Abstract:
An apparatus for demodulating digital video broadcast signals comprising data modulated on a plurality of spaced carrier frequencies, comprising a transform circuit and a timing synchronization circuit. The transform circuit may be configured to analyze digital sample values. The timing synchronization circuit may be configured to synchronize the transform circuit with symbol periods of the broadcast signal. The timing synchronization circuit may be further configured to operate in (i) a first mode for analyzing the digital sample values over a relatively wide timing range to establish synchronization and (ii) a second mode for analyzing the digital sample values over a relatively narrow range about a synchronization point.

Description:
This is a continuation of U.S. Ser. No. 09/071,573 filed May 1, 1998 now U.S. Pat. No. 6,205,988. 
     This application claims the benefit of U.S. Provisional Application No. 60/054,195, filed Jul. 30, 1997, United Kingdom Application No. 9709063.3, filed May 2, 1997, United Kingdom Application No. 9727112.6, filed Dec. 22, 1997, United Kingdom Application No. 9727113.4, filed Dec. 22, 1997, and United Kingdom Application No. 9808989.9, filed Apr. 27, 1998 which are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and/or architecture for demodulating digital video broadcast (DVB) signals. 
     BACKGROUND OF THE INVENTION 
     There are currently two major types of DVB, namely, terrestrial broadcasting and satellite/cable broadcasting. The invention is particularly, though not exclusively concerned with terrestrial broadcasting, which has special problems, particularly in communication channel impairment, arising from adjacent television channels, multipath, and co-channel interference, for example. A type of transmission which has been developed to meet these problems is known as Coded Orthogonal Frequency Division Multiplexing (COFDM)—see for example “Explaining Some of the Magic of COFDM” Stott, J. H.—Proceedings of 20th International Television Symposium, Montreux, June 1997. In COFDM, transmitted data is transmitted over a large number of carrier frequencies (1705 or 6817 for DVB) occupying a bandwidth of several MHz in the UHF spectrum, spaced (by the inverse of the active symbol period) so as to be orthogonal with each other; the data is modulated as QPSK or QAM and convolutionally coded, to enable soft-decision (Viterbi) decoding. Metrics for COFDM include Channel State Information (CSI) which represents the degree of confidence in each carrier for reliably transmitting data. 
     Modulation and Demodulation of the carriers may be carried out by a Fast Fourier Transform (FFT) algorithm performing Discrete Fourier Transform operations. Naturally, various practical problems arise in demodulation, firstly in translating the transmitted signal to a frequency at which demodulation can be carried out, and secondly by accurately demodulating the data from a large number of carriers in a demodulator which is not overly complex or expensive, which involves inter alia synchronizing the demodulator (receiver) in time to the incoming signal. This is important for the proper execution of the FFT algorithms. 
     The data signal on each carrier has a relatively long symbol period and this, in part, gives the signal its good performance in conditions of multipath propagation. The multipath performance is further enhanced by the inclusion of a guard interval in which a portion of the modulated signal waveform taken from the end of each symbol is also included at the beginning of the symbol period. Different fractions of the basic symbol period can be used in this way to provide immunity to multipath distortion of increasingly long delays. 
     The principal requirement for synchronization in a receiver is to obtain from the signal waveform a reliable time synchronization pulse related to the start of the symbol period. Such a pulse is then used to start, at the correct position in the waveform, the process of Fourier Transformation which accomplishes a major portion of the demodulation process. A second requirement for synchronization is to lock a digital sampling clock for analog to digital conversion in the receiver to an appropriately chosen harmonic of the symbol period. However, the modulated OFDM waveform produced by adding together all the modulated carriers is essentially noise-like in nature and contains no obvious features such as regular pulses which could be used to synchronize the circuitry receiver. Because of this, techniques for synchronization are based on correlation of the signal with a version of itself delayed by the basic symbol period. The similarity between the portion included to form the guard interval and the final part of the basic symbol is then shown as a region of net correlation while the remainder of the symbol period shows no correlation. Even so, the correlated waveform still reflects the noise-like nature of the signal waveform and can be impaired by signal distortions, so it is necessary to process the signal further to obtain reliable synchronization. 
     British patent application GB-A-2037155 describes time synchronization involving the use of correlation and a filter which exploits the periodicity of the waveform to form a complex symbol pulse. The modulus of the-pulse signal is used to derive a pulse related to the start of the symbol period and a signal to control a sampling clock frequency in the demodulator. 
