Demodulation apparatus and method for reducing time delay of on-channel repeater in terrestrial digital TV broadcasting system

Abstract

Provided is a demodulation apparatus capable of reducing time delay of an on-channel repeater in a terrestrial digital television (TV) broadcasting system and a demodulation method thereof. The demodulation method includes the steps of a) converting an analogue intermediate frequency (IF) signal into a digital IF signal; b) generating an in-phase (I) signal and a quadrature (Q) signal by converting the frequency of the digital IF signals and down-converting the frequencies of the pilot components of the I and Q signals so that the center frequencies of the I and Q signals could be shifted to 0; c) filtering the down-converted I and Q signals and maximizing signal-to-noise ratio; d) up-converting the filtered I and Q signals into baseband signals in which the center frequencies of the filtered I and Q signals are placed in baseband and shifting the frequency of the pilot components of the filtered I and Q signals to 0; and e) adding up the up-converted I and Q signals to restore the signals into baseband signals for broadcasting.

Description

FIELD OF THE INVENTION

The present invention relates to a digital television (TV) broadcasting service technology; and, more particularly, to a demodulation apparatus that can reduce time delay of an on-channel repeater (OCR) in a terrestrial digital TV broadcasting system and a method thereof.

DESCRIPTION OF RELATED ART

Generally, main transmitters and repeaters are positioned to provide broadcasting services based on the broadcasting coverage of each broadcasting station and the topological surroundings/natural features.

The repeaters are installed in the area where broadcasting signals from a main transmitter are weakly received to solve the broadcasting signal reception problems in the regions which are out of the broadcasting service coverage and to expand the signal transmission area of the main transmitter.

The repeaters that are used to provide the current terrestrial digital broadcasting services receive broadcasting signals from the main transmitters and transmit the broadcasting signals through different frequencies, each of which is allocated to each signal.

Referring to FIG. 1, which shows a conceptual diagram of broadcasting service using a conventional repeater, a main transmitter 11 transmits broadcasting signals through a transmission frequency A. Then, each of the repeaters 12 to 15 relays the broadcasting signals to another frequencies B, C, D and E. That is, in the terrestrial digital TV broadcasting service shown in FIG. 1, each of the repeaters 12 to 15 is given a different frequency B, C, D or E, thus solving the broadcasting problems in the regions out of the broadcasting service coverage of the main transmitter or expanding the broadcasting service coverage.

However, the use of different frequencies B, C, D and E for different repeaters 12 to 15 requires many frequency resources, because each of the repeaters 12 to 15 should use a plurality of frequency bands. This is very inefficient in the aspect of frequency usage, because the same frequency cannot be re-used in the neighboring area but it can be used in a far region where no interference between the same frequencies is generated.

If the repeaters 12 to 15 use the frequency A which is the frequency used in the main transmitter 11, the same frequency can be re-used in the neighboring area. As a result, the frequency usage efficiency is increased remarkably. This is shown in FIG. 2, which shows a conceptual diagram of a broadcasting service using an ordinary on-channel repeater (OCR). In the drawing, a main transmitter 21 transmits broadcasting signals through a transmission frequency A and on-channel repeaters 22 to 25 relay the broadcasting signals to the frequency A which is the same as the frequency of the main transmitter 21. This way, the frequency usage efficiency can be increased.

To provide a broadcasting service in this structure, receivers could discriminate the broadcasting signals transmitted from the main transmitter 21 and the on-channel repeaters 22 to 25. Generally, receivers are equipped with an equalizing portion which removes multi-path signals. The equalizing portion can remove signals time-delayed and inputted undesirably in the same frequency band, other than desired signals.

However, as shown in FIG. 2, in case where broadcasting signals are relayed using the on-channel repeaters 22 to 25 through the same frequency band A, interference may be generated between the same channels and the time-delayed multi-path signals cannot be removed in the equalization unit of a receiver.

That is, the signals transmitted from the main transmitter 21 and the on-channel repeaters 22 to 25 have a time delay that goes out of the multi-path signal removal capability of the equalizing portion in the receiver, the equalizing portion cannot remove the time-delayed signals.

