Method and device for regenerating by maxima detection, a clock signal punctuating the transmission of received digitized signals

In order to restore a clock signal that has been used for coding digitized signals transmitted on a transmission channel, such as a line, the case when the received signals are distorted, the times (ti) when the signals go through their maxima (M) are detected with precision through an analysis of a series of samples (Ep) taken from the signals received during a time "window" (W) positioned with reference to a local clock. A device determines the amplitude differences between the successive samples and compares the difference configuration as obtained with predetermined standard configurations representative of forms that are acceptable for the maxima. In case when the difference configuration corresponds to no recognized standard configuration, the frequency of the local clock is changed so as to re-establish a sufficient similarity with a standard configuration.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an improved method and device for 
regenerating, from signals received on a transmission channel, a clock 
signal marking the transmission, on the channel, of digitized and coded 
data. 
2. Description of the Prior Art 
In data transmission using clock signals, the digitized data to be 
transmitted are coded from a predetermined number of different voltages of 
constant amplitude. 4, 8, 16 or coding symmetrical voltages are for 
example selected on either side of the zero voltage. With 2.sup.n levels 
for example, n bits can thus be transmitted simultaneously and the 
transmission rate of a transmission channel can therefore be increased. An 
application of transmission on a cable using a multilevel coding is 
described for example in the Assignee's French Patent 2,675,974 for 
optimizing the signal transfer rates on transmission cables. 
The Assignee's French Patent 2,683,411 (EP-541,431) describes a process and 
a device for restoring a clock signal marking the transmission of signals 
received on a transmission channel, notably for the radio transmission of 
seismic signals. 
In order to restore the clock signal, the transition times of the signals 
are detected with precision through an analysis of a series of samples 
taken from the signals for at least a fraction of the clock period. The 
amplitude differences between the various successive samples are detected 
and the obtained difference configuration is compared with standard 
configurations observed on the transmission channel by means of previous 
tests and stored in a memory. If the similarity between the measured 
difference configuration and one of the standard configurations is 
sufficient, the window is correctly centered. If it is not, a local clock 
is used for resynchronization. 
This prior method works correctly in all the cases where the transitions 
between the successive levels are sufficiently marked. In the opposite 
case, which notably occurs when the passband of the transmission channel 
is incompatible or poorly compatible with the desired transmission rate, 
the transition zones are too degraded for the configurations of samples 
taken in each "window" to correspond to a stored standard configuration. 
The resynchronization of a local clock on the basis of the comparisons 
performed is therefore more uncertain. 
In geophysical exploration operations intended to connect downhole 
servicing equipments: measuring instruments, well sondes, etc, to a 
surface control and recording station, transmission cables with a limited 
passband are generally used, such as multi-conductor electrocarrying 
cables. 
By using the optimization process described in the Assignee's French Patent 
2,675,974, the transmission rate of the transmission lines can be 
optimized in this type of cable. To that effect, correction circuits are 
placed at the end of a line so that the transfer function of the corrected 
line corresponds at least within a certain frequency interval to that of a 
reference filter such as a Bessel filter for example. The presence of 
these correction circuits allows the transmission rate of the transmission 
lines to be improved considerably. 
However, it has been noticed during transmission tests on this type of 
cable, either because the transmission rate is not compatible with the 
natural passband of the lines, or because the correction applied thereto 
is not optimal, the signals exhibit a horizontal instability (jitter) as 
they approach zero, as shown in FIG. 1, which makes the transitions 
between different levels hardly detectable, and their vertices are rounded 
in the shape of sinusoid vertices, however with a relatively stable 
position. FIG. 1 highlights this phenomenon. 
On the other hand, it has been observed that it is easier to discern when 
the signals go through their maximum amplitude in a relatively narrow 
sampling window. 
It is well-known that a local clock can be synchronized with the clock that 
marks the transmission of a signal by detecting the times when the signal 
derivative with respect to time becomes equal to zero and by adjusting the 
local clock by means of a locking loop. 
However, in the case when the received signal is of the multilevel type, 
such a detection can lead (see FIG. 3) to an erroneous adjustment of the 
local clock to intermediate times where the derivative changes sign 
(points m in FIG. 3) and not to the really representative vertices 
(vertices M). 