     An implementation of such an arrangement in the 8K mode of COFDM would require about 369 Kbits of memory, which is far too great for implementation in a single chip. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a demodulator for digital terrestrial broadcast signals which can demodulate data transmitted by a COFDM system but which my be manufactured simply and inexpensively, preferably in a single integrated circuit chip. 
     The present invention is based on the recognition that in addition to timing synchronization, correct demodulation requires inter alia both automatic frequency control (AFC) and channel equalization (CE), both of which have memory requirements, but neither of which processes can be initiated until the timing synchronization process is locked. The present invention envisages using the memory intended for use by AFC and/or CE in an initial hunt mode to establish synchronization where a large amount of memory is required to hunt over a wide timing range, and then subsequently maintaining synchronization in a zoom mode, where only a narrow timing range about the synchronization point is checked for timing variation, requiring a small amount of memory. In the zoom mode, AFC and CE may come into operation, making use of the memory no longer required by timing. Should synchronization be lost in the zoom mode, the system reverts to hunt mode, with suspension of AFC and CE. 
     Usually, the relatively wide timing range will be a symbol period, preferably a full OFDM sampling period. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 
     FIG. 1 is a schematic block diagram of a digital terrestrial front end converter incorporating the present invention; 
     FIG. 2 is a more detailed block diagram of demodulating apparatus according to the invention forming part of the converter of FIG. 1; 
     FIG. 3 is a more detailed block diagram of automatic frequency control and channel equalization units of FIG. 2; 
     FIG. 4 is a diagram of adjacent OFDM symbols having active and guard intervals; 
     FIG. 5 is a block diagram of a correlator unit for extracting timing information, together with waveform diagrams showing its principle of operation; 
     FIG. 6 is a schematic diagram of a complex pulse recovered from the correlator; 
     FIG. 7 is a block diagram together with appropriate waveform diagrams of one form of timing synchronization employed for carrying out the present invention; 
     FIGS. 8 and 9 are diagrams illustrating the principle underlying the present invention; 
     FIGS. 10 and 11 are block diagrams illustrating memory usage in accordance within the preferred embodiment of the timing synchronization apparatus of the present invention; 
     FIG. 12 is a block diagram of timing synchronization apparatus according to the present invention; 
     FIG. 13 is a schematic block diagram of a high pass filter used in time synchronization; 
     FIG. 14 is a schematic block diagram of an inter symbol filter used in the timing synchronization; 
     FIG. 15 is a schematic block diagram of an adaptive slicer used in the timing synchronization; 
     FIG. 16 is a schematic block diagram of a pulse processor used in the timing synchronization; 
     FIG. 17 is a schematic block diagram of a symbol counter used in the timing synchronization; and 
     FIG. 18 is a window associated with FIG.  17 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of the present invention comprises a front end for digital terrestrial television transmitted according to the DVB-T specification. The front end generally comprises two components. First, an analog down-converter converts the input signal from UHF to a low IF. Second, an integrated circuit chip accepts the analog signal from the down-converter and performs the required DSP operations, which include synchronization and demodulation, to form a stream of soft decisions suitable for presentation to an FEC decoder (Forward Error Correction decoder). 
     Full compliance to the DVB-T specification means that the chip is capable of decoding signals transmitted in the following modes: 
     1) A signal that contains either 1705 or 6817 active carriers, commonly referred to as 2K and 8K respectively. The chip includes the functionality and memory required to perform the FFT algorithm in both modes. 
     2) Non-hierarchical QPSK, 16-QAM and 64-QAM constellations. 
     3) Hierarchical 16-QAM and 64-QAM constellations, either uniform or non-uniform with the possible scale factors α=2 and α=4. 
     4) Guard intervals 1/4,1/8,1/16 and 1/32 of the OFDM symbol length. 
     5) Viterbi code rates 1/2, 2/3, 3/4, 5/6 and 7/8. 
     Referring now to FIG. 1, a block diagram of the front end system, a down-converter  2  receives the input UHF broadcast signal from an antenna  4  and converts the carrier signal to a first IF frequency of 30-40 MHz and then to a second IF frequency of 4.57 MHz. Since the bandwidth of the modulated data is about 7.6 MHz, this second IF signal is sufficiently low in frequency to present the signal as a base signal to a demodulator chip  6 . The demodulator chip  6  digitizes the incoming signal at a rate determined by a voltage controlled oscillator  8 , and provides an Automatic Frequency Control on a line  9  to down-converter  2 . The output of demodulator  6  represents demodulated data and is fed to a FEC decoder  10  (Forward Error Correction or Viterbi decoder) for recovering the data. The decoded data is fed to a transport stream demultiplexer  12  and then to an audio-visual decoder  14 . The front end is controlled by a system microcontroller  16 . 
