GB1596377A

GB1596377A - Apparatus and method for testing a communication channel

Info

Publication number: GB1596377A
Application number: GB91/77A
Authority: GB
Original assignee: Communications Patents Ltd
Current assignee: Communications Patents Ltd
Priority date: 1977-01-04
Filing date: 1977-01-04
Publication date: 1981-08-26
Also published as: DE2759157A1; IT7847526A0; NL7800043A; IT1111354B

Description

(54) IMPROVEMENTS IN OR RELATING TO AN APPARATUS AX METHOD FOR TESTING A COMMUNICATION CHANNEL (71) We, COMMUNICATIONS PATENTS LIMITED, a British Company of Carlton House, Lower Regent Street, London SWI 4LS, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to an apparatus and a method for testing a communication channel.

In for example wired broadcasting systems it is necessary to be able to test the wired network over the range of frequencies which are applied to the network. Ideally the losses on the network should be uniform over the frequency range.

The technique of vertical interval sweep testing and recording (VISTAR) has been devised to test television channels over a range of frequencies but is applicable to communications channels generally. This technique comprises applying at the sending end of a communications channel during pauses occurring in the transmitted signals a signal the frequency of which over a succession of pauses sweeps back and forth across the frequency range in discrete steps. The frequency sweep signal is detected at a point where it is desired to make a test and displayed on an oscilloscope as signal level versus frequency.

Testing techniques used at present which avoid the need to interrupt signals being carried over the communication channel often require complex and expensive oscilloscopes and auxiliary apparatus to form a useful display from the received test signals, and even sophisticated oscilloscopes produce a faint trace. This is of real significance, particularly in the case of wired broadcasting systems where testing has to be carried out in the field rather than under laboratory conditions.

It is an object of the present invention to allow testing to be carried out without interrupting signals being carried over the communication channel and which will permit information provided by the test signals to be displayed by means of readily available and relatively simple equipisient.

According to the present invention, there is provided an apparatus for testing a communications channel by analysing a test signal received from the channel, the frequency of the test signal being varied in a periodic manner, characterised in that the apparatus comprises a counter, means for applying bursts of the test signal of predetermined duration to the counter, the counter registering the number of cycles in the burst to provide a measure of the frequency of the burst, means for determining the amplitude of the test signal burst, means for storing information representative of the determined frequency and amplitude, and means for reading out the stored information.

Preferably, the test signal bursts of predetermined duration are applied to the counter via a slicer. The contents of the counter may be stored in for example a random access memory and the stored information may be read out under the control of a further counter in a form suitable for display on an oscilloscope.

The further counter may also be used to generate a graticule for interlacing with the oscilloscope display.

According to the present invention there is also provided a method of testing a communications channel by analysing a test signal received from the channel, the frequency of the test signal being varied in a periodic manner, characterised in that bursts of the test signal of predetermined duration are applied to a counter which registers the number of cycles in each burst to provide a measure of the frequency of the burst, the amplitude of the test signal burst is determined, information representative of the determined frequency and amplitude is stored and the stored information is read out.

The method according to the present invention enables a television communication chant nel to be tested when the channel is carrying a programme signal without adversely affecting reception of the programme.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a first section of a testing apparatus for receiving a test signal via a television programme signal communication channel; Figure 2 schematically illustrates a second section of the testing apparatus of Figure 1; and Figure 3 illustrates an oscilloscope graticule generated by the apparatus of Figure 2.

The apparatus described hereinafter with reference to the drawings is adapted for use in testing a wired broadcasting network on which high frequency (H.F.) television programme signals in accordance with C.C.I.R. system I are broadcast, the signals being within an 8MHz band extending from 3MHz to 11MHz.

When it is desired to test the network, a test signal is applied to the channel with a broadcast television programme signal. The test signal is generated by a wobbulator (not shown) which provides a frequency sweep signal that sweeps the 8MHz band up and down at a maximum rate of 2MHz per second i.e. four seconds per sweep minimum in each direction.

An insertion timer (not shown) inserts a l4ps + 2ps burst sample from the wobbulator on the 19th and 332nd lines of each field of the television programme signal which is being broadcast, the burst samples starting 1 Indus i: 2us into the respective lines. The sending end burst sample level is -8dB relative to the unpreemphasised vision carrier synchronising signals.

When it is desired to- test for signal losses between the sending end and any point of the network, a test apparatus is connected to extract the test signal from the network at that point.

Such a test apparatus is illustrated in the accompanying drawings.

