WO2002082790A1

WO2002082790A1 - Sound card based measurements for testing telephone lines

Info

Publication number: WO2002082790A1
Application number: PCT/US2002/010233
Authority: WO
Inventors: Gerald C. Hammond
Original assignee: Porta Systems Corporation
Priority date: 2001-04-03
Filing date: 2002-04-03
Publication date: 2002-10-17

Abstract

A device for testing a telephone line (Z) includes a personal computer (PC) and a sound card electronically communicating with the personal computer (PC). The sound card is capable of full duplex operation and includes an audio frequency signal output (Vs), a left channel signal input (L) and a right channel signal input (R). The audio frequency signal output (Vs) of the sound card is connected to one end of a sense resistor (Rs) and to the left channel signal input (L). The other end of the sense resistor (Rs) is connected to the telephone line (Z) and to the right channel signal input (R). An audio frequency signal (Vs) generated by the sound card is applied to the telephone line (Z) through the sense resistor (Rs). The parameters of the audio frequency signal (Vs) are measured by using the left channel signal input (L) of the sound card. The parameters of the response of the telephone line (Z) to the audio frequency signal (Vs) are measured using the right channel signal input (R) of the sound card. The parameters of the response of the telephone line (Z), with respect to the parameters of the audio frequency signal (Vs) generated by the sound card, are analyzed.

Description

SOUND CARD BASED MEASUREMENTS FOR TESTING TELEPHONE LINES

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on U.S. Patent Application Serial No. 60/281,126, filed on April 3, 2001 and entitled "Sound Card Based Measurements for Testing Telephone Lines".

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to methods and apparatus for testing telephone lines.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for testing telephone lines for faults.

It is another object of the present invention to provide apparatus for testing telephone lines for faults.

It is yet another object of the present invention to provide apparatus for testing telephone lines, which apparatus is inexpensive and readily available and which may be used in conjunction with a personal computer (PC).

It is a further object of the present invention to provide apparatus which tests telephone lines for faults, which apparatus may be easily carried by a technician and used at any location where telephone lines are accessible.

In accordance with one form of the present invention, a method for testing a telephone line includes the steps of applying an audio frequency stimulus, such as a voltage, through a sense resistor to one of the tip and ring line of the telephone line, and simultaneously measuring the response, such as a voltage, appearing on the telephone line. A sound card, connected to a personal computer, is preferably used to apply the audio frequency stimulus and to measure the response. More specifically, the sound card generates an audio frequency voltage which is applied through a series sense resistor to the tip or ring line. The voltage is measured by connecting the end of the sense resistor which is connected to the voltage output to one of the left and right channel input of the sound card. The response of the telephone under test is measured by connecting the other end of the sense resistor which is connected to the tip or ring line to the other of the left and right channel input of the sound card. The complex impedance of the telephone line is preferably measured at several frequencies. The telephone line is then analyzed using the impedances measured from the tests conducted using the sound card. This provides a model of the telephone line from which faults and line quality may be determined. Also, the broadband performance of the telephone line under test may be predicted using the line model developed through the analysis of the impedances of the telephone line measured using the sound card.

In another form of the present invention, apparatus for testing a telephone line includes a personal computer, a sound card electronically communicating with the personal computer and a sense resistor. The sound card is capable of full duplex operation in that it can act as a signal generator and a recorder simultaneously. The sound card includes an audio frequency voltage output and at least a left and a right channel signal input. The audio frequency voltage output is connected to either the ring or the tip line of the telephone line being tested through the sense resistor i.e., the sense resistor is serially interposed between the audio frequency signal output of the sound card and the tip or ring line of the telephone line under test. The left and right channel input of the sound card are connected across the sense resistor. If desired, multiplier resistors may be included between the left and right channel inputs of the sound card and the opposite ends of the sense resistor in order to increase the input impedance of the sound card.

These and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments thereof, which is to be read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a block diagram of a preferred form of apparatus used in accordance with the present invention to test telephone lines.

Figure 2 is an illustration of a computer display illustrating the spectrum analysis of the signals received on the left and right channels of the sound card used in the apparatus of the present invention.

Figure 3 is an illustration of the display of a computer showing a graph of the phase measurement of a telephone line circuit under test and plotting phase against frequency.

