MXPA96004223A - Carrier independent timing recovery system for a vestigial sideband modulated signal - Google Patents

Abstract

A television signal receiver for processing an HDTV signal transmitted in a vestigial sideband (VSB) format includes input complex filters shared by a timing recovery network (30) and a carrier recovery network (50). The filter network includes a pair of upper and lower band edge filters (20, 22) mirror imaged around the upper and lower band edges of the VSB signal for producing suppressed subcarrier AM output signals. The timing recovery network includes a phase detector (28, 38, 62) and responds to an AM signal derived from the two filters (via 26) for synchronizing a system clock (CLK). The carrier recovery network (50) also includes a phase detector (54, 60, 62, 64), and responds to outputs from one or both of the filters for producing an output error signal ('DELTA') representing a phase/frequency offset of the VSB signal. The error signal is used to reduce or eliminate the offset to produce a recovered baseband or near baseband signal. A subsequent equalizer eliminates any resid ual phase offsets in the recovered signal.

Description

SYSTEM OF TIME RECOVERY INDEPENDENTLY OF THE CARRIER FOR A MODULATED SIGNAL IN VESTIGIAL SIDEBAND 5 Field of the Invention This invention relates to a digital signal processing system. In particular, the invention relates to a time recovery system for use P "4," in a receiver of a vestigial sideband (VSB) signal, such as can be modulated with high definition television information (HDTV), for example.

Background of the Invention 15 The recovery of data from a vestigial sideband signal or quadrature-modulated amplitude (QAM) in a receiver requires the implementation of three functions: time recovery for symbol synchronization, recovery of the carrier (0 frequency demodulation) and compensation. Time recovery is the process by which the receiver clock is synchronized (time base) with the transmitter clock. This allows the received signal to be sampled at the optimum time to reduce the opportunity for an associated splice error 5 with the processing directed to the decision of the values of the received symbols. Carrier recovery is the process by which a received radiofrequency signal, after changing its frequency to a lower intermediate frequency bandpass, is changed in frequency to the baseband to allow information retrieval of the base band modulation. Compensation is a process that compensates for the effects of the alterations of the transmission channel on the received signal. More specifically, the compensation removes the interference between symbols (ISI) caused by the alterations of the transmission channel. Interference between symbols causes the value of a given symbol to be distorted by the values of the previous and next symbols. For quadrature-modulated amplitude signals, time recovery is usually the first function implemented in a receiver. The time is recovered, either from the intermediate passband signal or from the signal almost in the baseband, i.e., a baseband signal with a carrier phase shift that is corrected by the carrier recovery network. In any case, the time can be set before demodulation of the baseband. A quadrature-modulated amplitude signal that transports digital information is represented by a constellation of two-dimensional data symbols defined by the Real and Imaginary axes. In contrast, a vestigial sideband signal is represented by a constellation of one-dimensional data symbols where only one axis contains quantized data to be retrieved at a receiver. The recovery of time for a vestigial baseband sideband signal can be achieved using symbols or synchronization components (sync) periodically. This technique is used by a Grand Alliance terrestrial broadcasting high-definition television system recently proposed for the United States. A significant drawback of this technique is that the use of these synchronization symbols reduces the load capacity of the data transmission channel. The Grand Alliance high definition television system employs a vestigial sideband (VSB) digital transmission format to transmit a packet data stream. This high definition television system is a proposed transmission standard that is being considered in the United States by the Federal Communications Commission through its Advisory Committee of Advanced Television Service (ACATS). In this system, the data is configured as a sequence of data fields. Each field includes 313 segments: a field synchronization segment (which does not contain load data) followed by 312 data segments. A synchronization component is associated with each data segment. A description of the Grand Alliance high definition television system, as presented to the ACATS Technical Subgroup, on February 22, 1994 (project document) is in 1994 Proceedings of the National Association of Broadcasters, 84th Annual Broadcast Engineering Conference Proceedings, March 20-24, 1994.

