Signal receiving apparatus and signal transmitting system

A signal receiving apparatus 2 has a memory circuit 22, writing of data contained in a digital input signal transmitted from a signal transmitting apparatus 1 is performed using a clock signal separated and created by a PLL circuit 21 from the digital input signal received, and reading is performed using a reference clock signal with quartz accuracy from a reference clock generating circuit 24. To reproduce the digital input signal by correcting the shift between the clock signal and the reference clock signal, the signal receiving apparatus detects the shift between the two clock signals. When the signal receiving apparatus 2 side lags behind the signal transmitting apparatus 1, the data contained in the digital input signal undergoes thinning out, and when it leads, a signal generated from previous and subsequent digital input signal is interpolated.

TECHNICAL FIELD

The present invention relates to a signal receiving apparatus and signal transmitting system suitable for asynchronous digital transmission between a CD transport and a D/A converter, for example.

BACKGROUND ART

In a field of asynchronous digital transmission, a digital signal is transmitted asynchronously between a signal transmitting apparatus such as a CD transport specialized for playback of a CD (Compact Disc) and a signal receiving apparatus including a D/A (Digital/Analog) converter.

To achieve high quality playback in this case, it is essential to absorb a shift (time difference) between clocks used by the signal transmitting apparatus and the signal receiving apparatus, respectively.

A lot of control systems to achieve it have been proposed conventionally. For example, as shown inFIG. 5, a PLL system is generally known that provides a signal receiving apparatus20with a PLL (Phase Locked Loop) circuit201to separately generate a clock signal synchronized with a signal transmitting apparatus10from a digital input signal transmitted via a signal transmission line30.

Incidentally, the signal receiving apparatus20includes a D/A converter (D/A circuit). The D/A circuit202converts the digital input signal from the PLL circuit201to generate an analog output signal that is supplied to an audio-visual playback system not shown via a signal line40.

In addition, as shown inFIG. 6, for example, an SRC system is also known that places at a post-stage of a PLL circuit201an SRC (Sampling Ratio Converter) circuit204, to which a reference clock signal is supplied from a reference clock generating circuit203, and that generates a clock signal at quartz accuracy anew using the SRC circuit204. Furthermore, as shown inFIG. 7, for example, a two-way twin link system is known that employs a signal receiving apparatus20including a D/A circuit202as a master, and that establishes synchronization by supplying a reference clock signal from a reference clock generating circuit203to the D/A circuit202and by sending it back to a signal transmitting apparatus10such as a CD transport via a signal transmission line50.

Moreover, as shown inFIG. 8, a mass buffering system is known that places at a post stage of a PLL circuit201a mass memory circuit205, to which a reference clock signal is supplied from a reference clock generating circuit203, and that picks out a clock signal from a digital input signal stored in the mass memory circuit205at quartz accuracy.

Incidentally, inFIG. 6-FIG.8, the same components as those ofFIG. 5are designated by the same reference numerals and their duplicate description is omitted here.

On the other hand, in an image playback field, an application for a time axis correcting apparatus is filed that corrects time axis fluctuations efficiently by avoiding repetition of quantization by reading a signal at every prescribed interval while thinning it out or repeatedly reading it to adjust the time axis of digital data transferred between an image memory and an image input device, thereby generating and outputting a high quality image signal (see Patent Document 1, for example).

Prior Art Document

Patent Document

DISCLOSURE OF THE INVENTION

However, according to the conventional PLL system as shown inFIG. 5, tracking the central frequency and removing jitter components must be performed at the same time. Accordingly, it has its limit because the removal of low-frequency jitter components near the central frequency inherent in the clock signal will deteriorate the trackability, thereby causing deterioration in performance and sound quality such as that in the SN ratio (Signal to Noise Ratio). In addition, according to the SRC system shown inFIG. 6, as for the reference clock generating circuit203of the D/A circuit202, although its accuracy can be increased to the quartz accuracy, as for the jitter components contained in the transmission system, they cannot be separated from the signal component, and hence it makes little difference from the foregoing PLL system.

In addition, as for the twin link system shown inFIG. 7, both the jitter concerning the transmission system and the jitter of the reference clock signal of the D/A circuit202can be reduced to the quartz accuracy in performance, the signal transmitting apparatus10must be matched to an external clock signal. Accordingly, when there are a lot of playback sources, it requires signal transmission lines30and50corresponding to the number of the playback sources, thereby being restricted in the system configuration.