Figure 4 is a block diagram of an alternative form of apparatus used in accordance with the present invention to test telephone lines.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The sound card or equivalent subsystem in a modern personal computer (PC) has the following characteristics:

a. The ability to simultaneously generate and capture ("record") stereo signals in the audio spectrum. This means that the sound card can generate the stimuli and perform the measurements associated with telephone line testing.

b. Generates and records audio signals using high resolution digital-to- analog and analog-to-digital conversion. Typically, sound cards use 16 bit resolution which gives the same quality as compact disc (CD) recordings and can provide the accuracy required for telephone network testing. This is the same resolution as the hardware in a standard test unit product sold by Porta Systems Corporation, Syosset, New York, U.S.A., i.e., the CL680 MKIII or Sherlock™ Remote Test Unit (RTU). Tests using sample sound cards show comparable accuracy to the MKIII RTU.

c. Is driven by software that runs on the PC. This allows conventional applications to use the sound card features, e.g., Microsoft Windows Media Player™, RealPlayer™ etc., as well as specialized applications such as that described herein. By applying an audio frequency stimulus (e.g., a voltage) through a sense resistor and simultaneously measuring the voltage appearing on a telephone line, it is possible to:

1. Measure the complex impedance of the telephone line at several frequencies.

2. Analyze the telephone line circuit using the impedances measured from step 1 above. This provides a model of the telephone line from which faults and line quality can be determined. The method by which the impedances are analyzed is described in Robert DeTullio et al. PCT Published Application No. WO 99/62188, published on December 2, 1999, Serial No. PCT/US99/11617, entitled "Apparatus and Method for Testing a Telecommunications System," the disclosure of which is incorporated herein by reference.

3. Predict the broadband performance of the telephone line under test using the line model determined from step 2 above. This forms the basis for "qualifying" the line for high-speed digital data transmission using various digital subscriber line (DSL) technologies such as asymmetric DSL (ADSL), high speed DSL (HDSL), symmetric high speed DSL (SHDSL), etc.

A PC based testing solution in accordance with the present invention can be realized using:

I. The sound card to perform the measurements on the telephone line.

II. Simple hardware (not shown, but readily implemented by one skilled in the art) that externally switches the sound card connections in test and calibration modes. In "test" mode, the sound card output is connected through a sense resistor to the telephone line circuit under test to provide an audio frequency stimulus to the telephone line. Measurements are then performed using the sound card inputs connected to either side of the sense resistor to determine the complex impedance of the line, as disclosed in detail in the aforementioned published PCT Application No. PCT/US99/11617. This can be repeated at several frequencies. In "calibration" mode, the telephone line is not connected to the sense resistor and measurements are used to determine hardware correction co-efficients and offsets. More specifically, before a test on a telephone line is conducted, the sound card and external sense resistor should be calibrated. The calibration circuit would look just like that shown in Fig. 1, except that the telephone line, represented by the letter Z, is disconnected from the sense resistor, Rs. A test is run by having the sound card generate an audio frequency, time varying signal, which is monitored on one side of the sense resistor with one input channel (e.g., the left channel) of the sound card, and monitoring the signal on the other side of the sense resistor with the other input channel (e.g., the right channel) of the sound card. The offsets and intrinsic capacitances of the sound card and sense resistor may then be determined. These correction factors can then be applied to obtain accurate measurements in the "test" mode, i.e., when the telephone line is connected to the sense resistor, Rs. Additionally, the simple hardware, which is external to the personal computer (PC), may include other components. These may be used to protect the sound card from any harmful voltages that may be present on the telephone line and/or provide copy protection ("dongle").

III. Signals from the PC that control the hardware in II above. These signals can be derived from any of the standard PC communications interfaces, e.g. serial (Com), parallel (Printer) or universal serial bus (USB).

IV. Application software that drives the sound card and controls the simple hardware required for calibration and testing. This software may be easily written by one skilled in the art.

A typical application would be the use of a laptop or portable PC that can be easily carried by a technician and used at any location where telephone lines are accessible. These locations would include the exchange e.g., the central office (CO), the main distribution frame (MDF), cross connection cabinets and the customer's premises. With appropriate software, which may be easily written by one skilled in the art, the system could then be used for fault location or digital subscriber line (DSL) qualification testing.

