METHOD AND APPARATUS FOR PROCESSING AN ANALOG INTERMEDIATE FREQUENCY TELEVISION SIGNAL

FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for processing an analog intermediate frequency television signal. Further, the present invention relates to a computer program comprising program code means for causing a computer to carry out the steps of the aforementioned method. Finally, the present invention relates to a television receiver comprising a tuner and the aforementioned apparatus.

BACKGROUND OF THE INVENTION

TV (television) frontend architectures, which are based on can tuner solutions, require relatively large space, because its design includes a plurality of components. In particular, the use of SAW (surface acoustic wave) filters in addition to the use of integrated circuits results in a fundamental limitation for size.

SUMMARY OF THE INVENTION

It is an object of the present invention to simplify the processing of the analog intermediate frequency television signal.

It is a further object of the present invention to reduce the effort for filtering.

Still further, it is an object of the present invention to allow an easier implementation of analog integrated filters and in particular to avoid the use of SAW filters.

Still further, it is an object of the present invention to reduce the size of the apparatus, in particular by a better integration with higher density.

In order to achieve the above mentioned and further objections, in accordance with a first aspect of the present invention, there is provided a method for processing an analog intermediate frequency television signal, comprising the steps of providing a first signal comprising an analog intermediate frequency television signal,

deriving from said first signal a second signal including at least a picture signal component and a sound carrier component, and demodulating said second signal so as to generate a video signal, wherein said sound carrier component is essentially removed from said second signal before said demodulating step.

In accordance with a second aspect of the present invention, there is provided a computer program comprising program code means for causing a computer to carry out the steps of the aforementioned method when the computer program is carried out on a computer.

According to a third aspect of the present invention, there is provided an apparatus for processing an analog intermediate frequency television signal, comprising means for providing a first signal comprising an analog intermediate frequency television signal, means for deriving from said first signal a second signal including at least a picture signal component and a sound carrier component, and means for demodulating said second signal so as to generate a video signal, and further comprising means for essentially removing said sound carrier component from said second signal before entering said demodulating means.

Preferred embodiments of the present invention according to the above first to third aspects are defined in the dependent claims.

In a preferred embodiment, said analog intermediate frequency television signal is an analog low intermediate frequency television signal, or said first signal is processed so that it comprises an analog low intermediate frequency television signal. In case of the latter alternative, the analog intermediate frequency television signal can be down- converted to said analog low intermediate frequency television signal. In this embodiment, in-band filtering, in particular all required in-band filtering can be moved into a low intermediate frequency domain. The low intermediate frequency is preferably defined as being in a range of about 0.5 to 10 MHz.

In a further preferred embodiment, the removal of said sound carrier component from said second signal is done by essentially filtering said sound carrier

component out of said second signal. For such filtering a notch filter means can be used. In an advantageous modification of this embodiment, wherein said picture signal component comprises a picture carrier, a notch frequency of said notch filtering is controlled in order to substantially match with the tuning of the frequency of said picture carrier. The notch frequency and characteristic of said notch filtering can be switchable in accordance with the television standard to be used.

During said demodulating step a low pass filtering can be carried out essentially only for reducing lower adjacent channel components from said second signal.

In a still further preferred embodiment, said video signal is generated as a colour video base band signal.

During the step of deriving said second signal from said first signal, said second signal can be filtered out of said first signal.

In a still further preferred embodiment, there is provided a further step of deriving from said first signal a third signal including at least a sound signal component. In particular, said third signal can be filtered out of said first signal.

Accordingly, the invention solves the problem of tuners, where the "in band" selectivity is done by SAW filters in conjunction with integrated or external base band filter applications, by moving all required "in band" filtering into the low IF (intermediate frequency) domain. By doing this, not only the SAW filters are becoming obsolete, but also the signal demodulation on low IF requires, due to bandwidth limitation, less linearity, and the base band filtering for removal of demodulation harmonics can be achieved with lower degree. The total effort for integrated filtering is even reduced, as mainly the base band filters of the conventional concept can be re-used in the concept of the present invention.

