Discrimination and attenuation of pre-eehoes in a digital audio signal The invention relates to a method and a device for discriminating and processing the attenuation of the pre-echos in the decoding of a digital audio signal. For the transmission of digital audio signals over telecommunication networks, whether they are fixed or mobile networks for example, or for the storage of the signals, compression (or source coding) processes are used that implement coding systems which are generally of the linear predication time coding or transform frequency coding type. The field of application of the method and the device that are the subjects of the invention is therefore the compression of the sound signals, in particular Ihe digital audio signals coded by frequency transform. Figure 1 represents, by way of illustration, a theoretical block diagram of the coding and the decoding of a digital audio signal by transform including an overlap/addition analysis-synthesis according to the prior art. Some music sequences, such as percussions and certain speech segments such as the plosives {IkJ, /t/,...), are characterized by extremely abrupt onsets which are reflected by very rapid transitions and a very strong variation of the dynamic range of the signal in the space of a few samples. One example of transition is given in figure 1 based on the sample 410. For the coding/decoding processing, the input signal is decomposed into blocks of samples of length L whose boundaries are represented in figure 1 by vertical dotted lines. The input signal is denoted x{n), in which n is the index of the sample. The breakdown into successive blocks (or frames) leads to the definition of the blocks X^(n) - [ JC(N.L) ... x(N.L+L-l) ] - [JCN(0) ... XN(L-1)3, where N is the index of the block (or of the frame), L is the length of the frame. In figure 1, there are L—160 samples. In the case of the modified discrete cosine transform MDCT, two blocks XN(n) and XN+1(n) are analyzed jointly to give a block of transformed coefficients associated with the frame of index N and the analysis window is sinusoidal. The division into blocks, also called frames, applied by the transform coding is totally independent of the sound signal and the transitions can therefore appear at any point of the analysis window. Now, after transform decoding, the reconstructed signal is affected by "noise" (or distortion) generated by the quantization (Q)- inverse quantization (Q"1) operation. This coding noise is temporarily distributed relatively uniformly over all the temporal support of the transformed block, that is to say over the entire length of the window of length 2L of samples (with overlap of L samples). The energy of the coding noise is generally proportional to the energy of the block and is a function of the coding/decoding bit rate. For a block including an onset (like the block 320-480 of figure 1), the energy of the signal is high, the noise is therefore also of high level. In transform coding, the level of the coding noise is typically lower than that of the signal for the high energy segments which immediately follow the transition, but the level is higher than that of the signal for the lower energy segments, in particular over the part preceding the transition (samples 160-410 of figure 1). For the abovementioned pari, the signal-to-noise ratio is negative and the resulting degradation can appear very disturbing in the listening. The coding noise prior to the transition is called pre-echo and the noise following the transition is called post-echo. It can be seen in figure 1 that the pre-echo affects the frame preceding the transition and the frame where the transition occurs. Psycho-acoustic experiments have demonstrated that the human ear performs a temporal pre-masking of the sounds that is fairly limited, of the order of a few milliseconds. The noise preceding the onset, or pre-echo, is audible when the duration of the pre-echo is greater than the pre-masking duration. The human ear also performs a post-masking of a longer duration, from 5 to 60 milliseconds, upon the transition from high-energy sequences to low-energy sequences. The rate or level of disturbance that is acceptable for the post-echos is therefore greater than for the pre-echos. The pre-echo phenomenon, more critical, is all the more disturbing when the length of the blocks in terms of number of samples is great. Now, in transform coding, it is well known that, for the standing signals, the more the length of the transform increases, the greater the coding gain. At a fixed sampling frequency and at a fixed bit rate, if the number of points of the window (therefore the length of the transform) is increased, there will be more bits per frame to code the frequency rays deemed useful by the physchoacoustical model, hence the advantage of using blocks of great length. The MPEG AAC (Advanced Audio Coding) coding, for example, uses a window of great length which contains a fixed number of samples, 2048, i.e. over a duration of 64 ms if the sampling; frequency is 32 kHz; the problem of the pre-echos is managed therein by making it possible to switch from these long windows to 8 short windows through intermediate windows (called transition windows), which