Furthermore, according to the mass buffering system shown inFIG. 8, since it causes time delay, the time shift between the digital input signal and analog output signal gradually increases with time. Thus, it has a problem of being unable to be applied to audio-visual playback that demands real-time performance that requires synchronization between the audio and video in particular.

In addition, according to the technique disclosed in the foregoing Patent Document 1, although it tries to prevent a visual problem by scattering image data corresponding to noise extracted per unit time at random, when it is applied to the audio-visual playback, it causes deterioration in sound quality.

The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a signal receiving apparatus and signal transmitting system that have minimum time shift between the digital input signal and analog output signal, that can reduce the jitter to the quartz accuracy, and that is suitably applied to the audio-visual playback.

To solve the foregoing problems, the signal receiving apparatus in accordance with the present invention includes: a PLL circuit for separating and generating a clock signal from a digital input signal; a memory circuit into which the digital input signal is written according to the clock signal generated by the PLL circuit, and out of which the digital input signal is read out according to a reference clock signal of a DA converter; a time difference detecting circuit for detecting time difference between the clock signal and the reference clock signal; and a data processing circuit for processing, when the time difference detecting circuit detects the time difference between the clock signal and the reference clock signal, the digital input signal in accordance with a condition of the time difference detected, and for writing the digital input signal passing through the processing into the memory circuit.

In addition, a signal transmitting-receiving system in accordance with the present invention includes a signal transmitting apparatus for transmitting a digital input signal; and a signal receiving apparatus that is connected to the signal transmitting apparatus via a signal transmission line, and that comprises: a PLL circuit for generating a clock signal from the digital input signal transmitted via the signal transmission line; a DA converter for converting the digital input signal to an analog output signal; a memory circuit into which the digital input signal is written according to the clock signal separated and generated by the PLL circuit, and out of which the digital input signal is read according to a reference clock from a reference clock generating circuit; a time difference detecting circuit for detecting time difference between the clock and the reference clock; and a data processing circuit for processing, when the time difference detecting circuit detects the time difference between the clock signal and the reference clock signal, the digital input signal in accordance with conditions of the time difference detected, and for writing into the memory circuit.

According to the present invention, it can provide a signal receiving apparatus and a signal transmitting system that is capable of minimizing the time shift between the digital input signal and the analog output signal and of reducing the jitter to quartz accuracy, and that is particularly suitable for the application to the audio-visual playback.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention will now be described with reference to the accompanying drawings to explain the present invention in more detail.

[Explanation of Signal Transmitting System]

FIG. 1is a block diagram showing a configuration of a signal transmitting system of an embodiment 1 in accordance with the present invention.

As shown inFIG. 1, a signal transmitting-receiving system of the embodiment 1 in accordance with the present invention comprises a signal transmitting apparatus1and a signal receiving apparatus2connected to the signal transmitting apparatus1via a signal transmission line3. Incidentally, as for the signal transmission line3, it can be either wired or wireless.

As shown inFIG. 1, the signal receiving apparatus2comprises a PLL circuit21, a memory circuit22, a D/A circuit23, a reference clock generating circuit24(clock B), a time difference detecting circuit25, and a data processing circuit26.

The PLL circuit21separates and generates a clock A from the digital input signal transmitted from the signal transmitting apparatus1via the transmission line3, and supplies it to the memory circuit22and time difference detecting circuit25.

The memory circuit22is a comparatively small memory that writes the digital input signal in response to the clock A generated by the PLL circuit21and reads it out in response to the reference clock B. The D/A circuit23is a D/A converter that converts the digital input signal read out of the memory circuit22to an analog output signal, and supplies it to a playback system not shown via an output signal line4.

The time difference detecting circuit25detects the time difference (shift) between the clock A generated by the PLL circuit21and the reference clock B from the clock generating circuit24, and supplies it to the data processing circuit26.

When the shift between the clock A and the reference clock B is detected by the time difference detecting circuit25, the data processing circuit26processes the digital input signal in accordance with a shift condition detected, and writes to the memory circuit22. Details of the internal configuration of the data processing circuit26and the like will be described later with reference toFIG. 2.

As shown inFIG. 1, the signal transmitting system of the embodiment 1 in accordance with the present invention provides the signal receiving apparatus2with the memory circuit22, writes the data signal using the clock A generated from the digital input signal by the PLL circuit21, and reads it using the quartz accuracy reference clock B generated from the reference clock generating circuit24. However, as the conventional mass buffering system shown inFIG. 8, if the configuration is used as it is, the shift between the digital input signal and the analog output signal will gradually increase with time, resulting in an overflow if the memory capacity is not enough.

