GB1584592A - Setting of clock pulse sources - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE SETTING OF CLOCK PULSE SOURCES (71) We, TREND COMMUNICA TIONS LIMITED, a British Company, of St. Alphage House, 2 Fore Street, London, and LARRY FREDERICK LIND, a citizen of the United States of America, of 1 Almond Close, Wivenhoe, Colchester, Essex, 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: This invention concerns equipment for use on binary character systems, such as telegraphic and data transmission networks.

In particular, this invention relates to methods of and apparatus for the setting of a clock pulse source to deliver a pre-selected number of clock pulses per binary element period of a binary waveform. After the setting of the clock pulse source, tests may be made employing the set source, such as the measurement of distortions in both continuous (i.e. isochronous) binary waveforms and in "stop/start" waveforms - which latter are normally employed in telegraphy, telex and so on.

When a binary waveform is transmitted over telegraph lines, it is especially liable to two types of distortions; namely, bias distortion and fortuitous distortion. However, when a binary waveform is received, a third distortion can arise in the receiving equipment itself. This is speed distortion, which results from an incorrect setting of the clock pulse source in the receiving apparatus. This form of distortion can be corrected by an appropriate adjustment of the clock pulse source so that the clock pulse rate in the receiving equipment is synchronised with the clock pulse rate at the transmitter of the waveform.

Once speed distortion has been eliminated, only bias and fortuitous distortion are significant, for although other distortions can occur in binary waveforms, they are usually of little importance in telegraphy and data transmission binary waveforms transmitted at relatively low speeds over telegraph lines. Bias distortion is regular in nature, and manifests itself as a constant change at reception in the mark-space ratio from the transmission mark-space ratio of 1 : 1. Bias distortion can be present in positive- or negative-going edges arriving later than expected, though sometimes with this distortion an edge arrives earlier than expected. Fortuitous distortion is random in nature and can occur with positive- or negative-going edges which arrive either earlier or later than expected, notwithstanding the presence of bias distortion.Bias distortion is often of greater interest to a user, for if it is present on a received binary waveform, it is possible electronically to correct for it, by suitable adjustment of the reception equipment. Because fortuitous distortion is random in nature, it usually has to be tolerated provided it does not exceed levels, and thus often there is no point in assessing accurately the precise level of fortuitous distortion in a received waveform.

Waveform receiving equipment has been designed which automatically sets an internal clock pulse source to be synchronised with the clock pulse rate of the transmission of binary characters. The setting of a clock pulse source is normally a pre-requisite to some other operation, such as the measurement of distortion. However, though automatic equipment is especially convenient to use, it is nevertheless complex and hence expensive, and at times, if other manual operations are required, the slightly longer time required to set the clock pulse source by operation of a manual control is not significant. It is an aim therefore of this invention to provide a method of setting a clock pulse source which is simple manually to effect.

According to one aspect of this invention, there is provided a method of setting a clock pulse source to deliver a pre-selected number of clock pulses per binary element period to an element clock requiring said preselected number of clock pulses to be driven: through one period, comprising: resetting the element clock at the arrival of a start edge of the binary waveform; detecting the first following edge of the same sense as the start edge; commencing the counting of clock pulses at the detection of the said first following edge; stopping the counting of clock pulses in the averaging counter when the element clock has counted a pre-set number of clock pulses following the completion of the counting of the immediately preceding element clock period, repeating the aforesaid averaging counter counting sequence for a chosen number of times; averaging the counts obtained thereby; and then displaying the averaged count as an indication of the error in the clock pulse source rate relative to the period of the binary waveform so that the clock pulse source rate may be adjusted dependent upon the displayed averaged count.

The effect of averaging the obtained count for a number of counting sequences serves to reduce the effect of fortuitous distortion, as well as to reduce the effects of receiving characters of different binary make-up, which will themselves prevent an accurate display of the precise error.

However, the display will show whether the clock pulse source is running too fast or too slow - and hence which way an adjustment should be made - and will show when the speed is exact.

Because bias distortion is regular in nature and each edge of the binary waveform of the same sense is subject to the same space bias distortion or the same mark bias distortion, the interval between any two edges of the same sense must be a multiple of the binary element period, ignoring the effect of any fortuitous distortion. In the above method, the speed of the clock pulse source is adjusted by operating only on edges of the same sense, so that effects of bias distortion are eliminated.

The element clock is reset at the arrival of a start edge - that is the first positive-going edge of a stop-start transmission character.

