Method and device for measuring the successive amplitude levels of signals received on a transmission channel

Measurement of the successive levels of a received signal is achieved by combining together the elements of groups of digitized samples (E1 to Em) which are taken from the signal during successive sampling windows (W) positioned by a local clock (H) kept thoroughly synchronous with the signal. The combination consists of a weighted average so as to take account of the transmission conditions: passband of the channel, required flow rate, background noise, etc, likely to distort the signals transmitted and therefore to lead to errors in the measurement of the actual levels thereof. Such weighting is achieved by using the set of digital samples (E1 to Em) corresponding to a single group, for addressing a memory (10) in which the values to be selected for each one of the digital words have been previously stored.

FIELD OF THE INVENTION 
The object of the invention is a method and a device for measuring with 
precision the successive amplitude levels of signals received on a 
transmission channel. 
The method in accordance with the invention may have applications in many 
instances where signals of variable amplitude are received. It may notably 
be applied to the transmission of amplitude-modulated signals. It will be 
described hereafter when applied to a type of transmission where the 
signals to be measured are punctuated by a clock signal and whose 
amplitude is subjected to variations, notably because of the coding mode 
that has been selected for the transmission thereof on the transmission 
channel. 
BACKGROUND OF THE INVENTION 
Precise measurement of the successive levels of a signal received by a 
reception device may be achieved, for example, according to a technique 
known in itself, by taking from the signal at discrete intervals, several 
successive samples and by calculating an average of the respective 
measurements thereof. These groups of samples may be taken, for example, 
at each period of the signal received. The results are valid providing 
that the time interval or window during which the samples of a single 
group are taken has been correctly positioned, on a determined fraction of 
the signal, half a period for example. Precise positioning of the sampling 
window with respect to the signal may be achieved with precision if a 
local clock thoroughly synchronous with the clock signal conveyed with the 
signal received is available. 
This technique for measuring levels from a group of successive samples 
provides precise results if the transmission conditions are satisfactory. 
If the signals transmitted are distorted by the propagation thereof on a 
transmission channel whose passband is inadequate for the rate of data to 
be transmitted, or if the background noise superimposed on the signal 
received is relatively high, the level measurements may be altered. In 
case of a multi-level type transmission applied to the transmission of 
digitized data where the difference between two successive levels may be 
relatively small, a level measurement error may lead to confusions in the 
logic symbols received. The analog signals restored after decoding are 
therefore distorted. 
SUMMARY OF THE INVENTION 
The method in accordance with the invention enables precise measurement of 
the successive amplitude levels of a periodic signal of variable amplitude 
received on a transmission channel, while avoiding the drawbacks of the 
prior methods. The method comprises: 
--restoring a clock signal synchronous with said periodic signal, 
--sampling the periodic signal at least during sampling time intervals each 
equal to a definite fraction of the period thereof and at a sampling 
frequency sufficient to obtain successive groups each containing a 
definite number of distinct samples during said time interval, and 
--digitizing on a certain number n of bits each of the samples contained in 
each group, to form a corresponding set of digital words. 
The method is characterized in that it comprises: 
--allocating to each set of digital words a weighted average value taking 
account of the transmission conditions on said transmission channel and/or 
the fraction of the signal included in the sampling time interval. 
This allocation is, for example, achieved by comparing each set of digital 
words with a pre-established list of digital words and, according to the 
result of each comparison: 
--associating with each group of digital words a single digitized value 
representative of the effective level of the periodic signal during the 
sampling time interval, the weighting applied taking account of the 
fraction of the periodic signal sampled and of the propagation conditions 
on said transmission channel. 
According to one preferred embodiment, said allocation of a weighted 
average value is achieved by reading directly a single digital word 
representative of the level of the signal during the sampling time 
interval, said digital single word being located in a memory location, the 
address of said location being set by the set of digital words obtained at 
each sampling. 
The allocation or comparison operation mentioned above makes it possible to 
adapt to a particular transmission channel by taking account of the 
distortions it may apply at a relatively high rate for example, or to 
change as one wants the fraction of the period of the signal to be sampled 
to measure the amplitude thereof, by adapting to imposed operating 
conditions. It is possible, with the method, to allocate to amplitude 
stages of the signal received a value that is correct despite level 
deviations exhibited by some of the samples in the sampling windows with 
respect to the signal. 
When the signals received are, for example, coded digitized signals whose 
transmission is punctuated by a clock signal, the method comprises 
positioning of the sampling time interval by a local clock synchronized 
with said clock signal. 
When the signals received are, for example, amplitude-modulated signals, 
the method comprises positioning of the sampling time interval by a clock 
synchronized with the frequency of the modulated signals. 
