Television – Receiver circuitry – Demodulator
Reexamination Certificate
1998-05-01
2001-11-20
Eisenzopf, Reinhard J. (Department: 2614)
Television
Receiver circuitry
Demodulator
C375S340000, C375S344000
Reexamination Certificate
active
06320627
ABSTRACT:
This invention relates to demodulating digital video broadcast (DVB) signals.
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), spaced (by the inverse of the active symbol period) so as to be orthogonal with each other; the data is convolutionally coded, to enable softdecision (Viterbi) decoding. Metrics for COFDM are more complex than those of single frequency networks in that they 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.
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 may be manufactured simply and inexpensively, preferably in a single integrated circuit chip.
The present invention provides as a first aspect, apparatus for demodulating digital video broadcast signals comprising data modulated on a multiplicity of spaced carrier frequencies, including:
down-conversion means for converting an input broadcast signal to a frequency sufficiently low to enable analog to digital conversion of the signal;
analog to digital conversion means for converting the broadcast signal to a series of digital samples, real to complex conversion means for converting each digital sample to a complex number value, Fourier Transform means for analysing the complex number values to provide a series of signal values for each carrier frequency, frequency control means, comprising means responsive to the output of said Fourier Transform means for producing a signal for controlling the frequency of the signal formed by said complex number values, and signal processing means for receiving the signal values and providing an output for decoding, the signal processing means including channel equalisation means and channel state information processing means.
In accordance with the invention, the input broadcast signal which is normally a UHF signal, say 700 MHz, is down converted, preferably in two stages, firstly to about 30-40 MHz and secondly to about 4.5 MHz. Since the bandwidth of the signal is about 7.6 MHz, an IF frequency of 4.5 MHZ represents essentially a DC or base band signal which can then be sampled by means of an analog to digital converter. Subsequent to analog to digital conversion, the sampled signal is converted to complex number values, in order to represent a true DC signal centred on 0 Hz. This facilitates the operation of the Fourier transform device which as mentioned above is normally an FFT performing a DFT on each carrier signal. The result of the transform is a series of data values for the data encoded on each carrier wave.
The data is processed, principally for channel equalisation and for weighting the contribution of each channel by the derived Channel State Information.
Another signal processing employed is correction for common phase error. As will become clear below, phase error in COFDM signals is present in two components, a random component and a component which is common to all carriers, arising from local oscillator phase noise. Such common phase error may be removed by a technique as described in more detail below.
The process of demodulation requires very accurate tracking of the input signal and to this end automatic frequency control and timing control are desirable. Timing control is necessary in order to ensure that the timing window for the FFT is correctly positioned in relation to the input waveforms. Thus, the sampling by the ADC must be synchronised with the input wave forms. For an input signal centred on 4.57 MHz, an ADC operating frequency of 18.29 MHz (4.57×4) is preferred. The ADC is maintained in synchronisation by a loop control wherein the complex signal value at the input of the FFT is applied to a time synchronisation unit whose output is converted in a digital to analog converter (DAC) to an analog value, which is employed to control a voltage controlled oscillator providing a clock signal to the ADC.
Automatic frequency control (AFC) is necessary to maintain the demodulation process in synchronisation with down-conversion, otherwise a gradually increasing phase error occurs in the recovered signals. To this end, a signal derived subsequent to the FFT, from the demodulated signals may be fed back to the local oscillator for IF generation in order to maintain frequency synchronisation. However, such control has disadvantages of complication in that a control signal must be fed back to the IF generation means and the control signal must adjust the reference crystal within the search range of the AFC. As an alternative therefore, AFC may be provided as a digital control applied to a digital frequency shifter coupled the input of the FFT device. The process of automatic frequency control (AFC) is described in more detail below. However, it will be shown that AFC requires a coarse control and a fine control. The fine control is dependent upon measuring the phase difference (first difference) between two adjacent continual pilot signal samples, whereas the coarse control requires the determination of rate of change of phase (or second difference) i.e., the difference between two consecutive phase differences between adjacent samples.
An important consideration in designing a demodulator for incorporation in an integrated circuit chip is reducing the separating requirements for memory. Bearing in mind the chip may only contain about 1 M Bit of memory, and that signal values for up to about 7000 carrier frequencies may be processed in the chip, this requires tight control over the use of available memory. Certain operations such as Fourier transformation and symbol interleaving require fixed amounts of memory (about 50% of the total). However, other operations such as timing synchronisation, common phase error (CPE) correction, and channel equalisation require some memory but the amount of memory can be adjusted.
In particular, as will become clear below, common phase error requires at least one symbol delay (for each carrier) and channel equalisation may use three symbols delay (for each carrier). However, as pointed out above, automatic frequency control requires measuring phase differences, as does common phase error control. It has been found that by using the signal for second difference in phase error, it is possible to adjust the channel equalisation to use only two symbols delay. Since the common phase error determination already employs two symbols delay, it has been found, in accordance with the invention, that memory required for such two symbol delays may be shared between CPE correction and channel equalisation, so that data is stored in the same memory area for the two operations in different phases of operation of the demodulator.
Accordingly, the present invention provides in a further aspect, apparatus for demodulating a digital video broadcast signal compr
Clarke Christopher Keith Perry
Guyot Jean-Marc
Haffenden Oliver Paul
Lauret Régis
Mitchell Justin David
Eisenzopf Reinhard J.
LSI Logic Corporation
Maiorana P.C. Christopher P.
Yenke Brian P.
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