Digital type VSB modulation apparatus

Abstract: In this invention, in order to generate a VSB modulation signal from a digital baseband signal, an in-phase component of a modulation signal having a frequency spectrum having a high-frequency component whose level is half that of its low-frequency component, and an orthogonal component of the modulation signal of a high-frequency component having a frequency spectrum whose level is half that of the low-frequency component are generated from the baseband signal. The in-phase component and the orthogonal component are multiplied with carrier signals to obtain a carrier in-phase component and a carrier orthogonal component. The carrier in-phase component and the carrier orthogonal component are synthesized to obtain a VSB modulation signal.

Description: The present invention relates to a digital type VSB modulation apparatus for generating a VSB (vestigial sideband) modulation signal from a digital baseband signal. As is well known, a video modulation means used in a conventional television transmission apparatus AM-modulates an analog baseband signal to convert it into a DSB (double sideband) modulation signal, and limits the bandwidth of the DSB modulation signal using a band-pass filter included in an intermediate frequency circuit or a transmission frequency circuit, thereby obtaining a VSB modulation signal (see, e.g., "Broadcast Technology Library 10, Broadcast Equipment, Chapter 3, Television Broadcast Equipment" issued on April 20, 1983 by Nippon Hoso Publishing Inc.). In this case, as the band-pass filter, a surface acoustic filter is generally used. However, the surface acoustic filter is economically disadvantageous since separate filters must be prepared in units of use frequencies. In addition, the surface acoustic filter suffers from large ripples of group delay time characteristics due to its structure. For this reason, the group delay time characteristics of a transmission apparatus itself depend on the surface acoustic filter. Recently, strong demand has arisen for a digital VSB modulation apparatus. However, it is very difficult in terms of a structure to simply replace circuit sections constituting a conventional VSB modulation means with digital circuits having the same functions. For example, when the band-pass filter is realized by a digital filter, a high-speed circuit operation is required, and the number of taps becomes very large since the attenuation characteristic curve of the stop band is very steep, resulting in a large circuit scale. Document FR-A-2 633 127 discloses a numerical method and an apparatus for VSB modulation comprising a signal processor as complex filter means. Furthermore, a SSB generator is known from document EP-A-0 323 675 to provide a digitized SSB signal having a desired carrier frequency. In this generator, a digitized analog signal is processed in accordance with Weavers method. Document US-A-3 559 070 discloses a data transmission system using carrier modulation for transmitting digital data signals. In the apparatus of this document the band of a digital signal is limited by a premodulation low-pass filter in advance. Then, the digital signal is supplied to a low-pass filter and a high-pass filter, and thereby divided into low frequency components and high frequency components. By use of the two filters, however, the size of the circuit increases. The present invention has been made in consideration of the above situation, and has as its object to provide a digital type VSB modulation apparatus, which can generate a VSB modulation signal from a digital baseband signal with a simple arrangement. According to one aspect of the present invention, there is provided a digital type VSB modulation apparatus comprising: generation means having filter means for extracting a low-frequency component from a digitalized baseband signal, delay means for executing a delay process of delaying the baseband signal for a period of time corresponding to a processing time of the filter means, and subtracter means for obtaining a high-frequency component of the baseband signal by subtracting a low-frequency component extracted by the filter means from an output of the delay means, for generating an in-phase component of a modulation signal having a frequency spectrum having a high-frequency component whose level is a half of that of a low-frequency component thereof, and an orthogonal component of the modulation signal of a high-frequency component having a frequency spectrum whose level is a half of that of the low-frequency component; carrier wave component generation means for multiplying the in-phase component and the orthogonal component output from the generation means by carrier wave signals in order to obtain a carrier wave in-phase component and a carrier wave orthogonal component; and synthesis means for synthesizing the carrier wave in-phase component and the carrier wave orthogonal component output from the carrier component generation means in order to obtain a VSB modulation signal. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figs. 1A to 1E are graphs for explaining the principle of the present invention; Fig. 2 is a block diagram showing the first embodiment of a digital VSB modulation apparatus according to the present invention; Figs. 3A to 3E, and Figs. 4A to 4C are graphs for explaining the operation of the embodiment shown in Fig. 2; Figs. 5 and 6 are block diagrams showing arrangements of partial modifications of the first embodiment; Fig. 7 is a block diagram showing the second embodiment of a digital type VSB modulation apparatus according to the present invention; Figs. 8 and 9 are block diagrams showing arrangements of partial modifications of the second embodiment. Prior to the description of an embodiment of the present invention, the principle of the present invention will be described below. More specifically, Fig. 1A shows the frequency spectrum of a VSB modulation signal. In this VSB modulation signal, DSB (double sideband) modulation is performed for low-frequency components of a baseband signal falling within a range between 0 and f L (the lower limit of an occupied bandwidth) Hz, and SSB modulation (single sideband) modulation is performed for high-frequency components of the baseband signal falling within a range between f L and f H (the upper limit of the occupied bandwidth) Hz. For this reason, the VSB modulation signal can be expressed as a function S(t) of time t as follows: S(t) = (1 + kcos2πf 1 t)cos2πfct + k 2 cos2π(fc + f 2 )t where k is the modulation index, fc is the carrier frequency, and f 1 and f 2 are the modulation signal frequencies. These frequencies satisfy relations 0 < f 1 ≦ f L and f L ≦ f 2 < f H . The first term of the right-hand side of equation (1) corresponds to the DSB modulation signal, and the second term thereof corresponds to the SSB modulation signal. Fig. 1B shows the frequency spectrum of the DSB modulation signal, and Fig. 1C shows the frequency spectrum of the SSB modulation signal. Equation (1) can be converted as follows: S(t) = (1 + kcos2πf 1 t)cos2πfct + k 2 (cos2πf 2 t·cos2πfct - sin2πf 2 t·sin2πfct) = (1 + kcos2πf 1 t + k 2 cos2πf 2 t)cos2πfct - k 2 sin2πfct·sin2πf 2 t The first term of the right-hand side of equation (2) represents a carrier in-phase component, and the second term thereof represents a carrier orthogonal component. Fig. 1D shows the frequency spectrum of the carrier in-phase component, and Fig. 1E shows the frequency spectrum of the carrier orthogonal component. As can be understood from these figures, a synthesized spectrum of the carrier in-phase component and the carrier orthogonal components becomes the frequency spectrum of the VSB modulation signal shown in Fig. 1. An embodiment of the present invention based on the above-mentioned principle will be described below with reference to the accompanying drawings. In Fig. 2, reference numeral 11 denotes an input terminal, which is supplied with a baseband signal digital-converted at a sampling frequency fs. The sampling frequency fs satisfies relations fs > 2f H and fs > 2fc. The digital baseband signal supplied to the input terminal 11 is multiplied with the modulation index k by a multiplier 12 so as to be converted to a frequency spectrum shown in Fig. 3A. Thereafter, the baseband signal is supplied to a distributor 13. One output of the distributor 13 is supplied to an LPF (low-pass filter) 14, so that the high-frequency components of the baseband signal are cut, thereby extracting low-frequency components falling within a range between 0 and f L Hz, as shown in Fig. 3B. The output from the LPF 14 is distributed to two outputs by a distributor 15, and one output is supplied to the negative input terminal "-" of a subtracter 16. The other output from the distributor 13 is delayed by a processing time of the LPF 14 by a delay circuit 17, and the delayed output is supplied to the positive input terminal "+" of the subtracter 16. The subtracter 16 subtracts the low-frequency components of the baseband signal (Fig. 3B) supplied to its negative input terminal "-" from the original baseband signal (Fig. 3A) supplied to its positive input terminal "+", thereby extracting high-frequency components of the baseband signal falling within a range