JP4623628B2 - Digital terrestrial broadcast receiving headend device and terrestrial digital broadcast hearing system - Google Patents

Description

The present invention relates to a terrestrial digital broadcast receiving headend device and a terrestrial digital broadcast hearing system. Specifically, the terrestrial digital broadcast is received, the level is adjusted for each channel, and converted to a frequency other than the terrestrial digital broadcast channel band such as the VHF band, the MID band, or the CATV band, and supplied to the joint receiving facility. The present invention relates to a terrestrial digital broadcast receiving headend apparatus and a terrestrial digital broadcast hearing system.

  The frequency band used for terrestrial digital broadcasting is the UHF band (470 MHz to 770 MHz) common to terrestrial analog broadcasting. Similarly to terrestrial analog broadcasting, the bandwidth is divided into 50 equal parts of 6 MHz, and each of them is “13”. It is used as a broadcast channel up to “62”.

  In terrestrial analog broadcasting, broadcasting is performed on at least every other channel in order to avoid adjacent channel interference, but in terrestrial digital broadcasting, orthogonal frequency division multiplexing (OFDM), which is resistant to adjacent channel interference, is employed. Therefore, such intermittent channel assignment is not performed.

  However, even in the case of terrestrial digital broadcasting, in the actual field, there may be a deviation in the level of the received signal for each channel. If this deviation is large, the reception quality is affected by the nonlinear distortion of the receiver amplification stage. May deteriorate. For this reason, level adjustment for each reception channel is performed using a device called a headend device.

  The head-end device is applied to a joint reception facility in a building or a condominium, in particular, but in a joint reception facility that supports only a frequency band below the UHF band, for example, a VHF band or a mid band (MID band). In addition to the level adjustment function for each channel, a function for converting to another frequency such as a VHF band or an MID band and outputting it is also required.

  The above-mentioned two functions, that is, a level adjustment function for each channel and a frequency conversion function (a function of converting a UHF band terrestrial digital broadcast signal into another frequency such as a VHF band or an MID band and outputting it) As a conventional head end device, for example, the one described in Non-Patent Document 1 is known.

FIG. 4 is a block diagram of a conventional headend device. However, the illustrated configuration representatively shows only a portion for adjusting the level of an arbitrary channel for the sake of simplicity. In an actual head end apparatus, the illustrated configuration is connected in parallel by the number of channels, and finally all the channels are mixed and supplied to the joint receiving facility.
In this figure, a terrestrial digital broadcast signal in the UHF band is input to a filter 3 via a preamplifier 1 and a premixer 2. The filter 3 removes frequency components other than a specific channel (for example, 13ch of terrestrial digital broadcasting), and then supplies the signal to a joint reception facility (not shown) via a rear-stage mixer 4 and a rear-stage amplifier 5.

  The front-stage mixer 2 and the rear-stage mixer 4 mix individual local oscillation frequencies f1 and f2 produced by dedicated local oscillators 6 and 7, respectively, and an input frequency, and take out a signal having a difference or sum frequency. The local oscillation frequency f1 for the front-stage mixer 2 is for converting to an intermediate frequency suitable for the filter 3, and the local oscillation frequency f2 for the rear-stage mixer 4 is for converting to a frequency in the VHF band, for example. is there. Further, the channel level is adjusted by changing the amplification degree of the pre-stage amplifier 1 and the post-stage amplifier 5.

  In such a configuration, for example, an intermediate frequency suitable for the filter 3 is set to 56.875 MHz for convenience. The specific channel is, for example, 13 channels, and the band of 13 channels of terrestrial digital broadcasting is ± 3 MHz of the center frequency 473.142857 MHz. F1 = 530.017857 MHz. The former mixer 2 can convert the signal to an intermediate frequency signal of ± 3 MHz with a center frequency of 56.875 MHz. In addition, when the signal that has passed through the filter 3 is converted to a frequency in the VHF band, the local oscillation frequency f2 for the subsequent mixer 4 may be set to Fx + 56.875 MHz. Here, Fx is a predetermined frequency corresponding to the VHF band (90 to 222 MHz).

"Japan CATV Technology Association Standard (JCTEA STD-011-1.0) Digital Cable Television Broadcasting, Terrestrial Digital Television Broadcasting, Pass-Through Transmission Version 1.0" issued by Japan CATV Technology Association, 2003 July 1

  However, in the above-described conventional head end device, the two mixers (the front-stage mixer 2 and the rear-stage mixer 4) located before and after the filter 3 are individually formed by dedicated local oscillators 6 and 7, respectively. Since the local oscillation frequencies f1 and f2 are provided, there are the following disadvantages.