With osochronous transmission, any edge may be regarded as a start edge - either a positive- or a negative-going edge. The first edge of the same sense as the start edge may arrive two binary element periods after the start edge at the soonest, though of course it may not arrive for some number of periods thereafter. Because the counted period error will increase as a function of the number of periods between the start and first detected edges, this method cannot give an accurate absolute indication of the speed error but can nevertheless show whether the clock pulse source is running slow or fast.

When operating on stop/start waveforms, it is advantageous for only one count to be made on each character. To this end, the detection of a start edge should be inhibited after the detection of one such edge for the duration of a number of successive binary element periods, the precise number depending upon the character alphabet chosen. This inhibiting may of course be used with isochronous waveforms, though there is no advantage in doing so.

The element clock requires said preselected number of clock pulses to be driven through one element period, so that when the clock pulse source has been properly adjusted the period of the element clock is precisely one binary element period. Thus, the element clock may give an indication of the termination of one binary element period and the commencement of the next.

Preferably, the preset number of clock pulses at which the counting by the averaging counter is stopped is precisely the count in the element clock at the mid-point of the period of the element clock. The period of the element clock 13 conveniently set at 100 clock pulses, and thus the counting is stopped when 50 clock pulses have been dehvered following the commencement of an element period. To achieve this, the element clock can give the required output half-way through its period, though a separate counter could be provided for this purpose. If the clock pulse source (and hence the element clock) is running at the correct rate, the detection of the said first edge will occur at precisely the correct moment as determined by the element clock and so the count of the clock pulses will be precisely 50 - always ignoring effects of fortuitous distortion. An averaged count of 50 should then be displayed as zero speed distortion, with counts of less than 50 indicating that the clock pulse rate is too quick and counts of more than 50 indicating that the clock pulse rate is too slow.

In order to display the averaged count with the optimum effect it is preferred for the averaged count to be suitably encoded for driving a display comprising a linear array of lamps, and preferably of lightemitting diodes. An averaged count of 50 should illuminate the central lamp of the array, indicating no speed distortion, with counts of more than 50 illuminating an appropriate lamp to one side of the central lamp to show the clock pulse rate is too slow, and with counts of less than 50 illuminating lamps to the other side of the central lamp to show the clock pulse rate is too fast.

A manual control is provided for clock pulse speed, the operator using the manual control to adjust the rate of delivery of clock pulses until the centre lamp of the linear display is illuminated. For adjustment of the speed, the two lamps at the extremes of the display can be used to show that the speed error is so large that it is out of range of the other lamps, and the operator would have to adjust the control unit both end lamps are extinguished; then one intermediate lamp will be illuminated and the operator can see which way further to adjust the control to bring the speed of the clock pulse source to the required value, as shown by the illumination of the central lamp.

It is preferred for the counting for clock pulse rate to be effected 8 times and then averaged for display. It is found that the clock pulse source can be set with sufficient accuracy with a relatively low number of counts duly averaged, and a power of two is chosen for it is easy to divide by such a number when working with digital signals.

It is found that apparatus arranged to perform the method of setting a clock pulse source as described above, according to this invention, can be re-arranged relatively easily so as to allow the gauging of any fortuitous distortion present in the same waveform, by assessing total distortion.

Accordingly, according to another aspect of this invention, there is provided a method of assessing the total distortion present in a binary waveform, comprising: performing the abovementioned method of setting a clock pulse source to deliver to an element clock a preselected number of pulses per binary element period; re-setting the element clock on the arrival of an edge of the binary waveform; counting with the element clock delivered clock pulses; and at the arrival of each successive edge following the resetting edge, transferring the instantaneous count in the element clock to a display device, the early or late arrival of each successive edge relative to its expected arrival as determined by the element clock being displayed.

In this method assessing total distortion, no averaging takes place. Instead, an instantaneous display is provided of any total distortion (i.e. bias and fortuitous distortion) present in the arrival time of each edge, relative to its expected arrival time, from which any fortuitous distortion can be gauged. Preferably the display devices comprise a linear array of lamps, a particular instantaneous count illuminating a particular lamp of the array. Thus, the total distortion will be shown as lamps flickering depending upon the amount of distortion present. If only bias distortion is present, a single lamp will remain lit, spaced from the sole lamp lit if no total distortion is present.

This is because the count each time will be constant but not zero. In view of the random nature of fortuitous distortion, the lamps which are illuminated will also move around the display at random, and it is left to an operator to gauge the precise amount of fortuitous distortion by observing the display. In order to assist the operator, however, it is preferred for there to be included a maximum (or peak) detector which maximum detector provides to the display an indication of the maximum amount of total distortion present and maintains a lamp illuminated in correspondence with that maximum. Preferably, both early and late maximum total distortions (that is, the early arrival or the late arrival of an edge relative to its expected arrival and caused by fortuitous distortion) are displayed separately as two maxima, by two appropriate lamps.