For this reason, according to the signal transmitting system of the embodiment 1 in accordance with the present invention, the signal receiving apparatus2is configured in such a manner that it detects the shift between the write clock A and the read clock B, and that paying attention to the fact that the shift is 100 ppm (parts per million) or less in practice (which corresponds to one shift per 0.23 second when the shift is 100 ppm at the sampling frequency 44.1 kHz), it executes thinning out processing of the data when one delay per 0.23 second occurs and interpolation processing of a signal generated from previous and subsequent signals in a case of advance, for example, to achieve the time difference correction.

In addition, the signal transmitting system of the embodiment 1 in accordance with the present invention is configured in such a manner that to minimize sound quality deterioration in an acoustic feeling due to the above-mentioned thinning out processing or interpolation processing, the signal receiving apparatus2monitors the amplitude, the amount of change and the randomness of the digital input signal, detects a place where the absolute amplitude is small, a place where the amount of change is small or a place where the randomness is high, and extracts a score calculated by weighting at least two of them as a target signal sample for the thinning out or interpolation.

Furthermore, the signal transmitting system of the embodiment 1 in accordance with the present invention is configured in such a manner that to suppress the deterioration in the acoustic feeling after the thinning out or interpolation of the target signal sampling, the signal receiving apparatus2performs shorter cross-fade processing as the randomness increases at the places to which the thinning out processing or interpolation processing is applied and performs longer cross-fade processing as the randomness reduces, thereby attenuating interconnected points of the signal to create a smoother waveform and to make it more difficult to perceive discontinuities. This is due to the fact that the perception of the discontinuities is harder when the randomness is high and the recognition of the discontinuities is easier when the randomness is low.

As described above, according to the signal transmitting system of the embodiment 1 in accordance with the present invention, the signal receiving apparatus2has the memory circuit22, and writing of the data contained in the digital input signal transmitted from the signal transmitting apparatus1is performed according to the clock (clock A) generated by the PLL circuit21from the digital input signal received, and reading of the data is carried out using the quartz accuracy reference clock (clock B) from the reference clock generating circuit24.

Here, to correct the shift between the clock A and the reference clock B and to reproduce the digital input signal, the signal receiving apparatus2is configured in such a manner that it detects the time difference between them, that when the shift between the two clock signals is detected and the signal receiving apparatus2lags behind the signal transmitting apparatus1, it thins out the data contained in the digital input signal, and that when it advances, it interpolates a signal generated from the previous and subsequent digital input signal. Accordingly, it can provide a signal transmitting system that can minimize the time shift between the digital input signal and the analog output signal, that can reduce the jitter to the quartz accuracy, and that is suitable for the application to the audio-visual playback.

Incidentally, according to the signal transmitting system of the embodiment 1 in accordance with the present invention described above, the system is described on the assumption that the signal transmitting apparatus1is a CD transport and the signal receiving apparatus2includes a D/A converter. However, it is not limited to the combination. For example, it is applicable to any combination of signal transmitting and receiving apparatuses carrying out asynchronous digital transmission.

[Detailed Explanation of Signal Receiving Apparatus]

FIG. 2is a block diagram showing an internal configuration of the data processing circuit26included in the signal receiving apparatus2of the embodiment 1 in accordance with the present invention.

As shown inFIG. 2, the data processing circuit26comprises an amplitude monitoring unit261, a change amount monitoring unit262, a randomness monitoring unit263, a target signal sample extracting unit264, a cross-fade processing unit265and a memory updating unit266.

The amplitude monitoring unit261monitors the amplitude of the digital input signal written in the memory circuit22, the change amount monitoring unit262calculates the amount of change per unit time of the digital input signal written in the memory circuit22, and the randomness monitoring unit263calculates the randomness per unit time of the digital input signal written in the memory circuit22, and they all supply them to the target signal sample extracting unit264.

Incidentally, to calculate the randomness, the randomness monitoring unit263obtains variance (standard deviation), for example, decides that the randomness becomes higher as the variance of the digital input signal increases, and controls the target signal sample extracting unit264. Here, the term “variance” expresses the magnitude of the distance (square) between variables and the average, and refers to a well-known statistical method that means an error from the average value.