Programmable integrated filters can be used (e.g. in a balanced integrator technology) at low IF level, i.e. dependent upon the used TV system and standard at a frequency range of about 0.5 to 3MHz. The filters can be either used in a notch configuration to remove the sound carrier from a VIF (vision intermediate frequency) processing chain, or in a band-pass configuration to pass the sound to an intercarrier processing stage. The notch

filters, which are needed to be very accurate in frequency, can be optionally controlled in relation to a tuned picture carrier. This can either be done by using a picture carrier AFC (automatic frequency control) information or by signal tracking methods, with a tracking signal derived from the VIF PLL oscillator.

In accordance with a fourth aspect of the present invention, there is provided a television receiver comprising a tuner and an apparatus according to the aforementioned second aspect, wherein the said apparatus is connected to the output of said tuner.

In a preferred embodiment of the television receiver, said tuner is a silicon tuner.

Upcoming new silicon tuner designs, which provide a low IF output in a range 0.5 to 10MHz instead of e.g. 30MHz to 40MHz with can tuners, can be used for a higher integration level. Additional benefit from the low IF output of a silicon tuner is an easier implementation of analog integrated filters. A very small filter bandwidth and a high stop band attenuation can be achieved by such a low IF frequency range. The use of analog integrated technology enables the possibility for monolithic integration of the complete frontend, as the silicon tuner part is also based on analog integration. Thus, very small and economic single chip solutions can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings

Fig. 1 shows a schematical block diagram of a conventional system comprising a "can tuner";

Fig. 2 shows a schematical block diagram of a system including a low IF demodulator according to a preferred embodiment of the present invention, wherein this system is adapted to be used in conjunction with a conventional can tuner;

Fig. 3 shows a schematical block diagram of a system including a low IF demodulator according to a preferred embodiment of the present invention, wherein this system is adapted to be used in conjunction with a low IF tuner chip;

Fig. 4 shows a schematical block diagram of the low IF demodulator according to a preferred embodiment of the present invention; and

Fig. 5 schematically shows several wave forms diagrams according to a preferred embodiment of the present invention, wherein fig. 5a shows an arrangement of several channels in a row, fig. 5b shows a diagram of the amplitude over the frequency for a tuner with conventional intermediate frequency selectivity (solid line 1) and for a low intermediate frequency signal interface to be connected to the output of the tuner or for a tuner with low intermediate frequency selectivity as used for the present invention (dotted line 2), fig. 5c shows wave forms for vision intermediate frequency selectivity (solid line 3) and sound intermediate frequency selectivity (dotted line 4) for a conventional demodulation by using surface acoustic wave filters, fig. 5d shows wave forms for vision intermediate frequency selectivity in accordance with a preferred embodiment of the present invention by using sound carrier notch and adjacent N-I sound carrier notch and low pass filtering with adjacent N-I picture carrier notch (solid line 5) and for vision intermediate frequency selectivity using a demodulator with integrated Nyquist filtering (N-I selectivity) (dotted line 6), and fig. 5e shows the sound intermediate frequency selectivity using a sound carrier low pass or complex bandpass filtering in accordance with a preferred embodiment of the present invention (dotted line 7).

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a prior art system wherein an antenna 2 which receives a RF (radio frequency) TV (television) signal is connected to a so called "can tuner" 4 which converts the input RF (radio frequency) TV channel "N" into an intermediate frequency band, which is e.g. in a range between 30MHz to 40MHz. This intermediate signal, which has a frequency characteristic as exemplarily shown by the solid line 1 in figure 5b, is available at the output of the tuner 4 and fed to SIF (sound intermediate frequency) and VIF (vision intermediate frequency) SAW (surface acoustic wave) filters 6, 8 for sound and vision processing, respectively. For multi-standard TV receiver configurations, the SAW

characteristics are designed for passing the signals of all the different standards coming through channel N.