SUMMARY OF THE INVENTION 
The method according to the invention regenerates, from signals picked up 
on a transmission channel, a clock signal marking the transmission on the 
channel of digitized and coded data, so as to synchronize a local clock 
thereon, while avoiding the uncertainties of the prior method that are 
linked, under certain circumstances, to the location of the transition 
times between the signals, by avoiding any adjustment ambiguity. 
The invention can be used in bipolar coding transmission systems. It is 
particularly well-suited to systems for transmitting simultaneously 
several data elements such as multilevel transmission systems. 
The invention positions a time interval or read window of the received 
signals with reference to the local clock, and comprises locating the 
vertices of the received signals by selective detection of the times when 
their derivative becomes equal to zero or changes sign after an increase 
in an absolute value of the received signals, and possibly recentering the 
read window on these times by changing the frequency or the phase of the 
signal of the local clock. 
Locating these vertices is for example performed by: 
determining the amplitude differences between samples taken on signals 
during successive read windows, so as to obtain a difference configuration 
corresponding to each of these windows, 
comparing each difference configuration obtained with a list of previously 
recognized and catalogued standard difference configurations corresponding 
to windows including the sought vertices, and by 
optionally recentering each read window by applying, to the frequency or to 
the phase of the signal of the local clock, a predetermined correction 
associated with each standard configuration of the list, in the case when 
the difference configuration obtained corresponds to a catalogued standard 
configuration, this correction concerning for example a division factor of 
a reference frequency. 
The local clock modification can be performed by carrying out the 
comparison stage only on the received signals whose amplitude of the 
vertices is higher than a threshold value, or by selecting the vertices 
whose amplitude is equal to the highest level of the multilevel signals 
emitted on the transmission channel. 
The method can also advantageously comprise measuring the amplitude of the 
signals substantially at the time corresponding to the middle of each 
centered or recentered window. 
The device according to the invention comprises means for modifying the 
amplitude of the signals picked up, and means for sampling and for 
digitizing signals during time intervals or read windows of these signals 
determined with reference to the local clock, as well as centering means 
suited to locate vertices of the signals picked up by detection of the 
times when their derivative becomes equal to zero, and to recenter if need 
be the read window on these times by changing the frequency or the phase 
of the signal of the local clock. 
The centering means comprises a set intended to form during each window a 
difference configuration from the amplitude differences between samples 
taken successively from the signals during the window, and to select 
difference configurations including the signal derivative going through a 
zero value after an increase in absolute value of the signals, and 
comparison elements for comparing each set of differences obtained with a 
list of standard difference configurations, and for applying to the 
frequency or to the phase of the signal of the local clock a predetermined 
correction associated with each standard configuration of the list. These 
comparison elements of each set of differences can include, for example, 
means for forming digital words representative of difference 
configurations between the samples taken from the same window and storage 
means indicating the correction to be brought about to the frequency or to 
the phase of the local clock. 
The set for forming the difference configurations comprises for example 
means for determining the direction of the differences between samples 
that have been successively taken and digitized, and means for locating 
the maxima by taking account of the sign of the digitized samples. 
The local clock comprises in series for example a first division element 
for dividing a reference frequency by an adjustable division factor, 
connected to said comparison elements, a second division element with a 
division factor m, producing a signal at the bit succession frequency, and 
a third division element with a division factor n, producing a signal at 
the baud succession frequency. 
The device preferably comprises means for selecting the signals whose 
amplitude exceeds a fixed threshold value, as well as a phasing or locking 
indicator for a validated signal operating system. 
The invention can also include means for detecting the amplitude of the 
signals received substantially at the time corresponding to the middle of 
the centered or recentered window. 
With the discrimination between the times when the derivative becomes equal 
to zero while only those really corresponding to positive or negative 
vertices of the multilevel signals received are retained, it is certain 
that the reception window eventually centers unequivocally on the vertices 
and consequently properly phases the local clock.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A multilevel signal is constituted (FIG. 1) from a certain number of 
voltages of different amplitudes. As it is well known, a particular 
combination of binary signals that can be transmitted simultaneously on a 
transmission channel corresponds to each of the successive amplitude 
stages of the signal. The precise determination of the level of each of 
the successive stages of the signal received on this channel allows the 
transmitter binary signals to be restored. 
