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Analog filter for Teletax pulse attenuation and method for tone detection through decimation and selective bandpass filtering

IP.com Disclosure Number: IPCOM000016961D
Original Publication Date: 1999-Apr-01
Included in the Prior Art Database: 2003-Jul-21
Document File: 3 page(s) / 26K

Publishing Venue

Siemens

Related People

Alberto Canella: AUTHOR [+3]

Abstract

The detection of the Teletax (TTX) tones at 12 or 16 kHz is often a requirement for the line interface (e.g. Siemens ANIC-Chipset). But because the maximum TTX amplitude could reach 10.5 Vrms at the tip-ring it needs to be attenuate to the maximum allowed value by the input-stages (about 3 Vrms). Because of the two different frequencies a suitable analog filter must provide this attenuation at both the TTX frequencies. The structure proposed for the attenuation consists of a capacitance in series with a resistor (R 2 ), between Tip and Ring, and two coils in parallel with a resistor (R 1 ) along both Tip and Ring, as shown in Fig.1. The Lowpass action is provided by the capacitor C, in combination with loops over the ANIC-Chipset, while the coils are suitable to let the filter be transparent to DC and to the voice-band signals. At high frequencies the input impedance will converge to the sum 2*R 1 +R 2 , so it is possible to control this value by a suitable choice of R 1 and R 2 .

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Bauelemente

Analog filter for Teletax pulse attenuation and method for tone detectionthrough decimation and selective bandpass filtering

Idee: Alberto Canella, Villach (Österreich); Manish Bhardwat, Villach (Österreich);

Manfred Kogler, Villach (Österreich)

The detection of the Teletax (TTX) tones at 12 or 16 kHz is often a requirement for the lineinterface (e.g. Siemens ANIC-Chipset). But because the maximum TTX amplitude couldreach 10.5 Vrms at the tip-ring it needs to be attenuate to the maximum allowed value bythe input-stages (about 3 Vrms). Because of the two different frequencies a suitable analogfilter must provide this attenuation at both the TTX frequencies.

The structure proposed for the attenuation consists of a capacitance in series with a resistor(R 2 ), between Tip and Ring, and two coils in parallel with a resistor (R 1 ) along both Tip andRing, as shown in Fig.1. The Lowpass action is provided by the capacitor C, in combinationwith loops over the ANIC-Chipset, while the coils are suitable to let the filter be transparentto DC and to the voice-band signals. At high frequencies the input impedance will convergeto the sum 2*R 1 +R 2 , so it is possible to control this value by a suitable choice of R 1� and R 2 .

Another problem containing the two frequencies is the detection. Typically the detectionrequires a highly selective bandpass filter followed by a power detector. If the powerdetector registers power above a certain threshold (defined by the TTX-standard), aninterrupt to the central Digital Signal Processor is generated. In the low cost line interfacechips it is of utmost importance to implement tone detection functionality as efficently andas cheap as possible.

The current solutions needs to have programmable coefficents because of the need tohandle two different frequencies. Moreover the filter has to be extremely robust as it has tomanage a dynamic range up to 44dB in tone power. Because of these requirements WaveDigital structures are often employed.

The introduced idea solves the problem without using filter structure based on the WDFtheory and without modifying any coefficients to...