Therefore, in order to provide broadcasting services through the on-channel repeaters 22 to 25 in the terrestrial digital TV broadcasting system, the output signals of the on-channel repeaters 22 to 25 should be the same as those of the main transmitter 21 and the time delay between the output signal from the main transmitter 21 and the output signals from the on-channel repeaters should be small. In short, the time delay in the on-channel repeaters 22 to 25 should be minimized.

Korean Patent Laid-Open No. 10-2003-32007 published on May 20, 2003, discloses technology that can increase transmission output power of the on-channel repeaters by removing feedback signals generated due to low isolation of the transmitting and receiving antenna from the on-channel repeaters 22 to 25. According to the technology of the Korean Patent Laid-Open No. 10-2003-32007 which adopts the same on-channel repeaters of FIG. 3, the output signals of the on-channel repeaters 22 to 25 are the same as the output signal of the main transmitter 21, and the time delay between the output signal of the main transmitter and the output signals of the on-channel repeaters is small. Also, the characteristics of the output signals of the on-channel repeaters are superior to those of the input signals of the on-channel repeaters by removing the noise and multi-path signals generated by the transmission between the main transmitter 21 and the on-channel repeaters 22 to 25.

FIG. 3 shows the structure of the on-channel repeater of FIG. 2 more in detail. The on-channel repeaters 22 to 25 include a receiving antenna 31, a radio frequency (RF) receiving portion 32, an intermediate frequency (IF) down-converting portion 33, a demodulating portion 34, an equalizing portion 35, a modulating portion 36, an RF up-converting portion 37, a high-power amplifying portion 38, a transmitting antenna 39, and a local oscillator (LO) 40.

The RF receiving portion 32 receives RF broadcasting signals transmitted from the main transmitter 21 through the receiving antenna 31. The IF down-converting portion 33 converts the RF broadcasting signals into IF signals based on a first reference frequency. The demodulating portion 34 converts the IF signals obtained in the IF down-converting portion 33 into baseband signals.

The equalizing portion 35 removes noise, multi-path signals, and feedback signals from the baseband signals that are obtained in the demodulating portion 34 and generated between the main transmitter 21 and the on-channel repeaters 22 to 25. The modulating portion 36 converts the baseband signals obtained in the equalizing portion 35 into IF signals.

The RF up-converting portion 37 converts the IF signals from the modulating portion 36 into RF broadcasting signals based on a second reference frequency. The high-power amplifying portion 38 amplifies the RF broadcasting signals obtained in the RF up-converting portion 37 and relays the amplified signals to the transmitting antenna.

The transmitting antenna 39 transmits the broadcasting signals outputted from the high-power amplifying portion 38. The local oscillator (LO) 40 generates a first reference frequency in consideration that the demodulating portion 34 converts the IF signals into baseband signals to synchronize the frequencies and phases of transmitting and receiving signals, provides the first reference frequency to the IF down-converting portion 33, generates a second reference frequency based on the first reference frequency, and provides the second reference frequency to the RF up-converting portion 37.

The on-channel repeaters 22 to 25 are operated as follows. First, the receiving antenna 31 and the RF receiving portion 32 receive the RF broadcasting signals transmitted from the main transmitter 21. The received RF signals are converted into IF signals through the IF down-converting portion 33. The IF signals are converted into baseband signals in the demodulating portion 34.

The high-performance equalizing portion 35 removes noise and multi-path signals, which are caused by the transmission between the main transmitter 21 and the on-channel repeaters 22 to 25, and feedback signals generated due to low isolation of the transmission and receiving antennas 31 and 39.

The baseband signals free from the noise, the multi-path signals and the feedback signals are converted into IF broadcasting signals in the modulating portion 36. The IF signals are converted into RF signals in the RF up-converting portion 37, the RF signals are amplified in the high-power amplifying portion 38 and then transmitted out through the transmitting antenna 39. The frequency and phase of the receiving signal should be synchronized with those of the transmitting signal.

The frequencies and phases of the signals in the transmitting portion and the receiving portion are synchronized as follows. To synchronize the frequencies of the transmitting and receiving signals in the RF frequency band with each other, only a reference frequency is provided to the IF down-converting unit and the RF up-converting unit. Then, the frequencies of the transmitting and receiving signals are synchronized with each other in the IF frequency band.

