Sampling frequency down-converting apparatus

Abstract: The present invention is directed to a sampling frequency down-converting apparatus which low-pass converts the digital picture signal sampled through the sampling frequency of 8 Fsc by the thinning out operation into the digital picture signal sampled by 4 Fsc, and a sampling frequency up-converting apparatus which high-pass converts the digital picture signal of the 4 Fsc into the 8 Fsc by the interpolation. If the signal is wider in the required zone than the conventional NTSC signal and the PAL signal like the ATV signal, the transmission and the recording may be effected without the deterioration of the frequency characteristics.

Description: The present invention generally relates to a sampling frequency down-converting apparatus, which make it possible to effect recording operation on a recording medium as the digital signals of the comparatively low sampling frequencies or to effect transmission operations among the digital picture apparatuses in spite of the increase in the required frequency zone from the signals of the NTSC system (hereinafter referred to as NTSC signal) and the signal of the PAL system (hereinafter referred to as PAL signal), in the signals of systems called ATV (Advanced Television) system, HD-MAC (High Definition - Multiplexed Analogue Components) system and so on especially even among the television signals, namely, of the next generation television system which is designed so that the aspect ratio 3 : 4 of the NTSC system of the current television signal system or of the PAL system may be made as sideways long as 9 : 16, and the resolution may be improved. The NTSC signals only will be described hereinafter now that the same things may be said even about the PAL signals. Generally, in a digital picture apparatus such as a digital switcher handling the NTSC signals, a digital video tape recorder (hereinafter referred to as D - VTR) or the like, the NTSC signals of the analog are sampled four times the color sub-carrier frequency Fsc, i.e., 4 Fsc. From Fsc ≒ 3.58 MHz, the sampling frequency becomes 4 Fsc ≒ 14.3 MHz. At this time, the frequency zone of the NTSC signals to be digitalized is almost determined by a pre-low-pass filter (hereinafter referred to as LPF) provided on the input side of an A/D converter. Normally, the flat portion of the frequency characteristics of the LPF is to approximately 5.5 MNz from 0 (direct current). Since the required frequency zone of the NTSC signals is 4.2 MHz, the NTSC signals are digitalized with surplus by the sampling of 4 Fsc. An ACTV (Advanced Compatible Television) system is recently worth notice among an ATV system for improving the resolution with the aspect ratio of the above-described NTSC system some systems being made long sideways from 3 : 4 to 9 : 16, and some system. The ACTV system only is described hereinafter now that the same thing can be said even about the other ATV system. In the ACTV system, the field frequency and the color sub-carrier frequency are the same as in the NTSC system. But in the horizontal scanning frequency, there are a case where there is 15.75 / 1.00 1 kHz as that of the NTSC system, and a case where there is 31.5 / 1.00 1 kHz which is twice 15.75 so as to effect 525 sequential scannings for each one field. The former is called ACTV - E (Advanced Compatible Television - Entry) system, while the latter is called ACTV - 1. The frequency zone of the ACTV - E system increases by the oblong portion and the horizontal resolution improvement portion as compared with that of the NTSC system. In order to have sideways long portion, the required frequency zone 4.2 MHz of the NTSC system increases as follows. (16 / 9) / (4 / 3) X 4.2 MHz = (4 / 3) X 4.2 = 5.6 MHz In order to further improve the horizontal resolution as compared with that of the NTSC system, the ACTV - E system finally requires the flat frequency zone of 7 MHz. Also, in the case of ACTV - 1 system, the zone increases twice to effect the sequential scannings with respect to the interlace scanning of the ACTV - E system, thus resulting in 14 MHz. In a case of digitalizing the signals (hereinafter referred to as ACTV signals) of such an ACTV system, in the sampling of the conventional 4 Fsc, the zone to be converted is approximately 5. 