  (1) It is ideal that the frequency of the local oscillator is always constant, but in reality, since two local oscillators are used per channel, the frequency value after the conversion differs from the ideal frequency value after conversion. Occurs. In addition, some phase “fluctuations” are inevitable. This “fluctuation” is called phase noise. The frequency shift and phase noise in the conventional headend device described above cause the following inconveniences because the individual local oscillation frequencies f1 and f2 are used.

5 and 6 are diagrams for explaining the influence of the frequency shift. As shown in FIG. 5 (a), UHF band terrestrial digital broadcasting (in the example shown, a frequency arrangement of 13 to 15ch is representatively shown) sent from a broadcasting station shows interference between the channels. To prevent this, a guard band of about 0.4 MHz is provided.
When digital terrestrial broadcast signals arranged adjacent to each other via a guard band are converted for each channel, the adjacent channels cannot actually be cut completely due to the characteristics of the filter 3 used at the intermediate frequency, and leakage occurs. For example, in FIG. 5B, if a signal after converting a certain channel (here, 13ch) through the filter 3 is C13ch, the C13ch signal cannot completely cut the adjacent channels C14ch and C15ch, There is some leakage. Note that C14ch and C15ch are the outputs of the level adjustment units for 14ch and 15ch, respectively. Further, in the other 14ch level adjustment unit, as shown in the diagram after the conversion in FIG. 5C, the C14ch signal cannot completely cut the C13ch and C15ch of the adjacent channels, and some leakage occurs. Yes.

  In such a case, assuming that the difference between the ideal value and the actual value of the local oscillation frequencies f1 and f2 provided for each channel is zero, as shown in FIG. 6A, for example, for C13ch and C14ch When the output of the level adjuster is mixed, the phase of the leaked signal and the original signal of that channel are aligned, so there is no problem as a so-called SFN (Single Frequency Network). When f1 and f2 have a deviation from the ideal value, as shown in FIG. 6B, a deviation from the frequency band after the conversion to the ideal value occurs, and the leaked signal and the original signal of the channel are As a result, the leaked signal becomes an interference wave of the channel, and as a result, the digital terrestrial broadcast cannot be normally received.

  (2) In order to eliminate such inconvenience, it is necessary to use a high-performance filter 3 (so-called sharp characteristics). As such a filter, for example, a SAW (Surface Acoustic Wave) filter is known, but a SAW filter that can completely cut a leakage signal from an adjacent channel is considerably expensive. Since it cannot be cut in one stage, it must be configured in multiple stages. In addition, since SAW filters corresponding to the number of channels are required, there is a problem in that the cost of the headend device is increased.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a terrestrial digital broadcast receiving headend device that can use an inexpensive SAW filter that is less susceptible to frequency shift and phase noise, and that can improve broadcast quality and reduce costs. There is to do.

The terrestrial digital broadcast receiving headend device according to claim 1 includes n level adjusting units and one frequency converting unit provided corresponding to channels of the terrestrial digital broadcasting, and the level adjusting unit includes the level adjusting unit, A variable amplification amplifier that variably amplifies the terrestrial digital broadcast signal, a first mixer that converts the variably amplified terrestrial digital broadcast signal to a predetermined intermediate frequency, and a terrestrial digital broadcast signal converted to the intermediate frequency A filter for extracting a signal of a specific channel, a second mixer for converting the signal of the specific channel into the frequency of the original digital terrestrial broadcast signal, and a first local oscillation frequency commonly used by the first mixer and the second mixer A first local oscillator for generating the frequency converter, the frequency converting unit combining the outputs of the n level adjusting units, and the combining unit Thus the combined signal VHF band, a third mixer for converting the MID band or CATV band, characterized in that it comprises a second local oscillator for generating a second local oscillation frequency to be used in the third mixer.
The terrestrial digital broadcast hearing system according to claim 2 , wherein the terrestrial digital broadcast signal is received, the level is adjusted for each channel, and the level-adjusted terrestrial digital broadcast signal is VHF band, MID band or CATV band. In a terrestrial digital broadcast common listening system that converts the signal into a common reception facility and includes n level adjustment units and one frequency conversion unit provided corresponding to the channel of the terrestrial digital broadcast, the level adjustment unit Is a variable amplification amplifier that variably amplifies the terrestrial digital broadcast signal, a first mixer that converts the variably amplified terrestrial digital broadcast signal to a predetermined intermediate frequency, and a terrestrial digital broadcast signal converted to the intermediate frequency. A filter for extracting the signal of a specific channel from the inside, and the frequency of the original terrestrial digital broadcast signal from the signal of the specific channel And a first local oscillator that generates a first local oscillation frequency that is commonly used by the first mixer and the second mixer, and the frequency conversion unit includes the n level adjustment units. Synthesizer outputs, a third mixer for converting the signal synthesized by the synthesizer into a VHF band, MID band, or CATV band, and a second local oscillation frequency for use in the third mixer. And two local oscillators.
According to a third aspect of the present invention, in the terrestrial digital broadcast receiving headend device according to the first aspect, the filter is a SAW filter.
The digital terrestrial broadcast system according to claim 4 is the digital terrestrial broadcast system according to claim 2 , wherein the filter is a SAW filter.