Referring to Figure lithe signals appearing on the network are picked up by a high impedance probe 1, amplified by a pre-amplifier 2, and passed via a demodulator 3, a sync level detector 4, a sync stripper 5 and a pulse generator 6 to control logic 7. The control logic 7 controls the sequence of operations in the apparatus of Figure 1. The inputs to circuit elements shown in Figure 1 which are labelled C are control inputs derived from the control logic unless shown otherwise.

At the start of each field of the television signal being broadcast, a counter 8 is cleared by the control logic 7 and a digital switch 9 is set to feed line pulses from the pulse generator 6 to the counter clock input. A slicer 10 is held off and a wobbulator level detector 11 is reset by the control logic 7. A sync level decay unit 12 provides an output which is restored to sync level at the start of each field.

The counter 8 counts line pulses up to the 19th line of each field. The control logic 7 which receives the five most significant bits (MSB) of the counter content sets the digital switch 9 at the start of this line to feed the slicer output to the counter input, but the slicer is still held off. The counter 8 is then pre-loaded with a count corresponding with forty clock pulse inputs (binary 0101000) for a purpose to be described hereinafter.

For 8us during the wob burst the slicer 10 is switched on by the control logic 7 and the number of cycles in the burst is counted by counter 8. With an 8ps 'window', each cycle corresponds with 1/8 MHz, and using the six least significant bits (LSB) of the counter 8 for the frequency count, a range of 8MHz in sixty four 1/8 MHz steps can be counted. In the present case the range must extend from twenty four (3MHz) to eighty seven (10 7/8 MHz).

By pre-loading the counter with an appropriate number before the count begins the 8MHz range can start anywhere from 0 to 77/8 MHz.

In this case the counter is pre-loaded with a count of forty so that the count range is from forty (0101000) to one hundred and twenty seven (1111111). As the minimum count in addition to the pre-loading is twenty four (3MHz) the actual count range is from sixty four (1000000). As we take only the six LSB, the count range is from 3MHz (000000) to 107/8 MHz (111111). At the end of the 8duos burst the six bit frequency count is transferred to a latch 13. Simultaneously with the counting described above the wobbulator level is sampled and held by the wobbulator level detector 11.

At the start of the 20th line of each field counter 8 is cleared and the digital switch 9 is set to feed line pulses to the counter again. The output from the sync level decay unit is allowed to decay from the sync level at 1/8dB per line period, and this output is fed to the non-inverting input of a level comparator 14. The inverting input of the comparator is fed by the held wobbulator level sample from detector 11.

The line pulses are counted until the sync level has decayed to equal the wobbulator level. The input to the counter 8 is then blocked by the digital switch 9, and the count now held is the wobbulator level in l/8dB's relative to sync level to the nearest 1/8dB. A count of zero (00000000) is sync level and a count of one hundred and twenty eight (10000000) is -16dB relative to sync level. If the wobbulator level is lower than -16dB, the count is stopped at -16dB by the digital switch 9. The measurement range is thus 16dB in 1/8dB steps and since the wobbulator burst is sent at -8dB relative to the carrier this represents a transmission amplitude response range of +8do.

The frequency and amplitude of the wobbulator burst are thus measured digitally.

This information is now stored in the latch 13 and counter 8 respectively until the 332nd line of the field, whereupon the above sequence is repeated.

The processing of the stored information will now be described with reference to Figure 2.

The apparatus comprises a basic information store in the form of a 1024 x 1 bit static ran-' dom access memory (RAM) 15. The RAM is assigned to two channels each of sixty four 8-bit words. Each word has a 6 bit address which is indicative of frequency and the word is the corresponding amplitude level, e.g. address number forty five (101101) will contain the level measured at forty five divided by eight i.e. 5.625MHz above the 3MHz datum since the frequency counter has 1 /8MHz increments.

A 14-bit counter 16 (driven by clock 17) at 1.7MHz controls the operating sequence of the circuit components of Figure 2. The 6 M.S.B.

of counter 16 form the frequency address to the A3 to As inputs of memory 15 and are also fed to a comparator 18. The other input to the comparator 18 is the wobbulator frequency held in the latch 13 (Figure 1). If the level count held by counter 8 (Figure 1) has stopped at a number less than one hundred and twenty eight and a control switch 19 is in the 'record' position making contact with terminals 20, a 'write' cycle may be initiated by a digital switch 21 when the inputs to comparator 18 are equal.