To synchronize the frequencies of the transmitting and receiving signals in the IF frequency band with each other, frequency and timing offset information abstracted from resynchronization process in the demodulating portion 34 are used in the modulating portion 36 without any change. This way, the output signal from the transmitting unit of the on-channel repeaters 22 to 25 is synchronized with the receiving signal in the frequency and phase. Therefore, the frequency and phase of the output signal of the on-channel repeaters 22 to 25 can be synchronized with those of the signal generated in the main transmitter 21 without using any additional reference signal.

As described above, if the signals transmitted from the main transmitter 21 and the on-channel repeaters 22 to 25 have time delay that goes out of the multi-path signal removal capability of the equalizing portion of the receiver, the equalizing portion fails to remove the delayed signals. Therefore, the time delay between the output signals of the main transmitter 21 and the output signals of the on-channel repeaters 22 to 25 should be small to provide a digital broadcasting service through the on-channel repeaters 22 to 25.

Although the time delay should be minimized, the conventional demodulating portion 34 of FIG. 4 yields long time delay helplessly. The conventional demodulating portion 34 is described herein with reference to FIG. 4.

The receiving antenna 31 of an on-channel repeaters 22, 23, 24 or 25 and the RF receiving portion 32 receive RF broadcasting signals from the main transmitter 21. The received RF signals are converted into IF signals in the IF down-converting portion, i.e., a first down-converting portion, 33 to produce first down-converted analogue IF signals.

The first down-converted analogue IF signals are converted into digital IF singals in the analogue-to-digital converting portion (ADC) 41. The digital IF signals are converted into near baseband signals in a second down-converting portion 42. The second down-converting portion 42 is added here, because it is easier to design a matched filter 43 in near baseband than in the IF frequency band (around 44 MHz). In FIG. 4, a frequency f_NB down-converts a frequency of the IF band into a frequency of the near baseband.

Subsequently, the signals of the near baseband obtained in the second down-converting portion 42 pass through the matched filter 43 to maximize a signal-to-noise ratio. The signals that have passed through the matched filter 43 are down-converted by a third down-converting portion 44 so that the pilot components of the signals are located in a 0 frequency.

That is, the signals of the near baseband become baseband signals for vestigial sideband (VSB) broadcast, the center frequency of the signals being located in 2.69 MHz. In the drawing, f_B is a frequency that down-converts the frequency of the near baseband into a frequency of baseband for VSB broadcast. The baseband signals for VSB broadcast that are obtained in the third down-converting portion 44 lose image components in a low pass filter 45 to thereby become baseband signals for VSB broadcast.

In the VSB demodulation process, the conventional demodulating portion 34 includes two time delay devices, i.e., the matched filter 43 and the low pass filter 45. The length of time delay caused by the two filters 43 and 45 are determined according to the number of filter taps used in the filters.

When it is assumed that the number of taps used in the matched filter 43 is N and the number of taps used in the low pass filter 45 is M, the two filters 43 and 45 generate time delay of N/2 and M/2, respectively. The total length of time delay caused by the two filters 43 and 45 becomes (N+M)/2.

Therefore, a demodulation proposition that can minimize the time delay of the on-channel repeaters 22 to 25 is called for to provide digital broadcasting service through the on-channel repeaters 22 to 25. In short, the time delay between the output signals of the main transmitter 21 and the output signals of the on-channel repeaters 22 to 25 should be very small.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a demodulation apparatus that can reduce time delay caused by filters used for demodulation in an on-channel repeater in a terrestrial digital television (TV) broadcasting system, and a demodulation method thereof.

In accordance with an aspect of the present invention, there is provided a demodulation apparatus that can reduce time delay of an on-channel repeater in a terrestrial digital television broadcasting system, including: an analogue-to-digital conversion unit for converting an analogue intermediate frequency (IF) signal into a digital IF signal; a down-converting unit for generating an in-phase (I) signal and a quadrature (Q) signal by converting the frequency of the digital IF signal and for down-converting the frequencies of the pilot components of the I and Q signals so that the center frequencies of the I and Q signals could be shifted to 0; a filtering unit for filtering the down-converted I and Q signals and maximizing signal-to-noise ratio; an up-converting unit for up-converting the filtered I and Q signals into baseband signals in which the center frequencies of the filtered I and Q signals are placed in baseband and the pilot components of the filtered I and Q signals are shifted to 0; and an adding unit for adding up the up-converted I and Q signals to restore the signals into baseband signals for broadcasting.

