fig4 depicts a first method according to the invention of compensating readout channel distortions in an optical storage systems , such as defocus , spherical aberrations , and optical data carrier tangential tilt , said optical data carrier being intended to store a primary data signal . this method comprises an adaptive phase - filtering step ( w ap ) applied to a readout data signal ( rds ) derived from said primary data signal , for generating a phase - filtered data signal ( pfds ) from a measure ( mtt ) of the optical data carrier tangential tilt . this method comprises an adaptive amplitude - filtering step ( w fir ), adaptive to an adaptation data signal ( ads ) and applied to said phase - filtered data signal ( pfds ) for generating a fully - filtered data signal ( ffds ). this method comprises a first sampling rate conversion step ( src - pll ) for converting the sampling rate of said fully - filtered data signal ( ffds ) for generating an output data signal ( ods ). the sampling rate conversion step ( src - pll ), also called timing recovery step , aims at re - sampling ( i . e . interpolating ) the incoming bit - asynchronous stream of readout samples into a stream of samples synchronized with the data bits stored on the optical data carrier . the re - sampling is performed from an internal clock derived , for example , from the incoming bit - asynchronous stream itself by means of a pll ( phase locked loop ). to this end , the src - pll step comprises the set of processing steps depicted in fig8 . the sampling rate of the incoming bit - asynchronous stream is is converted by a re - sampling step src from the internal clock ic generated by the discrete - time oscillator step ( dto ). after rate conversion , the signal is successively fed through a noise filter and an equalizer eq designed for filtering the incoming noise - filtered data signal in view of shaping the readout channel response and shaping the noise component in order to match both of them better to the requirements of the bit detector ( det ), for generating the output re - sampled signal os . the pll comprises in series a phase detection step for detecting the phase of the output re - sampled signal os , a loop filtering step for ensuring the stability of the loop , and said discrete - time oscillator step ( dto ). since the adaptive phase - filtering step ( w ap ) and the adaptive amplitude - filtering step ( w fir ) are performed before the sampling rate conversion step ( src - pll ), this method is made fully bit - asynchronously so as to improve the signal quality at the input of the timing recovery subsystem ( src - pll ) and consequently improve the quality of the timing recovery step . this method comprises a second sampling rate conversion step ( src ) for converting the sampling rate of said phase - filtered data signal ( pfds ) so as to generate a sampling rate converted data signal ( srcds ) being bit - synchronous . the second conversion step src is a “ slave ” of the main conversion step ( src - pll ) since its sampling frequency and phase are derived from those computed by the loop comprised in the src - pll step . the purpose of the second src step is to synchronize the srcds data signal and the ids 1 data signal . let w ap α ( e jω ) denote the fourier transform representation of the group delay equalizer w ap , let w fir α ( e jω ) denote the fourier transform representation of the amplitude equalizer w fir , let a k denote data to be retrieved and forming the primary data signal stored on the optical data carrier , let â k denote the output estimated data of a k generated at the output of signal processing steps according to the invention , â k corresponding to the output of the bit decision step det in fig4 to 6 . let α k denote the value of the measured tangential tilt value mtt , let { circumflex over ( α )} k denote the estimated value of α k generated by the tangential tilt estimating step tte , let h k α denote the channel impulse response of the readout channel distorted by the tangential tilt , and corresponding to the given channel response ( not necessarily optimally equalized even when α k = 0 ). let g k denote the partial derivative of h k α with respect to the tangential tilt , evaluated at tangential tilt α k = 0 , let p k denote the target partial response of the readout channel , which can be expressed as a simple fir ( finite impulse response ) filter . this target partial response is chosen such that performance of the employed bit detector step det is optimized . let x k denote either the data signal srcds generated by the sampling rate converter step src if lms adaptation is used , or the data signal ids 1 if zf adaptation is employed . this method comprises an adaptation step ( adapt ) for generating said adaptation data signal ( ads ), from : said output data signal ( ods ), said bit - synchronous sampling rate converted data signal ( srcds ): this bit - synchronous signal is used as a clock signal . an information data signal ( ids 1 ): this signal is derived from a convolution between p k and â k . the so - called zero - forcing ( zf ) : the error signal ( the difference between ods and ids 1 data signals ) is correlated with the ids 1 data signal . this algorithm aims at forcing the error signal to zero . the so - called least mean square ( lms ): the error signal ( the difference between ods and ids 1 data signals ) is correlated with the srcds data signal . this algorithm aims at minimizing the mean squared error of the error signal . the filter coefficients of the group delay equalizer w ap are pre - computed for different tilt angles , for example stored in a lookup table , and updated