The memory is put into the 'write' mode by switch 19, thereby also enabling a multiplexer (MUX) 22. A diplexer 23 holds the channel address bit Ag of the memory 15 at '1' or 'O' as selected manually by a channel selector switch 24. The 8-bit level count held in the counter 8 (Figure 1) is then written serially into the memory 15 via the MUX 22, addressed by bits 4, 5 and 6 from the 14-bit counter 16.

In the above manner sixty four amplitude levels are written into the memory 15 in the correct frequency sequence in the time taken for the wobbulator at the sending end to complete one sweep. By means of a switch 24 a second set of sixty four amplitude levels corresponding with a different input to the probe 1 can be similarly written into the memory 15.

Readout (playback) is achieved by using bit 3 of the counter 16 to clock the output of memory 15 serially into an 8-bit static shift register 25, the outputs of which are transferred in parallel to an 8-bit latch 26 at every zero of bits 4, 5 and 6 (the MUX 22 address bits) of counter 16. This is achieved by use of a three input nor gate 27. Thus each dB level count is held temporarily for eight counts of bit 3. In the "read" mode the diplexer 23 connects bit 7 of counter 16 to the channel address bit A9 of the memory 15 so that for each frequency address two 8-bit words are read out, one for each channel. This gives an interlaced level data readout.

Each 8-bit word out of the latch 26 is fed in parallel to a two line to one line MUX 28 and for half the eight counts of bit 3 of counter 16 to a digital to analogue convertor 29 which in turn is connected to the 'Y' input of an oscilloscope (not shown). Thus a two trace interlaced display can be obtained on a single beam oscilloscope giving two conventional amplitude versus frequency plots.

For the other half of the eight counts of bit 3 of counter 16, the convertor 29 is fed with the output from a digitally coded graticule generator 30 to interlace a graticule into the display. The graticule is generated from bits 0 to 9 of the counter 16 and provides horizontal lines every 2dB from 0 to 14dB inclusive and vertical lines every MHz from 3 to 11 MHz inclusive, with double lines at 3, 7 and 11 MHz.

The generated graticule is shown in Figure 3.

Either channel can be erased by putting switch 19 into the "erase" position contacting terminals 31 and by pressing switch 32. This initiates the "write" cycle manually and writes 0's into the channel (selected by switch 24) which is to be erased from memory 15.

It will be appreciated that by for example using a 4us test signal burst the frequency range could be doubled if the resultant halving of accuracy was acceptable.

Although the described embodiment of the present invention is adapted to test an H.F.

communications channel, the invention is equally applicable to the testing of many other types of system where the transmission of repetitive signals such as data and p.c.m.

telephony is involved. For example, the invention may be used to test the performance of coaxial cable V.H.F. CATV systems and in particular to verify the frequency response of each picture channel in such systems. The vertical interval testing procedure of the present invention could be conveniently used for this purpose with measurements being made within a receiving device operating at intermediate frequency for example.

As a further example of the applicability of the present invention, a video channel extending from D.C. to around 5 MHz or more could also be tested by means of the present invention, providing one complete scanning line of the picture signal is blanked out and the resulting blanked out interval is employed for the transmission of test signals. The blanked out interval is sufficient to allow the transmission of one and a half cycles of a 50KHz wave, and these one and a half cycles are sufficient to enable a measurement to be made.

The use of test signals from 50KHz upwards would allow satisfactory verification of the response of the video channel to be made.

WHAT WE CLAIM IS: 1. An apparatus for testing a communications channel by analysing a test signal received from the channel, the frequency of the test signal being varied in a periodic manner, characterised in that the apparatus comprises a counter, means for applying bursts of the test signal of predetermined duration to the counter, the counter registering the number of cycles in the burst to provide a measure of the frequency of the burst, means for determining the amplitude of the test signal burst, means for storing information representative of the determined frequency and amplitude, and means for reading out the stored information.

2. An apparatus according to claim 1, comprising a slicer arranged to apply said bursts of the received test signal to the counter.

3. An apparatus according to claim 2, comprising a memory for storing the contents of the burst counter.

4. An apparatus according to claim 3, wherein the slicer output is applied to the clock input of the burst counter by a digital switch controlled by logic circuitry, the logic circuitry being arranged to clear the burst counter at the start of each field of a received television signal and thereafter to control the digital switch to feed line pulses to the burst counter clock input, to monitor the content of the burst counter and to control the digital switch to connect the slicer to the burst counter clock input when the counter content reaches a predetermined count, to preload the burst counter, and then to switch on the slicer to pass a burst of the received test signal, the burst being of said predetermined duration.