The output signals of the SIF SAW and VIF SAW filters 6, 8 are fed into an IF demodulator 10. In the sound processing chain the IF demodulator 10 comprises an SIF-AGC (automatic gain control) amplifier 12, a SIF mixer 14 connected to the output of the SIF AGC amplifier 12 which is provided to convert the sound signal to a 2 ^nd sound intercarrier and controlled by a programmable FPLL control unit 16. The output of the SIF mixer 14 is fed through a programmable SIF bandpass filter 18 and outputted from the IF demodulator 10 as 2 ^nd sound intercarrier 2S.

In the video processing chain, the IF demodulator 10 comprises a VIF AGC amplifier 20 and a VIF demodulator 22 which is provided for demodulating the output signal from the VIF AGC amplifier 20 and controlled by the programmable FPLL 16. The output signal from the VIF demodulator 22 is fed through a programmable sound carrier trap and equalizer unit 24 where the sound carrier is completely removed so as to have a color video base band signal CVBS at the output of the IF demodulator 10.

Thus, in the prior art system, the final selectivity is done inside the programmable SIF bandpass filter 18 and the programmable sound carrier trap and equalizer unit 24 after demodulation and conversion by the SIF mixer 14 and the VIF demodulator 22, respectively. The vision intermediate frequency selectivity, provided by SAW filters, of this prior art system is shown by the solid line 3 in figure 5c, and the sound intermediate frequency selectivity of this prior art system is shown by the dotted line 4 in figure 5c.

Figure 2 shows a schematical block diagram of a system according to a first preferred embodiment of the present invention wherein a low IF demodulator 100 is used whose construction is described later in greater detail. The low IF demodulator is provided to process the analog output signal from the tuner at low intermediate frequency level, i.e. an analog intermediate frequency television signal. The system of figure 2 is provided to process the output signal of a conventional can tuner. Since, as mentioned above, a conventional can tuner is able to convert the input RF TV channel into a relatively higher intermediate frequency band only, which is e.g. in a range between 30MHz to 40MHz, the system of figure 2 additionally comprises a low IF signal interface 30 which is connected between the

can tuner which is not shown in figure 2 and the low IF demodulator 100 and is provided to down-convert the IF output signal of the can tuner to a low intermediate frequency band of e.g. 0.5 to 10MHz. For doing so, in the example shown in figure 2 the low IF signal interface 30 comprises a mixer 32 which carriers out the down-conversion of the frequency and is controlled by a synthesizer 34. The synthesizer 34 generates a synthesizer frequency which is matched with the input frequency by the mixer 32 for producing a low IF output frequency range of approximately 0.5 to 10MHz. The output of the mixer 32 is fed through a polyphase filter 36. The output of the low IF signal interface 30 is a low IF TV signal LIF which has a wave form as exemplarily shown by the dotted line 2 in figure 5b and is input into the demodulator 100 for further processing as later described in greater detail. This signal LIF may be available in symmetrical and complex format. As shown by the dotted line 2 in figure 5b, the low IF selectivity is such that all channels higher than channel N are essentially cut a way, i.e. the cut off frequency is equal to the upper most frequency of channel N. This format can be advantageous for the implementation for analog filter stages in the demodulator 100.

Figure 3 shows a schematical block diagram of a system according to a second preferred embodiment of the present invention comprising a silicon tuner 40 which converts the RF input channel "N" to a frequency range of approximately 0.5 to 10MHz. For providing the low IF tuner signal LIF, the silicon tuner 40 a part of which is only schematically shown in figure 3 comprises a polyphase filter 36 corresponding to that as included in the low IF signal interface 30 of figure 2. Whereas the system of figure 2 requires the interconnection of a low IF signal interface 30 as an extra component, in the system of figure 3 a similar low IF signal interface is already part of the silicon tuner 40 so that the low IF signal interface of the silicon tuner 40 comprises components corresponding to those of the low IF signal interface 30 of the system of figure 2. So, the signal LIF of the system of figure 3 generally has the same wave form as the signal LIF of the system of figure 2 as shown by the dotted line 2 in figure 5b.