during the drive operation based upon the estimated tangential tilt angle from the measure of the tangential tilt . the correspondence between coefficients and the tilt value is established analytically or experimentally . the filter coefficients w j fir ( k ) of the amplitude equalizer w fir are adjusted such that no interference with the pll loop of the scr - pll step occurs . these filter coefficients are set via the adaptation data signal ads . to this end , filter coefficients verify the following symmetry relation : w j fir ( k )= w n − j + 1 fir ( k ) with j = 1 . . . n ( 1 ) the amplitude equalizer w fir can thus adaptively compensate the amplitude distortion of the optical readout channel while keeping its phase characteristics linear . the filter coefficients w j fir ( k ) of the amplitude equalizer w fir are defined by the adaptation step adapt which consists of a constrained lms algorithm verifying the following equations : w j fir ⁡ ( k + 1 ) = w j fir ⁡ ( k ) + μδ j ⁢ ⁢ with ⁢ ⁢ j ≤ n 2 ( 2 ) where ⁢ ⁢ δ j = e ⁡ ( k ) ⁢ x ⁡ ( k - j ) ⁢ ⁢ and ( 3 ) e ⁡ ( k ) = ∑ j = 1 n 2 ⁢ w j fir ⁡ ( k ) ⁢ ( x ⁡ ( k - j ) + x ⁡ ( k - n + 1 - j ) ) + w n 2 fir ⁡ ( k ) ⁢ x ⁡ ( k - n 2 ) - ∑ i ⁢ p ⁡ ( i ) ⁢ a ^ ⁡ ( k - i ) ( 4 ) the coefficient μ denotes an adaptation gain defining the stability and the speed convergence of the filter coefficients . an increased adaptation constant μ leads to a faster convergence . it should be noted on the symmetry condition in ( 1 ) that the processing can be performed only on n / 2 coefficients . the error generation in the adaptation step adapt is done in the bit - synchronous domain , while the filter coefficients update is done in the bit - asynchronous domain . if the frequency in the bit - asynchronous domain differs too much from that in the bit - synchronous domain , an additional interpolator ( not shown ) may be required to circumvent the malfunction of the adaptive equalizer . furthermore , because of the over - sampling in the bit - asynchronous domain , some out - of - band components ( i . e . spectral components of the signal whose frequency is higher than the optical readout channel cut - off frequency ) beyond the optical channel cut - off frequency may be introduced in the adaptation step adapt . fig5 depicts a second method according to the invention of compensating readout channel distortions in an optical storage systems , such as defocus , spherical aberrations , and optical data carrier tangential tilt , said optical data carrier being intended to store a primary data signal . this method differs from that of fig4 in that an adaptive amplitude - filtering step ( w ampl ) is inserted between the filtering steps w ap and w fir , said filtering step w ampl using a filter having coefficients that are adaptive to the measure ( mtt ) of the optical data carrier tangential tilt . amplitude frequency response distortions caused by tangential tilt are thus compensated . the filtering step ( w ampl ) receives the phase - filtered data signal pfds and generates a phase - and - amplitude filtered data signal pafds used as an input signal for the filtering step w fir . the filter coefficients of filtering step ( w ampl ) should be adjusted on the basis of the mtt signal such that the amplitude frequency channel response distortions caused by tangential tilt are compensated . this improved scheme allows to improve the robustness of the distortions compensation and the ramp - up behavior since the adaptation speed of the filtering step w ampl is faster than that of the adaptation step adapt . alternatively , the sampling rate conversion step src could receive the phase - filtered data signal pfds generated by the w ap filtering step as input signal . in this scheme , the coefficient update algorithm is given by the following relation : w j fir ⁡ ( k + 1 ) = w j fir ⁡ ( k ) + μδ j lms + γ ⁡ ( w j a ^ - w j fir ⁡ ( k ) ) ⁢ ⁢ with ⁢ ⁢ j ≤ n 2 ( 5 ) it can be shown that the average final coefficient value is thus given by the following relation : lim k → ∞ ⁢ e ⁢ { w j fir ⁡ ( k ) } = μσ x 2 ⁢ w j opt + γ ⁢ ⁢ w j a ^ μσ x 2 + γ ( 6 ) where σ x 2 = e { x k 2 } is the variance of the amplitude equalizer input signal , and w j opt is the optimal coefficient value in the sense of least mean square . filter coefficients w j { circumflex over ( α )} correspond to the w ampl filter coefficients . the adaptive filter w fir α ( e jω ) coefficients in ( 5 ) may either remain symmetrical as in ( 2 ) or be made asymmetric . if symmetrical , the stability of the adaptation is improved . if asymmetrical , the filtering step w ampl may be used not only for compensating amplitude distortions , but also for compensating residual phase distortions which may still remain after the tangential tilt correction . this introduced stabilization leakage scheme ensures that the interference with the is src - pll loop is significantly decreased , so that no stability or convergence problems occur . tuning of the leakage constant y can provide a trade - off between loop stability and adaptation flexibility , in tackling the residual phase distortions . large values of γ make the loop very stable , whereas small values of γ allow more flexibility . to avoid some possible interference between the adaptation step adapt and the tte step , the adaptation gain for the tte may be chosen higher than that for the lms - adaptation , or an adaptation constraint may be imposed on the w fir filter , for example , in limiting the variation range of the w fir filter coefficient . as for the group delay equalizer w ap α ( e jω ), an all - pass filter with n coefficients is mostly used . thus , the z - transfer function of filter