  According to the present invention, first, the terrestrial digital broadcast signal is variably amplified by the variable amplification amplifier, and then the variably amplified terrestrial digital broadcast signal is converted to a predetermined intermediate frequency by the first mixer. Then, a signal of a specific channel is extracted from the terrestrial digital broadcast signal converted to the intermediate frequency by the filter, and then the signal of the specific channel is converted to the frequency of the original terrestrial digital broadcast signal by the second mixer. Converted.

  Here, since the first mixer and the second mixer commonly use one local oscillation frequency (first local oscillation frequency) generated by the first local oscillator, the input in the first mixer and the second mixer The frequency and the output frequency are completely the same frequency. Therefore, the level adjustment for each channel is performed correctly, and when the channels are mixed, there is no frequency shift between the original signal and the leaked signal, and the signal is not adversely affected. Further, the phase noise is canceled out due to the same fluctuation, and is not affected at all by the phase noise.

  Therefore, since the frequency shift after the level adjustment can be eliminated, an inexpensive SAW filter (a SAW filter whose performance is slightly inferior to the ideal sharpness characteristic) can be used, and the cost can be reduced.

In addition, the number of filters and the number of local oscillators in the entire headend device can be set to n + 1 by adding the number of frequency conversion units (1) to the number of level adjustment units (the number of channels n). Since this can be significantly reduced as compared with the individual device, the cost of the head-end device can be reduced also in this respect.
Moreover, since the local oscillation frequency of the first mixer and the second mixer is made by the same local oscillator, the influence of the phase noise is eliminated, and the phase noise is generated only by the third mixer and the second local oscillator. Compared to the influence of the phase noise caused by the two local oscillators as in the prior art, it is possible to suppress the deterioration of the phase noise by about half.

  Embodiments of the present invention will be described below with reference to the drawings. It should be noted that the specific details or examples in the following description and the illustrations of numerical values, character strings, and other symbols are only for reference in order to clarify the idea of the present invention, and the present invention may be used in whole or in part. Obviously, the idea of the invention is not limited. In addition, a well-known technique, a well-known procedure, a well-known architecture, a well-known circuit configuration, and the like (hereinafter, “well-known matter”) are not described in detail, but this is also to simplify the description. Not all or part of the matter is intentionally excluded. Such well-known matters are known to those skilled in the art at the time of filing of the present invention, and are naturally included in the following description.

  1 and 2 are an external view and an internal structure diagram of a terrestrial digital broadcast receiving headend device (hereinafter simply referred to as “headend device”). In this figure, a head end device 10 is housed in a wall-hanging case 11, and the case 11 is a box-shaped case 12 having an open surface formed by bending or welding a metal plate. And a back plate 13 that closes the open surface of the case 12 and is attached to a wall surface (not shown) or the like.

  A large number of air holes 14 to 20 having a small area are formed in each surface (upper surface, left and right side surfaces, lower surface and front surface) other than the open surface of the case 12. In the figure, since the right side surface and the lower surface of the case 12 are shaded, the air holes drilled in those surfaces are not visible, but for convenience of explanation, these air holes are denoted by reference numerals (18 to 18). 20) only.