The construction of the low IF demodulator 100 is shown as a schematical block diagram in greater detail in figure 4. Similar to the conventional system of figure 1, the signal from the tuner is spit into a SIF (sound intermediate frequency) processing chain as shown in the upper half of figure 4 and into a VIF (vision intermediate frequency) processing chain as shown in the lower half of figure 4.

For sound processing in the SIF processing chain, the signal is branched to a programmable low SIF bandpass or low pass filter 102 for sound bandpass filtering. For the reference frequency of the bandpass filter only two cases have to be considered in the present example, so that IMHz or 2MHz can be used from a divided 16MHz system reference not shown here. The output of the bandpass filter 102 is applied to a gain controlled amplifier 104 (with a bandwidth of 0.2MHz to 3.5MHz). For FM/AM systems a SIF AGC control unit 106 operates on the basis of the detected SIF amplification output signal. In case of an AM modulated sound carrier the detection time constant is adapted to the lowest audio frequency of the AM sound. Thus, the amplitude detection equals already the AM audio signal and is fed to a sound output buffer amplifier 108 and then outputted as an AM demodulated audio signal. In case of quasi split sound (QSS), the sound is converted to a 2 ^nd sound intercarrier by an intercarrier mixer 110. To reduce mixer sum signals, a mixer with IQ processing is proposed, but with low requirements for suppression of the sum signal (2OdB), so that the effort of the IQ processing can be in accordance to this low requirements. The mixer 110 is provided to produce a suitable frequency spacing to the picture carrier and is controlled by a control unit 112. The output of the mixer 110 is fed through a programmable SSIF low pass filter 114 and then to the sound output buffer 118 whose output signal is outputted from the low IF demodulator 100 as 2 ^nd sound intercarrier. For a true split sound (TSS) the output of the amplifier 104 is switched by a SIF by-pass switch 116 directly to the sound output buffer 118 and then outputted as 2 ^nd sound intercarrier to be provided to a mono sound AM/FM demodulator (not shown here). The sound intermediate frequency selectivity is exemplarily shown by the dotted line 7 in figure 5e.

As further shown in figure 4, the VIF processing chain includes first a programmable bandpass filter 120 to make a coarse filtering of the low IF spectrum of the low IF signal LIF inputted into the demodulator 100. The low IF signal LIF is fed to the bandpass filter 120. The characteristic and design of this filter can be in accordance with a bandpass filter as used for DVB (digital video broadcast) wherein the bandwidth of this filter is adapted to 6MHz or 7MHz or 8 MHz DVB systems. The upper cut off frequency is shifted in accordance to the required bandwidth. The low IF signal LIF is set to picture carrier frequencies of 5.75MHz / 6.75MHz / 7.75MHz.The lower cut of the frequency of this bandpass filter 120 can, similar to the VIF SAW filter 8 of figure 1, used to support the

selectivity. The main task of this bandpass filter 120 however is to reduce lower adjacent RF input channels N-I, N-2, , N-M, corresponding to the VIF SAW filter 8 of figure 1.

For additional selectivity, a programmable notch filter 122 is provided following the programmable low VIF bandpass filter 120. This filter 122 is used as a N-I sound carrier trap and equalizer means to suppress the sound carrier of the adjacent N-I RF channel. This filter 122 can be made programmable as to its frequency position, wherein such a programmable function is of advantage compared to the fixed SAW filter characteristic of the system in figure 1.

In cascade to this filter 122 a programmable channel N sound carrier trap 124 follows. Because the sound at low IF output is around 1.25MHz the notch is set accordingly. In the preferred embodiment, the filter trap design is based on a triple notch approach. For both, the N sound carrier notch and the N-I sound carrier notch, it is optional possible to track the frequency position of the filter by using the AFC information from the picture carrier.