Summary of the Invention A described time recovery system, suitable for use with vestigial sideband signals, conveniently achieves time recovery without relying on synchronization components. This is done using the recovery of the edge time of the band with respect to a received vestigial sideband signal to develop double-sideband modulated amplitude signals, from which the time information is extracted. In an illustrated embodiment, a bandwidth filter network filters the upper and lower band edges of the vestigial sideband signal. The described system operates conveniently at the speed of the symbol, does not rely on a received signal containing synchronization components limiting the bandwidth to facilitate the recovery of time, and is able to achieve time synchronization regardless of a phase shift. the carrier when it is present. In a described embodiment, the filter network is a complex digital filter network with responses that are complementary to the frequency spectrum of the vestigial sideband signal received on the edges of bands filtered by the filter network. In accordance with a characteristic of the invention, the filter network is shared by a carrier recovery network for the frequency change of the received vestigial sideband signal towards the baseband.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a portion of an advanced television receiver, such as a high definition television receiver, including a time recovery system in accordance with the principles of the invention. Figures 2 to 6 illustrate the amplitude versus frequency responses of the signals associated with the operation of the system of Figure 1.

Detailed Description of the Drawings In Figure 1, an analogue high definition television signal modulated in vestigial sideband broadcasting received by an antenna 10, is processed by an input network 14 including radio frequency tuning circuits, a tuner double conversion to produce a suitable intermediate frequency step band to convert to digital form, and appropriate gain control circuits, for example. The vestigial sideband signal received illustratively is an 8-VSB signal with a symbol rate of approximately 10.76 Msymbols / second, which occupies a conventional 6 MHz NTSC frequency spectrum, in accordance with the Grand HD television specification Alliance In a specific manner, the vestigial sideband signal received in that example is an 8-VSB signal having a one-dimensional data constellation defined by the following eight data symbols: -7 -5 -3 -1 1 3 5 7 The Nyquist bandwidth for this system is nominally 5.38 MHz, for example, with an excess nominal bandwidth of 0.31 MHz on each band edge. The described system can also be used for 16-VSB signals, for example. The output signal from the input processor 14 is converted from the analog form to the digital form by an analog-to-digital converter 16, which operates at a sample rate of 2 samples / symbol. The received vestigial sideband signal may include a pilot component, and has been demodulated by unit 14, such that the center of the 6 MHz band is nominally located at 5.38 MHz. The frequency spectrum of this signal in the The input of the analog to digital converter 16 occupies a scale of 2.38 MHz to 8.38 MHz. When the time synchronization is established, the unit of analog-to-digital converter 16 samples this signal at 21.52 MHz, which is twice the speed of the analogue to digital converter. symbols. The pilot component, which represents the direct current point of the pulse-modulated amplitude (PAM) signal of the original baseband, is nominally at 2.69 MHz (the Nyquist frequency), which is 1/8 fsr. In the following discussion: fc is the carrier frequency of the transmitted signal (nominally 5.38 MHz), fst is the frequency of transmitted symbols (10.76 Msymbols / second, or four times the Nyquist frequency, and fgr is the sampling frequency of the receiver (21.52 MHz) In the time lock, fsr = 2fst.It results in a carrier lock when the baseband is demodulated, fc = 1/4 fsr.The digital signal from the unit of analog-to-digital converter 16 is applies to two complex band edge filters 20 and 22, which are mirror image filters around the Nyquist frequency, each filter exhibits real and imaginary functions, so that the output signals from these filters contain real and In Figure 1, a letter "C" designates the signal lines that carry complex signals with real and imaginary components, while other signal lines carry only real components. and 22 produce output signals without image components, i.e., the output signals contain positive or negative spectral components, but not both. This has the advantage of not generating noisy components that may be difficult to remove subsequently. In this system, filters 20 and 22 are designed to have complex analytical output signals with negative spectral components, as will be seen in Figure 2. The negative spectrum is arbitrary; The positive spectrum could also have been selected. Figure 2 illustrates the negative frequency spectrum encompassed by the bandpass responses of filters 20 and 22, and by the bandwidth of the received vestigial sideband signal applied to the inputs of filters 20 and 22. Actual input signal has positive and negative spectra. The positive spectrum is canceled using known techniques, leaving the negative spectrum. The filter 20 extracts the upper band edge of the negative spectrum of the vestigial sideband signal, and the filter 22 extracts the lower band edge of the negative spectrum of the vestigial sideband signal. The upper band edge is the one that contains the highest frequency components, regardless of whether they are positive or negative components. The lower band edge is associated with the lower frequency components The band edge responses of the filters 20, 22 and the vestigial sideband signal are intercepted at the Nyquist points in this example. In the following figures, a symbol "?" designates a frequency offset of the carrier, such as may be associated with a signal near the baseband, ie, a signal not completely changed in frequency to the baseband. The phase shift will be discussed in detail in relation to the carrier recovery network (baseband demodulation) The responses of filters 20 and 22 are complementary to the frequency spectrum of the input signal at the edge of the band that filters are extracting, as shown in Figure 2. This has the effect of producing a modulated amplitude signal (AM) of the suppressed dual-sideband carrier, when there is no pilot component in the received vestigial sideband signal (Figure 3), and which produces a modulated amplitude signal of the residual double sideband carrier when a pilot is present on that side edge. The response of the filter 20 to the left of the frequency f-L is not critical, and the response of the filter 22 to the right of the frequency f2 is not critical.