  That is, the air hole 14 is drilled in the upper surface of the case 12 (shown), the air hole 15 is drilled in the upper front part of the case 12 (shown), the air hole 16 is drilled in the upper part of the left side of the case 12 (shown), and the air hole 17 is drilled in the lower part of the left side of the case 12 (shown). The air hole 18 is drilled in the upper part of the right side surface of the case 12 (not shown), the air hole 19 is drilled in the lower part of the right side surface of the case 12 (not shown), The air hole 20 is formed in the lower surface of the case 12 (not shown).

  In addition, on the front surface of the case 12, an access hole 23 for level adjustment is provided in accordance with each position of n (for example, 10) level adjustment units 21 (level adjustment devices) housed in the case 12. Although not shown, a transmitted light window such as a power indicator lamp and an operation status indicator lamp is provided at an appropriate position.

  The back plate 13 has bolt fixing holes 24 to 27 drilled into wall surfaces or the like at arbitrary positions on the peripheral edge thereof (in the example shown, a plate portion extending up and down the case 12). A power supply unit 28 is attached near the top of the power supply unit 13, and ten level adjustment units 21 arranged side by side are attached on the signal distribution unit 29 and the frequency conversion unit 30 attached below the power supply unit 28. ing. The level adjustment unit 21 is for individually adjusting the signal level for each reception channel in the television joint reception facility (particularly for digital terrestrial broadcasting), and each level adjustment unit 21 includes each unit. Level adjustment elements 31 and 32 accessible from the front side. The metal plate 33 attached to the lower side of the power supply unit 28 is a shielding plate for cutting off the heat of the power supply unit 28.

  The UHF band terrestrial digital broadcast signal received by the antenna 35 is input to the terminal 34 of the signal distribution unit 29 via the cable 36, and the level for each channel from the terminal 37 of the frequency conversion unit 30. The terrestrial digital broadcast signal that has been adjusted and converted into, for example, a VHF band signal is supplied via a cable 38 to a joint reception facility 41 including a distributor 39 and a large number of television terminals 40.

  FIG. 3 is an overall configuration diagram of a digital terrestrial broadcasting system including the headend device 10. In this figure, a UHF band terrestrial digital broadcast signal received by an antenna 35 is supplied to a preamplifier 42 in a signal distribution unit 29 via a cable 36 and a terminal 34, and after being amplified by the preamplifier 42, a distributor N is distributed at 43.

  Here, “n” represents the number of level adjusting units 21 provided in the head end device 10. Since each level adjusting unit 21 adjusts the level of a specific channel of terrestrial digital broadcasting, in other words, “n” is the number of channels of terrestrial digital broadcasting that can be handled by the headend device 10. .

  The n level adjustment units 21 have the same configuration, and the level adjustment unit 21 includes an input bandpass filter 44, a first amplifier 45 (variable amplification amplifier), a first mixer 46, and a SAW filter 47 ( Filter), a second amplifier 48, a second mixer 49, an AGC circuit 50, and a first local oscillator 51.

  The input bandpass filter 44 is a filter that cuts frequency components other than the band (for example, 6 MHz bandwidth) of the designated channel in the UHF band (470 to 770 MHz). Good characteristics are not required. For example, an inexpensive filter such as an LC type may be used. The terrestrial digital broadcast signal centered on the designated channel that has passed through the input bandpass filter 44 is applied to the first mixer 46 via the first amplifier 45 having a variable amplification degree, and the first mixer 46 outputs a predetermined local oscillation frequency. (First local oscillation frequency) is mixed with fa and converted to an intermediate frequency. This intermediate frequency is a frequency adapted to the pass band of the SAW filter 47, and is, for example, 50 MHz.

  Here, the “SAW filter” is a type of device using a sound wave (surface acoustic wave) propagating on a solid surface. The SAW filter can produce a very small and fine filter. Generally available SAW filters have a low nominal pass frequency of about 30 MHz and a high pass number of about several hundred MHz. The lower the pass frequency, the greater the sharpness (that is, the better the frequency selectivity). On the other hand, such high-performance SAW filters are expensive (for example, about 30 MHz and tens of thousands of yen), and cannot be adopted in terms of cost except for special applications. .

  Therefore, in this embodiment, the level adjustment of terrestrial digital broadcasting in the UHF band is performed at the same local oscillation frequency for each channel, the input and output are the same frequency, each channel is mixed, and all channels are collectively processed into the VHF band or the like. As a configuration for converting to the above, it is possible to use an inexpensive SAW filter 47 having an appropriate frequency selectivity by suppressing the frequency shift, thereby achieving the improvement in performance and the cost reduction of the headend device 10. The purpose is.

