Error rate monitor

An error rate monitor having means to detect errors in a data stream and a counter for counting the quantity of data or elapsed time between two consecutive errors. The monitor may also caculate running totals of the number of counts falling within each of a plurality of bands of count value. A display shows the running totals as bars on a bar graph, together with two pointers indicating the counted data or time since the previous error and the counted data or time since beginning operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an error rate monitor and more 
particularly to an error rate monitor which displays the number of errors 
occurring in a data stream. 
2. Description of the Related Art 
Conventional error rate systems, as used on VTRs and audio tape recorders, 
count up the errors for a period of time and then display that figure as a 
fraction. For example, they count the number of errors for one thousand 
words and then display the result as a fraction, i.e. (Number of errored 
words/total number of words). Thus, if one sample is in error, the block 
error rate is 1.times.10.sub.-3. 
The size of the block, that is, the number of words in the divisor of the 
above expression has two effects. Firstly, the longer the block, the 
greater the resolution of the block error rate figure, and secondly, the 
shorter the block, the quicker an error rate may be displayed. However, 
these two requirements conflict, such that the error rate monitor cannot, 
at the same time, be both precise and rapidly responsive to the changes in 
error rate. 
An alternative system is to measure the error rate over a number of words, 
say N, but display the error rate more frequently, i.e. after each new N/k 
words. In this way, the display is updated k times more often than before. 
The calculation is still made on N words, such that with each calculation, 
the N/k oldest words are discarded from the calculation whilst another new 
N/k words are included. 
This system makes a running calculation, but still has the disadvantage 
that it takes N words before the display is able to show the error rate to 
a full 1/N precision. Before this time, if any display is to be made, it 
should indicate that it is of lower resolution. Thus, in essence, either 
the operator has to wait for the display or is given instant access to a 
set of figures which are calculated on a different basis than later 
values. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an error rate monitor 
which alleviates these problems. 
According to the present invention, there is provided an error rate 
monitoring system for displaying the error rate of a stream of data in 
which errors can be detected. The system comprises detection means for 
detecting an error in the data stream and counting means with at least one 
counter whose count value is progressively changed as data flows in said 
stream. It also comprises means for resetting the at least one counter in 
response to detection of an error by said detection means and a display 
for displaying a representation of a value counted by said at least one 
counter. 
An error rate system according to the present invention is thus capable of 
almost instant display, yet is able to represent the error rate to the 
greatest possible precision in any test. As an added advantage, it is more 
easily able to show cyclic phenomena, hi-lighting defects due to regular 
recurring conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be more clearly understood from the following 
description, given by way of non-limitative example, with reference to the 
accompanying drawings. 
Referring to FIG. 1, a stream of data DS, for instance from a video or 
audio tape playback apparatus, is input to an error detection circuit 2 
which detects errors in the data stream. The data DS may be of any 
predetermined form in which error may be inferred and may comprise, for 
instance bits, bytes, words, etc. 
First and second counters 41 and 42 are provided as part of counter 4 and, 
as illustrated in the present embodiment, also receive the data stream DS 
so as to count the number of, for instance, data words received. However, 
counters 41 and 42 may alternatively count elapsed time. 
In operation, counter 42 counts the number of words or time elapsed since 
the system was started up, in other words, since the current test 
operation was started. However, the counter 41 is reset by the error 
detection circuit 2 upon detection of an error in the data stream DS. 
Thus, the counter 41 counts the number of words or time elapsed since the 
last detected error. This count may be continuously monitored on display 
12 via display driver 10. 
The calculation and storing circuit 8 is provided to keep a running total 
of the number of occurrences of each value of the counter 41 when it is 
reset by the error detection circuit 2. Thus, when the counter 41 is 
reset, the final count before being reset, i.e. the time elapsed or number 
of data words between the last two errors, is passed to the calculation 
and storing circuit 8. This determines in which of a number of bands of 
count the final count falls, for instance, bands of data numbers 0-99, 
100-199, etc or bands of time 0-1 sec, 1-2 sec, etc. It then keeps a 
running total of how many final counts have occurred in each of the bands 
since the system was started up. 
If necessary and as illustrated in the present embodiment, a memory 6 may 
be provided so as to briefly store the final count when the counter 41 is 
reset such that the calculation and storing circuit 8 has time to retrieve 
that count. 