This discrimination of the different levels is more delicate in practice 
because, after propagation on a transmission channel, with a high 
transmission rate considering the channel passband, the stages of the 
initial multilevel signal (FIG. 2) are rounded as schematized in FIG. 3. 
The stages of the multiband signal are all the more difficult to establish 
since the number of distinct levels to be recognized is high and the 
differences between the levels are relatively slight. 
In order to avoid any errors in level recognition, it is important that the 
time when the maximum amplitude reached is measured is well-centered on 
the successive vertices. 
According to the invention, this condition is fulfilled mainly by 
positioning a window W for reading the received signals with reference to 
a local clock, by locating vertices M of these signals picked up by 
detecting the instants when its derivative changes sign or becomes equal 
to zero, and the read window is centered on these times (or corrected if 
need be) by changing the frequency or the phase of the signal of the local 
clock. 
The device according to the invention receives coded signals marked by a 
clock signal on a transmission channel L (FIG. 4) consisting for example 
of a transmission line, possibly optimized by correction circuits C in 
accordance with the process described in the above-mentioned patent French 
Patent 2,675,974. The correction circuits C result in the pulse response 
width of the cable, when they are connected thereto, having the same order 
of magnitude as the period of the clock punctuating the baud transmission 
on line L. 
These signals come in coded form, for example in the well-known ANSI code, 
with evenly distributed polarity reversals so that, within a predetermined 
time interval, an amplitude maximum and minimum are available, and their 
DC component is zero. 
The positioning of window W is performed by means of a local clock and the 
centering of the window is mainly achieved with respect to the time ti 
when the sign change of the signal derivative is detected. Since the 
horizontal instability or jitter is higher with low-amplitude signals, 
only those whose amplitude exceeds a certain set threshold T are 
preferably retained for calculating the derivative, and only the maxima of 
the absolute value are taken into account, and not the minima m, as shown 
in FIG. 3. 
The positioning of the windows is performed for example by the device 
described hereafter and schematized in FIG. 4. 
The device includes a synchronization set SA comprising three frequency 
division stages 1, 2, 3 connected in series, receiving a high-frequency 
signal H emitted by a quartz crystal Q. The first division stage 1 applies 
a division factor k that can range between three values k0, (k0-d) and 
(k0+d), and produces a first clock signal H1. The second and the third 
stages 2, 3 apply respectively a second division factor m and a third 
division factor n, both fixed, and they produce respectively a second and 
a third clock signal H2, H3. The quartz frequency and the division factors 
m and n are so selected that the frequency of signal H2 corresponds to the 
bit reception frequency on line L, and that of signal H3 corresponds to 
the reception frequency of the signal bauds. The width of window W is 
selected for example equal to the period of signal H3. 
After passing through circuits C, the signals received on line L, coded 
according to a bipolar code and without DC component, are applied to an 
automatic gain control (AGC) amplifier 4. After amplification, they are 
sampled (FIG. 3) and digitized in an analog-to-digital converter A/D-C 5 
controlled by clock signal H1. The frequency of this signal H1, which is 
applied to converter A/D-C 5, is selected to have a sufficiently fine 
sampled signal resolution. The same reference voltage VR is applied to 
amplifier 4 and to converter A/D-C 5. The digitized signals are then 
applied to a signal decoder 6 that measures the successive levels of the 
multilevel signals received and decodes them so as to reconstitute the 
data that have been transmitted on the line. It is controlled by the clock 
signals from set SA. 
The permanent recentering of the read window W through the phasing of the 
signals of the local synchronization set SA with the clock signal mark the 
transmission on line L is performed as described hereafter, by detecting 
the times when the derivative changes sign or at least becomes equal to 
zero, and therefore the position of the successive vertices of the 
signals. 