5 MHz as described hereinabove and is insufficient. Therefore, it is impossible to serve a double purpose the digital picture apparatus of the 4 Fsc sampling for the conventional NTSC system use, which is the original purpose of the ACTV system. Thus, conventionally it has been considered to increase the sampling frequency like 5 Fsc or 6 Fsc. But they are both in a direction of increasing the data rate. The D - VCR for recording them reduces the recording time as compared with the NTSC system in the recording operation on the recording medium of the same capacity. In our time of longer hours' image transcription, it is a big task to be solved for the D - VTR which records the signals in the next generation television system. US-A-4 302 776 discloses a sampling frequency down converting apparatus comprising a means for sampling original digital picture signals at a sampling frequency of 4 Fsc so as to thin-out the sampled picture signals and a means for low-pass converting the sampled picture signals into intermediate digital picture signals having a sampling frequency of 2 Fsc. SUMMARY OF THE INVENTION Accordingly, an essential object of the present invention is to provide an apparatus which provides affinity to the digital picture apparatus of the conventional 4 Fsc sampling, and is able to effect the sampling for satisfying the required frequency zone of the ACTV. In accomplishing the above-described object, according to one aspect of the present invention, there is provided a sampling frequency down-converting apparatus comprising: a first delay means for delaying by 262H an original digital picture signal having a sampling frequency of N Fsc (Fsc: color sub-carrier frequency and N is an integer = 8 or 16); a second delay means for delaying by 262H the output of the first delay means; a first filter for low-pass filtering the output of the first delay means; a second filter for band-pass filtering the original digital picture signal; a third filter for band-pass filtering the output of the second delay means; a first adder for adding the output of the first filter, the output of the second filter and the output of the third filter; a third delay means for delaying the output of the first delay means by n samples (n is a positive integer); a second adder for adding the output of the first adder and the output of the thid delay means; and a means for thinning out a processed original digital picture signal having a sampling frequency of N Fsc which is the output of the second adder into an intermediate digital picture signal, which has a sampling frequency of approximately N/2 Fsc; wherein the sampling processes occurring until the output of the second adder are made at a sampling frequency of N Fsc. Further advantageous embodiments are defined in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, in which; Fig. 1 is a block diagram of a preferred embodiment in a sampling frequency down-converting apparatus of the present invention; Fig. 2 is a detailed block diagram in the embodiment of a BPF of Fig. 1; Fig. 3 is a detailed block diagram in the embodiment of a LPF of Fig. 1; Fig. 4 shows a frequency spectrum in each type of sampling; Fig. 5 shows a two-dimensional model view of the various types of samplings; and Fig. 6 shows a frequency characteristic graph of the BPF and LPF of Fig. 1. DETAILED DESCRIPTION OF THE INVENTION Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. Referring now to the drawings, there is shown in Fig. 1, a block diagram in one embodiment of a sampling frequency down-converting apparatus in the present invention. An original digital picture signal which has sampled a signal (hereinafter referred to as ACTV - E signal) of the ACTV - E system by a 8 Fsc is inputted into a terminal 1. This signal is inputted into a band pass filter (hereinafter referred to as BPF 2) and also, is line shifted (is delayed by the number of the samples of 262H portion) by each 262H by two 262H delayers 3, 5. The respective outputs are inputted into a LPF 4, a BPF 6. Then, the respective outputs of the BPF 2, the LPF 4, the BPF 6 are added by an adder 8, and also, are delayed by one clock by a data latching circuit (D) 9. After the output of the 262H delayer 3 has been delayed by n clock (n is a positive integer) for the timing adjustment by the n data latching circuit (nD), it is added to the output of the data latching circuit 9 by the adder 10. The added output is delayed by one clock by the data latching circuit (D) 11. In accordance with the field identification signal (FLD) for distinguishing two fields in one frame, the addition output of the adder 10 or the delay output of the data latching circuit 11 is alternately selected for each field to output it (the output means of the FLD