In accordance with another aspect of the present invention, there is provided a demodulation method that can reduce time delay of an on-channel repeater in a terrestrial digital TV broadcasting service, including the steps of: a) converting an analogue IF signal into a digital IF signal; b) generating an I signal and a Q signal by converting the frequency of the digital IF signal and down-converting the frequencies of the pilot components of the I and Q signals so that the center frequencies of the I and Q signals could be shifted to 0; c) filtering the down-converted I and Q signals and maximizing signal-to-noise ratio; d) up-converting the filtered I and Q signals into baseband signals in which the center frequencies of the filtered I and Q signals are placed in baseband and shifting the pilot components of the filtered I and Q signals to 0; and e) adding up the up-converted I and Q signals to restore the signals into baseband signals for broadcasting.

The ultimate goal of the present invention is to relay digital broadcasting signals by using on-channel repeaters which yield low time delay and output signals of excellent characteristics.

With on-channel repeaters which yield low time delay and output signals of excellent characteristics, existing receivers are less affected by the relatively low system delay and the excellent characteristics of the output signals make it possible to expand relay area.

In order to reduce the time delay of the on-channel repeaters in the digital TV broadcasting system, RF broadcasting signals transmitted from the main transmitter to the on-channel repeaters are down-converted into IF signals during demodulation to obtain first down-converted analogue IF signals. The first down-converted analogue IF signals are converted into digital IF signals, and the digital IF signals are down-converted to obtain second down-converted signals so that the center frequency could be a 0 frequency. The second down-converted signals pass through a matched filter to maximize the signal to noise ratio of the second down-converted signals. The signals that have passed the matched filter are up-converted so that their pilot components could be positioned at a frequency of 0. Baseband signals are obtained by adding the up-converted I and Q signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:

FIG. 1 is a conceptual diagram showing a broadcasting service using a conventional repeater;

FIG. 2 is a conceptual diagram illustrating a broadcasting service using an ordinary on-channel repeater (OCR);

FIG. 3 is a block diagram depicting the on-channel repeater of FIG. 2;

FIG. 4 is a block diagram showing the demodulating portion used in the conventional on-channel repeater;

FIG. 5 is a block diagram illustrating a demodulating portion that can reduce time delay in an on-channel repeater in accordance with an embodiment of the present invention; and

FIG. 6 is a flowchart describing a demodulation process for reducing time delay in the on-channel repeater in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter.

FIG. 5 is a block diagram illustrating a demodulating portion that can reduce time delay in an on-channel repeater in accordance with an embodiment of the present invention. Referring to FIG. 5, the demodulating portion 34 suggested in the present invention includes an analogue-to-digital conversion unit (ADC) 51, a second down-converting unit 52, a matched filtering unit 53, an up-converting unit 54, and an adder 55.

The ADC 51 converts analogue intermediate frequency (IF) signals to digital IF signals, when the radio frequency (RF) broadcasting signals transmitted from a main transmitter 21 through a receiving antenna 31 of on-channel repeaters and an RF receiving portion 32 are inputted after down-converted in an IF down-converting portion 33, which is a first down-converting portion.

The second down-converting unit 52 generates in-phase (I) and quadrature (Q) signals by converting the frequency of the digital IF signals, down-converts the frequency of the pilot components of the I and Q signals, and shifts the center frequencies of the I and Q signals to a frequency of 0. The matched filtering unit 53 filters the down-converted I and Q signals to thereby maximize a signal-to-noise ratio.

The up-converting unit 54 up-converts the filtered I and Q signals so that the center frequencies of both signals could be baseband signals and shifts the pilot components of the I and Q signals to the frequency of 0. The adder 55 restores the up-converted I and Q signals to baseband signals for broadcast.

The demodulating portion 34 used in an on-channel repeater of the present invention is operated as follows. First, the receiving antenna 31 and the RF receiving portion 32 of the on-channel repeater receive RF broadcasting signals transmitted from the main transmitter 21. The RF signals are converted into IF signals in the IF down-converting portion 33, i.e., the first down-converting portion, to thereby produce first down-conversion analogue signals.