     Referring now to FIG. 2, the demodulator chip  6  is shown in more detail. The demodulator chip  6  is shown schematically in FIG.  3 . The low second IF fed to an analog digital converter which samples the IF signal at a frequency of 18.29 MHz (×4 the second IF frequency of 4.57 MHz), in an analog to digital converter  20 . The digital output samples are fed to a real to complex converter  22  which converts the digital samples to complex number values in order to provide a complex signal centered on zero frequency. This signal is fed to a Fast Fourier Transform device (FFT)  24  and to a timing synchronization unit  26  which derives timing synchronization pulse from the complex input signal which is fed on line  261  to FFT  24 . A frequency error signal is fed to a digital to analog converter  28  to supply an analog control voltage to a voltage controlled oscillator  8 , which provides a sampling clock digital converter  262  to analog to digital converter  20 , and the remaining units in the signal processing chain. 
     The FFT device  24  has four modes of operation. First, it is capable of performing either a 2048 point or an 8192 point transform. Second, it is capable of performing the transform in either direction. The inverse FFT functionality is provided so that the integrated circuit may be used in applications requiring OFDM modulation. In any event, the FET performs a series of discrete Fourier transforms on each carrier frequency to provide at an output the data symbols for each carrier frequency. These output signals are corrected in phase at a common phase error generator unit  30  and then passed to a channel equalizer  32 , a channel state information unit  34  and a deinterleaver  36 . The signal thus processed is then passed at an output from the demodulator to forward error correction unit  10 . The phase error correction block  30  calculates the common phase error of the signal and applies the necessary correction. The channel equalizer  32  first performs linear temporal equalization followed by frequency equalization using a high order interpolating filter. The equalizer outputs an equalized constellation to the channel state information unit  34 . Unit  34  generates 3 or 4 bit soft decisions which are suitable for presentation to a Viterbi decoder. Deinterleaver  36  performs firstly symbol deinterleaving followed by bit deinterleaving. 
     In addition, the output signals from FFT  24  are passed to a frequency synchronization unit  38  which converts it to a control signal on line  9  for automatic frequency control, which acts upon a local oscillator in down-converter unit  2  for adjusting the frequency of the first or second IF. As an alternative, unit  38  may control a DDFS unit  39  (indicated in dotted lines) to compensate digitally for the frequency drift by centering the signal around 0 Hz. 
     In addition, the output of FFT  24  is fed to a frame synchronization unit  40 . A microcontroller interface  42  is provided, and in addition RAM memory  44  is provided to which all the units  22 ,  24 ,  30 - 36  have access to in order to provide their required operations. 
     Temporal Response Versus Noise Averaging in Channel Equalizer  32   
     The signal from the FFT is affected by all the impairments caused by the channel; for example, in the presence of a single echo, the FFT output will suffer from frequency selective fading. The purpose of the channel equalizer  32  is to rotate and scale the constellation so that the constellations on all the carriers are of a known size (but not necessarily of the same reliability). The process is performed by using the scattered pilot information contained in the COFDM signal. The scattered pilots provide a reference signal of known amplitude and phase on every third OFDM carrier. Since this scattered pilot information is subject to the same channel impairments as the data carriers, the scattered pilots are noisy. 
     In the present invention, linear interpolation is performed between two received scattered pilots, and these interpolated values are used as the reference to equalize the data. Since scattered pilots at the same time duration are spaced 4 OFDM symbols apart, a compensating data delay of 3 OFDM symbols must be provided to permit this option. 
     Automatic Frequency Control (AFC) and Common Phase Error (CPE) Correction 
     The addition of local oscillator phase noise to an OFDM signal has two principal effects: 
     1) To rotate the received constellation by an amount which is the same for all carriers within one OFDM symbol, but varying randomly from symbol to symbol. This is called the common phase error (CPE) and primarily results from the lower-frequency components of the phase-noise spectrum; and 
     2) To add Inter-Carrier Interference (ICI) of a random character similar to additive thermal noise. ICI primarily results from the higher frequency components of the phase-noise spectrum. ICI cannot be corrected and must be allowed for in the noise budget. It can be kept small in comparison with thermal noise by suitable local oscillator design. 