necessitates a certain delay in the coding to detect the presence of a transition and adapt the windows. The length of these short windows is therefore 256 samples (8 ms at 32 kHz). At low bit rate, it is still possible to have an audible pre-echo of a few ms. The switching of the windows makes it possible to attenuate the pre-echo, but not to eliminate it. The transform coders used for the conversational applications, such as ITU-T G.722.1, G.722.1C or G.719, often used a frame length of 20 ms and a window of 40 ms duration at 16, 32 or 48 kHz (respectively). It can be noted that the ITU-T G.719 coder incorporates a window switching mechanism with transient detection, but the pre-echo is not completely reduced at low bit rate (typically at 32 Kbit/s). In order to reduce the abovementioned disturbing effect of the pre-echo phenomenon, various solutions have been proposed in the coder and/or the decoder. The window switching has already been cited; it necessitates transmitting an auxiliary information item to identify the type of windows used in the current frame. Another solution consists in applying an adaptive filtering. In the zone preceding the onset, the reconstructed signal is seen as the sum of the original signal and of the quantization noise. A corresponding filtering technique has been described in the article entitled High Quality Audio Transform Coding at 64 Kbit/s, IEEE Trans, on Communications Vol 42, No. 11, November 1994, published by Y. Mahieux and J. P. Petit. The implementation of such a filtering requires knowledge of parameters of which some, like the prediction coefficients and the variance of the signal corrupted by the pre-echo, are estimated in the decoder from noisy samples. However, information such as the energy of the original signal can be known only to the coder and must consequently be transmitted. This entails transmitting additional information, which, at constrained bit rate, reduces the relative budget allocated to the transform coding. When the received block contains an abrupt variation of the dynamic range, the filtering processing is applied to it. The abovementioned filter process does not make it possible to restore the original signal, but provides a strong reduction of the pre-echos. It does however entail transmitting the additional parameters to the decoder. Unlike the above solutions, various pre-echo reduction techniques without specific transmission of the information have been proposed. For example, a review of the reduction of pre-echos in the context of hierarchical coding is presented in the article by B. Kovesi, S. Ragot, M. Gartner, H. Taddei, entitled "Pre-echo reduction in the ITU-T G.729.1 embedded coder," EUSIPCO, Lausanne, Switzerland, August 2008. A typical example of pre-echo attenuation processing method without auxiliary information is described in the French patent application FR 08 56248. In this example, attenuation factors are determined for each sub-block, in the low-energy sub-blocks preceding a sub-block in which a transition or onset has been detected. The attenuation factor g(k) in the kth sub-block is calculated for example as a function of the ratio R(k) between the energy of the highest energy sub-block and the energy of the kth sub-block concerned: g(k) = /(*(*)) in which/ is a decreasing function with values between 0 and 1 and k is the number of the sub-block. Other definitions of the factor g{k) are possible, for example as a function of the energy En{Jz) in the current sub-block and of the energy En{k~\) in the preceding sub-block. If the energy of the sub-blocks varies little relative to the maximum energy in the sub-blocks considered in the current frame, no attenuation is then necessary; the factor g(k) is set at an attenuation value inhibiting the attenuation, that is to say 1. Otherwise, the attenuation factor lies between 0 and 1. In most cases, above all when the pre-echo is disturbing, the frame which precedes the pre-echo frame has a uniform energy which corresponds to the energy of a low-energy segment (typically a background noise). From experiments, it is neither useftil nor even desirable for, after pre-echo attenuation processing, the energy of the signal to become lower than the average energy (per sub-block) of the signal preceding the processing zone - typically that of the preceding frame, denoted En, or that of the second half of the preceding frame, denoted En*. For the sub-block of index k to be processed, the limit value, denoted lirn(£), of the attenuation factor can be calculated in order to obtain exactly the same energy as the average energy per sub-block of the segment preceding the sub-block to be processed. This value is of course limited to a maximum of 1 since it is the attenuation values that are of interest here. More specifically, the following is defined here: in which the average energy of the preceding segment is approximated by the value max (En,En*]. The \\m0(k) value thus obtained serves as a lower limit in the final calculation of the attenuation factor of the sub-block, it is therefore used as follows: g(A:) = max(g(A),limJf(A)) The attenuation factors (or gains) g(k) determined for the sub-blocks can then be smoothed by a smoothing function applied sample-by-sample