Subrange-type A/D converter

Abstract: The subrange-type A/D converter includes A/D converters in multiple stages (204, 212), with their driving clock frequencies being in certain relationship so that they are used commonly for the A/D conversion of the high definition television signal and the A/D conversion of a test signal for detecting the transmission characteristics.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a subrange-type A/D converter used for a decoding equalizer, such as described e.g. in EP-A- 0 308 776 and which is suited to implement the automatic equalization of the transmission characteristics of the transmission system. DESCRIPTION OF THE RELATED ART The conventional technique for equalizing the transmission path characteristics for the transmission of sampled values produced by offset sampling, such as in the MUSE system, employs standard receivers on the part of the receiver and implements pre-equalization by an encoder on the part of the transmitter so that the receivers have satisfactory reproduction characteristics. However, this technique fails in the achievement of effective pre-equalization if there exists the variability of characteristics among the standard receivers. Another problem is that it necessitates distinct equalization when supplying a signal to different information media such as a BS (broadcasting satellite) system and a 22 GHz ground system. In the technique of automatic equalization of transmission characteristics for the analog signal transmission of high definition television signal, in which a certain unit impulse signal is inserted as a test signal in the television signal and the characteristics of the transmission path is detected through the subsequent signal processing as disclosed in JP-A-62172826, it necessitates an A/D converter of 16.2 MHz clock rate for the trunk system and an A/D converter of 32.4 MHz clock rate for the equalizing system. Subrange-type A/D converters are known in the art from e.g. US-A-4 008 468, US-A-4 122 439 and DE-A-2 905 980. SUMMARY OF THE INVENTION It is an object of this invention to provide a subrange-type A/D converter used for the above-mentioned decoding equalizer and operative to implement both the A/D conversion of the television signal and the A/D conversion of the test signal for automatic equalization without increasing the complexity of the circuit arrangement. This object is achieved by the features indicated in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing an embodiment of a decoding equalizer as described in EP-A- 0 308 776 ; Fig. 2 is a block diagram of a further example of a decoding equalizer described in EP-A-0 308 776; Fig. 3 is a block diagram showing the overall arrangement of the subrange-type A/D converter suitable for the above indicated decoding equalizers; Figs. 4 and 5 are block diagrams showing circuit sections added to the subrange-type A/D converter; Fig. 6 is a waveform diagram showing, as an example, the unit impulse waveform signal for equalization; and Fig. 7 is a waveform diagram showing, as an example, the reception conversion waveform for equalization. DESCRIPTION OF A PREFERRED EMBODIMENT An embodiment of the invention will be described in detail with reference to the drawings. The embodiment will be described by taking an example of the MUSE transmission system developed for the high definition television signal, which is an analog signal transmission system for transmitting the analog signal obtained by offset sampling between frames, fields and lines. Fig. 1 is a block diagram showing an embodiment of a decoding equalizer. In Fig. 1, indicated by 1 is an A/D converter which samples the input analog signal in MUSE system at a frequency of 32.4 MHz. 2 is a sub-sampling circuit which converts the output signal of the A/D converter 1 into a signal of a trunk sampling clock rate of 16.2 MHz (or Ms/s: mega-samples per second). 3 is a delay circuit, and 4 is an adder. 5 is an equalizing filter with a variable tap, and is used to produce an equalization correction value. 6 is a VIT signal extraction circuit which extracts the VIT (Vertical Interval Test) signal with an impulse response waveform for measuring the distortion of the transmission path by being inserted in advance in the trunk system signal. 7 is a CPU which receives signal waveform data provided by the signal extraction circuit 6 and compares it with stored ideal waveform data to evaluate the distortion, and controls the variable tap of the equalizing filter 5 so that it produces the equalization correction value. The correction value is added to the trunk system signal by the adder 4, and the transmission characteristics of the transmission line are equalized. Fig. 2 is a block diagram showing another embodiment of a decoding equalizer. In Fig. 2, components identical to those of Fig. 1 are referred to by the same symbols, and their explanation will not be repeated. Indicated by 8 is an A/D converter which samples the input signal at 16.2 MHz, and its output signal is used for the trunk system. 