  The illustrated SAW filter 47 is an inexpensive filter having an appropriate frequency selectivity (for example, having a nominal pass frequency of about 50 MHz available for several thousand yen). “Moderate frequency selectivity” basically means that frequency components other than a specific channel of digital terrestrial broadcasting can be cut, but the frequency selectivity is not perfect and some signals from adjacent channels are not. It is the one that can tolerate leakage.

  As described at the beginning (see FIG. 6A), even if a leak signal from an adjacent channel exists, there is a “shift” in the frequency between the original signal of the channel and the leak signal from the adjacent channel. If not (or if the “deviation” is negligible), there is no adverse effect from the leakage signal (or it is suppressed to an acceptable level).

  In the present embodiment, after the signal passing through the SAW filter 47 is amplified by the second amplifier 48, the amplified signal and a predetermined local oscillation frequency fa are mixed by the second mixer 49, for example, the original UHF band. Return to the signal. The local oscillation frequency fa used in the second mixer 49 is generated by a signal generation source (first local oscillator 51) common to the local oscillation frequency fa used in the first mixer 46. is there.

  Therefore, since there is only one signal generation source (first local oscillator 51), the frequency shift between the original signal of the channel and the leakage signal from the adjacent channel is greatly suppressed as in the prior art at the beginning. Can do. As a result, as described above, even if an inexpensive SAW filter 47 having an appropriate frequency selectivity is used, the adverse effect of a leakage signal from an adjacent channel can be avoided, and the cost of the headend device 10 can be reduced. It is.

Next, the head end device 10 has a level adjustment function for each channel and a frequency conversion function to a VHF band or the like.
The former level adjustment function is realized by the AGC circuit 50 and the variable first amplification amplifier 45. That is, the AGC circuit 50 generates an AGC voltage having a magnitude corresponding to the output signal of the second amplifier 48, and the amplification degree of the first amplifier 45 is changed in response to the AGC voltage. The level of the output signal of the second amplifier 48 is always maintained at a predetermined level.

  The latter frequency conversion function includes a synthesizer 52 (synthesizing unit) that synthesizes the outputs of the n level adjusting units 21, a third amplifier 53 that amplifies the output of the synthesizer 52, and a third amplifier 53. A third mixer 54 that mixes the output with a predetermined local oscillation frequency (second local oscillation frequency) fb and converts it to, for example, a signal in the VHF band; and an output bandpass filter 55 that cuts a signal other than the VHF band; Are realized by a fourth amplifier 56 that amplifies the signal that has passed through the output bandpass filter 55 and a second local oscillator 57 that generates a local oscillation frequency fb. The digital terrestrial broadcast signal converted into all channels at the same time is supplied to the joint reception facility 41 via the terminal 37 and the cable 38.

  As described above, in the present embodiment, the mixer (first mixer 46 and second mixer 49) provided before and after the SAW filter 47 is made with a common signal generation source (first local oscillator 51). Since only one local oscillation signal fa is provided, the adjusted frequencies of the level adjusting units 21 can be matched. Therefore, unlike the conventional apparatus, it is possible to suppress the generation of an interference wave due to a frequency shift using the two local oscillation signals f1 and f2, so that an inexpensive SAW filter 47 can be used. The performance of the head end device 10 can be improved and the cost can be reduced.

In addition, the number of local oscillators mounted on the head-end device 10 can be a total of “n + 1” of n first local oscillators 51 and one second local oscillator 57, which is “2n” of the prior art. Therefore, the cost of the head end device 10 can be reduced also in this respect.
Furthermore, in the present embodiment, since the frequency conversion of the level adjustment unit is output by returning to the same frequency with one local oscillation signal fa, the phase noise due to “fluctuation” of fa is canceled. Therefore, only the phase noise due to only the local oscillation signal fb of the frequency conversion unit 30 is affected as a whole of the headend device 10, and the influence is compared with the phase noise due to the two conventional local oscillation signals f1 and f2. Can be halved.