between f L and f H Hz, as shown in Fig. 3C. The high-frequency baseband signal output from the subtracter 16 is multiplied with a fixed coefficient "0.5" by a multiplier 18 so as to be converted into a frequency spectrum shown in Fig. 3D. Thereafter, the baseband signal is supplied to a distributor 19. One output from the distributor 19 is supplied to an orthogonal transformer 20. The orthogonal transformer 20 has a function, known as the Hilbert transformation, of shifting the phase of a signal in the entire target frequency band through 90°. More specifically, if the input to the orthogonal transformer 20 is given by: (k/2)·cos2π·f 2 ·t then, the transformer transforms its output to: -(k/2)·sin2π·f 2 ·t The other output from the distributor 19, which output is given by: (k/2)·cos2π·f 2 ·t is delayed by a processing time of the orthogonal transformer 20 by a delay circuit 23, and the delayed output is supplied to one input terminal of an adder 22. The other output from the distributor 15, which output is given by: k·cos2π·f 1 ·t is delayed by a processing time of the orthogonal transformer 20 by a delay circuit 23, and the delayed output is supplied to the other input terminal of the adder 22. The adder 22 adds the low-frequency baseband signal shown in Fig. 3B to the high-frequency baseband signal shown in Fig. 3D, thereby generating a baseband signal having a frequency spectrum shown in Fig. 3E. The output from the adder 22 is added to a constant "1" by an adder 24, and the sum output is multiplied, by a multiplier 25, with a carrier component given by: cos2π·fc·t As a result, a carrier in-phase component shown in Fig. 4A, i.e., the first term of the right-hand side of equation (2) described above, is obtained, and is supplied to one input terminal of an adder 26. The output from the orthogonal transformer 20, which output is given by: -(k/2)·sin2π·f 2 ·t is multiplied, by a multiplier 27, with the carrier component given by: sin2π·fc·t As a result, a carrier orthogonal component shown in Fig. 4B, i.e., the second term of the right-hand side of equation (2) described above is obtained, and is supplied to the other input terminal of the adder 26. Upon multiplication with the carrier components: cos2π·fc·t and sin2π·fc·t the spectrum levels at two sides of a carrier become half the spectrum level of the original baseband signal. The adder 26 adds the carrier in-phase component and the carrier orthogonal component shown in Figs. 4A and 4B, thereby generating a VSB modulation signal having a frequency spectrum shown in Fig. 4C. Thereafter, the VSB modulation signal output from the adder 26 is converted into an analog signal by a D/A (digital-to-analog) converter 28, and is output through an output terminal 29. The VSB modulation signal is then supplied to a power amplifier 10 for, e.g., transmission. Therefore, according to the arrangement of the above embodiment, since the carrier in-phase component and the carrier orthogonal component are generated from a digital baseband signal, and these components are added to each other to obtain the VSB modulation signal, the arrangement can become very simple and compact as compared with the prior art wherein circuit sections are simply replaced with digital circuits having the same functions. The embodiment shown in Fig. 2 exemplifies a case wherein the condition fs > 2fc is satisfied. Fig. 5 shows a modification when this condition is not satisfied. In the following description, the same reference numerals in Fig. 5 denote the same parts as in Fig. 2. In Fig. 5, interpolators 30 and 31 are respectively inserted between the adder 24 and the multiplier 25, and between the orthogonal transformer 20 and the multiplier 27. These interpolators 30 and 31 have a function of converting the sampling frequency fs to a frequency corresponding to an integer multiple of the frequency fs while preserving the spectrum of an input signal. If an output sampling frequency fs 1 of the interpolators 30 and 31 is set to satisfy: fs1 > 2fc the above embodiment can also be applied to a case wherein the carrier frequency fc is high, and the sampling frequency fs of the baseband signal is lower than a frequency twice the carrier frequency fc. Fig. 6 shows another modification of the embodiment shown in Fig. 2. In Fig. 6, the outputs from the adder 24 and the orthogonal transformer 20 are converted into analog signals respectively through D/A (digital-to-analog) converters 32 and 33. For this reason, the multipliers 25 and 27 and the adder 26 after the D/A converters 32 and 33 are constituted by analog circuits, and the D/A converter 28 after the adder 26 is omitted. In