is not shown). As shown in the same drawing, the data latching circuits 7, 9, 11 provided so far are all latched by the clock of 8 Fsc from the terminal 16 (one sample is delayed by one clock). Then, the output of the MUX 12 is inputted into the data latching circuit (D) 13 and is delayed by one clock by the clock of the 4 Fsc. The clock of the 4 Fsc makes the clock of 8 Fsc from the terminal 16 through the frequency division by the counter 17. The output of the data latching circuit 13, together with the clock of 4 Fsc of the terminal 15 as the intermediate digital picture signal of the sampling frequency of the approximately 4 Fsc, is outputted into the digital interface (not shown) from the terminal 14. The means for thinning out the original digital picture signals is composed of the data latching circuit 11, the MUX 12, the data latching circuit 13. It is to be noted that the clocks of 8 Fsc to be inputted into the 262H delayers 3, 5 and the BPFs 2, 6 and the LPF 4 are also the clocks from the terminal 16. The BPF 2 and BPF 6 are completely the same in construction. The detailed block diagram in one embodiment thereof is shown in Fig. 2. In Fig. 2, only the data latching circuit (D) 21 is a circuit for delaying by one clock. The other data latching circuits (2D) 23, 25, 27, 29, 31, 33 are the circuits for delaying two clocks. The signals from the terminal 20 are shifted by these data latching circuits. The output of each data latching circuit is multiplied respectively by coefficients -1, 4, -7, 8, -7, 4, -1 by the coefficient multipliers 22, 24, 26, 28, 30, 32, 34. Seven outputs of these coefficient multipliers 22, 24, 26, 28, 30, 32, 34 are added by the adder 35 and are outputted into the terminal 36. Also, Fig. 3 is a detailed block diagram in one embodiment of the LPF 4. In Fig. 3, the signals from the terminal 40 are shifted by the data latching circuits (2D) 42, 44, 46, 48, 50, 52, 54 which delay two clocks. Also, the signal of the terminal 40 and the output of each data latching circuit is multiplied respectively by the coefficients 2, -3, -4, 37, 37, -4, -3, 2 by the coefficient multipliers 41, 43, 45, 47, 49, 51, 53, 55, and eight outputs of these coefficient multipliers 41, 43, 45, 47, 49, 51, 53, 55 are added by the adder 56 and are outputted into the terminal 57. The operation of the sampling frequency down-converting apparatus (see Fig. 1) composed of such BPF, LPF and so on will be illustrated by the use of Fig. 4, Fig. 5. Fig. 4 (a) through (c) are the frequency spectrum by the various types of samplings. The scale of the quadrature axis is shown in Fsc unit and MHz unit. Fig. 4 (a) shows a case where the sampling has been effected by the sampling frequency of 4 Fsc, wherein the zone of the analog ACTV signal to be sampled in advance by the LPF and so on before the analog/digital (A/D) conversion is effected is limited to 2 Fsc (7.16 MHz), thereafter the A/D conversion is effected, so that a Y signal component of base band spectrum 60 (oblique line portion surrounded in solid lines) containing a C signal component 61 around the color sub-carrier wave and a folded spectrum 62 (oblique line portion surrounded by dotted lines) around the 4 Fsc may not be overlapped with the 2 Fsc provided as the boundary. Fig. 4 (b) shows a case where the sampling has been effected by the sampling frequency of the 8 Fsc. Since the folded spectrum 64 around the 8 Fsc is sufficiently higher than the base band spectrum 63, the zone restriction is not required to be effected in advance as in the (a). The zone of the ACTV signal to be sampled may be extended over necessary 2 Fsc. Therefore, the flat frequency characteristics may be obtained till 2 Fsc, i.e., 7. 16 MHz as shown. Fig. 4 (c) shows the frequency spectrum in a case where the original digital signal sampled by the sampling frequency of the 8 Fsc as in the (b) is thinned out (sampling) every other sample and is converted into the digital signal (hereinafter referred to as intermediate digital signal) of approximately 4 Fsc of the sampling frequency. At this time, the base band spectrum 66 (right-rising oblique lines portion surrounded by solid lines) and the folded spectrum 68 (left-rising oblique line portion surrounded by dotted lines) are overlapped on each other in the piled up region 67 (cross portion). Since the frequency spectrum of the picture signal normally has the energies concentrated at the interval between the horizontal frequency Fh and the field frequency Fv, the base band spectrum 66 and the folded