  In this embodiment, as shown in FIG. 3, the level adjustment function for each channel is implemented in the level adjustment unit 21, and other functions are implemented in the signal distribution unit 29 and the frequency conversion unit 30. . That is, in the illustrated example, the preamplifier 42, the distributor 43, and the synthesizer 52 are mounted on the signal distributor 29, and the third amplifier 53, the third mixer 54, the output bandpass filter 55, the fourth amplifier 56, and the second amplifier 56 are mounted. Although the local oscillator 57 is mounted on the frequency conversion unit 30, this is an example of design and is not limited to this. The signal distribution unit 29 and the frequency conversion unit 30 may be combined into one, or may be divided into three or more parts.

In any case, as a product form to be put on the market, (a) the head end device 10 itself including all components (level adjustment unit 21, signal distribution unit 29, frequency conversion unit 30, etc.), (b) The level adjustment unit 21 itself, (c) the signal distribution unit 29 itself, (d) the frequency conversion unit 30 itself may be used.
Further, the frequency conversion unit 30 may be a separate component (separate unit), and the frequency conversion unit 30 appropriately selected according to the frequency to be converted may be incorporated in the headend device 10.
Further, the frequency conversion unit 30 may have a circuit capable of converting into a plurality of types of frequencies, and may be switched by selecting with a switch or the like.

  In the present embodiment, the preamplifier 42 is included in the head end device 10, but the present invention is not limited thereto, and the preamplifier 42 may be a separate component (separate unit), for example, inserted directly under the antenna 35. Good.

1 is an external view of a head end device 10. FIG. 2 is an internal structure diagram of a head end device 10. FIG. 1 is an overall configuration diagram of a digital terrestrial broadcasting system that includes a head-end device 10. FIG. It is a block diagram of the conventional headend apparatus. It is a figure explaining the influence of a frequency shift. It is a figure explaining the influence of a frequency shift.

Explanation of symbols

fa first local oscillation frequency fb second local oscillation frequency 10 terrestrial digital broadcast receiving headend device 21 level adjustment unit (level adjustment device)
30 Frequency converter 45 First amplifier (variable amplifier)
46 First mixer 47 SAW filter (filter)
49 Second mixer 51 First local oscillator 52 Synthesizer (synthesizer)
54 Third mixer 57 Second local oscillator

Claims (4)

N level adjustment units and one frequency conversion unit provided corresponding to channels of terrestrial digital broadcasting,
The level adjustment unit
A variable amplification amplifier that variably amplifies the terrestrial digital broadcast signal;
A first mixer for converting the variably amplified terrestrial digital broadcast signal into a predetermined intermediate frequency;
A filter for extracting a signal of a specific channel from the terrestrial digital broadcast signal converted to the intermediate frequency;
A second mixer for converting the signal of the specific channel into the frequency of the original digital terrestrial broadcast signal;
Including a first local oscillator that generates a first local oscillation frequency commonly used in the first mixer and the second mixer,
The frequency converter is
Combining means for combining the outputs of the n number of level adjustment units;
A third mixer for converting the signal synthesized by the synthesis means into a VHF band, a MID band, or a CATV band;
And a second local oscillator that generates a second local oscillation frequency used in the third mixer. Digital terrestrial broadcasting that receives terrestrial digital broadcast signals, performs level adjustment for each channel, converts the terrestrial digital broadcast signals after level adjustment into VHF band, MID band, or CATV band and supplies them to the joint receiving facility In the hearing system,
N level adjustment units and one frequency conversion unit provided corresponding to channels of terrestrial digital broadcasting,
The level adjustment unit
A variable amplification amplifier that variably amplifies the terrestrial digital broadcast signal;
A first mixer for converting the variably amplified terrestrial digital broadcast signal into a predetermined intermediate frequency;
A filter for extracting a signal of a specific channel from the terrestrial digital broadcast signal converted to the intermediate frequency;
A second mixer for converting the signal of the specific channel into the frequency of the original digital terrestrial broadcast signal;
Including a first local oscillator that generates a first local oscillation frequency commonly used in the first mixer and the second mixer,
The frequency converter is
Combining means for combining the outputs of the n number of level adjustment units;
A third mixer for converting the signal synthesized by the synthesis means into a VHF band, a MID band, or a CATV band;
A terrestrial digital broadcast hearing system, comprising: a second local oscillator that generates a second local oscillation frequency used in the third mixer.   The terrestrial digital broadcast receiving headend device according to claim 1, wherein the filter is a SAW filter. The digital terrestrial broadcasting system according to claim 2 , wherein the filter is a SAW filter.

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