Accordingly, in addition to the function of the polyphase filter 36 which is either included in an extra low IF signal interface 30 as shown in figure 2 or already included in the silicon tuner 40 as shown in figure 3, a further selectivity is obtained by the programmable bandpass filter 120. For sound removal, the programmable channel N sound carrier trap 124 follows which is proposed to be a real filter so that the trap is effective of both sides, i.e. on a negative and positive frequency axis. Preferably, the characteristic of the trap 124 is taken from a triple notch approach. The sum of the functions of the filters 120, 122 and 124 is applied to the spectrum of the input signal LIF, and the result of this is shown by the solid line 5 in figure 5d. A comparison between figure 5c and 5d shows that the notch of the frequency characteristic indicative by the solid line 5 is exactly positioned at the location of the sound carrier SC _N - Due to the trap filtering, a high reduction of the input spectrum can be achieved. Thus, there is almost no signal left in the range from 0 to e.g. about 2MHz. This is of benefit for the VIF demodulation, where due to the low picture carrier frequency the double picture carrier frequency, which is associated with the demodulation process, is released from these signal components.

The sound carrier is removed by the programmable notch filter 122 at low IF level instead of doing this after VIF demodulation (as done the programmable sound carrier trap and equalizer unit 24 in the conventional system of figure 1). This is not only advantageous due to an easier integrated notch implementation at low frequency, namely dependent upon the used TV standard and system at about 0.5 to 3MHz instead of about 4.5 to 7 MHz in the conventional system of figure 1 , but also advantageous due to linearity and less demanding spectral signal distribution of the demodulation harmonics. To achieve the required frequency accuracy, the programmable notch filter 124 can be calibrated via a detector in the light of manufacturing process deviations. If required by process tolerances, the calibration can be done in accordance to the physical characteristics of the analog integration circuit design of the notch. The notch characteristic is switchable in accordance with the used TV system and standard, and the characteristic is designed for sufficient suppression of the sound carrier frequencies and levels, which are defined per standard. At low IF, the sound carrier frequency depends on the tuned frequency of the picture carrier, so that the notch frequency can be shifted in order to match with the tuning of the picture carrier frequency. For notch frequency control which is derived from the picture carrier frequency, either the AFC (automatic frequency control) information or a synthesized carrier can be used in a control system for notch tracking, which is derived from the tuned picture carrier frequency minus a 2 ^nd SIF frequency (frequency TV standard related). The frequency characteristic of the output signal of the programmable notch filter 124 and thus the vision intermediate frequency selectivity at the output of the programmable notch filter 124 is exemplarily shown by the solid line 5 in figure 5d.

Placing the sound trap at low IF output is important because by this way it is possible to get rid of part of the folded double picture carrier spectrum. In other words, eliminating the lower part of the low IF spectrum generates after demodulation a required gap between an upper video bandwidth limit and a lower edge of the folded spectrum around a two times picture carrier. This gap is needed to carry out low pass video filtering with low effort.

For further VIF processing, an AGC (automatic gain control) amplification is carried out by an AGC amplifier 126, and an I/Q demodulation is carried out in a VIF mixer 128 with the subsequent use of a Nyquist slope in a Nyquist passive polyphase filter 130 for Nyquist slope filtering and an adjacent N-I channel suppression. Thereby the signal obtains a

frequency characteristic as exemplarily shown by the dotted line 6 in figure 5d. The output signal of the polyphase filter 130 is not only input into a programmable CVBS low pass filter, but also into an IF gain control unit 132 which detects the picture carrier level and in accordance with the detected picture carrier level controls the gain of the AGC amplifier 126. Due to the removal of the sound carrier already at the low IF instead of a removal after demodulation as in the conventional system of figure 1, the effort on low pass filtering carried out in a programmable CVBS low pass filter 134 following the polyphase filter 130 is required only for removal of the remaining demodulation harmonics. In particular, the programmable CVBS low pass filter 134 is provided to remove the double intermediate frequency (2 ^• βp) components from the signal.

After demodulation a circuit 136 for standard dependent overall group delay correction provides the necessary pulse and colour wherein and a 4 ^th order low pass filtering removes the rest of the two picture carrier spectrum. The output of the circuit 136 is fed through an output buffer 138 and outputted as a CVBS signal in the shown embodiment.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.