The device in accordance with the invention makes it possible to measure 
with precision the successive amplitude levels of a periodic signal of 
variable amplitude received on a transmission channel. It comprises clock 
restitution means adapted for generating a local signal synchronous with 
the frequency of the periodic signal received, analog-to-digital 
conversion means for taking a fixed number m of samples from the signal 
received during a determined sampling time interval and for digitizing 
said samples in the form of n-bit digital words, and selection means for 
allocating to each set of digital words a weighted average value taking 
account of the transmission conditions on said transmission channel. 
According to an advantageous embodiment, the selection means comprises an 
addressable memory containing digital words representative of amplitude 
values and means for forming memory reading addresses from at least part 
of the content of the storage means. 
These means for forming addresses to read the memory comprise, for example, 
storage elements for the whole of the digital words taken during each 
sampling time interval. 
The device comprises, for example, means for adapting the level of the 
signals received on said transmission channel before the application 
thereof to the analog-to-digital converter. 
According to a preferred embodiment, the clock restitution means comprise a 
local clock, storage means connected to said analog-to-digital converter, 
for digitized samples taken during time intervals (W') including instants 
of transition of said signal between amplitude levels, means for 
determining the relative level deviations between the samples taken during 
each time interval and for constituting with each one of them a set of 
deviation values and a control unit for generating correction signals as a 
function of the set of deviation values obtained, this control unit being 
adapted to produce correction signals allowing the local clock to be 
resynchronized with the clock signal. 
Determination of the relative deviations is preferably achieved by using 
two registers adapted to contain the successive digital words, means for 
comparing two by two the successive digital words contained in the two 
registers, and storage means to contain said sets of relative deviations. 
According to one embodiment, the local clock comprises a high-frequency 
oscillator, first frequency dividing means for dividing the high frequency 
of the oscillator signal by several distinct factors around one central 
value, and for selecting one of the factors as a function of said 
correction signals received from the control unit, and second dividing 
means connected to the first dividing means for generating a first signal 
at the same frequency (f) as the clock signal received and a 
synchronization signal at a frequency (F) which is a multiple of that of 
the first signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A signal such as that shown in FIG. 1 is obtained from a certain number of 
electric voltages v1, v2, . . . of determined amplitude (eight or sixteen, 
for example). A particular combination of binary signals which may be 
transmitted simultaneously on a transmission channel, as it is well-known 
by specialists, corresponds to each one of the successive stages of the 
signal. Thus, precise determination of the level of each of the successive 
stages of the signal received on this channel allows the binary signals 
transmitted to be restored. Measurement of each of the levels is achieved 
during a window W which has to be positioned with precision so as to avoid 
level recognition errors. 
The method in accordance with the invention firstly comprises the formation 
of groups of digital words each representative of a group of samples taken 
successively from a determined portion of the signal received. This 
formation of digital word groups previously requires precise positioning 
of a measurement time interval or window with respect to the selected 
signal portion, which is achieved by adjusting a clock element on the 
clock signal received with the signals. 
The method in accordance with the invention then comprises a step of 
calculation of the weighted average (averaging) of the digital words of 
each group, enabling accurate recognition of the levels successively 
measured. It appears in fact that a simple combination of the words of a 
single group may lead to errors on determining the actual levels. When the 
distortions undergone by the signals on the transmission channel are 
extensive (see FIG. 2 for example), the measurement window W may coincide 
with a stage portion exhibiting considerable amplitude variations. The 
digitized samples formed in this window exhibit notable deviations in 
relation to one another. Error risks on the average level allocated to 
such a group increase considerably. It is all the more frequent since the 
number of distinct levels to be recognized is large and the deviations 
between the levels are relatively small. This weighted averaging operation 
makes it possible to easily take account of the real transmission 
conditions on the channel used: rate, noise level, etc. 
This weighted average operation may be achieved by comparing each group of 
digitized samples to a pre-established list of digital words provided with 
appropriate weightings; according to the result of each comparison, each 
group of digitized samples is allocated a single digitized value 
representative of the level of the periodic signal during the sampling 
time interval, the weighting applied taking account of the fraction of the 
sampled periodic signal and of the propagation conditions on the 
transmission channel. It can, for example, be seen in FIG. 2 that a simple 
average achieved on the samples E1 to E8 taken from a window W would lead 
to an average value O.sub.m and to the allocation of a level Vk+1 to the 
samples of the group. If it is known that, considering the passband of the 
transmission channel and the required transmission rate, the signals will 
be distorted, with considerable lagging, it will be decided to allocate to 
the group of samples the level Vk for example, which is the level of the 
stage reached with a given delay with respect to the transition. 