5. An apparatus according to claim 4, comprising a demodulator to which the test signals are applied, a sync level detector connected to the demodulator output, a sync stripper connected to the outputs of the demodulator and the sync level detector, and a pulse generator connected to the output of the sync stripper, the pulse generator providing pulses at the field and line rates to the logic circuitry at the line rate to the digital switch.

6. An apparatus according to claim 5, comprising a sync level decoy unit connected to the output of the sync level detector, a level detector connected to the output of the demodulator, and a level comparator connected to the outputs of the level detection and the sync level decoy unit, the output of the level comparator controlling the digital switch to block the input to the burst counter when the output of the sync level decoy unit has decoyed to the level of the level detector output.

7. An apparatus according to claim 6, wherein the memory is a random access memory from which information is read out under the control of the output of a further counter.

8. An apparatus according to claim 7, wherein the memory is connected to the burst counter by a multiplexer such that the information represented by the content of the burst counter can be read into the memory serially.

9. An apparatus according to claim 7 or 8, wherein the memory output is connected via a shift register and a latch to a digital to analogue convertor the output of which is suitable for application to an oscilloscope.

10. An apparatus according to claim 9, wherein the output of the further counter is applied to a digitally coded graticule generator, the output of the graticule generator and the latch being applied to the said digital to analogue converter by a two line to one line multiplexer.

11. A method of testing a communications channel by analysing a test signal received from the channel, the frequency of the test signal being varied in a periodic manner, characterised in that bursts of the test signal of predetermined duration are applied to a counter which registers the number of cycles in each burst to provide a measure of the frequency of the burst, the amplitude of the test signal burst is determined, information representative of the determined frequency and amplitude is stored and the stored information is read out.

12. An apparatus for testing a communications channel substantially as hereinbefore described with reference to the accompanying drawings.

13. A method of testing a communications channel substantially as hereinbefore described with reference to the accompanying drawings.

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Claims

**WARNING** start of CLMS field may overlap end of DESC **.

the received test signal to the counter.

3. An apparatus according to claim 2, comprising a memory for storing the contents of the burst counter.

4. An apparatus according to claim 3, wherein the slicer output is applied to the clock input of the burst counter by a digital switch controlled by logic circuitry, the logic circuitry being arranged to clear the burst counter at the start of each field of a received television signal and thereafter to control the digital switch to feed line pulses to the burst counter clock input, to monitor the content of the burst counter and to control the digital switch to connect the slicer to the burst counter clock input when the counter content reaches a predetermined count, to preload the burst counter, and then to switch on the slicer to pass a burst of the received test signal, the burst being of said predetermined duration.

5. An apparatus according to claim 4, comprising a demodulator to which the test signals are applied, a sync level detector connected to the demodulator output, a sync stripper connected to the outputs of the demodulator and the sync level detector, and a pulse generator connected to the output of the sync stripper, the pulse generator providing pulses at the field and line rates to the logic circuitry at the line rate to the digital switch.

6. An apparatus according to claim 5, comprising a sync level decoy unit connected to the output of the sync level detector, a level detector connected to the output of the demodulator, and a level comparator connected to the outputs of the level detection and the sync level decoy unit, the output of the level comparator controlling the digital switch to block the input to the burst counter when the output of the sync level decoy unit has decoyed to the level of the level detector output.

7. An apparatus according to claim 6, wherein the memory is a random access memory from which information is read out under the control of the output of a further counter.

8. An apparatus according to claim 7, wherein the memory is connected to the burst counter by a multiplexer such that the information represented by the content of the burst counter can be read into the memory serially.

9. An apparatus according to claim 7 or 8, wherein the memory output is connected via a shift register and a latch to a digital to analogue convertor the output of which is suitable for application to an oscilloscope.

10. An apparatus according to claim 9, wherein the output of the further counter is applied to a digitally coded graticule generator, the output of the graticule generator and the latch being applied to the said digital to analogue converter by a two line to one line multiplexer.

11. A method of testing a communications channel by analysing a test signal received from the channel, the frequency of the test signal being varied in a periodic manner, characterised in that bursts of the test signal of predetermined duration are applied to a counter which registers the number of cycles in each burst to provide a measure of the frequency of the burst, the amplitude of the test signal burst is determined, information representative of the determined frequency and amplitude is stored and the stored information is read out.

12. An apparatus for testing a communications channel substantially as hereinbefore described with reference to the accompanying drawings.

13. A method of testing a communications channel substantially as hereinbefore described with reference to the accompanying drawings.