The first down-conversion analogue signals are converted into digital IF signals in the analogue-to-digital conversion unit (ADC) 51. The digital IF signals are shifted so that the center frequency could be positioned at a frequency of 0, that is, the pilot components could be positioned at −2.69 MHz.

The second down-converting unit 52 includes a first down-converter 521 and a second down-converter 522. The first down-converter 521 generates I components by multiplying cos(2π·f_C·nT) by the digital IF signals. The second down-converter 522 generates Q components by multiplying sin(2π·f_C·nT) by the digital IF signals. In FIG. 5, f_C is a frequency that down-converts the frequency of the IF pilot signals to a frequency of −2.69 MHz.

Subsequently, the signals shifted in the second down-converting unit 52 pass through the matched filtering unit 53 to maximize the signal-to-noise ratio. The matched filtering unit 53 includes a matched filter for I signals and a matched filter for Q signals. The two matched filters 531 and 532 have the same structure.

The signals that have passed through the mated filtering unit 53 are up-converted in the up-converting unit 54 so that the pilot components could be positioned at a frequency of 0, that is, the center frequency of the signals could be 2.69 MHz, thus making the signals baseband signals.

The up-converting unit 54 includes a first up-converter 541 for I signals which is multiplied by cos(2π·f_D·nT) and a second up-converter 542 for Q signals which is multiplied by sin(2π·f_D·nT). In FIG. 5, f_D is a frequency that positions the center frequency of the signals at a frequency of 2.69 MHz. That is, it positions the pilot components of the signals at a frequency of 0.

Finally, the up-converted I and Q signals are added up in the adder 55 to thereby produce baseband signals for vestigial sideband (VSB) broadcast, which are to be restored.

Since the baseband matched filtering unit 53 plays the role of the low pass filter 45 of FIG. 4, the VSB demodulating portion 34 of the present invention has only one time delay device, that is, the matched filtering unit 53. Here, the length of the time delay caused by the matched filtering unit 53 is determined according to the number of filter taps.

When it is assumed that the number of taps used in the matched filter 531 and 532 is N, time delay of N/2 is generated in each filter 531 and 532. Since the total length of time delay caused by the two filters 531 and 532 is the same as the time delay generated by the one matched filtering unit 43 of FIG. 4, the total time delay is generated as much as N/2.

Therefore, the on-channel repeaters 22-25 generating short time delay can be manufacture by using the demodulating portion having the structure described above. In the on-channel repeaters 22-25, the time delay between the output signals of the main transmitter 21 and the output signals of the on-channel repeaters 22 to 25 is very small.

FIG. 6 is a flowchart describing a demodulation process for reducing time delay in the on-channel repeater in accordance with an embodiment of the present invention.

At step S601, the on-channel repeater 31 and the RF receiving portion 32 receives RF broadcasting signals transmitted from the main transmitter 21. At step S602, the IF down-converting portion 33, which is the first down-converting portion, converts the RF signals into IF signals. Then, the ADC 51 converts the first down-converted analogue IF signals, which are obtained in the IF down-converting portion 33, i.e., the first down-converting portion, into digital IF signals.

Subsequently, the second down-converting unit 52 shifts the frequency of the digital IF signals obtained in the ADC 51 so that their center frequency could be positioned at a frequency of 0, that is, the pilot components of the digital IF signals are positioned at −2.69 MHz. In short, the second down-converting unit 52 generates I components by multiplying cos(2π·f_C·nT) by the digital IF signals and generates Q components by multiplying sin(2π·f_C·nT) by the digital IF signals. Thus, it down-converts the pilot frequency from the IF bandwidth to a frequency of −2.69 MHz.

At steps S604 and S605, the matched filtering unit 53 maximizes the signal to noise ratio of each I and Q signal which are obtained after shifted in the second down-converting unit 52. Then, the up-converting unit 54 up-converts the signals outputted from the matched filtering unit 53 so that the pilot components could be positioned at a frequency of 0, that is, the signal could be baseband signals with their center frequencies being positioned at 2.69 MHz. The up-converting unit 54 positions the center frequencies at 2.69 MHz by multiplying cos(2π·f_D·nT) by the filtered I signals and multiplying sin(2π·f_D·nT) by the filtered Q signals. In short, the pilot components of the signals are positioned at a frequency of 0.