to avoid abrupt variations of the attenuation factor at the boundaries of the blocks. For example, the gain per sample can first of all be defined as a piecewise constant function: gp„(n) = g(k),n = kL',-,(k + Y)L,-l in which L' represents the length of a sub-block. The function is then smoothed according to the following equation: gpre (») - Mgpre (« ~ 0 + 0 ~ <*) gpre («) * « = \ \n) resulting from this filtering therefore contains predominantly low-frequency components of the decoded signal. the second sub-signal xrac ss2 (n) is obtained by complementary high-pass filtering by using an FIR filter with 3 coefficients and with zero phase of transfer function -c{ri)z~~l + 2c(n) - c(n)z, in which [~~c(ri),2c(n),-c(n)\ are the coefficients of the high-pass filter; this filter is implemented with the differences equation: ^^2(M) = ~c(«)*w("~0 + 2c(M)Jcn«(w)-c(wW" + 1)- The sub-signal xw2(") resulting from this filtering therefore contains predominantly high-frequency components of the decoded signal. Note that xrccM (n)+x^s2 («) - xrtiC (n) . It is therefore also possible to obtain xrec 2 (n) by subtracting xrec , (n) from xrec {n) wn'ch reduces the complexity of the calculations: xrecss2 (n) ~ xn,c (n)~XyecM (n) The combination of the attenuated sub-signals to obtain the attenuated signal Sa is done by simple addition of the attenuated sub-signals in the step E608 described below. So as not to use a future signal for these filterings, it is for example possible to complement the decoded signal with a 0 sample at the end of the block. In the case of the decoded signal complemented with a 0 sample at the end of the block for n—L-l, the sub-signal xiw vvl (;?) is obtained by: It can be noted that the two sub-signals here still have the same sampling frequency as the decoded signal. A step E606 of calculation of pre-echo attenuation factors is implemented in the computation module 606. This calculation is done separately for the two sub-signals. These attenuation factors are obtained for each sample of the pre-echo zone determined in E602 as a function of the frame in which the onset has been detected and of the preceding frame. The factors g ss] (n) and g ss2 (n) are then obtained in which n is the index of the corresponding sample. These factors will, if necessary, be smoothed to obtain the factors gpre^{?i) and gpress2(-n) respectively. This smoothing is important above all for the sub-signals containing the low-frequency components (therefore for gpnjlsl (n) in this example). An example of realization of the attenuation calculation is described in the patent application FR 08 56248. The attenuation factors are calculated for each sub-block. In the method described here, they are, in addition, calculated separately for each sub-signal. For the samples preceding the detected onset, the attenuation factors &pre,xs\ (») and («) are therefore calculated. Next, these attenuation values are, if necessary, smoothed to obtain the attenuation values for each sample. The calculation of the attenuation factor of a sub-signal (for example gpKM2 in)) can be similar to that described in the patent application J?R Qg 56248 for the decoded signal as a function of the ratio R(k) (used also for the detection of the onset) between the energy of the highest energy sub-block and the energy of the kth sub-block of the decoded signal. gprejmi W is initialized as: in which/ is a decreasing function with values between 0 and 1, for example f=0 if R(k) <= 16, f= 0.1 if 16 > R(k) >= 32 and f=0.01 if r(k) >32. If the variation of the energy relative to the maximum energy is low, no attenuation is then necessary. The factor is then set at an attenuation value inhibiting the attenuation, that is to say 1. Otherwise, the attenuation factor lies between 0 and 1. This initialization can be common for all the sub-signals. The attenuation values are then refined for each sub-signal to be able to set the optimal attenuation level per sub-signal as a function of the characteristics of the decoded signal. For example, the attenuations can be limited as a function of the average energy of the sub-signal of the preceding frame because it is not desirable for, after the pre-echo attenuation processing, the energy of the signal to become lower than the average energy per sub-block of the signal preceding the processing zone (typically that of the preceding frame or that of the second half of the preceding frame). This limitation can be done in a way similar to that described in the patent application ^ ^8 56248. For example, for the second sub-signal xriiC>s,v2 \nj ^ the energy in the K sub-blocks of the current frame is first of all calculated as: Also known from memory are the average energy of the preceding frame Ena2 and that of the second half of the preceding frame Eriss2' which can be calculated (on the preceding frame) as: in which the sub-block indexes from 0 to K correspond to the current frame. For the sub-block k to be processed, the limit value of the factor lim Tv2 (k) can be calculated in order to obtain exactly the same energy as the average energy per sub-block of the segment preceding the sub-block to be processed. This value is of course limited to a maximum of 1 since the interest here is on the attenuation