Accordingly, a standard 16-bit sound card can provide accurate measurements to support line qualification using line modeling algorithms, such as those disclosed in the aforementioned PCT Application No. PCT/US99/11617. The conclusion from tests (described below) using two different sound card products is that PC sound cards do appear to provide a viable hardware platform for voice frequency based line qualification.

Sound cards have become a standard feature of both portable and desktop PC's. The apparatus of the present invention requires little or no additional hardware and is therefore highly cost effective for a PC based test system.

Description of Tests Performed

A mid range desktop PC was used that had an integrated Crystal Audio sound card manufactured by Cirrus Logic, Inc. of Austin, Texas, U.S.A. Additionally a "Sound Blaster™ Live! Value" card manufactured by Creative Labs, Inc. of Milpitas, California, U.S.A. was used to provide comparative testing. These cards are typical of low to medium range sound cards retailing for between $20 and $50. The PC specification was:

• DELL Model 400M workstation with Windows 2000 SP 1

• CPU speed 300 MHz/66MHz bus

• 64MB Ram

All measurements were performed using SpectraPLUS™ FFT Spectrum Analysis System software manufactured by Sound Technology, Inc. of San Jose, California, U.S.A. This application uses Windows™ drivers and is sound card independent. The only requirement for this application is that the sound card is capable of full duplex operation, i.e., it can act as a signal generator and recorder simultaneously. This appears to be standard for all "modern" cards.

The application provides many features including dual channel spectrum analysis, real time waveform display and phase comparison between Left and Right channels. The signal source phase cannot be guaranteed since many cards use wave table and/or DSP technology. In the tests described, phase measurements rely upon two sensing inputs and a continuous swept source covering the measurement range (although testing at discrete frequencies may be suitable). A white noise source was also effective when performing phase measurements but required longer averaging due to the lower power spectral density (PSD).

The application will capture raw 16-bit PCM data for off line processing. However, the SPECTRUM and PHASE display screens (shown in Figs. 2 and 3, respectively) with cursors to measure the signal amplitude and phase respectively were used to simplify the study.

An initial look at the noise profile using the spectrum analysis feature showed a floor at around -95dBm, and with averaging, it was possible to detect signals as low as -1 lOdBm. This is comparable with results obtained using the aforementioned Porta Systems CL680 MKIII RTU.

Test Configuration

Both sound cards had the following external connections:

• Mono microphone - high sensitivity low impedance 5mV/2k Ohms.

• Stereo Line input - max sensitivity around 100 mV FSD (full scale deflection) and 10k-20k Ohm impedance.

• Speaker output - low impedance with a full scale range of around 2-3 volts pk-pk.

The Line inputs were used with 100k Ohm multiplier resistors, Rin(L) and Rin(R), to increase the input impedance. The gain of the line inputs was adjusted to compensate for the associated attenuation.

No attempt was made to check or calibrate the absolute voltage levels - the impedance calculations require only relative amplitude and phase of the source and measured voltages. The readings in Appendix A are therefore "nominal", not absolute.

Measurement Method

The Left and Right line inputs were connected as shown in Fig. 1. Vs (source) is generated by the left or right speaker output on the sound card. The voltage is measured by the Left channel, and the voltage across the network under test, Z, is measured by the Right channel. Rin(L), Rin(R) and Rs are resistors. Each Rin = 100k Ohms, and Rs = Ik Ohm. This configuration allowed measurement of relative phase and amplitude of source and load voltages. The phase offset was measured with no load connected. Of course, it should be understood that the connection of the left and right channel inputs to the sense resistor, Rs, may be reversed.

Results

Appendix A gives the results of tests with Z = lOOnF and Z = 100 nF in parallel with 5k Ohms. This corresponds to the line capacitance of a telephone line circuit of around 1.5 km in length. The measurement frequency was also varied between 100 Hz and 5kHz, a 50: 1 ratio.

The test configuration was capable of measuring the load capacitance with an accuracy of better than + 0.4% at frequencies between 500Hz and 2000Hz, with some loss of accuracy at 100Hz and 5000Hz. Limitations of the SpectraPLUS test software were considered to contribute to the errors. For example, it was difficult to manually set amplitude and phase measurement cursors to the same frequency as the source, and on screen amplitude results were rounded to 3 significant figures. These errors would be eliminated if the computations were performed directly on the 16-bit raw data.

