol rate can't and won't be any higher than 3400 or 3400
baud.  You will need a damm good line for that also.  The main reason that
there are few if any reliable connections actually at 56K.

>
>Encyclopaedia Britannica says...
>
>"In addition to the elementary forms of digital modulation described
>above, there exist more advanced methods that result from a
>superposition of multiple modulating signals. An example of the latter
>form of modulation is quadrature amplitude modulation (QAM). QAM signals

56K doesn't use digital modulation priciples.  Nyquist would and does come
into play if you go down that root hence the 33k limit on V34 which is
digital modulation.

56K uses the decrete level to represent a symbol.  I need would need to
read up a little but of the 256 possible levels about 160 are used.

Now back to HP48G emulator and linux :)


Tony Clark

Digital and System Groups Manager
ERG Telecommunications

Mobile Asia +852 93168907
Australia   +61 411577715