Before setting the time and carrier insurance, these modulated amplitude signals contain frequency (and phase) shifts that can be used for time and carrier recovery. In a specific manner, the center of the modulated amplitude signal obtained from the output of the upper band edge filter 20 is located at -fc-l / 4 fst. If a pilot signal is present (such as in the Grand Alliance high definition television system), it would appear on this frequency. The 1/4 frequency fst is a quarter of the symbol frequency, if the signal is treated as a vestigial sideband signal. In a similar manner, the center of the modulated amplitude signal obtained from the output of the lower band edge filter is set to -fc + 1/4 fgt (the Nyquist frequency). Time synchronization is achieved when the sampling clock input frequency (CKL) of the analog-to-digital converter unit 16 is four times the frequency difference between the carriers of these two band-width modulated signals. of upper and lower suppressed carrier (Figure 3). The time recovery system operates as follows. The output signal from the filter 22 is conjugated by the unit 25 to send the spectrum of the filter output signal 22 from negative to positive. This is illustrated by Figure 4. Conjugation is a well-known process performed in unit 25 by first separating the signal into its real and imaginary components using well-known methods. The imaginary component is inverted by multiplying it by a negative unit factor. The inverted imaginary component and the original real component are recombined. The modulated amplitude signal of the recombined lower band edge is multiplied with the modulated amplitude signal of the upper band edge from the filter 20 in the multiplier 26. A modulated amplitude signal produced in the output of the multiplier 26 has the component frequency of carrier fc removed, as illustrated by Figure 5. The removal of this component fc results because the negative carrier of the component of the upper bandwidth cancels the positive carrier of the amplitude-modulated signal conjugate from the lower band edge. The amplitude signal modulated at a suppressed carrier center frequency of 1/2 f3t is maintained because both multiplied amplitude modulated signals are double "sideband" signals In the frequency domain, these signals are represented in the baseband as even functions (ie, as only real components), and its convolution 'is represented in the base band as functions valued in par.The bandwidth of the amplitude output signal modulated from the multiplier 26 has been doubled by the convolution process (the multiplication in time produces the convolution in the frequency.) By driving each third sample to zero, the sampling clock frequency of the CLK receiver can be synchronized with the symbol frequency of the sideband signal vestigial input regardless of any shifting of the carrier (?) This is done by a phase detector 28 in a network of 30 time recovery as follows. The imaginary component of the double-sideband modulated amplitude output signal from the multiplier 26 (Figure 5) is an indication of the magnitude of the bad weather of the signal. The real component indicates the direction of bad weather (the signals of the suppressed carrier of modulated amplitude have an ambiguity of 180 degrees that must be resolved). If this signal of modulated amplitude is perfectly set in time, the imaginary component is absent. The double-sideband modulated amplitude signal from the multiplier 26 is separated into its constituent real and imaginary components by means of a unit 32 in the phase detector 28 using known separation techniques. Employing known techniques, a translation unit 34 determines the sign of the real component (for the address information), and multiplies this sign by the separate samples of the imaginary component. The output of the multiplier 36 represents an error signal that is driven to zero by the action of the time control cycle when the time lock is reached. Since the frequency of the carrier of the double-sideband signal is nominally 1/2 fgr, in the safe of the imaginary component of the output signal from the multiplier 36 will be zero. The multiplication of the imaginary component with the sign of the real component gives the phase detector 28 the ability to differentiate between positive and negative frequency phase shifts. The output signal from the phase detector 28 is filtered by a low pass cycle filter 38 which contains both an integral line and a proportional line, as is known, and is set to clock at a frequency of 1/2 fsr. The cycle filter 38 is set to clock to process every third sample of the input signal, since the purpose of the cycle is to drive every third sample of the imaginary component to zero. The output of the filter 38 is a direct current voltage that is applied to a controlled voltage oscillator (VCO) 40. The oscillator 40 provides the sampling clock of the CLK receiver for the analog-to-digital converter unit 16 as a function of the Direct current voltage. Time synchronization is achieved when the sampling clock of the analog-to-digital converter provided by the described time