The device in accordance with the invention receives coded signals 
punctuated by a clock signal on a transmission channel 1 consisting for 
example of a transmission line. It firstly comprises (FIG. 3) means for 
positioning with precision a time interval or window on a determined 
fraction of the signal, such as a portion of each of the successive stages 
thereof for example. These means comprise a reception set 2 appropriate to 
the signals received which, in case of a transmission line, comprises for 
example (FIG. 3A) a low-pass filter 3. Attenuation of the signals from 
filter 3, due to transmission, is compensated by an automatic gain control 
unit 5 comprising a variable attenuator 4, an amplifier 5 and an envelope 
detector 6 connected to the output of amplifier 5, which controls the 
attenuation factor of attenuator 4. The signals coming from reception set 
2 are applied to a clock regulation assembly 7 of a well-known type 
adapted to produce a first reference signal H(f), of frequency synchronous 
with the clock signal punctuating the received signals, and a second 
signal H(F) of frequency F which is multiple of the previous one and 
signals defining a time window W during which samples will be selected. 
Such a clock regulation assembly comprises a PLL type loop including a 
local oscillator, such as that described, for example, in the French 
patent specification FR-2,624,32 registered by the applicant. 
The signal from reception set 2 is also applied to an analog-to-digital 
converter 8 which samples the signal and digitizes the successive samples, 
at a rate set by the signal H(f) from assembly 7. Converter 8 produces 
n-bit digital words (n=4 or 8 for example). The digital words E1, E2, . . 
. Em (m is 8 for example) produced during the window W imposed by assembly 
7 are loaded in series into a register 9. The parallel outputs of register 
9 are connected respectively to the inputs of the address register R of a 
memory 10. 
Comparison is achieved here by reading directly, in a location of the 
memory whose address is fixed at each sampling by the group of digitized 
samples obtained, the single digital word to which it has been agreed to 
allocate the signal level in the measurement window. Level allocation is 
easily done by loading a PROM type memory according to the conditions of 
use of the transmission channel. 
According to the embodiment of FIG. 4, synchronization of a local clock 
enabling precise positioning of window W is based on an analogous 
principle of analysis of the groups of samples and of comparisons with a 
predetermined-list in memory 10. In the diagrams of FIGS. 3 and 4, the 
elements referenced 1 to 6 and 8 to 10 are identical. Clock regulation 
assembly 7 is here connected to the output of converter 8, and it is 
analogous to that described in the French patent application EN 91/13,770. 
Assembly 7 comprises a local clock consisting of a high-frequency 
oscillator 11, a dividing assembly 12, adapted to divide the frequency of 
the signal of oscillator 11 by a factor 7, a factor 8 or a factor 9 for 
example, and to select one of the three resulting signals as a function of 
correction signals CR0 or CR1, and dividing elements 13 receiving the 
selected signals and, after division, producing the clock signals H(f), 
H(F) and signals defining window W and defining another window W' centered 
on the successive transitions of the signals received. Frequency F is 
chosen, for example, equal to 8f or to 16f. 
Assembly 7 comprises means for comparing the digitized samples taken during 
the sampling window W' centered on the transitions of the signal received. 
These means comprise a register 14 connected to the output of converter 8, 
which receives, at the frequency F, the digital words issuing from the 
converter. Register 14 is connected in series to a register 15 so that the 
digital words pass successively into one and the other. These consecutive 
words or samples are compared to one another at the same frequency by a 
comparison means 16 adapted, on the one hand, to determine the relative 
deviation thereof, and, on the other hand, to compare this relative 
deviation to a threshold deviation. It is given a value 1 if the relative 
deviation between any two digitized samples is acceptable, and 0 if it is 
not. The binary values coming successively from comparison means 16 are 
transferred into a shift register 17. The digital words formed by all the 
binary values obtained in each window W' are stored in a register 18. Each 
of the words corresponds to a deviation configuration representative of a 
transition that has been sampled. Assembly 7 further comprises comparison 
means to recognize whether this configuration is acceptable. These means 
comprise an EPROM type memory 19 whose address register 20 is connected in 
parallel to register 18. The digital words therein address directly memory 
19 and allow 2-bit digital words CR0, CR1 stored previously, to be read in 
the memory. If the bit configuration in register 18 is acceptable, i.e. if 
the synchronism between the clock detected on the transmission channel and 
the local clock is considered to be correct, the digital word read causes 
no corrective action. Others correspond to cases where the local clock is 
ahead of the clock signal transmitted. Other configurations yet correspond 
to cases where the local clock is late. The logical signals CR0 and CR1 
from the memory are applied to dividing means 12. 
If the signal of the local clock appears to be ahead with respect to the 
clock transmitted, application of the corrective signal CR0, for example, 
leads to the selection of the division factor 8 by dividing assembly 12, 
so as to decrease the frequencies f and F produced by dividing elements 3. 
On the other hand, the local clock being late leads to the selection of 
the division factor 7 by dividing assembly 12 and to an increase in the 
same frequencies. 
The invention has been described as applied to the measurement of the 
successive levels of a coded signal punctuated by a clock signal. However, 
without departing from the scope of the invention, the method may be 
applied to the periodic measurement of the envelope of a modulated signal 
received on a transmission channel. 
The transmission channel on which the coded signals are received may be a 
transmission line. It may also consist of a radio channel made up of a 
modulated carrier with a modulation of a well-known type suited to the 
signals to be transmitted.