spectrum 68 become the relation of the frequency interleaving just in the piled up region 67 if the sub-sampling frequency for obtaining the intermediate digital signal is shifted by Fh/2 or Fv/2 with respect to the 4 Fsc. In order to restore it into the analog ACTV signal, the frequency characteristics may provide the flat ACTV signal as far as the 2 Fsc if the digital-along conversion (D/A) is effected after the signal has been restored to the original digital signal of the sampling frequency 8 Fsc through the interpolation of the thinned out samples. Normally the color sub-carrier wave is selected into a proper frequency (odd times of one half of the fH), the C signal is modulated by the color sub-carrier wave to effect the frequency interleaving. Thus, the piled up region 67 is better not to be applied upon the zone of the C signal component 65. The sub-sampling method will be described hereinafter by the use of Fig. 5. In Fig. 5, the sampling position of the two field portions and the thinning out position are represented respectively " * " and " . " in the two-dimensional terms. The solid lines and the dotted lines respectively show scanning lines. The difference in the field is represented by the difference between the solid lines and the dotted lines. Fig. 5 (a) shows a case of the sampling by the sampling frequency of the 8 Fsc. Fig. 5 (b) shows a case of the sampling by the sampling frequency of the 4 Fsc to be thinned out for every other sample from the (a). Fig. 5 (c) shows a case of a so-called line offset type, where one sample is shifted every other line in the thinning out method. Fig. 5 (d) is a case of a so-called field offset type, where one sample is shifted every other field in the thinning out method. At this time, the sampling frequency of (c) is shifted by fh/2 from the 4 Fsc, while the sampling frequency of the (d) is shifted by fv/2 from the 4 Fsc. One embodiment of the sampling frequency down-converting apparatus in the present invention of Fig. 1 realizes the field offset type of Fig. 5 (d). Especially it is not restricted to this. In Fig. 1, a circuit from the terminal 1 to the output of the adder 10 is a so-called zone restricting filter. When the original digital signal has been sub-sampled by the sampling frequency of approximately 4 Fsc so as to become such a frequency spectrum as shown by Fig. 4 (c), the component of the folded spectrum 68 in the piled up region 67 is previously restricted in zone by the chasing filter of the Fv period only in only the passing zone of the BPF so that the component distribution is not be leaked into the component within the base band spectrum 66 separated by Fv/2. The frequency characteristics of the zone restricting filter (composed of the terminal 1 through the adder 10) is shown in Fig. 6 in a case where the BPFs 2, 6 and the LPF 4 shown in Fig. 2, Fig. 3 are used. The frequency characteristics shown in the dotted lines become the direct current (DC) in the 2 Fsc (7.16 MHz) of the central frequency, and zero in the Fsc. Thus, the bad influence to be applied upon the C signal component becomes minimum. The chasing filter in the passing zone of the BPF is omitted, since the filtering is effected at the fv period, which is difficult to illustrate. Namely, the envelope of the frequency characteristics of the chasing filter shape looks as if it is the frequency characteristics of the BPF shown in the dotted lines. The overall characteristics containing the LPF shown in the solid lines become the flat characteristics of approximately 1 dB from the DC to the 2 Fsc. In the circuit from the output of the adder 10 to the terminal 14 of Fig. 1, the original signals sampled by the sampling frequency of the 8 Fsc are thinned out every other sample to generate the intermediate digital picture signal of approximately 4 Fsc in the sampling frequency. The field offset type is realized by the passing of the output of the data latching circuit 11 every other field in the MUX 12. If a circuit (which is composed of the terminal 1 through the adder 10) constituting the zone limiting filter is omitted here, the fundamental performance of sampling with the 4 Fsc the original digital signal sampled by the 8 Fsc in accordance with the present invention may be achieved. If the coefficients of the coefficient multipliers of the BPF and the LPF are set respectively -5, 16, -27, 32, -27, 16, -5, and 8, -10, -18, 148, 148, -18, -10, 8, the frequency characteristics are further improved and the circuit scale increases somewhat. In the