     It is possible to remove the common phase-error component caused by phase noise added in the down-conversion broadcast signal by digital processing in the chip. This processing is performed by the common-phase-error correction block  30 . 
     The common phase-error correction block  30  is able to remove the common phase error because all carriers within a given symbol suffer the same common phase error. By measuring the continual pilots, whose intended phase is the same from symbol to symbol, the common phase error is determined and then subtracted from the phase of all the data cells in the same symbol. There are sufficient continual pilots (which in any case are transmitted with a power approximately 2.5 dB greater than data cells) that the effect of thermal noise on this measurement can be rendered negligible by averaging. 
     As explained above, AFC is necessary for strict control over the frequency of the down-converted broadcast signal. This control may be exerted over the local oscillator in the down-converter, or by means of a DDFS  39  prior to the FFT unit  24 . In the present invention it is recognized that the frequency error may be determined by computing phase variation between adjacent symbol intervals in continual pilot signals (see copending application GBP12427A), a first phase difference representing fine frequency error (less than one carrier spacing) and a rate of change of phase difference (second phase difference) being used to determine coarse frequency error (number of whole carrier spacings offset). Thus two delay elements, in which consecutive data symbols are held, are required to compute the second phase difference. In addition, the computed first phase difference is employed for CPE correction. 
     Referring now to FIG. 3, this shows a more detailed block diagram of the common phase error correction circuit  30  and channel equalizer  32 . The common phase error correction circuit  30  receives an output from FFT  24  which is applied to a first and second delay elements  50 ,  52 , each being 16K memory locations for holding 8K data symbols in complex format. Signals from the inputs and outputs of delay elements  50 ,  52  are provided to subtraction circuits in a unit  54  in order to derive phase error signals. These phase error signals are employed for fine frequency error correction. The assessed common phase error is applied to correction circuits  58 ,  60 , the output to correction circuit  60  being applied via a further delay element  62 . The signal outputs from memory elements  50 ,  52  are thus corrected for phase in circuits  58 ,  60 , and are applied to an interpolator  66  in channel equalizer circuit  32 . The output circuit from correction circuit  58  is applied directly to the interpolator  66 , but the signal from correction circuit  60  is applied to the interpolator  66  firstly directly at a tapping and then via first and second memory elements  70 ,  72 , each of 6252×20 bits symbols in complex format. Thus, although as explained above a three symbol delay is necessary in order for CE to operate on scattered pilots, only two delay elements are required, since the third delay is provided by element  62 . 
     A significant problem for demodulator integrated circuit  6  is the amount of RAM that the chip requires. It is necessary to make the best possible use of the RAM. Some of the blocks of memory, such as the FFT and symbol deinterleaver, require fixed amounts of RAM and it is not possible to reduce them (except by reducing the word widths and so degrading the performance). The timing synchronization however is designed for reducing the amount of memory but without degrading the performance. 
     Timing Synchronization 
     Coded Orthogonal Frequency Division Multiplexing (COFDM) has been specified for terrestrial television as defined in the DVB-T ETSI specification. The OFDM symbol produced by adding all the modulated carriers is essentially noise-like in nature and contains no obvious features such as regular pulse which could be used to synchronize the receiver. The addition of the guard interval is the key to the usefulness of COFDM in multipath environment. It is also the key to the proposed synchronization method. As shown in FIG. 4, each OFDM symbol is extended by a period T G  (or Δ) (the guard interval) which precedes the useful or active symbol period T A , so that the whole symbol now lasts T S  in total. T A  is the reciprocal of the carrier spacing F A , and is the duration of the time domain signal analyzed by the Fast Fourier Transform. Each carrier is continuous over the boundary between the guard interval and the active part of the same symbol, keeping the same amplitude and phase. If we consider the signal as complex baseband, with all the carriers not only spaced by F A  but also equal to multiple F A , then the signal in the guard interval is effectively a copy of the segment of the signal occupying the last T G &#39;s worth of the active part. It follows that the signal has the same value at any instant which are separated by T A  but lie within the same symbol. 