The reduced accuracy at 5000Hz may be due the test software or series resistance losses in the capacitor under test.

Conclusions

The sound cards tested were capable of accurate measurement without any form of calibration using an accurate sense resistor, Rs, as the reference. Further optimization is possible. The purpose of the tests was to establish the basic feasibility of the invention. A solution for three terminal impedance measurement could be realized using both output channels connected via sense resistors to the circuit under test. In other words, the tip and ring line of the telephone line may be tested simultaneously with the sound card. Most sound cards have a left speaker output and right speaker output. These may be used as audio frequency voltage sources, Vsi and Vs₂, for example. Also, most sound cards have a left and right line input, such as shown in Figure 1 , but also have a left and right microphone input, M_L and M_R, for example.

Two sense resistors, Rsi and Rs₂, may be used. One end of resistor Rsi is connected to the left speaker output (Vsi), and to the left line input of the sound card, while the other end of resistor Rsi is connected to the tip (or ring) line and to the right line input of the sound card. One end of resistor Rs₂ is connected to the right speaker output (Vs₂) and to the left microphone input M_L, and the other end of resistor Rs₂ is connected to the ring (or tip) line and to the right microphone input M_R. Of course, the left and right speaker outputs may be reversed, as well as the left and right line and microphone inputs. Also, input resistors Rin (1) - Rin (4) may be used on the line and microphone inputs to increase the input impedance, if necessary, in the same manner as resistors Rin (L) and RIN (R) are used in the previous embodiment shown in Fig. 1. Measurements are made in a similar manner as described previously, and the characteristics of the telephone line may be determined using the methods and algorithms disclosed in PCT Application No. PCT/US99/11617, described previously. This alternative circuit of the present invention is shown in Fig. 4.

Although the illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.

Appendix A: Measurements Sound Blaster Live! Value

Meter

Rs 996 impedance = 122000 Sampling Rate 48k measured ci 100 nF cap. (TX3) = 1.0020E-07 32k FFT Avg. 10

100nF

Phs

F Hz Vs VI Phase ofst. Phase Rad. Vlreal Vlimag Ireal limag Yreal Yimag Rparallel Xparallel Cparallel Cerr %

9.99295E- 101 0.270.269 3.6 0.01 6.2657E-02 2.6847E-01 1.6844E-02 1.5340E-06 -1.6911 E-05 1.7549E-06 6.3102E-055.6984E+05 15847.48 08 0.27

5000.2170.206 17.29 0.02 3.0142E-01 1.9671 E-01 6.1156E-02 2.0369E-05 -6.1402E-05 5.9308E-06 3.1398E-04 1.6861 E+05 3184.889.9944E-08 0.26

9.98047E-

10000.2480.209 31.8 0.04 5.5432E-01 1.7770E-01 1.1001E-01 7.0578E-05 -1.1045E-04 8.9572E-06 6.2709E-04 1.1164E+05 1594.66 08 0.39

9.98787E- 20000.2210.137 50.89 0.09 8.8663E-01 8.6588E-02 1.0617E-01 1.3495E-04 -1.0659E-04 1.9629E-05 1.2551E-035.0944E+04 796.74 08 0.32

1.00242E-

50100.2690.081 71.36 0.21 1.2418E+00 2.6170E-02 7.6656E-02 2.4380E-04 -7.6964E-05 7.3277E-05 3.1555E-03 1.3647E+04 316.91 07 -0.04

100nF in parallel with 5k Ohms Rparallel = 5.0000E+03

F Hz Vs VI

101 0.271

5000.217

10000.248

20000.221 0.127 46.02 0.09 8.0163E-01 8.8333E-02 9.1248E-02 1.3320E-04 -9.1615E-05 2.1119E-04 1.2553E-034.7351 E+034.9263E+03 796.62 9.9894E-08 0.31 f_l

50100.284 0.083 68.16 0.21 1.1860E+00 3.1159E-02 7.6929E-02 2.5386E-04 -7.7238E-05 2.8570E-04 3.1841E-033.5002E+033.6036E+03 314.06 1.0115E-07 -0.95 -27.li