9 is an A/D converter which samples the input signal at 32.4 MHz, and it is used as a circuit component of the equalization system. Figs. 3, 4 and 5 show, as an example, the arrangement of the subrange-type A/D converter suitable for the foregoing decoding equalizer. The conventional subrange-type A/D converter generally includes two A/D converters of 16 MHz clock rate and 32 MHz clock rate, whereas the inventive subrange-type A/D converter covers the A/D conversion for both the VIT signal and high definition television signal, resulting in a smaller scale of circuit. Generally, each A/D conversion stage of the subrange-type A/D converter needs to operate faster than the operating speed of the overall converter. It is relatively easy to design a high-speed A/D converter with a relatively low resolution, e.g. 6 bits or less. Accordingly, a high-resolution A/D converter of around 10 bits used for the subsampling transmission of high definition television signal is arranged favorably in the subrange type with each A/D conversion stage having a resolution of 6 bits or less, whereby a high-speed, high-resolution A/D converter is realized relatively simply. The subrange-type A/D converter of this embodiment utilizes the above-mentioned advantages of the subrange type. As shown in Fig. 3, the fore-stage A/D converter 204 for converting the high-order 6 bits out of 10-bit conversion output digital signal, for example, has its driving clock rate set to 32.4 MHz twice the television signal sampling clock rate 16.2 MHz for the trunk system, and the back-stage A/D converter 212 for converting the low-order 5 bits has its driving clock rate left unchanged at 16.2 MHz, so that the 32.4 MHz A/D converter 204 is used commonly for the conversion of upper digits of television signal and the formation of the signal for correcting the transmission characteristics. Accordingly, the A/D converter of this embodiment is capable of high-speed A/D conversion at as high resolution as around 10 bits, for example, and the accuracy of A/D conversion is determined from the input to the back-stage A/D converter 212. Eventually, the accuracy is determined by the A/D converter 212 driven at a 16.2 MHz clock rate, and therefore it is identical to the conventional subrange-type A/D converter operating at a 16.2 MHz clock rate for both the fore stage and back stage. Next, the arrangement and operation of the A/D converter shown in Fig. 3 will be explained. Clock pulses of 32.4 MHz and 16.2 MHz for driving the upper and lower conversion stages, respectively, are produced as shown in Fig. 4. A clock generator (not shown) provides on a terminal 219 a clock of 16.2 MHz having a duty cycle below 50%, which is fed to an exclusive-OR gate 221 directly and also through a delay element 220, and a 32.4 MHz clock is produced at the output of the gate 221. The 16.2 MHz clock is delivered through another exclusive-OR gate 222. The 32.4 MHz and 16.2 MHz clocks are fed through respective OR gates 223 and 225, and led out of the converter on output terminals 224 and 226 for external uses, as shown in Fig. 5. In the subrange-type A/D converter shown in Fig. 3, an analog input signal such as the high definition television signal on the input terminal 201 is introduced to a sampler 202, by which the signal is sampled by the 32.4 MHz clock supplied on the clock terminal h. The sampled signal level held by a capacitor 203 for one sampling period is fed to the A/D converter 204, by which the voltage level is converted into a high-order 6-bit digital signal by being driven by the 32.4 MHz clock supplied on the clock terminal h. The high-order 6-bit digital signal is led out through an OR gate 205 on an output terminal 206 as a digital test waveform signal at a 32.4 MHz clock rate made up of unit impulses as shown in Fig. 6 which is an example of the digital test waveform signal against the analog test signal having a waveform shown in Fig. 7 applied to the input terminal 201. The high-order 6-bit conversion output signal of 32.4 MHz provided by the fore-stage A/D converter 204 is fed to a flip-flop 207 driven by the 16.2 MHz clock so that the signal is converted to have a 16.2 MHz clock rate, and it is converted back to an analog signal by a D/A converter 208 and fed to a subtractor 211. The subtractor 211 has another input receiving a signal which is derived from the analog input signal on the input terminal 201, which is sampled at the 16.2 MHz clock by a sampler 209 and held by a capacitor 210 for one sampling period. Accordingly, the subtractor 211 produces a differential analog signal which is the analog input signal subtracted by the analog signal equivalent to the high-order conversion digital signal, and the differential analog signal is fed to the back-stage A/D converter 212 driven by the 16.2 MHz clock supplied on the clock terminal ℓ, thereby producing a 5-bit low-order conversion digital signal. In the illustrated circuit arrangement, which is intended to convert an analog input signal into a 10-bit digital signal, it is sufficient for the lower conversion stage to produce a low-order 4-bit digital signal left aside from the 6-bit conversion output produced by the upper conversion stage. However, there is little difference in the complexity and easiness of fabricating each A/D conversion stage of up to 6 bits, and therefore the A/D converter 212 is designed to have a 5-bit resolution, with its low-order 4 bits providing