     The similarity between the portion included to form the guard interval and the final part of the basic symbol is then shown as a region of net correlation while the remainder of the symbol period shows no correlation. Because of this, techniques for timing synchronization are based on correlation of the signal with a version of itself delayed by the active symbol period. A correlator  74  is shown in FIG. 5, comprising a one symbol delay  76  and a feed forward path  77  coupled to respective inputs of a complex multiplier  78 . 
     The correlator takes the complex baseband signal Y, delays it by T A  and multiplies the delayed version X with the complex conjugate of Y to form the product XY*. For those instants where the two samples X and Y belong to the same symbol, then the means of the product XY* is proportional to σ 2  (the square of the variance); otherwise, the two samples X, Y being independent, the product XY* has zero mean. Because the output XY* of the complex multiplier is noisy (see FIG.  5 ), it is necessary to filter to make visible the underlying pulse structure before the slicing process to detect the symbol pulse. 
     An implementation is shown in FIG.  7 . Two types of filters are used. The first filter, a recursive high pass filter  80  is included to remove the arbitrary DC component introduced when an OFDM signal experiences interferences. The second filter, an inter symbol recursive filter  82  reduces the noise component by averaging the signal value from one symbol to another. After these two filters, slicer  84  extracts the timing information in a way that it can be used to control the loop governing the sampling frequency and symbol timing. Because the timing synchronization process takes place before frequency control, possible frequency offset caused by tuner error may occur. The complex filtered signal product input to slicer  84  is a complex pulse shape signal, as shown in FIG.  6 . This complex filtered signal product is effectively projected over the real and imaginary axes in slicer  84 . The magnitudes of the real and imaginary projections are compared and the larger one is chosen. The selected one is then sliced and further processed to check and isolate the leading edge of the pulse. 
     This sample pulse is then fed to a pulse processor  86  which generates a single pulse at the leading edge of the pulse and prevents the generation of spurious pulses. The timing of pulses produced by the pulse processor  86  is compared with the timing of pulses produced from the sampling clock frequency in a symbol counter  88 . The comparison produces an error signal “sampling frequency error” applied to oscillator  8  (FIG.  2 ). 
     The correlation function is performed over a full active period of the OFDM symbol; in 8K mode, a full version of the delay line in the correlator would require up 8192 complex samples store locations (each of 20 bits) leading to a large memory of 163840 bits. 
     The inter symbol recursive filter performs its averaging over a full symbol period (active+guard interval period). In 8K mode with a guard interval of one quarter of the active period this results to 10240 complex samples per OFDM symbol, each complex samples of 20 bits, 12 bits for both real and imaginary parts, leading to a memory requirement of 204800 bits. Thus, a full version of the timing algorithm requires about 369 Kbits of memory. 
     Some architecture trade-offs and some algorithm alterations in the timing synchronization are necessary for the sole purpose of reducing this huge amount of memory but without degrading the performance. 
     To reduce dramatically this memory and still keeping the same performance, the timing synchronization in accordance with the invention generally comprises two processes, an initial first (e.g., HUNT) and a subsequent second (e.g., ZOOM) process, as illustrated in FIG.  8 . 
     The estimation of the frequency offset in AFC requires two OFDM symbol delays of memory, each memory of 6911 words of 20 bits whereas the Channel Equalizer for the channel response estimation requires two OFDM symbol memories, each of 6252 words of 20 bits. As the Frequency Synchronization and the Channel Equalizer units cannot work properly until the Timing Synchronization has reached its steady state, these memories are to the timing synchronization process in order to perform the beginning of the Timing Synchronization operations. Once the Timing Synchronization process has reached its perform convergence point the timing processing will zoom only around the region of interest where the rising edge of the correlated signal is supposed to be located, resulting in a much smaller memory requirement, the frequency synchronization and the channel equalizer memories being given back to their respective units. If the synchronization gets lost (unlock state), then the Frequency Synchronization and Channel Equalization memories would be reallocated to the Timing Synchronization process. The zoom technique employed for the Timing Synchronization process is illustrated in FIG.  9 . 
     The Timing Synchronization process according to the invention consists in two states, the HUNT timing synchronization state, and the ZOOM timing synchronization state. In the HUNT state, the timing process is not locked, and the timing process tracks the rising edge of a correlated signal pulse. The state machine moves from HUNT to ZOOM when ALPHA consecutive pulse lock events occur otherwise stay in the HUNT state. In the ZOOM state, the timing process is pulse locked (the guard interval is properly detected and does not move). In ZOOM state, the state machine goes back to HUNT state when DELTA consecutive pulse unlock event occurs. In practice, ALPHA and DELTA are usually less than 10, say 4 or 5, and are preferably programmable. 