Advantageously, the counting of clock pulses is stopped after the arrival of a pre-chosen number - and preferably seven of binary element periods and then the element clock is reset on the arrival of the next edge. Seven is conveniently chosen so that the method can be used with both isochronous and stop/start binary waveform transmissions using a seven element alphabet; for the latter it is found that the method will, in time, align itself so as to start itself to be performed at the commencement of each character. For other alphabets, the appropriate number of element periods should be chosen.

In this method of assessing total distortion, the element clock preferably advances one count for each received clock pulse and advances to zero on receipt of said preselected number of clock pulses - assuming that at the start of delivery of clock pulses the element clock was reset to zero. Thus, when the clock pulse rate has been set to the preferred value of 100 per binary element period, the element clock conveniently is a clock of period 100. Alternatively, the element clock could take the form of an up/down counter, the counter reversing the direction of count half-way through an element period as well as at the ends of the element period. Thus, for a preferred clock pulse rate of 100, the element clock may be an up/down counter which counts from 00 to 49 and back to 00, the direction of count being changed each time the counter reaches 00 or 49.

With the preferred form of element clock the display is arranged to show early total distortion with counts of 50 to 99 and late total distortion with counts of 00 to 48. For an up/down counter, 00 represent no total distortion, and the display device must be arranged to decode appropriately for the element clock employed.

Apparatus arranged to perform the method of setting a clock pulse source in accordance with this invention can further be re-arranged relatively easily to allow the assessment of bias distortion of a binary waveform. Methods of and apparatus for the assessment of a bias distortion are described and claimed in our co-pending Application No. 23343/76 (Serial No.

1585121). In its broadest aspect, this method of assessing any bias distortion present in a received binary waveform comprises: adjusting a clock pulse source to deliver to an element clock a preselected number of clock pulses for each binary element period, the element clock requiring said pre-selected number of pulses to be driven through one element period; resetting the element clock on the arrival of one of a positive-or negative-going edge of the waveform; starting an averaging counter to count clock pulses on the arrival of the next following edge of the opposite sense; stopping the counting of clock pulses in the averaging counter when a pre-set number of clock pulses have been counted by the element clock following the point at which said next following edge should have arrived if no distortion is present; repeating the counting sequence by starting and stopping the averaging counter in the aforesaid manner for a chosen number of times; averaging the counts obtained thereby; and then displaying the averaged count to indicate the degree of any bias distortion present in the waveform.

As in the present invention, when assessing bias distortion it is preferred for the clock pulse source to be adjusted to deliver clock pulses at a rate of 100 for each binary element period, and for the counting of clock pulses to be stopped when the clock pulse source has delivered precisely 50 pulses following the point at which said next following edge should have arrived as determined by the element clock. It will be seen therefore that the method of assessing bias distortion in this way has several common features with the above described method for setting clock pulse rates of this invention, and - to a lesser extent - with methods of assessing total distortion, following clock pulse rate setting. For display of bias distortion, the same linear array of lamps can be employed as is used with clock pulse rate setting and assessment of total distortion.

Thus, an averaged count of 50 (with the preferred clock pulse rate) should illuminate the central lamp of the array, indicating no bias distortion, with counts of more than 50 illuminating lamps to one side of the central lamp and designated mark bias distortion, and with counts of less than 50 illuminating lamps to the other side of the central lamp and designated space bias distortion.

This invention further extends to apparatus for setting a clock pulse source to provide a preselected number of clock pulses for each binary element of a binary waveform. Accordingly, a further aspect of this invention provides apparatus for setting a clock pulse source to deliver a preselected number of clock pulses per binary element period of a binary waveform, comprising: a clock pulse source adjustable to provide a preselected number of clock pulses per binary element period; an element clock for counting clock pulses so as to be driven repeatedly through element periods, the element clock requiring said preselected number of pulses to be driven through each element period; first and second edge detectors for the binary waveform, the first edge detector serving to detect a positive-going start edge of a binary character and providing an output to reset the element clock and the second edge detector providing an output on detection of the next positive-going edge following an output from the first edge detector; an averaging counter arranged to start counting clock pulses on receipt of an output from the second edge detector and to stop counting pulses when the element has reached a pre-set point in its period; and means to initiate a chosen number of counting sequences by the averaging counter and then to transfer the average of the counts to a display device to give an indication of any required adjustment in the clock pulse source rate.

Said means preferably comprises a counter in the form of a measurement clock (i.e.

a counter with a preset period), which measurement clock counts the number of outputs from the first edge detector and at the completion of a period causes the total count in the averaging counter to be averaged and transferred to a display device.