values. More specifically: attenuations associated with the samples of the sub-block of the onset are all set to 1 even if the onset is situated toward the end of this sub-block. In another variant embodiment, the start position of the onset pos is refined in the sub-block of the onset, for example by subdividing the sub-block into sub-sub-blocks by observing the trend of the energy of these sub-sub-blocks. Assuming that the onset start position is detected in the sub-block k, k>0 and the start of the refined onset pos is located in this sub-block, the attenuation values for the samples of this sub-block which are located before the pos index can be initialized as a function of the attenuation value corresponding to the last sample of the preceding sub-block: All the attenuations from Xhepos index are set to 1. For the first sub-signal containing the low-frequency components of the decoded signal, the calculation of the attenuation values based on the sub-signal xrecM (/?) can be similar to the calculation of the attenuation values based on the decoded signal xrec(p) ■ Thus, in a variant embodiment, in the interests of reducing the complexity of calculation, the attenuation values can be determined based on the decoded signal xrec (n). In the case where the detection of the onsets is made on the decoded signal, it is therefore no longer necessary to recalculate energies of the sub-blocks because, for this signal, the energy values per sub-block are already calculated to detect the onsets. Since, for the great majority of the signals, the low frequencies are much more energy-intensive than the high frequencies, the energies per sub-block of the decoded signal xrai(«) and the sub-signal xrcc , (ft) are very close, this approximation gives a very satisfactory result. The attenuation factors gpKa\{ti) and gpress2(.n) determined for each sub-block can then be smoothed by a smoothing function applied sample-by-sample to avoid abrupt variations of the attenuation factor at the boundaries of the blocks. This is particularly important for the sub-signals containing low-frequency components like the sub-signal xrec ss\ ip) but not necessary for the sub-signals containing only high-frequency components like the sub-signal xrcc vv2 (n). Figure 7 illustrates an example of application of an attenuation gain with smoothing functions represented by the arrows L. This figure illustrates in a), an example of original signal, in b), the signal decoded without pre-echo attenuation, in c), the attenuation gains for the two sub-signals obtained according to the decomposition step E605 and in d), the signal decoded with pre-echo attenuation of the steps E607 and E608 (that is to say after combination of the two attenuated sub-signals). It can be seen in this figure that the attenuation gain represented by dotted line and corresponding to the gain, calculated for the first sub-signal comprising low-frequency components, comprises smoothing functions as described above. The attenuation gain represented by solid line and calculated for the second sub-signal comprising high-frequency components does not comprise any smoothing gain. The signal represented in d) clearly shows the pre-echo has been attenuated effectively by the attenuation processing implemented. The smoothing function is for example defined preferably by the following equations: with the convention that g;)reiJl (n) n = -(w-l),"*,-l are the last u-1 attenuation factors obtained for the last samples of the sub-block preceding the sub-signal xrec^ {n) . Typically u = 5 but another value could be used. Depending on the smoothing used, the pre-echo zone (the number of the samples attenuated) can therefore be different for the two sub-signals processed separately, even if the detection of the onset is made in common on the basis of the decoded signal. The smoothed attenuation factor does not go back up to i at the time of the onset, which implies a reduction of the amplitude of the onset. The perceptible impact of this reduction is very low but should nevertheless be avoided. To mitigate this problem, the attenuation factor value can be forced to 1 for the u-1 samples preceding the/705 index where the start of the onset is situated. This is equivalent to advancing the pos marker by u-1 samples for the sub-signal where the smoothing is applied. Thus, the smoothing function progressively increases the factor to have a value 1 at the moment of the onset. The amplitude of the onset is then preserved. In this embodiment with decomposition of the signal, the verification of the increase in energy of the'.pre-echo zone according to the invention is performed for at least one sub-signal or for each of these sub-signals, The comparison threshold used can be different according to the sub-signals and according to the number of sub-blocks available before the onset. If, in at least one sub-signal, the normalized leading coefficient bin is below the threshold of this sub-signal, the attenuation of the pre-echoes is inhibited for all the sub-signals. In the case of pre-echoes in a signal deriving from an inverse MDCT transform, the energy of the pre-echo component increases or is at least stable in all the sub-signals. The inhibition of pre-ecbo processing can