     In the HUNT state, the timing synchronization process is unlocked and a rising edge pulse is tracked in the correlation signal. The timing process performs correlation and the filtering over the full OFDM signal with all the necessary memories located in the Frequency Synchronization and Channel Equalizer units. The full 8K memory needed for the correlation operation is allocated to the first delay element  50  (FIG. 10) and a part of the, second delay element  52  of the frequency synchronization while the 10K memory needed for recursive inter symbol filtering is allocated to the first delay element  70  and a part of the second delay element  72  of the channel equalizer unit. 
     Only after ALPHA consecutive events confirming that the timing synchronization process has properly detected the rising edge of the correlated signal (pulse locked condition) than the timing process moves from HUNT to ZOOM state, otherwise the timing stays in the HUNT state. 
     In the ZOOM state mode (FIG.  11 ), the timing synchronization is pulse locked since the region of interest where the rising edge of the pulse is sitting is properly detected. In this mode, all the memories are given back to their respective units, the frequency synchronization and channel equalizer. Only two small buffers  92 ,  94  each of 384 words of 20 bits for the correlation and the inter symbol filtering reside in the timing synchronization unit. After DELTA consecutive loss of pulse synchronization, the timing synchronization process goes back to the HUNT state mode where the Frequency Synchronization and Channel Equalizer memories are reallocated to the timing process. FIG. 11 illustrates the dataflow processing with the memory usage when in ZOOM state mode. 
     The preferred embodiment as shown in FIG. 12 includes a correlator  74 , operating on a full version (HUNT state) or only on a subset (ZOOM state) of the signal (3 time slots—384 samples), performs the correlation between the delayed version of the noisy-like OFDM signal and its complex conjugate. 
     The high pass recursive filter  80  operating on all samples removes the offset caused by interference. This is followed by 8 IIR symbol filter  82  operating over a full version (HUNT state) or only over a subset (ZOOM state) of the signal which exploits the periodic nature of the waveform to suppress noise and other impairments to produce a rectangular pulse, the duration of which is related to the guard interval. 
     Then the adaptive slicer  84  projects the incoming complex input along the four axes (real, -real, imaginary, -imaginary), compares the four axis values, chooses the larger one and then slices it at a predetermined value. The one bit signals from the adaptive slicer unit are finally passed to the pulse processor unit  86  which check and isolate the desired leading edge of the correlation pulse. Finally, the Edge_Pulse signal produced by the pulse processor unit is presented to the symbol counter  88 . The role of the symbol counter is to obtain both the sampling frequency and the symbol timing synchronization. The symbol counter provides a zoom window signal on line  90 , the start pulse for the FFT processing and the sampling frequency error for external analog oscillator. 
     The correlator  74  performs the correlation between the delayed version of the noisy-like OFDM signal and its complex conjugate. In HUNT state, the correlator considers all the signal and gets the direct samples from its input and the delayed version of the signal from the Frequency Synchronization unit memories  52 ,  54  (FIG.  10 ). In the ZOOM state, the correlator considers only a subset of the signal defined by the zoom window and gets direct samples from its input and its delayed signal from its internal small buffer. 
     After the correlator unit  74 , the high pass filter unit  80  comprises an intra symbol recursive low pass filter which extracts the average level of the complex product signal and a subtractor which subtracts the average level from the complex input signal. Referring to FIG. 13, the high pass filter comprises a subtractor  1302  for subtracting the input signal from an averaged version of the input signal, the averaged value being provided by a delay element  1304  which provides an output both to subtractor unit  1302  and to a further subtractor unit  1306  coupled to receive the input signal and providing a difference value to a constant multiplier α 1  (1/8192 in 2 K mode and l/32768 in 8 K mode)  1108 , the scaled version being provided to an adder  1310  where it is added with the output of delayed unit  1304 , thereby providing a long term average complex input signal. 
     The high pass filter unit  80  is followed by the inter symbol IIR filter unit  84  which suppresses noise to produce a recognizable pulse. This filter works on the full symbol (HUNT mode), or a subset (ZOOM mode) and implements the following equation from symbol to symbol for incoming complex samples: 
     