Advantageously, the period of the measurement clock is 8, and, for a binary count in the averaging counter, the averaging is effected by transferring the binary count except for the 3 least significant binary bits this having the effect of dividing the count by 8.

The display device is preferably in the form of a linear array of lamps, and conveniently light-emitting diodes, arranged such that the central lamp is lit when the clock pulse rate is correct, and a lamp to one side or the other of the central lamp being lit when the clock pulse rate is too fast or too slow, respectively. If the speed is greatly in error, the two end lamps may be illuminated simultaneously, and a manual control for the clock pulse source operated until a lamp to one side or the other of centre is lit; further adjustment is then effected until the central lamp is lit.

Preferably, the apparatus is in the form of a self-contained test-set including means to assess total distortion to allow the gauging of fortuitous distortion, which set can be connected to any desired binary waveform transmission line and used initially to set the clock pulse rate and then to assess any total distortion present in the binary waveform being transmitted therealong. Advantageously, the apparatus also includes the means necessary for assessing any bias distortion present in the binary waveform being transmitted, employing the method as described and claimed in our co-pending Application No. 23343/76 (Serial No.

1585121).

A test-set of this invention and employing the methods described above can be constructed so as to be especially simple to operate. With a display device in the form of a linear array of lamps - and preferably light-emitting diodes - and with a threeposition switch provided to adjust the apparatus respectively for the setting of the internal clock pulse source, the assessment of bias distortion and the gauging of fortuitous distortion by assessing the total distortion, the test-set is simple to operate and reliable results can be obtained. A further control has to be provided to allow the manual adjustment of the clock pulse source dependent upon the display indication when the switch is in the appropriate position, and re-set button can be provided for the peak total distortion hold display when the set is adjusted to assess total distortion.

By way of example only, one specific embodiment of a test-set having an internal clock pulse source for setting to provide a preselected number of pulses per binary element of a binary waveform, in accordance with this invention and useable for assessing total distortion to allow gauging of fortuitous distortion in accordance with preferred aspects of the invention, will now be described with reference to the accompanying drawings, in which: Figure 1 is a block diagram of that part of the test-set employed to adjust the clock pulse source thereof to the required element delivery rate for the waveform being tested; Figure 2 is a diagram showing the method of adjustment of the speed of the clock pulse source; Figure 3 is a block diagram showing that part of the test-set employed for assessing the total distortion together with a peak hold arrangement; and Figure 4 is a diagram showing the method of assessing total distortion.

Referring initially to Figure 1, there is shown a block diagram of those parts of the test-set which are employed in the adjustment of the test-set so that its internal clock pulse source runs at the required rate; namely, 100 clock pulses per element period of the binary waveform being tested. In the figure there is shown a clock pulse source 10 having a manual control 11 to adjust the rate of delivery of clock pulses. The clock pulse source 10 drives an element clock C2 arranged to count from 0 to 99 and which then provides an output indicating the end of one element period and the commencement of the next, the element clock returning to 0 for the next count sequence.

Two edge detectors for the waveform to be tested are provided, detector 12 of which being arranged to detect a start edge (relevant to stop/start transmission only) and detector 13 of which being arranged to detect the first positive-going edge following the detection of a start edge. Edges of the same sense are used to cancel out measurement errors due to any bias distortion present in the incoming waveform. For the case of an isochronous waveform, the start edge is any positive-going edge, though for stop/start transmissions, the start edge is the first edge of a character made up from typically - 7 binary elements. Detector 12 inhibits the action of detector 13 until a start edge has been detected; after this, the detector 13 serves to detect the next positive-going edge of the waveform being tested.The start edge detector 12 serves to re-set the element clock C2 to zero, the clock running at the clock pulse rate set by control 11 as soon as the test-set is turned on. Of course, the rate set by control 11 probably is not the required rate for the waveform being tested. The clock pulses from the source 10 are supplied to an averaging counter C1 which however is inoperative until it is started by detector 13 detecting the first positive-going edge following a start edge. As soon as this first positive-going edge is detected, the averaging counter C1 commences to count clock pulses from the source 10 and continues to count until averaging counter C1 is stopped by a signal from detector 14, this detector serving to detect when the element clock C2 has reached a count of 50.Detector 12 looks once more for a start edge. This cycle is repeated 7 more times, so that averaging counter C1 accumulates in it the result of 8 separate counting cycles. A measurement clock C3 serves to count the 8 cycles of operation and after the eighth cycle, causes the accumulated count in the averaging counter C1 to be transferred to an output device, after the count has been divided by eight in order to effect the averaging thereof.Because eight results are counted, it is especially easy to divide the count in averaging counter C1 by eight to obtain the average; all that is necessary is to "lose" the three least significant bits of the binary count in averaging counter C1 to effect the divide-by-eight, the remaining bits of the count in averaging counter C1 being trans ferred to an output device 16. If the averaged count is abnormally low or high, an 'out-of-range' indicator is operated. An averaged count of 50 implies frequency synchronism between the incoming data and the element clock.