Finally, a display driver 10 receives signals from counters 41 and 42 and 
the calculation and storing circuit 8 so as to display on the display 12 
the counter values and the recorded number of occurrences. 
Operation of the system will be further described by reference to FIG. 2 of 
the accompanying drawings. 
The error rate monitor records not "how many errors in a block" but "how 
long between errors" in terms of time or words. In other words, it records 
a count for the time between consecutive detected errors. At the start of 
a measurement, this count is zero and two display pointers, one measuring 
a count since the last error and one measuring the total count, are both 
at the zero location. As the test progresses, with no errors having 
occurred, the pointers move up the scale together. Thus, if one thousand 
words are input without an error being detected, then both pointers reach 
the 1/1000 index and, if ten thousand error free words pass the test, then 
both pointers reach the 1/10000 mark. 
This situation continues until an error is detected, whereupon the pointers 
diverge. One of the pointers, i.e. that corresponding to counter 42 
showing the maximum resolution of the test so far, continues along the 
scale. The other of the pointers, i.e. that corresponding to counter 41 
indicating the number of words since the last error, is reset to zero to 
start again its journey along the scale. 
Simultaneously with the pointers diverging, according to the embodiment 
shown in FIG. 2, a bar is drawn orthogonally from the direction of the 
pointers movement to show that one error has been detected. Its place on 
the scale then shows that the detected error was separated from the 
previous error by the number of samples (data words) or the time indicated 
by its place on the scale. 
After a detected error, the pointers then again progress along the scale 
until the next error. At this time, another bar is placed at the new 
position of the pointer representing the number of words since the last 
error. This pointer is then again reset to zero. If this newly added 
result happens to coincide with the previous value then the bar size is 
increased by one at that point. In this way, as illustrated in FIG. 2, a 
bar graph of the distribution of time or samples between errors is drawn. 
Thus, one pointer shows the duration of operation, i.e. how long the error 
monitoring has been running, and thereby shows the best possible 
performance, of the system under test. In other words, this pointer shows 
the total time or number of samples that have occurred during the test and 
therefore shows the total time or number of samples which could have been 
free from errors; a time or number of samples between errors longer than 
this total is impossible. 
Clearly, this pointer is limited by the length of the scale and cannot show 
a total time or number of samples beyond the maximum value of the scale. 
In this regard, it is proposed that a scale is fixed or selected by the 
operator such that its maximum value is greater than any interval between 
errors which might be of interest. Alternatively, an autoscaling system 
could be used such that the scale is changed each time its maximum value 
is reached. In this way, the resolution of the monitor would be 
automatically reduced as the test progressed. 
The other pointer shows the elapsed time or number of words since the last 
error, thereby allowing the viewer instant access to the present 
situation, necessary for adjustments that are error dependant. In the 
illustrated example, the pointers move right to left in a linear fashion 
until error monitoring stops, or there is an error. If there is an error, 
the relevant point resets to the right hand side of the scale. 
The bars indicate the number of "time or data between errors" recorded at 
that value since monitoring started. This type of display allows the 
balance of errors to be observed. A bar on the right hand side indicates 
errors occurring in quick succession which may suggest effects such as 
error propagation. On the other hand, a very tall bar at some later point 
may indicate a mechanical or repetitive effect and its frequency may show 
whether, for example on a tape machine, it is caused by a tape reel or a 
head drum. 
Given an equal probability random error distribution, the resultant bar 
graphs form an approximately triangular display. In view of this, the 
display may be weighted such that the area beneath the graph is constant 
for random errors after a given period of measurement (this also makes 
scaling the display easy). In other words, the weighting is arranged such 
that one long gap between errors is equivalent to many short ones. 
The precise nature of the weighting may be different for different 
applications. For example, noise is often "pink" rather than "white" and 
can have a 1/f distribution. In particular, certain applications, for 
instance certain transmission channels, may have known particular noise 
characteristics. In these circumstances, the weighting may be adjusted to 
give a rectangular display for the expected noise distribution, thereby 
facilitating recognition of abnormal noise. 
The horizontal axis which has the scale for counts of data or time can be 
made logarithmic as well as linear. A logarithmic scale is less useful for 
maintenance, but is more suitable for an operational display and functions 
such as tape analysis. In addition, by using a logarithmic scale, the 
maximum displayed test duration can easily encompass normal tape lengths 
of for instance 2 hours. 
The type of display scale is preferably selectable depending on use.