recovery system including the network 30, is four times the frequency difference between the carriers of the two amplitude output signals. modulated from filters 20 and 22 (Figure 3). The proportional and integral control portions of the filter 38 can be adjusted as known, by utilizing gain control climbers Kl and K2, respectively. These climbers are set to large values to facilitate acquisition of the signal in the acquisition mode, and can be reduced in value during the tracking mode to increase noise immunity. The time required to achieve time insurance varies as a function of the amount of noise and distortion of multiple lines present in the signal, the bandwidth of the control cycle, and the time constants of the control cycle, for example . The carrier can be recovered using two different methods in the system of Figure 1. One method uses both band-width modulated signals from the outputs of the filters 20 and 22 in a manner that is similar to that described above. for the recovery of time. The second method uses only one bandwidth of the received signal. In the second case, typically the band edge that is used is that which may contain the pilot. The extra energy associated with the pilot component improves the performance of the carrier recovery cycle in conditions of low signal to noise. However, it is noted that both methods conveniently do not require the presence of a pilot component. The carrier recovery method employing both band edges multiplies the outputs of the filters 20 and 22 together in a multiplier 45 without conjugating the signal, as was done for time recovery. This multiplication produces a carrier modulated amplitude signal suppressed at the output of the multiplier 45 with a carrier frequency of -2fc. The symbol velocity component fst has been completely removed from this modulated amplitude signal. If there is a carrier phase shift (?), The carrier frequency is -2fc-2? as illustrated in Figure 6. Up to this point, the carrier recovery process is independent of the sampling clock frequency of the receiver demodulator, fgr. In digital signal processing applications, it is generally desirable to design controlled voltage oscillators (VCOs) or spectral exchangers that conveniently produce signals that are harmonically related to the clock frequency of the digital signal processor. In this regard, it is noted that the complex double-sideband modulated amplitude signal at the output of multiplier 45 (Figure 6) was centered on a frequency of -2fc (neglecting any carrier phase shift), or on -2f - 2? (including phase shifting) This means that this modulated amplitude signal is being mounted in the symbolic overlay region More specifically, in practice, the left sideband portion of the modulated amplitude signal shown in Figure 6 is actually "wrap around" in the positive frequency spectrum No symbolism occurs, because this modulated amplitude signal is a complex signal where the adjacent positive frequency component of the first negative repeat band has been removed. the design of the carrier recovery network, an associated phase detector 54 is essentially the same as the phase detector 28 used in the time recovery network 30. In order to achieve this, the carrier of the input signal of amplitude modulated up to the phase detector 54 should be boosted to 1/4 fgr The carrier of the amplitude-modulated signal input to the phase detector 54 is nominally changed up to a frequency of 1/4 fsr by using a complex spectral changer that operates at +1/4 fsr. The spectral changer comprises complex multipliers 52 and 64. The multiplier 64 responds to a sampling signal of 1/4 fgr to change the output signal of the controlled voltage oscillator 62 by +1/4 fgr. The response of the controlled voltage oscillator 62 is similar to that of the controlled voltage oscillator 40 in the time control cycle, and responds to a direct current voltage produced by a low pass cycle filter 60 that is similar to the filter 38 in the time recovery cycle. A complex output signal resulting from the multiplier 64, which contains the frequency offset generated by the cycle plus the fixed 1/4 fsr, is applied to an input of the complex multiplier 52. The other input of the multiplier 52 receives the amplitude signal modulated centered at -2fc from the output of the complex multiplier 45. The phase detector 54 operates the same as the phase detector 28 in the time recovery cycle. The phase detector 54 includes a real / imaginary component separator 55, a translation network of the sign function 56, and an output multiplier 57. The phase detector 54 operates by multiplying the imaginary component of the output signal from the multiplier 52 with the sign of the real component. This causes the value of every third sample of the imaginary component to be driven to zero. Since the phase detector 54, like the phase detector 28, operates on the same series of samples (ie, nons or pairs, but not both), the cycle filter 60 (such as the filter 38) is required to supply output samples at symbol speed rather than at twice the symbol rate. This significantly reduces the complexity of the cycle