this case, the multipliers 25 and 27 and the adder 26 have a function equivalent to that in digital processing, and the adder 26 can output a VSB modulation signal given by equation (2) described above. The analog multipliers 25 and 27 can be easily realized by balanced modulators, and the analog adder 26 can be easily realized by a resistor mixing circuit, a transformer, or the like, thus simplifying the arrangement. Fig. 7 shows the second embodiment of the present invention. More specifically, reference numeral 34 denotes an input terminal, which is supplied with a baseband signal digital-converted at a sampling frequency fs. The sampling frequency fs satisfies relations fs > 2f H and fs > 2fc. The digital baseband signal supplied to the input terminal 34 is multiplied with a modulation index k/2 by a multiplier 35, and is then supplied to a distributor 36. One output from the distributor 36 is supplied to an LPF 37, so that the high-frequency component of the baseband signal are cut, thereby extracting low-frequency components falling within a range between 0 and f L Hz. The output from the LPF 37 is distributed to two outputs by a distributor 38, and one output is supplied to the negative input terminal "-" of a subtracter 39. The other output from the distributor 36 is delayed by a processing time of the LPF 37 by a delay circuit 40, and the delayed output is distributed to two outputs by a distributor 41. One output from the distributor 41 is supplied to the positive input terminal "+" of the subtracter 39. The subtracter 39 subtracts the low-frequency components of the baseband signal from the original baseband signal supplied to its positive input terminal "+", thereby extracting high-frequency components of the baseband signal falling within a range between f L and f H Hz. The high-frequency baseband signal output from the subtracter 39 is supplied to an orthogonal transformer 42. The orthogonal transformer 42 has a function, known as, e.g., the Hilbert transformation, of shifting the phase of a signal in the entire target frequency band through 90°. More specifically, if the input to the orthogonal transformer 20 is given by: (k/2)·cos2π·f 2 ·t then, the transformer transforms its output to: -(k/2)·sin2π·f 2 ·t The other outputs from the distributors 38 and 41 are added by an adder 43, thus generating a baseband signal having the frequency spectrum previously shown in Fig. 3E. The baseband signal is delayed by a processing time of the orthogonal transformer 42 by a delay circuit 44, and the delayed signal is added to a constant "1" by an adder 45. Thereafter, the sum signal is multiplied, by a multiplier 46, with a carrier component given by: cos2π·fc·t As a result, a carrier in-phase component shown in Fig. 4A, i.e., the first term of the right-hand side of equation (2) described above is obtained, and is supplied to one input terminal of an adder 47. On the other hand, the output from the orthogonal transformer 42, which output is given by: -(k/2)·sin2π·f 2 ·t is multiplied, by a multiplier 48, with a carrier component given by: sin2π·fc·t As a result, a carrier orthogonal component shown in Fig. 4B, i.e., the second term of the right-hand side of equation (2) described above, is obtained, and is supplied to the other input terminal of the adder 47. The adder 47 adds the carrier in-phase component and the carrier orthogonal component shown in Figs. 4A and 4B, thereby generating a VSB modulation signal having a frequency spectrum shown in Fig. 4C. Thereafter, the VSB modulation signal output from the adder 47 is converted into an analog signal by a D/A converter 49, and is output through an output terminal 50. The VSB modulation signal is supplied to a power amplifier for transmission. Therefore, according to the second embodiment shown in Fig. 7, since the carrier in-phase component and the carrier orthogonal component are generated from a digital baseband signal, and these components are added to each other to obtain the VSB modulation signal, the arrangement can become very simple and compact as compared with the prior art wherein circuit sections are simply replaced with digital circuits having the same functions. The second embodiment shown in Fig. 7 exemplifies a case wherein the condition fs > 2fc is satisfied. Fig. 8 shows a modification when this condition is not satisfied. In the following description, the same reference numerals in Fig. 8 denote the same parts as in Fig. 7. In Fig. 8, interpolators 51 and 52 are respectively inserted between the adder 45 and the multiplier 46, and between the orthogonal transformer 42 and the multiplier 48. These interpolators 