w ap can be expressed as follows : w ap α ⁡ ( z ) = y ap ⁡ ( z ) u ap ⁡ ( z ) = a n α + a n - 1 α ⁢ z - 1 + ⋯ + z - n 1 + a 1 α ⁢ z - 1 + ⋯ + a n α ⁢ z - n ( 7 ) y ap ( k )= u ap ( k − n )− a 1 α ( y ap ( k − 1 )− u ap ( k − n + 1 )− . . . − a n α ( y ap ( k − n )− u ap ( k )) ( 8 ) which implies that the number of the filter coefficient multipliers is only n instead of 2n . also , as for the symmetrical fir filter with m coefficients , it can be easily seen from the symmetry condition of the filter that the number of the filter coefficient multipliers is m / 2 instead of m . fig6 depicts a third method according to the invention of compensating readout channel distortions in an optical storage systems , such as defocus , spherical aberrations , and optical data carrier tangential tilt , said optical data carrier being intended to store a primary data signal . this method comprises an adaptive phase - filtering step ( w ap ) applied to a readout data signal ( rds ) derived from said primary data signal for generating a phase - filtered data signal ( pfds ) from a measure ( mtt ) of the optical data carrier tangential tilt . the filter coefficients of the group delay equalizer w ap and of the filter w ampl are pre - computed for different tilt angles , for example stored in a lookup table , and updated during the drive operation on the basis of the estimated tangential tilt . the correspondence between the coefficients and the tilt value is established analytically or experimentally . this method comprises a sampling rate conversion step ( src - pll ) for converting the sampling rate of said phase - filtered data signal ( pafds ) so as to generate a sampling rate converted data signal ( srcds ). this processing step serves to perform the same function as depicted for the first method according to the invention in fig4 . in a preferred fourth method depicted in fig7 , an adaptive amplitude - filtering step ( w ampl ) is inserted between the filtering step w ap and the sampling rate converter step src - pll , said filtering step w ampl using a filter having coefficients adaptive to the measure ( mtt ) of the optical data carrier tangential tilt . amplitude frequency response distortions caused by tangential tilt are thus compensated . the filtering step ( w ampl ) receives the phase - filtered data signal pfds and generates a phase - and - amplitude filtered data signal pafds used as an input signal for the sampling rate converter step src - pll . this method comprises an adaptive filtering step ( w fir ), adaptive to an adaptation data signal ( ads ), applied to said sampling rate converted data signal ( srcds ), for generating an output data signal ( ods ). the filtering step w fir may comprise either a symmetrical or an asymmetric filter and may correct both amplitude and phase distortions if it is placed in a fully decoupled way with respect to the src - pll loop . since the adaptive filtering steps w ap and w ampl are performed bit - asynchronously before the sampling rate conversion step ( src - pll ), and the adaptive filtering step ( w fir ) is performed bit - synchronously after the sampling rate conversion step ( src - pll ), this method is a mixed asynchronous / synchronous solution . this method comprises an adaptation step ( adapt ) for generating said adaptation data signal ( ads ), from : said output data signal ( ods ), said bit - synchronous sampling rate converted data signal ( srcds ): this bit - synchronous signal is used as a clock signal . an information data signal ( ids 1 ): this signal is derived from a convolution between p k and â k . in each method according to the invention , a measure mtt of the tangential tilt is required . such a measure may be obtained optically as described in the patent application u . s . pat . no . 6 , 525 , 332 disclosing a method of estimating the tangential tilt of an optical data carrier . however , signal - processing - based methods are preferred . for example , a tangential tilt measure may be derived from a cross - correlation between an information data signal ids 2 and a decision data signal dds . the information data signal ids 2 derives from a convolution between g k and â k , while the decision data signal dds derives from bit decisions generated by the detection step det . the bit detection step ( det ) aims at generating bit decisions , for example in forcing the incoming stream ods to 0 or 1 according to thresholds in the case of two - level signals , or in using a maximum - likelihood sequence detection if a viterbi - type bit detector is used . the det step is designed to match the nominal transmission channel , i . e . a data channel in which the effects of tangential tilt , defocus , and spherical aberrations have been compensated . the various methods according to the invention may be implemented by means of a computer program comprising code instructions for implementing the individual processing steps described above , and executed by a signal processor . in an optical data carrier reader and / or writer , such various methods may be implemented in an optical disc drive ( e . g . an electronic module or as an integrated circuit ) for compensating the distortions of an optical data carrier intended to store a primary data signal , said device comprising processing means , such as signal processors executing the instructions of a computer program , for implementing the steps of the methods according to the invention . this optical disc drive may advantageously be implemented in an apparatus for reading and / or writing an optical disc . the verb “ comprise ” does not exclude the presence of other elements than those listed in the claims .