Character clock 15 is associated with the element clock C2 and serves to inhibit the operation of the start edge detector 12 for 7 binary periods as defined by element clock C2 following the detection of a start edge.

This character clock has no significant function when operating on an isochronous binary transmission, though when operating in the stop/start mode the character clock 15 serves to prevent more than one result in the averaging counter C1 being made per character of the binary waveform. With a stop/start transmission having a different number of binary element periods per character, the character clock 15 should be adjusted accordingly, so as to prevent the detector 12 re-setting the element clock C2 more than once for each character.

The output device 16 comprises a decoder for the binary count of averaging counter C1, and a latch arrangement driving a linear display of 43 light-emitting diodes (LEDs).

The decoder and latch are arranged to illuminate the centre LED of the row thereof if the averaged count is 50, for in this case the clock is running at the correct speed. For counts of less than 50 an appropriate LED to one side of the centre is illuminated to show that the element clock (and hence the clock pulse source) is running too quickly relative to the received binary waveform, whereas for counts of greater than 50 an appropriate LED is illuminated to the other side of the centre, showing that the clock is running too slowly.

By observing which LED is illuminated, an operator may adjust control 11 to adjust the speed of the clock pulse source and hence the element clock until the centre LED of the array is illuminated.

The display is arranged so that the two extreme LEDs serve as the out-of-range indicator and when both LEDs are illuminated the operator adjusts the control 11 until both are extinguished; after this, the sense in which the control should be operated is indicated by the LED then illuminated and the operator can properly adjust the speed until the centre LED is illuminated.

Referring to Figure 2, there is shown an example of a character of a stop/start binary waveform with which the test-set element clock is to be synchronised. This character consists of seven binary elements, there being a positive-going start edge 20 at the commencement of the character, the waveform remaining at level ZERO (in telegraph notation) for three binary element periods before returning to level ONE for one binary element period. Following the period at level ONE, the first positive-going edge 21 following the start edge occurs, with the waveform remaining at level ZERO for two binary element periods and then- returning back to the rest state ONE at the end of the character. The test-set clock element counting is shown by the lower line 23 in the Figure.

The start edge 20 is detected by detector 12 and at this point the element clock C2 is re-set to zero as shown by the lower line 23.

The element clock is shown as running too fast, and in this case the clock has advanced from a count of 99 to 00 before the completion of the first binary element period. Here, the dock pulse source is shown as providing about 11 further pulses before the completion of the binary waveform element period. By the time the first positive-going edge is detected by detector 13, the element clock C2 has run through four complete binary element periods, plus a further 45 counts. The arrival of the first positive-going edge starts averaging counter C1 counting clock pulses until detector 14 stops the counting because the element clock has reached a count of 50; for this case therefore the averaging counter C1 will reach a count of only 5 before being stopped by detector 14.

This process is repeated for the next 7 characters, and the 8 results averaged to provide the output measurement to the display of LEDs. It will be appreciated that the count in averaging counter C1 will be dependent both upon the relative periods of the test-set clock and the binary waveform as well as on the number of binary element periods between the start edge and the first positive-going edge following the start edge.

The count will also be effected by any fortuitous distortion present. By averaging over 8 counts, the effects of fortuitous distortion can largely be eliminated, and provided that a succession of different data characters is being transmitted, the effect of the spacing between the start edge and the first following positive-going edge will also tend to average out. However, the test-set is not intended to provide a calibrated reading of the speed error but is instead simply intended to show whether the element clock is running too quickly or too slowly, so that the operator may suitably adjust the speed of the clock. When the element clock is synchronised properly, the centre LED of the display thereof will be illuminated, quite irrespective of the spacing'between the start edge and the first following positive-going edge, though of course fortuitous distortion can still upset the counting.

Referring now to Figures 3 and 4, there is respectively shown the apparatus and method used to assess the total amount of distortion (which includes bias distortion and fortuitous distortion) present in a received binary waveform. The same clock pulse source 10 is used as has been described above - which of course must be set to deliver 100 clock pulses per binary element period in the manner described above with the incoming waveform. The element clock C2 again provides a count from 00 to 99, then advancing to 00, each binary element period.