       
           y   k ( l+l )=α 2 ( x   k ( l+l )−y k ( l )+y k ( l ) 
       
     
     Where k represents the temporal index and l the symbol number. 
     Referring to FIG. 14, the intersymbol IIR filter unit  82  comprises a delay line  1402  having a full one symbol delay, representing the active OFDM symbol pulse and the guard interval. The output of the delay is applied to a subtractor  1404  where it is subtracted from the input signal, the difference version being scaled by a factor α 2 , the scaled signal being added in an adder  1408  to the output from delay unit  1402 , the output from the adder being applied to the input of delay unit  1402  and also being provided as an output signal. 
     The delay line is organized as a first buffer  70 ,  72  (FIG. 10) in the HUNT mode of 10 K. In the ZOOM mode (FIG. 11) the buffer is organized as a buffer  94  of 384 words, each of 20 bits. 
     Referring back to FIG. 6, the shape of the pulse from the filters may not be optimum because there may be local oscillator frequency error in the down-converter. Such an error causes the complex pulse to be rotated from its nominal position on the real axis to another position in the Argand diagram, so that the argument of the pulse can take any value. In addition, the modulus of the pulse can vary, and the shape of the pulse can vary in view of multipath in the propagation. The basis of the method shown in FIG. 6 is that, if the plane of the argument of the symbol pulse is close to the real or imaginary axis, than that signal component represents a good approximation to the signal. Thus, referring to FIG. 15, the input signal is converted to a sign and magnitude value in converter  1502 , and the output is applied via a filtering mechanism to registers  1504  storing the magnitude values on the 4 axes. The sign bit from converter  1502  controls switches  1506  to select either the negative or positive part of the axes for application to a subtractor  1508  where the stored value is compared with the input signal, the sign of the comparison being applied to a multiplexer unit  1510 . The output of the subtractor  1508  is applied to a scaler  1512  and thence to an adder  1514  where the scaled input signal is added to the stored value, and the updated value or the previous stored value is chosen by multiplexer  1510 , in accordance with the result of the subtraction in unit  1508 . A decrementing circuit  1516  is provided which subtracts  1  from the stored magnitude every  512  clock cycles at 10 MHz in 2 K mode and every 2,048 clock cycles at 9 MHz in 8 K mode. The scaler  1512  scales by a factor 1/4. 
     The four stored values representative of the peak for the four axes (X≧0, X&lt;0, Y≧0, Y&lt;0) are compared at  1520  and the greater axis in magnitude (Top) is chosen. This determines whether the real or imaginary component has the greater peak magnitude and controls the selection either of all the real samples from the input if X≧0 or X&lt;0 axes are chosen or all are imaginary otherwise. Also the sign of the greater axis in magnitude is used at  1522  to control the inversion of the selected input samples in such a way that the output of the True/Invert unit always provides a positive rectified value. At the same time, the positive axis (X≧0 or Y≧0) with the greater magnitude (Top) has subtracted three times (12) the value of the axis labeled Bottom, this axis corresponding to the opposite direction of the axis Top (units  1524 ,  1526 ), the result of the subtraction being divided by 4 at  1528 . This result is then subtracted at  1530  from the rectified selected input sample and the sign bit of the subtraction provides a one bit signal to the pulse processor unit  86 . 
     The pulse processor unit  86  takes the one bit signal coming from the adaptive slicer unit and produces a single short pulse from the rising edge of the symbol pulse as shown in FIG. 16. A majority logic  1602  detects the rising edge of the input pulse, and produces by a monostable  1604 , a pulse in the region of the rising edge of the pulse, which is provided as an output single block pulse. The pulse processor logic also prevents any further pulses from being conveyed to the output until the region of the next symbol pulse has been reached. To do so, an Active Period Symbol counter  1606  is free-incrementing up to its maximum. Only when the free-running counter has reached its maximum is a pulse from monostable  1604  presented to the output (the Edge-Pulse signal). This single pulse also reinitiates the counting process of the free-running counter. When the maximum is not reached, any new incoming single pulse is ignored, thus preventing single pulses to be closer than the active period TA. 
     The symbol counter unit  88  receives the Edge_Pulse signal from the pulse processor unit  86 . The symbol counter unit  88  comprises (FIG. 17) an upper counter  1702  for receiving Edge_Pulse signal, and a lower counter  1704  receiving a clock signal. Upper counter  1704  provides a Zoom-Window signal for defining a zoom window in the ZOOM mode. Upper counter  1704  provides a Pulse Sync signal when timing lock is achieved. 
     The lower counter  1704  completes a whole number of count cycles in a symbol period, while the upper counter advances by one each time the lower stage completes a count cycle. The lower stage counts over the range 0 to 127, simply incrementing at every incoming CLK 18  clock pulse and never reset. At every Edge_Pulse, the lower counter content is stored in a register  1706 . The output of this register Vout is interpreted as a signal for controlling the frequency of the control oscillator. 
     The upper counter  1704  counts from 0 to 32, 0 to 33, 0 to 35 or 0 to 39 in 2K mode while from 0 to 131, 0 to 135, 0 to 143 or 0 to 159 in 8K mode depending on whether the guard interval is 1/32, 1/16, 1/8 or 1/4. In Zoom state mode, this upper counter is only reset when it is completing its count range, whereas in Hunt state mode, this upper counter is reset when it is completing its count range or when a pulse-start occurs. 
     The Zoom Window provided by the signal Zoom_Window is defined by the upper counter value which is specified in Table 1. This Zoom Window is only relevant for the synchronization process only in Zoom state mode. 
     The Pulse-Sync signal specified if the demodulator is properly locked to the beginning of the OFDM symbol otherwise not. This signal is updated by the Edge_Pulse signal. In Zoom state mode, only if the upper counter content and the lower counter are in the range specified in Table 1 when a Edge_Pulse occurs, then the PULSE_SYNC signal is asserted high, otherwise the signal is low. When in HUNT state mode, only if the upper counter content is zero when a Edge_Pulse occurs than the Pulse-Sync signal is asserted high, otherwise the signal is low. 
     The signals Zoom_Window and Pulse Sync are processed in a counter (not shown) so that a change of state from HUNT to ZOOM only occurs after ALPHA consecutive Pulse_Sync signals are provided (or Pulse_Sync exists in ALPHA consecutive symbol periods). Similarly a change of state from ZOOM to HUNT only occurs after loss of the signal Zoom_Window for DELTA consecutive symbol intervals. As an example the counter is arranged such that both ALPHA and DELTA are equal to three. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 1/32 
                 1/16 
                 1/8 
                 1/4 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 2K 
                 upper 
                 32 to 1 
                 33 to 1 
                 35 to 1 
                 39 to 1 
               