be done for example by setting the attenuation factors at 1 or by not discriminating the zone as a pre-echo zone, the pre-echo attenuation processing module then not being invoked as illustrated by way of example in the embodiment of figure 5 by the link between the block 604 and 602. In variants, the attenuation will be inhibited separately for each sub-signal as soon as the normalized leading coefficient bin is below the threshold of this sub-signal. The inhibition will be able to be implemented for example by setting the attenuation factors at 1 or by not invoking the pre-echo module for the sub-signal considered. Thus, in the particular embodiment described above with decomposition into two sub-signals, if the number of sub-blocks before the onset makes it possible to make this verification, the trend of the energy of the sub-blocks preceding the sub-block where the onset has been detected is verified, in the two sub-signals, by linear regression. This verification can be done according to the steps E603 and E604, at any moment after the division of the decoded signal into sub-signals (E605) and before the application of the attenuation factors of the pre-echoes (E607). The verification is possible if at least two sub-blocks precede the sub-block where the onset has been detected. If the onset is detected in the first or second sub-block, the verification according to the invention is not possible. In variants, it will be possible to re-use the leading coefficients) possibly calculated in the preceding frame if the onset is detected in the first or second sub-block of the current frame. If the onset is detected in the third sub-block, the energy of two sub-blocks in the pre-echo zone is then available to make this verification. By experimentation, with two points, the verification is not sufficiently reliable in the low- frequency sub-signal xrec Ml («) . Only the high-frequency sub-signal xrecss2(n) is then verified, and only that the energy does not decrease. The leading coefficient of the high-frequency sub-signal xrec Hs2 («) is compared to the 0 value threshold. Only its sign is important here, no normalization is needed. It is therefore sufficient to calculate, in the step E603, a single leading coefficient (without normalization) as: If b!ss2 is less than 0, the attenuation of the pre-echoes for this pre-echo zone is inhibited for all the sub-signals. If the onset is detected in the fourth sub-block or a sub-block of index higher than 4, the trend of the energy of the last 3 sub-blocks in the pre-echo zone preceding the sub-block where the onset has been detected is verified. The leading coefficient of the low-frequency sub-signal xrecssl («) is compared to 0, only its sign is important and there is no need to normalize this coefficient. It is therefore sufficient to calculate a single leading coefficient. If the onset has been detected in the sub-block of index id with id >= 3, this coefficient is determined as: busi = En(id - 1) - Enss2(id - 3) If b, SSj is less than 0, the attenuation of the pre-echoes is inhibited for this pre-echo zone, and for all the sub-signals. The leading coefficient of the high-frequency sub-signal xrcc vv2 (/?) is compared to a threshold of value 0.2. The normalized leading coefficient is calculated. If the onset has been detected in the sub-block of index id with id>^3, this coefficient is determined as: thus avoiding a division operation to reduce the complexity and to facilitate the implementation on a DSP processor (Digital Signal Processor) with fixed point arithmetic. The module 607 of the device 600 of figure 5 implements the step E607 of pre-echo attenuation in the pre-echo zone of each of the sub-signals by application to the sub-signals of the attenuation factors thus calculated. The pre-echo attenuation is therefore done independently in the sub-signals. Thus, in the sub-signals representing different frequency bands, the attenuation can be chosen as a function of the spectral distribution of the pre-echo. Finally, a step E608 of the obtaining module 608 makes it possible to obtain the attenuated output signal (the decoded signal after pre-echo attenuation) by combination (in this example by simple addition) of the attenuated sub-signals, according to the equation: Unlike a conventional decomposition into sub-bands, it can be noted here that the filterings used are not associated with sub-signal decimation operations and the complexity and the delay ("lookahead" or future frame) are reduced to the minimum. An exemplary embodiment of an attenuation discrimination and processing device according to the invention is now described with reference to figure 8. Physically, this device 100 within the meaning of the invention typically comprises a processor uP cooperating with a memory block BM including a storage memory and/or working memoiy, and a buffer memory MEM mentioned above as means for storing all the data necessary to the implementation of the discrimination and attenuation processing method as described with reference to figure 5. This device receives as input successive frames of the digital signal Se and delivers the signal Sa reconstructed with pre-echo attenuation in the discriminated pre-echo zones, with, if appropriate, reconstruction