As with the speed synchronisation, detector 12 is arranged to detect a start edge - that is a positive-going edge of a waveform of a stop/start transmission. For isochronous transmissions, any positive-going edge is sufficient. The output from detector 12 serves to re-set the element clock C2 to zero. Character clock 15 is arranged as described with reference to Figure 1, so that the element clock will be synchronised with the start of each character of a stop/start waveform as each character is received.

Detector 13 is modified so that it detects any edge of the character waveform being received, either negative-going or positivegoing. Each time an edge is detected by detector 13, the count in the element clock is transferred to the display arrangement 16, so that on the linear array of LEDs the error in the arrival time of the edge can instantaneously be displayed. If there is no error, the centre lamp of the display will be illuminated, whereas if the edge arrives either early or late, another appropriate lamp of the display will be illuminated. The 'information transferred to the display device 16 is arranged to show whether the edge arrived early or late, as determined by the element clock count, early edges (counts of 50 to 99) being displayed instantaneously to one side of the centre LED. The display does not indicate whether the edge was negative-or positive-going.

In addition to the above display, a peak detector and store 30 with a manual re-set is arranged to receive the count each time an edge is detected by detector 13, this peak detector providing a constant output to the display arrangement of the array of LEDs so as to illuminate the LEDs corresponding to peak amounts of early and late distortions.

The manual re-set is provided so that the effect of one particular but abnormal amount of fortuitous distortion may be cancelled. This re-set facility not only cancels the reading in the peak distortion store, but also the reading in the instantaneous distortion store. The latter action prevents a false display occurring between start-stop characters.

It will be appreciated that if bias distortion is present on the received waveform, but no fortuitous distortion, a single lamp of the display is constantly illuminated indicating that an edge contantly arrives early or late because a non-zero count will occur each time the edge arrives. This will be immediately apparent from the display, because fortuitous distortion is random in nature whereas bias distortion is not.

Figure 4 shows a data character of a stop/start transmission having some fortuitous distortion present, but no bias distortion. Following the start edge 35 of the character, there is a negative-going edge 36 arriving early, followed by a positive-going edge 37 on time, followed by negative-going edge 38 arriving late and then a positivegoing edge 39 arriving early and finally a negative-going edge 40 also arriving early.

The element clock C2 is shown by the lower line in Figure 4, and from this it can be seen that the element clock is re-set to zero at the arrival of the start edge 35 and that the arrival of the early negative-going edge 36 gives a positive count of, say, 90, whereas if there were no fortuitous distortion the count would have advanced to 00. The next positive-going edge 37 is on time and thus there is a count of 00 when this is detected by detector 13, though when the next negative-going edge 38 is detected, the counter has a low positive value in it, indicating a late edge arrival.

The display is arranged to show which side of the zero count the edge in question arrives by illuminating lamps either to the left of the centre - for instance for early edges - or to the right of the centre- for instance for late edges. The further the lamp which is illuminated is from the centre, the greater is the total distortion. This assessment is however not absolute and it is left to the operator's experience to decide what is a reasonable average level of total distortion.

Reference is directed to our copending Application No. 23343/76 (Serial No.

1585121), in which is described and claimed modified forms of the above apparatus, for assessing bias distortion of a received binary waveform.

WHAT WE CLAIM IS: 1. A method of setting a clock pulse source to deliver a pre-selected number of clock pulses per binary element period to an element clock requiring said preselected number of clock pulses to be driven through one period, comprising: resetting the element clock at the arrival of a start edge of the binary waveform; detecting the first following edge of the same sense as the start edge; commencing the counting of clock pulses at the detection of the said first following edge; stopping the counting of clock pulses in the averaging counter when the element clock has counted a pre-set number of clock pulses following the completion of the counting of the immediately preceding element clock period; repeating

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

Claims (31)

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

   distortion (which includes bias distortion and fortuitous distortion) present in a received binary waveform. The same clock pulse source 10 is used as has been described above - which of course must be set to deliver 100 clock pulses per binary element period in the manner described above with the incoming waveform. The element clock C2 again provides a count from 00 to 99, then advancing to 00, each binary element period.

   As with the speed synchronisation, detector 12 is arranged to detect a start edge - that is a positive-going edge of a waveform of a stop/start transmission. For isochronous transmissions, any positive-going edge is sufficient. The output from detector 12 serves to re-set the element clock C2 to zero. Character clock 15 is arranged as described with reference to Figure 1, so that the element clock will be synchronised with the start of each character of a stop/start waveform as each character is received.