filters, compared to what would be required for implementations of two samples per symbol. The output of the multiplier 45, at the input to the carrier recovery network 50, is a modulated carrier-amplitude signal of complex double-sideband centered at a frequency -2fc-2 ?. The output of the controlled voltage oscillator 62 in the carrier cycle is a signal approximately equal to 2 ?. To produce this signal at the output of the controlled voltage oscillator 62, a 1/4 fsr signal is added to the carrier cycle by means of the multiplier 64 to cancel the component 2fc. The complex signal 2? in the output of the controlled voltage oscillator 62, is it moved to an exit signal? of the carrier recovery network 50 by means of a frequency divider that divides between 2, 70. The output signal? is a tone (without a frequency spectrum) that represents phase / frequency phase shifting of the carrier. The vestigial sideband output signal from the analog-to-digital converter unit 16 is applied to an input of the multiplier 64 after being delayed by a unit 72, which compensates for the signal delay through the filters 20 and 22. The output of the delay unit 72 is a vestigial sideband signal, symmetric, double sideband, close to the baseband, complex, as illustrated by the frequency spectrum diagram adjacent to block 72. This signal is changed to be closer to the base band by multiplier 74, which is clocked at 1/8 fsr to produce an upper vestigial sideband near the baseband at its output, as illustrated by the frequency spectral diagram in the output of the multiplier 74. The vestigial sideband signal near the base band from the output of the multiplier 74 is applied to an input of a complex multiplier 71, and the output signal (representative of the phase shift)? from the output of the carrier recovery network 50 is applied to another input of the multiplier 71. The function of the multiplier 71 is to substantially cancel the phase shift? in the vestigial sideband signal, such that a vestigial band-sideband signal results. A complex demodulated vestigial sideband signal appearing at the output of the multiplier 71 must be in baseband, and often it is. However, in practice, this signal may contain residual phase shifts which may have to be compensated. This is done by a compensator 75, which can be of a known configuration. The compensator 75 compensates for the alterations of the channel as is known, and produces a compensated output signal that is decoded by the unit 76, and is processed by an output processor 78. The decoder 76 may include, for example, the trelli decoder, the deinterleaver, the Reed-Solomon error correction, and the audio / video decoder networks, as they are known. The output processor 78 may include audio and video processors, and audio and video playback devices. The recovery of the carrier can also be performed using a single band edge of the input signal, as follows. The lower bandwidth filter 22 produces a double-sideband modulated amplitude output signal with a carrier frequency of -fc + 1/4 fR. This is done by setting the input to the multiplier 45 from the filter 20 to a value of the unit. This can be done by placing a multiplexer on the signal line between the output of the filter 20 and the upper input of the multiplier 45. One input of the multiplexer receives the output signal from the filter 20, and another input receives a signal of unit value. The last signal is conveyed to the input of the multiplier 45 in response to a control signal applied to a control input of the multiplexer. The output signal from the upper bandwidth filter 20 is decoupled from the multiplier 45 when the unit value signal is used. In a system that uses two band filters, the time and carrier insurance may occur at approximately the same time. In a system that uses only a one-band filter, carrier insurance may occur after time insurance, depending on a variety of factors, such as noise, cycle gain and bandwidth. cycle. If the time lock has been established by network 30, then fst = 1/4 fsr, and the carrier fgt component can be removed by changing the modulated amplitude output signal of filter 22 with a spectral exchanger of -1 / 8 fsr. This spectral change is made in Figure 1 by changing the illustrated clock input of 1/4 fsr to the multiplier 64, in a clock of -1/8 fsr. After this spectral change, the signal carrier of amplitude modulated at the output of the multiplier 52 will be at a frequency -fc (in contrast to -2fc in the case of the double bandwidth method discussed above). In this embodiment, the carrier frequency of the modulated amplitude signal fc can be easily boosted to -1/4 far using a phase detector similar to the unit 54 used for the dual edge carrier recovery example, and forcing every third sample of the real component to zero. The output of the phase detector is integrated by a low pass cycle filter that drives a voltage controlled oscillator in a cycle that acts to drive the carrier frequency of the modulated amplitude signal up to -1/4 fsr of in a manner analogous to that described above in relation to the example of dual edge bank carrier recovery.