In addition, the randomness monitoring unit263may perform linear prediction of the digital input signal, and employ the prediction error as a scale of randomness decision. In this case, it decides that the greater the prediction error, the higher the randomness. Here, the term “linear prediction” relates to a time series of the digital input signal measured per unit time, and is a well-known arithmetic method that obtains the digital input signal of the next time series according to a prescribed formula using a time series up to now, calculates a prediction error when actually obtaining measured data at the next time, and aims to optimize the subsequent prediction by adjusting weighting coefficients to make the error smaller. Incidentally, as for calculating the randomness, the method based on the linear prediction can achieve more accurate calculation than the method of obtaining the variance.

The target signal sample extracting unit264calculates the score by weighting operation of at least two parameters from among the amplitude (level) of the digital input signal monitored by the amplitude monitoring unit261, the amount of change calculated by the change amount monitoring unit262and the randomness calculated by the randomness monitoring unit263, extracts target signal samples to be subjected to the thinning out or interpolation processing of the digital input signal according to the score calculated here, and supplies to the cross-fade processing unit265.

The cross-fade processing unit265executes the cross-fade operation with the length corresponding to the randomness of the target signal samples extracted by the target signal sample extracting unit264, and performs the smoothing processing of the digital input signal.

More specifically, the cross-fade processing unit265multiplies signals at two adjacent points of time of the target signal samples by a windowing function gradually decreasing with time elapse and by a windowing function gradually increasing with the time elapse, and adds them to obtain a smooth waveform by attenuating the signals at the two points of time, thereby making it difficult to perceive the discontinuities. Here, it applies the shorter cross-fade processing as the randomness of the target signal samples increases and the longer cross-fade processing as the randomness reduces.

Incidentally, the memory updating unit266overwrites in the memory circuit22the digital input signal after the processing, which passes through the smoothing processing of the places undergoing the thinning out or interpolation by the cross-fade processing unit265.

FIG. 3andFIG. 4are flowcharts showing the operation of the signal receiving apparatus of the embodiment in accordance with the present invention, which show basic operation (FIG. 3) and applied operation (FIG. 4), respectively. The term “basic operation” here refers to a basic flow up to the thinning out or interpolation processing of the target signal samples extracted from the digital input signal, and the term “applied operation” refers to the processing for minimizing the sound quality deterioration after the thinning out or interpolation processing.

The operation of the signal receiving apparatus of the embodiment 1 in accordance with the present invention shown in

FIG. 1andFIG. 2will be described below with reference to the flowcharts ofFIG. 3andFIG. 4.

In the flowchart ofFIG. 3, the signal receiving apparatus2waits for receiving the digital input signal transmitted from the signal transmitting apparatus1via the signal transmission line3(“YES” at step ST301). The PLL circuit21separates and generates the clock A from the digital input signal (step ST302), and writes the data contained in the digital input signal into the memory circuit22(step ST303).

On the other hand, the time difference detecting circuit25detects the time difference between the clock A generated and output from the PLL circuit21and the reference clock B from the reference clock generating circuit24. For example, when it detects the shift that occurs once per 0.23 second, it calculates the amount of the shift (including both lag and lead) and supplies it to the data processing circuit26(step ST304).

The data processing circuit26receiving it, executes, when a decision is made that the shift is a lag (“lag” at step ST305), the thinning out processing of the data at the timing (step ST306). In contrast, when a decision of a lead is made (“lead” at step ST305), it executes the interpolation processing of the data (step ST307). Then the data processing circuit26overwrites the data processed, which is generated as a result of the thinning out or interpolation processing, on the memory circuit22, thereby updating the contents of the digital input signal (step ST308).

To minimize the sound quality deterioration due to the thinning out or interpolation, the data processing circuit26further monitors the amplitude, the amount of change and the randomness of the digital input signal with the amplitude monitoring unit261, the change amount monitoring unit262and the randomness monitoring unit263, respectively, as shown in the flowchart ofFIG. 4, detects places with a small amplitude absolute value, places with a small amount of changes and places with high randomness, and delivers them to the target signal sample extracting unit264(step ST401).

Receiving them, the target signal sample extracting unit264calculates the score by the weighted operation of at least two of the amplitude, the amount of changes and randomness (step ST402), extracts the places with a high score as the target signal samples to be subjected to the thinning out or interpolation, and delivers them to the cross-fade processing unit265(step ST403).

The cross-fade processing unit265executes the cross-fade operation in the length corresponding to the randomness of the signal samples extracted by the target signal sample extracting unit264, thereby carrying out the smoothing processing of the digital input signal.