   Detector 13 is modified so that it detects any edge of the character waveform being received, either negative-going or positivegoing. Each time an edge is detected by detector 13, the count in the element clock is transferred to the display arrangement 16, so that on the linear array of LEDs the error in the arrival time of the edge can instantaneously be displayed. If there is no error, the centre lamp of the display will be illuminated, whereas if the edge arrives either early or late, another appropriate lamp of the display will be illuminated. The 'information transferred to the display device 16 is arranged to show whether the edge arrived early or late, as determined by the element clock count, early edges (counts of 50 to 99) being displayed instantaneously to one side of the centre LED. The display does not indicate whether the edge was negative-or positive-going.

   In addition to the above display, a peak detector and store 30 with a manual re-set is arranged to receive the count each time an edge is detected by detector 13, this peak detector providing a constant output to the display arrangement of the array of LEDs so as to illuminate the LEDs corresponding to peak amounts of early and late distortions.

   The manual re-set is provided so that the effect of one particular but abnormal amount of fortuitous distortion may be cancelled. This re-set facility not only cancels the reading in the peak distortion store, but also the reading in the instantaneous distortion store. The latter action prevents a false display occurring between start-stop characters.

   It will be appreciated that if bias distortion is present on the received waveform, but no fortuitous distortion, a single lamp of the display is constantly illuminated indicating that an edge contantly arrives early or late because a non-zero count will occur each time the edge arrives. This will be immediately apparent from the display, because fortuitous distortion is random in nature whereas bias distortion is not.

   Figure 4 shows a data character of a stop/start transmission having some fortuitous distortion present, but no bias distortion. Following the start edge 35 of the character, there is a negative-going edge 36 arriving early, followed by a positive-going edge 37 on time, followed by negative-going edge 38 arriving late and then a positivegoing edge 39 arriving early and finally a negative-going edge 40 also arriving early.

   The element clock C2 is shown by the lower line in Figure 4, and from this it can be seen that the element clock is re-set to zero at the arrival of the start edge 35 and that the arrival of the early negative-going edge 36 gives a positive count of, say, 90, whereas if there were no fortuitous distortion the count would have advanced to 00. The next positive-going edge 37 is on time and thus there is a count of 00 when this is detected by detector 13, though when the next negative-going edge 38 is detected, the counter has a low positive value in it, indicating a late edge arrival.

   The display is arranged to show which side of the zero count the edge in question arrives by illuminating lamps either to the left of the centre - for instance for early edges - or to the right of the centre- for instance for late edges. The further the lamp which is illuminated is from the centre, the greater is the total distortion. This assessment is however not absolute and it is left to the operator's experience to decide what is a reasonable average level of total distortion.

   Reference is directed to our copending Application No. 23343/76 (Serial No.

   1585121), in which is described and claimed modified forms of the above apparatus, for assessing bias distortion of a received binary waveform.

   WHAT WE CLAIM IS: 1. A method of setting a clock pulse source to deliver a pre-selected number of clock pulses per binary element period to an element clock requiring said preselected number of clock pulses to be driven through one period, comprising: resetting the element clock at the arrival of a start edge of the binary waveform; detecting the first following edge of the same sense as the start edge; commencing the counting of clock pulses at the detection of the said first following edge; stopping the counting of clock pulses in the averaging counter when the element clock has counted a pre-set number of clock pulses following the completion of the counting of the immediately preceding element clock period; repeating

