Dear Duane and Ben, It is indeed possible to have a modem approved without filters in them. They don't even check whether you remain connected, only whether your impedance isn't too low at 16KHz. It is unclear to me how well a modem would perform without it. The ones that had no filter but were used for demos worked pretty well with V.42 switched on. A difference between Germany and for instance Switzerland is that in Germany the metering tones are enabled by default on all lines, one has to request them to be disabled. Switzerland, as far as I know doesn't have them enabled on all the lines by default.
Best regards, Rene ---------- From: [email protected] on behalf of Duane J Marcroft Sent: Tuesday, March 25, 1997 3:31 AM To: [email protected] Cc: [email protected] Subject: Re: Filter for Germany Ben, I have designed a number of PCMCIA cards in the last several years and none them have billing tone filters for Germany. I believe it is now possible to have a modem approved in Germany sans filters. The modem is now allowed to make some errors in the presence of metering tones, but it must not drop the line. Also, if you use the inductance of the transformer in the correct way it is possible to increase the Rockwell modem tolerance of metering tones beyond 0.7 vrms and in the majority of connections the modem will not hiccup. By adjusting modem firmware it is possible to bridge hiccup periods and not disconnect. Regards, Duane __________________________________________ On Mon, 24 Mar 1997, Ben Wrigley wrote: > Section 1.7.8(D) of NET4 specifies that a +21 to +23dB (950mV) 16KHz metering > pulse signal should not impair communication. The best way to ensure this would > presumably be to filter out these signals using an LC filter. > > Some experimentation shows that a Rockwell 288ACFL chipset can live with 0.7V > r.m.s of 16KHz and possibly more, when the signal is AC-coupled to the line. > This shows that the bandpass filter on the DSP input stage is quite efficient > at removing 16KHz sine waves. The main obstacle to getting a good connection > is the fact that if the AC 16KHz signal is greater than the DC voltage, then > the DC voltage over the electronic inductor will drop to a level below which it > does not function during some of the cycle which will clip the modulated data > signal. What is needed is an LC filter with enough attenuation to stop this. I > expect 20dB would suffice. > > Does anyone out there know of a ready made filter that can be bought and > inserted between the DAA and the German wall socket to achieve this? > > Regards > Ben > > ------------------------------ > > Ben Wrigley > Approvals Specialist > > KTL > Newlands Science Park > Inglemire Lane > Hull > HU6 7TQ > UK > > Phone: +44 (0)1482 801801 > Fax: +44 (0)1482 801806 > Web: http://www.ktl.co.uk/ktl > Email: [email protected] >