In the described embodiments, the recovery of time (safe) can be achieved even in the presence of a carrier phase shift, and the time insurance does not rely on synchronization components in the vestigial sideband signal for this purpose. The choice to operate phase detectors 28 and 54 at the midpoint of the Nyquist region is a possible implementation. However, phase detectors could also be operated in the baseband with the same results. The use of the negative frequency spectrum is arbitrary. The positive spectrum could also have been used with analogous results, by using a different implementation of the circuits described. For example, the conjugate filters 20 and 22 would be used, and the input to the multiplier 64 would be -1/4 fgr.

Claims (11)

1. In a system for receiving a high definition television (HDTV) signal transmitted as a modulated vestigial side band (VSB) signal formatted as a one-dimensional data constellation of symbols representing digital image data, and subject to displaying a carrier delay, a time recovery network to establish the time synchronization between a symbol clock of the local receiver and a symbol clock of the transmitter, the time recovery network being able to achieve synchronization independently of the phase shift of the carrier when present in the received signal, this time recovery network comprising: an input network (14, 16) for receiving the vestigial sideband signal; a filter network that responds to an output signal from the input network, comprising a first bandwidth filter (20) and a second bandwidth filter (22) having bandwidth responses respectively associated with the upper and lower band edges of a frequency spectrum of the vestigial sideband signal, to produce double-sideband modulated amplitude (AM) signals at the respective outputs of the first and second filters; a phase detector network (28, 38) for processing a double-sideband modulated amplitude output signal from the filter network to produce a control signal representing a time error; a clock signal generator (40) that responds to the control signal to generate the symbol clock.

2. A system according to claim 1, wherein: the first and second filters are complex digital filters with responses that are complementary to the frequency spectrum of the vestigial side-band input signal at the band edges of the respective filters . A system according to claim 1, wherein: the received vestigial sideband signal exhibits a frequency spectrum with band-edge responses at frequencies fc - 1/4 fst and fc + 1/4 fst, respectively, with respect to a medium band carrier frequency fc +/- ?; and the first filter exhibits a bandwidth response at a Nyquist frequency fc - 1/4 fgt, and the second filter exhibits a bandwidth response at a Nyquist frequency fc - 1/4 fst, where fc is the frequency of the carrier of the transmitted vestigial sideband signal; fst is the frequency of the symbols transmitted; Y ? it is a shifting of the carrier when it is present. A system according to claim 1, wherein: the input network includes an analog-to-digital converter (16) for converting a received signal to a digital form, and this converter responding to the symbol clock. A system according to claim 1, and further comprising: a multiplier (26) having first and second inputs for receiving the output signals from the filters, respectively, and an output coupled with an input of the network of the phase detector. 6. A system according to claim 5, and further comprising: an element (25) for conjugating one of the output signals from the first and second filters before being applied to the multiplier. A system according to claim 5, wherein: a multiplier output signal is a suppressed carrier dual-band modulated amplitude signal centered around a frequency of 1/2 fgt, where fgt is the frequency of the symbols transmitted. A system according to claim 1, wherein: - the input signal to the network of the phase detector is a complex signal having real and imaginary components; and the phase detector network includes an element for canceling the imaginary component in an output signal from the phase detector network. 9. A system according to claim 1, wherein: the received vestigial sideband signal is an 8-VSB signal having a constellation of data symbols of a dimension defined by the following eight data symbols: -7- 5 -3 -1 1 3 5 7. 10. In a system for receiving a high definition television (HDTV) signal transmitted as a signal modulated in vestigial sideband (VSB) formatted as a data constellation of a symbol dimension which represent digital image data, an apparatus comprising: an input network (14, 16) to receive the vestigial sideband signal; a filter network that responds to an output signal from the input network, comprising a first bandwidth filter (20) and a second bandwidth filter (22) having bandwidth responses respectively associated with the upper and lower band edges of a frequency spectrum of the vestigial sideband signal, to produce le-sideband modulated amplitude (AM) signals at the respective outputs of the first and second filters; a time recovery network (30) that responds to the output signals from the first and second filters, to establish the time synchronization between a symbol clock of the local receiver and a symbol clock of the transmitter; and a carrier recovery network (50) that responds to the output signals from the first and second filters to change the frequency of the received vestigial sideband signal to the baseband. 11. A system according to claim 10, wherein: the first and second filters are digital filters with responses that are complementary to the frequency spectrum of the vestigial input sideband signal at the band edges of the respective filters.