   the aforesaid averaging counter counting sequence for a chosen number of times; averaging the counts obtained thereby; and then displaying the averaged count as an indication of the error in the clock pulse source rate relative to the period of the binary waveform so that the clock pulse source rate may be adjusted dependent upon the displayed averaged count.
2. 2. A method according to claim 1, in which the element clock is reset at the arrival of a positive-going edge of the binary waveform.
3. 3. A method according to claim 1 or claim 2, in which the detection of a start edge is inhibited after the detection of one such edge for the 'duration of a number of successive binary element periods depending upon the character alphabet of the binary waveform.
4. 4. A method according to any of the preceding claims, in which the preset number of clock pulses at which the counting by the averaging counter is stopped is precisely the count in the element clock at the mid-point of the period of the element clock.
5. 5. A method according to any of the preceding claims, in which the period of the element clock is set at 100 clock pulses.
6. 6. A method according to any of the preceding claims, in which a separate counter is associated with the element clock to provide an output when the element has advanced said pre-set number of clock pulses'.
7. 7. A method according to any of the preceding claims, in which the averaged count is suitably encoded for driving a display comprising a linear array of lamps.
8. 8. A method according to claim 7, in which each lamp of the array is a lightemitting diode.
9. 9. A method according to claim 7 or claim 8, in which an averaged count indicating no speed distortion causes the central lamp of the array to be illuminated, an appropriate lamp to one side of the central lamp being illuminated when the clock pulse rate is too slow, and an appropriate lamp to the other side of the central lamp being illuminated when the clock pulse rate is too fast.
10. 10. A method according to any of claims 7 to 9, wherein the two lamps at the extremes of the display are illuminated when the speed error is so large that it is but of range of the other lamps.
11. 11. A method according to any of the preceding claims wherein the speed of the clock pulse source is adjustable by means of a manual control.
12. 12. A method according to any of the preceding claims, in which the counting for the clock pulse rate is effected 8 times, and then averaged for display.
13. 13. A method according to claim 1 and substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.
14. 14. A method of setting a clock pulse source to deliver a pre-selected number of clock pulses per binary element period as claimed in any of the preceding claims, followed by assessing total distortion present in the binary waveform by: re-setting the element clock on the arrival of an edge of the binary waveform; counting with the element clock delivered clock pules; and at the arrival of each successive edge following the resetting edge, transferring the instantaneous count in the element clock to a display device, the early or late arrival of each successive edge relative to its expected arrival as determined by the element clock being displayed.
15. 15. A method according to claim 14, in which the instantaneous count is displayed on a linear array of lamps, a particular instantaneous count illuminating a particu lar lamp of the array.
16. 16. A method according to claim 14 or claim 15, in which there is included a maximum detector which maximum detector provides to the display an indication of the maximum amount of total distortion present and maintains an indication of that maximum.
17. 17. A method according to claim 16, in which both early and late maximum total distortions are displayed separately as two maxima.
18. 18. A method according to any of claims 14 to 17, in which counting is stopped after the arrival of a pre-chosen number of binary element periods and the element clock is reset on the arrival of the next edge.
19. 19. A method according to any of claims 14 to 18, in which the element clock advances one count for each received clock pulse and advances to zero on receipt of said preselected number of clock pulses, provided that at the start of delivery of clock pulses the element clock was reset to zero.
20. 20. A method according to any of claims 14 to 18, in which the element clock is in the form of an up/down counter, the counter reversing the direction of count half-way 'through an element period as well as at the ends of the element period.
21. 21. A method according to claim 14 and substantially as hereinbefore described with reference to Figures 1 to 4 of the accom panying drawings.
22. 22. Apparatus for setting a clock pulse source to deliver a preselected number of clock pulses per binary element period of a binary waveform, comprising: a clock pulse source adjustable to provide a preselected number of clock pulses per binary element period; an element clock for counting clock pulses so as to be driven repeatedly through element periods, the element clock requiring said preselected number of pulses to be driven through each element period; first and second edge detectors for the binary waveform, the first edge detector serving to detect a positive-going start edge of a binary character and providing an output to reset the element clock and the second edge detector providing an output on detection of the next positive-going edge following an output from the first edge detector; an averaging counter arranged to start counting clock pulses on receipt of an output from the second edge detector and to stop counting pulses when the element clock has reached a pre-set point in its period; and means to initiate a chosen number of counting sequences by the averaging counter and then to transfer the average of the counts to a display device to give an indication of any required adjustment in the clock pulse source rate.
23. 23. Apparatus according to claim 22, wherein said means comprises a counter in the form of a measurement clock, which measurement clock counts the number of outputs from the first edge detector and at the completion of a period causes the total count in the averaging counter to be averaged and transferred to a display device.
24. 24. Apparatus according to claim 23, wherein the period of the measurement clock is 8 clock pulses.
25. 25. Apparatus according to claim 24, wherein the averaging is effected by transferring the binary count in the averaging counter except for the 3 least significant binary bits.
26. 26. Apparatus according to any of claims 22 to 25, wherein the display device is in the form of a linear array of lamps arranged such that the central lamp is lit when the clock pulse rate is correct, and a lamp to one side or the other of the central lamp being lit when the clock pulse rate is too fast or too slow, respectively.
27. 27. Apparatus according to claim 26, wherein the two end lamps of the display are illuminated simultaneously if the speed is greatly in error.
28. 28. Apparatus according to any of claims 22 to 27, wherein a manual control is provided to adjust the clock pulse source rate until zero speed error is displayed.
29. 29. Apparatus according to claim 23 and substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.
30. 30. Apparatus according to any of claims 22 to 29. arranged in the form of a self-contained test-set including means to assess total distortion, which set is connectible to a binary waveform transmission line and used initially to set the clock pulse rate and then to assess any total distortion present in the binary waveform being transmitted therealong.
31. 31. Apparatus according to claim 30, wherein the test-set also includes means for assessing any bias distortion present in the binary waveform being transmitted.