In a message dated 11/22/2002, Jeffrey Race writes:
> I have two USR 5560 PC Card modems hooked up back to back over a very > clean circuit (basically just two battery-feed circuits connected by > two 1 uF capacitors). My terminal program always reports connection > at 31200 V42bis whether I enable or disable compression. Why might I not > get 33600? Could it be something about the modems? > Hi Jeffrey: It might be the modems themselves, the laptops they are in, or some other subtle problem. Remember that everything has to be almost perfect to achieve the 33,600 rate. In my experience, PC card modems usually perform less well than external modems, due to compromises that the designers made in order to fit into a small form factor. In your test circuit, are the two 1 uF caps in series, yielding effectively 0.5 uF? One thing you might try is changing the capacitors to get a minimum of 10 uF coupling the modems. The top speeds in V.34 have energy all the way down to about 200 Hz, so the coupling cap(s) have to be pretty large to avoid influencing the performance. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 http://www.randolph-telecom.com