Finally, at step S610, the adder 55 adds up the up-converted I and Q signals, which are obtained in the up-converting unit 54, to thereby generate baseband signals for VSB broadcasting.

The demodulating portion and method of the present invention generates relatively low system delay in on-channel repeaters. By using such on-channel repeaters, the relay area can be expanded and the limited frequency resources can be used efficiently.

While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

Claims

1. A demodulation apparatus that can reduce time delay of an on-channel repeater in a terrestrial digital television broadcasting system, comprising:

an analogue-to-digital conversion means for converting an analogue intermediate frequency (IF) signal into a digital IF signal;

a down-converting means for generating an in-phase (I) signal and a quadrature (Q) signal by converting the frequency of the digital IF signal and for down-converting the frequencies of the pilot components of the I and Q signals so that the center frequencies of the I and Q signals could be shifted to 0;

a filtering means for filtering the down-converted I and Q signals and maximizing signal-to-noise ratio;

an up-converting means for up-converting the filtered I and Q signals into baseband signals in which the center frequencies of the filtered I and Q signals are placed in baseband and the pilot components of the filtered I and Q signals are shifted to 0; and

an adding means for adding up the up-converted I and Q signals to restore the signals into baseband signals for broadcasting.

2. The demodulation apparatus as recited in claim 1, wherein the down-converting means includes:

a first down-converter for generating an in-phase (I) component by multiplying cos(2π·fC·nT) by the digital IF signal, fC denoting a frequency that down-converts the pilot frequency of IF bandwidth to −2.69 MHz; and

a second down-converter for generating a quadrature (Q) component by multiplying sin(2π·fC·nT) by the digital IF signal.

3. The demodulation apparatus as recited in claim 1, wherein the up-converting means includes:

a first up-converter for shifting the center frequency to 2.69 MHz by multiplying cos(2π·fD·nT) by the filtered I signal, fD being a frequency that shifts the center frequency of the filtered I signal to 2.69 MHz, that is, fD shifts the pilot component of a signal to 0; and

a second up-converter for shifting the center frequency to a frequency of 2.69 MHz, thus placing the pilot component of a signal at a frequency of 0 by multiplying sin(2π·fD·nT) by the filtered Q signal.

4. The demodulation apparatus as recited in claim 1, wherein the filtering means includes a first matched filter for the I signal and a second matched filter for the Q signal, performs the function of a low pass filter, and determines time delay according to the number of filter taps used in each filter, and the summation of the time delays caused by the first and second matched filters is the same as the time delay caused by one matched filter.

5. A demodulation method that can reduce time delay of an on-channel repeater in a terrestrial digital television (TV) broadcasting service, comprising the steps of:

a) converting an analogue intermediate frequency (IF) signal into a digital IF signal;

b) generating an in-phase (I) signal and a quadrature (Q) signal by converting the frequency of the digital IF signal and down-converting the frequencies of the pilot components of the I and Q signals so that the center frequencies of the I and Q signals could be shifted to 0;

c) filtering the down-converted I and Q signals and maximizing signal-to-noise ratio;

d) up-converting the filtered I and Q signals into baseband signals in which the center frequencies of the filtered I and Q signals are placed in baseband and shifting the pilot components of the filtered I and Q signals at 0; and

e) adding up the up-converted I and Q signals to restore the signals into baseband signals for broadcasting.

6. The demodulation method as recited in claim 5, wherein, at the step b), an in-phase (I) component is generated by multiplying cos(2π·fC·nT) by the digital IF signal, fC being a frequency that down-converts the pilot frequency of IF bandwidth to −2.69 MHz; and a quadrature (Q) component is generated by multiplying sin(2π·fC·nT) by the digital IF signal.

7. The demodulation method as recited in claim 5, wherein, at the step d), the center frequency of the filtered I signal is shifted to a frequency of 2.69 MHz by multiplying cos(2π·fD·nT) by the filtered I signal, fD being a frequency that shifts the center frequency to 2.69 MHz, that is, shifts the frequency of the pilot component of a signal to 0; and the center frequency is shifted to a frequency of 2.69 MHz, thus placing the pilot component of a signal at a frequency of 0 by multiplying sin(2π·fD·nT) by the filtered Q signal.