Appendix A: Measurements

Crystal Audio Meter

Rs 996 impedance = 122000 Sampling Rate 48k measured

Cl 100 nF cap. (TX3) = 1.0020E-07 32k FFT Avg. 10

100nF

Phs

F Hz Vs VI Phase ofst. Phase Rad. Vlreal Vlimag Ireal limag Yreal Yimag Rparallel Xparallel Cparallel Cerr%

100 0.554 0.544 3.09 -0.51 6.2832E-02 5.4293E-01 3.4158E-02 1.1118E-05 -3.4295E-05 1.6439E-05 6.4202E-05 6.0833E+04 15575.94 1.02E-07 -1.98

500 0.541 0.509 17.03 -0.07 2.9845E-01 4.8650E-01 1.4967E-01 5.4720E-05 -1.5027E-04 1.5946E-05 3.1378E-04 6.2712E+04 3186.93 9.99E-08 0.32

1004 0.54 0.452 31.88 0.01 5.5624E-01 3.8386E-01 2.3865E-01 1.5677E-04 -2.3961E-04 1.4647E-05 6.3332E-04 6.8272E+04 1578.97 1E-07 -0.19

2006 0.539 0.332 51.09 0.09 8.9012E-01 2.0893E-01 2.5801E-01 3.3139E-04 -2.5905E-04 2.1785E-05 1.2668E-03 4.5903E+04 789.42 1.01E-07 -0.30

5002 0.509 0.152 71.86 0.24 1.2500E+00 4.7928E-02 1.4425E-01 4.6292E-04 -1.4483E-04 5.6124E-05 3.1906E-03 1.7818E+04 313.42 1.02E-07 -1.32

100nF in parallel with 5k Ohms Rparallel = 5.0000E+03

Phs Cerr

F Hz Vs VI Phase ofst. Phase Rad. Vlreal Vlimag Ireal limag Yreal Yimag Rparallel Rparallel' Xparallel Cparallel % Rerr %

4.7927E+ 100.5 0.554 0.455 2.43 -0.51 5.1313E-02 4.5440E-01 2.3337E-02 9.9999E-05 -2.3431 E-05 2.1685E-04 6.2701 E-05 4.6115E+03 03 15948.769.93E-08 0.90 -4?^*

4.8150E+ 500 0.541 0.431 14.47 -0.07 2.5377E-01 4.1720E-01 1.0821 E-01 1.2430E-04 -1.0864E-04 2.1588E-04 3.1640E-04 4.6321 E+03 03 3160.60 1.01E-07 -0.51 -3r7

4.7788E+ 1004 0.54 0.394 27.43 0.01 4.7857E-01 3.4974E-01 1.8144E-01 1.9103E-04 -1.8217E-04 2.1745E-04 6.3369E-04 4.5987E+03 03 1578.06 1E-07-0.25 -4f4.

4.6522E+ 2006 0.539 0.307 45.97 0.09 8.0076E-01 2.1372E-01 2.2039E-01 3.2658E-04 -2.2128E-04 2.2315E-04 1.2654E-03 4.4814E+03 03 790.23 1E-07 -0.20 - i

3.9069E+

5002 0.509 0.149 68.54 0.24 1.1921 E+00 5.5092E-02 1.3844E-01 4.5573E-04 -1.3900E-04 2.6415E-04 3.1868E-03 3.7857E+03 03 313.80 1.01 E-07 -1.19 -21,8

Claims

WHAT IS CLAIMED IS:

1. Apparatus for testing a telephone line, which comprises: a personal computer; a sound card electronically communicating with the personal computer; and a sense resistor having first and second ends; the sound card being capable of duplex operation and having an audio frequency signal output and a left channel signal input and a right channel signal input, the audio frequency signal output is connected to the first end of the sense resistor and to the one of the left channel signal input and the right channel signal input, the second end of the sense resistor being connected to the telephone line and to the other of the left channel signal input and the right channel signal input.

2. A method for testing a telephone line, which comprises the steps of: applying an audio frequency signal generated by a sound card to a telephone line through a sense resistor, the sound card having an audio frequency signal output which is connected to the sense resistor on a first side thereof, and a left line input and a right line input, measuring the parameters of the audio frequency signal by using one of the left and right line inputs of the sound card; measuring the parameters of the response of the telephone line to the audio frequency signal by using the other of the left and right line inputs of the sound card; and analyzing the parameters of the response of the telephone line with respect to the parameters of the audio frequency signal generated by the sound card.