Hi Gullik, Real world modems need channels a little wider than the symbol rate, this is called the "excess bandwidth". In our case I use an excess bandwidth of 50%, which means a 75Hz wide signal for a 50 Hz symbol rate.
I am a bit fuzzy on multipath issues myself, but my understanding is: 1/ The low symbol rate (20ms) means the channel delay (say a few ms) is a small fraction of the symbol period, so doesn't influence the demodulated symbol much. 2/ The sum of the direct and multipath signals can cause frequency selective fading. With a FDM (parallel tone) modem this means that just one carrier will get wiped out. Thanks, David On Tue, 2012-06-12 at 13:05 +0200, Gullik Webjörn wrote: > It strikes me that the intercarrier spacing is 75 hz, > but the modulation rate is 50 baud. Either the speed > could be increased to 75 baud, or the number of > carriers could be increased, and still maintain separation > between the carriers? > > Is this the elusive "guard interval", i.e. that we have > 20 mS (1/50) to decode, instead of 14 mS (1/75) which > will buy us another 6 mS to sort out multipath issues? > > Regards, > > Gullik > ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