               
                 mode 
                 counter 
               
               
                   
                 (Zoom 
               
               
                   
                 Win- 
               
               
                   
                 dow 
               
               
                   
                 range) 
               
               
                   
                 upper 
                 lower ≧ 
                 lower ≧ 
                 lower ≧ 
                 lower ≧ 
               
               
                   
                 counter 
                 64 when 
                 64 when 
                 64 when 
                 64 when 
               
               
                   
                 (Zoom 
                 upper = 
                 upper = 
                 upper = 
                 upper = 
               
               
                   
                 Win- 
                 32 lower &lt; 
                 33 lower &lt; 
                 35 lower &lt; 
                 39 lower &lt; 
               
               
                   
                 dow 
                 64 when 
                 64 when 
                 64 when 
                 64 when 
               
               
                   
                 range) 
                 upper = 1 
                 upper = 1 
                 upper = 1 
                 upper = 1 
               
               
                 8K 
                 upper 
                 131 to 1 
                 135 to 1 
                 145 to 1 
                 159 to 1 
               
               
                 mode 
                 counter 
               
               
                   
                 (Zoom 
               
               
                   
                 Win- 
               
               
                   
                 dow 
               
               
                   
                 range) 
               
               
                   
                 lower 
                 lower ≧ 
                 lower ≧ 
                 lower ≧ 
                 lower ≧ 
               
               
                   
                 counter 
                 64 when 
                 64 when 
                 64 when 
                 64 when 
               
               
                   
                   
                 upper = 
                 upper = 
                 upper = 
                 upper = 
               
               
                   
                   
                 131 lower &lt; 
                 135 lower &lt; 
                 145 lower &lt; 
                 159 lower &lt; 
               
               
                   
                   
                 64 when 
                 64 when 
                 64 when 
                 64 when 
               
               
                   
                   
                 upper = 1 
                 upper = 1 
                 upper = 1 
                 upper = 1 
               
               
                   
               
             
          
         
       
     
     When in 2K mode, the upper stage value is 2, 4, 8 or 16 for a guard interval of 1/32, 1/16, 1/8 or 1/4 then the starting signal is asserted high otherwise low while in 8K mode only, the upper stage value is 8, 16, 32 or 64 for a guard interval of 1/32, 1/16, 1/8 or 1/4 then the starting signal is asserted high. Hold Off logic  1708  prevents two consecutive START signals to be present within a time period less than an active period. 
     Thus in operation, in the initial hunt mode, the edge of the pulse from filter  82 ,  84  is detected in slicer  84 , to generate in pulse processor  86  and Edge_Pulse signal which is applied to OR gate  1701  of symbol counter  88 . This clears counter  1702 , to provide a START signal from Hold Off circuit  1708 . In addition a Pulse_Sync signal is generated to indicate lock has been achieved, but this is not available for ALPHA consecutive symbol intervals in order that a reliable lock is achieved. 
     The Edge_Pulse is also applied to register  1706  to set the register at the current value of the lower counter  1704 , representing sampling frequency error. Thus the clock oscillator  8  is controlled in a control loop, such that a value of 0 is a large negative signal and a value of 127 is a large positive signal. A count of 64 gives zero frequency error, so that ultimately the loop will settle so that the average position of the incoming symbol pulses coincides with the count 64 in the lower counter. Thus coarse lock is achieved rapidly from the upper counter, and fine lock gradually by the action of the control loop. 
     The lower counter  1704  received the 18 MHz clock signal. The count of upper counter  1702  is adjusted for the particular mode. Thus when coarse lock has been achieved, and the Pulse_Sync signal is stable, a signal Zoom_Window is issued by counter  1702 . Referring to Table 1, this signal occupies three count values of the upper counter, e.g. 32, 0, 1 for the 2K mode, 1/32 guard interval. In addition, this window is generated only when the lower counter is above its mid point of 64 in the interval  32  and below its mode point in interval  1 , as indicated in FIG.  18 . In the zoom mode, as previously indicated, the timing circuitry operates on just 3 symbols, a fraction of the incoming symbols, and therefore the memory requirements is reduced to buffers  90 ,  92 . Timing lock is retained as long as the Edge_Pulse signal continues to arrive within the window defined as shown in FIG.  18 . 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.