More specifically, to suppress the deterioration in the acoustic feeling after the thinning out or interpolation processing of the target signal samples, the cross-fade processing unit265executes shorter cross-fade processing as the randomness increases of the places subjected to the thinning out or interpolation processing (step ST404), and executes longer cross-fade processing as the randomness decreases to make a smoother waveform by attenuating the connecting point of the signals, thereby making it difficult to discern the discontinuities.

Incidentally, the memory updating unit266overwrites on the memory circuit22the data after the processing, which passes through the smoothing processing of the places subjected to the thinning out or interpolation processing by the cross-fade processing unit265(step ST405), and completes a series of applied operation to suppress the deterioration in the acoustic feeling involved in the thinning out or interpolation processing of the digital input signal described above.

After that, the D/A circuit23reads the data written in the memory circuit22one after another, converts to the analog signal and supplies to the playback system. Thus, the audio-visual playback is carried out.

As described above, according to the signal receiving apparatus2of the embodiment 1 in accordance with the present invention, it has the memory circuit22, writes the data contained in the digital input signal transmitted from the signal transmitting apparatus1according to the clock (clock A) generated from the received digital input signal using the PLL circuit21, and reads out the data according to the reference clock (clock B) with the quartz accuracy supplied from the reference clock generating circuit24. Here, to reproduce the digital input signal by correcting the shift between the clock A and the reference clock B, the signal receiving apparatus2is configured in such a manner that it detects the shift between the clock signals, thins out the data contained in the digital input signal when the signal receiving apparatus2lags behind the signal transmitting apparatus1, and interpolates the signal generated from the previous and subsequent digital input signal when it leads. Accordingly, it can provide the signal receiving apparatus that can minimize the time shift between the input and output signals, that can reduce the jitter to the quartz accuracy and that is suitable for the application to the audio-visual playback.

In addition, according to the signal receiving apparatus2of the embodiment 1 in accordance with the present invention, to minimize the sound quality deterioration due to the thinning out or interpolation processing, it carries out thinning out or interpolation by extracting samples whose deterioration is difficult to be perceived in the acoustic feeling, thereby being able to offer an advantage of being able to perform, without constraints of the apparatus, playback with the sound quality nearly at the same level as the twin link system having large constraints of the equipment.

Furthermore, it can further reduce the deterioration in the acoustic feeling by applying, after the thinning out or interpolation processing of the samples, the cross-fade processing with a shorter length as the randomness increases at the places subjected to the thinning out or interpolation, and the cross-fade processing with a longer length as the randomness reduces.

Incidentally, as for the functions of the data processing circuit26shown inFIG. 2, all of them can be implemented by hardware or at least part of them can be implemented by hardware .

For example, the following processing can be implemented on a computer using one or a plurality of programs, or at least part of them can be carried out by hardware: (1) the data processing in which when the time difference detecting circuit25detects the time difference between the clock A and the reference clock B, the data processing circuit26processes the digital input signal according to the conditions of the time difference detected, and writes it into the memory circuit22; (2) the data processing in which when the time difference detecting circuit25detects the lag of the reference clock B, the data processing circuit26thins out the digital input signal, and when it detects the lead, it interpolates the signal generated from the previous and subsequent digital input signal; (3) the data processing in which the data processing circuit26monitors the level and the amount of change of the digital input signal, extracts the samples at which the level of the digital input signal is low and the amount of change is small, and makes them the processing target; (4) the data processing in which the data processing circuit26monitors the randomness of the digital input signal, extracts the portions at which the randomness is high, and makes them the processing target; (5) the data processing in which the data processing circuit26calculates the variance of the digital input signal, and makes the variance calculated the scale of the randomness decision; (6) the data processing in which the data processing circuit26performs the linear prediction operation of the digital input signal, and employs the magnitude of the prediction error as the scale of a randomness decision; (7) the data processing of carrying out weighted operation of at least two of the level, the amount of change and the randomness of the digital input signal, and extracts the samples according to the score calculated; and (8) the data processing of performing the cross-fade processing with the length corresponding to the randomness of the samples extracted.

Industrial Applicability

A signal receiving apparatus and signal transmitting system in accordance with the present invention can offer a signal receiving apparatus and signal transmitting system capable of minimizing the time shift between the digital input signal and the analog output signal, and of reducing the jitter to the quartz accuracy, thereby being suitable for the application to the audio-visual playback in particular. Accordingly, it is preferably used for a signal receiving apparatus and a signal transmitting system, which are suitable for the application to the asynchronous digital transmission between the CD transport and D/A converter, for example.