the lower conversion output at a 16.2 MHz clock rate through an OR gate 213 and output terminal 214 and with its highest 1 bit providing a carrier or borrower to an adder 216. The adder 216 also receives the output of the flip-flop 207 through another flip-flop 215 driven by the 16.2 MHz clock. The 6-bit high-order digital signal at the 16.2 MHz clock rate added by the carrier or borrower bit from the A/D converter 212 is fed to an OR gate 217 together with an overflow bit OF from the adder 216, and the OR gate 217 delivers the high-order 6-bit conversion output digital signal at 16.2 MHz clock rate to the output terminal 218. The subrange-type A/D converter arranged as described above, in fact, forms an arrangement which embodies the circuit section of the A/D converter 1 and subsampling circuit 2 included in the decoding equalizer shown in Fig. 1. The subrange-type A/D converter has on its terminals 214 and 218 the output signals at the trunk system sampling rate delivered to the delay circuit 3, and the output signal at the equalization system sampling rate on the terminal 206 is delivered to the equalizing filter 5. It is possible for the A/D converter shown in Figs. 3, 4 and 5 to be modified variously as in conventional subrange-type A/D converters. For example, in the illustrated arrangement, the output digital signal from the fore-stage A/D converter is converted to the low-rate digital signal with the flip-flop 207 and then converted back to the analog signal with the D/A converter 208, whereas the high-order output digital signal of 32.4 MHz from the A/D converter 204 may be converted back to the analog signal directly with the D/A converter 208. Instead of subtracting the analog signal produced by the D/A converter 208 from the input analog signal on the input terminal 201 with the subtractor 211, the back-converted analog signal is added to the reference level of the A/D converter 212 and, in this state, the input analog signal on the input terminal 201 is fed directly to the A/D converter 212 so that their difference is rendered A/D conversion. For the back-stage A/D converter 212, five bits are used to obtain the low-order 4-bit conversion output, i.e., the dynamic range twice as needed, and conceivably such an excessive range need not be afforded. As regards the clock rate, only the low-rate clock is supplied and it is multiplied within the converter as shown in Fig. 4, but instead both the 16.2 MHz and 32.4 MHz clocks may be supplied from separate clock sources, although the illustrated scheme is more preferable for the avoidance of possible influence of the phase relation between the clocks on the conversion characteristics. The number of stages of A/D converters is not confined to two stages, but any plurality of stage is possible.

Claims: A subrange-type decoder equalizing A/D converter used in a transmission system for television signals, said transmission system comprising: a sampling means for sampling a high definition television signal at a first sampling frequency to obtain a sampled value signal, a transmitter to transmit said sampled value signal and a test signal which is time-multiplexed in the sampled value signal for detecting the transmission characteristics of a transmission path, a decoder to decode the sampled value signal, an equalizer to equalize the transmission characteristics of the transmission path using said test signal, a receiver to receive the sampled value signal and the test signal, wherein the subrange-type decoder equalizing A/D converter comprises first sampling means (202) which samples the received sampled value signal at a second sampling frequency twice the first sampling frequency, a first A/D converter (204), connected to the first sampling means (202), having a first driving clock frequency corresponding to the second sampling frequency, a D/A converter (208) connected to the first A/D converter (204), second sampling means (209) which samples the received sampled value signal at the first sampling frequency, a subtractor (211) which produces an output diffe-rence between the outputs of the D/A converter (208) and the second sampling means (209), a second A/D converter (212), connected to the output of the subtractor (211), having a second driving clock fre-quency which is a fraction of the first driving clock frequency, and which corresponds to the first sampling frequency, an adder which adds the output of the second A/D converter (212) to the output of the first A/D converter (204). A subrange-type A/D converter according to claim 2, wherein the first A/D converter (204) has a 32.4 MHz driving clock frequency and the second A/D converter (212) has a 16.2 MHz driving clock frequency. A subrange-type A/D converter according to claim 1, wherein the subrange-type A/D converter outputs a k-bit signal, the first A/D converter outputs an m-bit signal and the second A/D converter outputs an n-bit signal, providing that m + n > k and excessive bits are used for correcting a total conversion output digital value.

Inventors: IWADATE YUICHI, NINOMIYA YUICHI, IWADATE, YUICHI, NINOMIYA, YUICHI
Applicants: JAPAN BROADCASTING CORP, NIPPON HOSO KYOKAI
Application year: 1988
Main classification: H04N7
IPC codes: H03M1, H04N7, H04N5