above-described embodiment of the present invention, the 8 Fsc sampling of the ACTV - E signal has been considered the original digital signal, but the sampling frequency of the 8 Fsc is too low, because the required zone becomes 14 MHz in the ACTV - 1 signal. In the case of the ACTV - 1 signal, the signal sampled by the sampling frequency of the 16 Fsc is to be the original digital picture signal. It is low-pass converted into the intermediate digital picture signal of approximately 8 Fsc by the thinning out method (sub-sampling) of the field offset or the like. Also, the intermediate digital picture signal of the approximately 8 Fsc is restored into the original digital picture signal of the 16 Fsc by the interpolation. In the embodiment, all the clocks in the embodiment where the above-described original digital signal has been made the ACTV - E signal have only to be made twice. At this time, the central frequency of all the above-described BPFs becomes 4 Fsc. As described hereinabove, in accordance with the present invention, the digital interface provided between the digital picture apparatuses, by the use of the intermediate digital signal of approximately 4 Fsc, with respect to the ACTV - E signal requiring the flat signal zone of the 7 MHz, so that most of the digital picture apparatuses including a digital VTR using the sampling frequency of the conventional 4 Fsc may be used as they are. Furthermore, the recording time and the format of the DVCR may be provided as in a case where the recording signal is the NTSC signal. The flat signal may be provided as far as 2 Fsc = 7.16 MHz if the digital/analog is effected through the restoration into the original digital picture signal of the 8 Fsc by the interpolation in the restoration into the analog signal. Also, the realization may be effected likewise by the use of the intermediate digital signal of the approximately 8 Fsc with respect to the ACTV - 1 requiring the flat zone of the 14 MHz, so that the large effect may be expected.

Claims: A sampling frequency down-converting apparatus comprising: a first delay means (3) for delaying by 262H an original digital picture signal having a sampling frequency of N Fsc (Fsc: color sub-carrier frequency and N is an integer = 8 or 16); a second delay means (5) for delaying by 262H the output of the first delay means (3); a first filter (4) for low-pass filtering the output of the first delay means (3); a second filter (2) for band-pass filtering the original digital picture signal; a third filter (6) for band-pass filtering the output of the second delay means (5); a first adder (8) for adding the output of the first filter (4), the output of the second filter (2) and the output of the third filter (6); a third delay means (7) for delaying the output of the first delay means (3) by n samples (n is a positive integer); a second adder (10) for adding the output of the first adder (8) and the output of the third delay means (7); and a means (11, 12, 13) for thinning out a processed original digital picture signal having a sampling frequency of N Fsc which is the output of the second adder (10) into an intermediate digital picture signal, which has a sampling frequency of approximately N/2 Fsc; wherein the sampling processes occurring until the output of the second adder (10) are made at a sampling frequency of N Fsc. A sampling frequency down-converting apparatus in accordance with claim 1, wherein the central frequency of the frequency characteristics of the second filter (2) is made twice the color sub-carrier frequency. A sampling frequency down-converting apparatus in accordance with claim 2, wherein the characteristics of the second filter (2) becomes 0 in amplitude at the color sub-carrier frequency. A sampling frequency down-converting apparatus in accordance with claim 2, wherein the coefficients of the coefficient multiplier (22, 24, 26, 28, 30, 32, 34) constituting a second filter has coefficients which are made the seventh of -1, 4, -7, 8, -7, 4, -1, in the order of the flow of the original digital picture signal. A sampling frequency down-converting apparatus in accordance with claim 2, wherein the coefficient multiplier constituting the second filter (2) has coefficients which are made the seventh of -5, 16, -27, 32, -27, 16, -5 in the order of the flow of the signals.

Inventors: KOYA TOSHIAKI, SEKIMOTO KUNIO, SUESADA KUNIO, KOYA, TOSHIAKI, SEKIMOTO, KUNIO, SUESADA, KUNIO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: H04N7
IPC codes: H04N7, H04N9, H04N11, H04N5