51 and 52 have a function of converting the sampling frequency fs to a frequency corresponding to an integer multiple of the frequency fs while preserving the spectrum of an input signal. If an output sampling frequency fs 1 of the interpolators 51 and 52 is set to satisfy: fs1 > 2fc the above embodiment can also be applied to a case wherein the carrier frequency fc is high, and the sampling frequency fs of the baseband signal is lower than a frequency twice the carrier frequency fc. Fig. 9 shows another modification of the second embodiment shown in Fig. 7. In Fig. 9, the outputs from the adder 45 and the orthogonal transformer 42 are converted into analog signals respectively through D/A converters 53 and 54. For this reason, the multipliers 46 and 48 and the adder 47 after the D/A converters 53 and 54 are constituted by analog circuits, and the D/A converter 49 after the adder 47 is omitted. In this case, the multipliers 46 and 48 and the adder 47 have a function equivalent to that in digital processing, and the adder 47 can output a VSB modulation signal given by equation (2) described above. The analog multipliers 46 and 48 can be easily realized by balanced modulators, and the analog adder 47 can be easily realized by a resistor mixing circuit, a transformer, or the like, thus simplifying the arrangement. In each of the above embodiments described above, the low-frequency components are extracted from the original baseband signal by cutting the high-frequency components, and the extracted low-frequency components are subtracted from the original baseband signal, thereby obtaining the high-frequency components. In order to separate the baseband signal into low- and high-frequency components, it can be passed through low- and high-pass filters. The VSB modulation signal generated like in each of the above embodiments is amplified by the power amplifier 10. The present invention is not limited to the above embodiments, and various other changes and modifications may be made within the scope of the appended claims.

Claims: A digital type VSB modulation apparatus comprising: generation means (11-22; 34-44) having filter means (14; 37) for extracting a low-frequency component from a digitalized baseband signal, the generation means outputting an in-phase component and an orthogonal component of a modulation signal; carrier wave component generation means (24, 25, 27; 45, 46, 48) for multiplying the in-phase component and the orthogonal component output from said generation means (11-22, 34-44) by carrier wave signals in order to obtain a carrier wave in-phase component and a carrier wave orthogonal component; and synthesis means (26; 47) for synthesizing the carrier wave in-phase component and the carrier wave orthogonal component output from said carrier wave component generation means (24, 25, 27; 45, 46, 48) in order to obtain a VSB modulation signal, characterized in that the generation means (11-22; 34-44) further comprises: delay means (17; 40) for executing a delay process of delaying said baseband signal for a period of time corresponding to a processing time of said filter means (14; 37), and subtracter means (16; 39) for obtaining a high-frequency component of the baseband signal by subtracting a low-frequency component extracted by said filter means (14; 37) from an output of said delay means (17; 40), for generating the in-phase component of the modulation signal having a frequency spectrum having a high-frequency component whose level is a half of that of a low-frequency component thereof, and the orthogonal component of the modulation signal of a high-frequency component having a frequency spectrum whose level is a half of that of the low-frequency component. The apparatus according to claim 1, characterized in that said generation means (11-22) comprises: first multiplier means (12) for multiplying the baseband signal by a predetermined modulation index: filter means (14) for extracting a low-freauency component of an output of said first multiplier means (12); delay means (40) for executing a delay process of delaying said baseband signal for a period of time corresponding to a processing time of said filter means (14); subtracter means (16) for obtaining a high-frequency component of the baseband signal by subtracting an output of said filter means (14) from an output of said delay means (17); second multiplier means (18) for multiplying an output level of said first subtracter means (16) by 1/2; orthogonal transform means (20) for executing an orthogonal transformation process of an output of said second multiplier means (18) to obtain the orthogonal component; and