of the attenuated signal by combination of the attenuated sub-signals. The memory block BM can comprise a computer program comprising code instructions for the implementation of the steps of the method according to the invention when these instructions are executed by a processor u,P of the device and in particular the steps of calculation of a leading coefficient of the energies for at least two sub-blocks preceding the sub-block in which an onset is detected, of comparison of the leading coefficient to a predefined threshold and of inhibition of the pre-echo attenuation processing in the pre-echo zone in the case where the calculated leading coefficient is below the predefined threshold. Figure 5 can illustrate the algorithm of such a computer program. This discrimination and attenuation processing device according to the invention can be independent or incorporated in a digital signal decoder. Such a decoder can be incorporated in digital audio signal storage or transmission equipment items such as communication gateways, communication terminals or servers of a communication network. 1. A method for discriminating and attenuating pre-echo in a digital audio signal generated from a transform coding, in which, upon decoding, for a current frame decomposed into sub-blocks, the low-energy sub-blocks preceding a sub-block in which a transition or onset is detected (E601) determine a pre-echo zone (E602) in which a pre-echo attenuation processing is carried out (E607), the method being characterized in that, in the case where an onset is detected from the third sub-block of the current of the current frame, it comprises the following steps: - calculation (E603) of a leading coefficient of the energies for at least two sub-blocks of the current frame preceding the sub-block in which an onset is detected; - comparison (E604) of the leading coefficient to a predefined threshold; and - inhibition (E602) of the pre-echo attenuation processing in the pre-echo zone in the case where the calculated leading coefficient is below the predefined threshold. 2. The method as claimed in claim 1, characterized in that it further comprises a step of decomposition of the digital audio signal into at least two sub-signals as a function of a frequency criterion and in that the comparison calculation steps are performed for at least one of the sub-signals. 3. The method as claimed in claim 1, characterized in that it further comprises a step of decomposition of the digital audio signal into at least two sub-signals as a function of a frequency criterion and in that the computation and comparison steps are performed for each of the sub-signals, the inhibition of the pre-echo attenuation processing in the pre-echo zone of all the sub-signals being performed when a calculated leading coefficient is below the predefined threshold for at least one sub-signal. 4. The method as claimed in claim 3, characterized in that a different threshold is defined for each sub-signal. 5. The method as claimed in one of claims 1 to 4, characterized in that the leading coefficient is calculated according to a least squares estimation method. The method as claimed in one of claims 1 to 5, characterized in that the leading coefficient is normalized. The method as claimed in claim 1, characterized in that, in the case where an onset is detected in the first or second sub-block of the current frame, a leading coefficient calculated for the preceding frame is used for the comparison step. . A device for discriminating and attenuating pre-echo in a digital audio signal generated by a transform coder, the device being associated with a decoder and comprising a transition or onset detection module (601), a pre-echo zone discrimination module (602) and a pre-echo attenuation processing module (607), an echo attenuation processing being performed for a current frame decomposed into sub-blocks, in the low-energy sub-blocks preceding a sub-block in which a transition or onset is detected determining a pre-echo zone, the device being characterized in that it further comprises: - a computation module (603) calculating a leading coefficient of the energies for at least two sub-blocks of the current frame preceding the sub-block in which an onset is detected, m the case where an onset is detected from the third sub-block of the current frame; - a comparator (604) capable of performing a comparison of the leading coefficient to a predefined threshold; and -a discrimination module (602) capable of inhibiting the pre-echo attenuation processing in the pre-echo zone in the case where the calculated leading coefficient is below the predefined threshold. ). A digital audio signal decoder comprising a pre-echo discrimination and attenuation device as claimed in claim 8. [0. A computer program comprising code instructions for implementing, the steps of the method as claimed in one of claims 1 to 7, when these instructions are executed by a processor. 11. A. storage medium that can be read by a pre-echo discrimination and attenuation processing device on which is stored a computer program comprising code instructions for executing the steps of the pre-echo discrimination and attenuation processing method as claimed in one of claims 1 to 7.