Each digitized signal sample coming from converter A/D-C 5 is fed into a 
register 7 and compared with the immediate next one in a comparator 8. The 
results of these successive comparisons are delivered on three successive 
outputs according to whether the increments are positive (i+), negative 
(i-) or zero (i=). The increments i+ and i-, as well as the sign bit sb 
coming from converter 5 are applied to a maxima detector 9. When it 
detects a passage to a zero derivative value (i=) following an increase in 
absolute value of the signal, detector 9 emits a logical 1 indicative of a 
maximum M, that is applied on an input of an AND gate 10. 
The digitized words from converter A/D-C 5 are also applied in parallel to 
two comparators 11, 12 with respective thresholds T+ and T- (FIG. 3). The 
threshold detection signals coming from the two comparators are applied, 
through an OR gate 13, to a second input of AND gate 10. A third input 
thereof receives the signals i= coming from comparator 8. 
The successive bits delivered through the AND gate 10 are formed into 
digital words (of 12 bits for example) in a shift register R2 (14). 
Considering the selection achieved by the previous elements 7 to 13, these 
words are representative of the amplitude differences between the m.n 
samples a1 to am.n that are taken successively in each of the windows W 
that frame a maximum M (FIG. 3). 
At a frequency set by the clock signal H2, corresponding to that of each 
baud transmitted, the words are transferred into a third register R3 (15). 
The operation of positioning window W is achieved by checking that the 
configuration of logical 1 and 0 of each word in register 13 is correct by 
comparing it with a stored standard list of configurations. If it is not, 
the frequency or the phase of the local set SA that controls the centering 
of the window is changed so as to find one later. 
This operation is performed here directly by seeking in a memory 16 of the 
EPROM type, at the address indicated by the word in the third register R3, 
2-bit or 4-bit words for example representative of the correction to be 
possibly applied to the division factor k of frequency divider 1. A 
monostable multivibrator 17 is connected to memory 16 and it delivers a 
signal LCK indicating a correct adjustment of the local synchronization 
set SA with respect to the clock signal received. Three cases may arise: 
a) the configuration obtained is not catalogued, in which case the division 
factor k is not changed and keeps its previous value, 
b) the configuration obtained corresponds to a good centering of the window 
and the difference d is eliminated, if it has not been yet, factor k being 
equal to k0, and 
c) the configuration is catalogued and the correction d, that is read in 
the memory location of the EPROM memory and is capable of recentering 
window W on vertex M according to the signal received by changing the 
frequency or the phase of the local clock, is applied to factor k. 
Certain configurations correspond to cases where the local clock SA is fast 
in relation to the clock signal transmitted. Other configurations 
correspond to cases where the local clock is slow. The digital words 
coming from memory 16 are applied to divider 1 and they determine the good 
variation in the range d around the value k0 defined above to be applied 
to the division factor k in order to restore the synchronism. 
The height of the threshold T+ and T- chosen for selecting the vertices of 
the signal can be changed at will according to the quality of the signals 
received. It is for example possible for a multilevel signal to detect 
only when the signal goes through the maximum level. The frequency of the 
possible recentering operations is thus decreased but, on the other hand, 
the selection device can be simplified since the minima are automatically 
excluded. 
The detection of each maximum is performed by signal decoder 6. As it 
receives the adjustment signal emitted by monostable multivibrator 17, it 
dephases by a half-period the signal H3 marking the bauds succession and 
it acquires for example the signal sample that is the closest to the 
rising front of the phase-shifted signal H3 that marks each time ti (FIG. 
3). 
The EPROM memory used for storing the corrections to be applied to the 
signal of the local clock SA lends itself well and easily to modifications 
to the list of standard configurations and to possible corrections to be 
brought. However, a processor programmed to perform the required 
corrections can for example be used instead without departing from the 
scope of the invention. 
The sensitivity of the device to the phase-shift variations between the 
signals of the local clock and of the clock detected on line L is 
determined through the difference d applicable to the division factor k of 
every baud and therefore every n.m.k period of the signal H of the local 
clock. The factor k has to be very great so that the effect of the 
variation d=1 applied remains limited (of the order of 2% to 5%), which 
implies a very high frequency for the signal H. It is however possible to 
limit this frequency providing that the correction is only applied every 
n.m of the signal H3 (baud periods) so as to reduce the division factor k 
in the same proportion.