first adder means (22) for adding an output of said second multiplier means (18) and an output of said filter means (14) to obtain the in-phase component. The apparatus according to claim 2, characterized in that said carrier component generation means (24, 25, 27) comprises: second adder means (24) for adding an output from said first adder means (22) to a constant 1; third multiplier means (25) for multiplying an output from said second adder means (24) with a carrier signal; and fourth multiplier means (27) for multiplying an output from said orthogonal transformer means (20) with a carrier signal having a 90° phase difference from the carrier signal multiplied by said third multiplier means (25). The apparatus according to claim 3, characterized by further comprising: interpolators (30, 31), inserted between said second adder means (24) and said third multiplier means (25), and between said orthogonal transformer-means (20) and said fourth multiplier means (27), for converting a sampling frequency so as to generate the VSB modulation signal even when the sampling frequency of the baseband signal is lower than a frequency twice a carrier frequency. The apparatus according to claim 3, characterized by further comprising: digital-to-analog converter means (32, 33), arranged between said second adder means (24) and said third multiplier means (25), and between said orthogonal transformer means (20) and said fourth multiplier means (27), so that said third multiplier means (25), said fourth multiplier means (27), and said synthesis means (26) are constituted by analog circuits. The apparatus according to claim 1, characterized in that said generation means (34-44) comprises: first multiplier means (35) for multiplying the baseband signal by a predetermined modulation index; filter means (37) for extracting a low-frequency component of an output of said first multiplier means (35); delay means (40) for executing a delay process of delaying said baseband signal for a period of time corresponding to a processing time of said filter means (37); subtracter means (39) for obtaining a high-frequency component of the baseband signal by subtracting an output of said filter means (37) from an output of said delay means (40); orthogonal transform means (42) for executing an orthogonal transformation process of an output of said subtractor means (39) to obtain the orthogonal component; and first adder means (43) for adding an output of said delay means (40) and an output of said filter means (37) to obtain the in-phase component. The apparatus according to claim 6, characterized in that said carrier component generation means (45, 46, 48) comprises: second adder means (45) for adding an output from said first adder means (43) to a constant 1; second multiplier means (46) for multiplying an output from said second adder means (45) with a carrier signal; and third multiplier means (48) for multiplying an output from said orthogonal transformer means (42) with a carrier signal having a 90° phase difference from the carrier signal multiplied by said second multiplier means (46). The apparatus according to claim 7, characterized by further comprising: interpolators (51, 52), inserted between said second adder means (45) and said second multiplier means (46), and between said orthogonal transformer means (42) and said third multiplier means (48), for converting a sampling frequency so as to generate the VSB modulation signal even when the sampling frequency of the baseband signal is lower than a frequency twice a carrier frequency. The apparatus according to claim 7, characterized by further comprising: digital-to-analog converter means (53, 54) arranged between said second adder means (45) and said second multiplier means (46), and between said orthogonal transformer means (42) and said third multiplier means (48), so that said second multiplier means (46), said third multiplier means (48), and said synthesis means (47) are constituted by analog circuits.

Inventors: AOKI TOSHIO, HAMADA MASATOSHI, HASHIMOTO YASUHIRO, ISHIKAWA TATSUYA, KAWAI NOBUHIKO, MIKI NOBUYUKI, TAKAHASHI YASUO, UEOKA KENJI, AOKI, TOSHIO, HAMADA, MASATOSHI, HASHIMOTO, YASUHIRO, ISHIKAWA, TATSUYA, KAWAI, NOBUHIKO, MIKI, NOBUYUKI, TAKAHASHI, YASUO, UEOKA, KENJI, Aoki, Toshio, c/o Intellectual Property Div., ISHIKAWA, TATSUYA, C/O INTELLECTUAL PROPERTY DIV., Miki, Nobuyuki, c/o Intellectual Property Div., Takahashi, Yasuo, c/o Intellectual Property Div.
Applicants: JAPAN BROADCASTING CORP, TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA, NIPPON HOSO KYOKAI
Application year: 1992
Main classification: H04L27
IPC codes: H04L27, H03C1