Motion video signal processing for recording or reproducing – Local trick play processing – With randomly accessible medium
Reexamination Certificate
1998-05-22
2002-01-01
Nguyen, Huy (Department: 2615)
Motion video signal processing for recording or reproducing
Local trick play processing
With randomly accessible medium
C386S349000, C386S349000
Reexamination Certificate
active
06336001
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a digital recording/reproduction apparatus for reproducing information in a digital signal recorded on a recording medium.
(2) Description of the Prior Art
In a digital video tape recorder, there is known a partial response/class 4 as a method for digitalizing and recording a video signal on a recording medium such as a magnetic tape and the like. With this method, the video signal digitalized and recorded can be decoded using Viterbi decoding.
However, since the Viterbi decoding is for processing a digital signal, when the Viterbi decoding is performed, in order to perform quantization, it is necessary to convert the reproduction signal reproduced from the recording medium from the analog signal to the digital signal, using a sampling clock of a predetermined sampling rate.
In this case, if the phase of the sampling clock is not proper, the reproduction signal in the analog form cannot be quantized properly. Particularly, in the case of the Viterbi decoding, since it is for discriminating the decoded data and correcting errors according to the amplitude level, if the phase of the sampling clock is shifted and the amplitude level is not correctly converted, a code error occurs.
Specifically speaking, for example in
FIG. 21
, when the recording signal is “0, 1, 0, 1, 0, 0, 1, 0”, if the reproduced waveform is simply detected with the threshold of “1” and “−1” (reproduction without Viterbi decoding), the detection result becomes “0, 1, 1, 1, 0, 1, 1, 0”.
On the contrary, if the reproduced waveform in
FIG. 21
is detected using the Viterbi decoding, when there is a “peak” where the amplitude level is higher than point B or a “valley” where it is lower than point B, after the amplitude level at point B exceeds the threshold “1” and from point C downward, “1” will be determined as the data at point B. At point C, the amplitude level is higher than the threshold “1”, but lower than point B, hence the data at point C is not determined.
At the next point D, the amplitude level becomes lower than “−1”, and for the first time, “1” is determined as the data at point B, and “0” is determined as the data at point C. However, the data at point D is not determined at this moment. In order to determine the data at point D, it is necessary to have a “valley” where the amplitude level is lower than the threshold point D or a “peak” where it is higher than “1” at point E or downward.
Then, at point E, since the amplitude level does not exceed “1”, the data at point D is not yet determined. At the next point F, since the amplitude level is not lower than the amplitude level at point D, the data at point D is not yet determined. It is not until the amplitude level at the next point G exceeds “1” that “−1” is determined as the data at point D, “0” is determined as the data at point E, and “0” is also determined as the data at point F, respectively.
As described above, with the Viterbi decoding, decoding is performed by detecting the amplitude level of the reproduced waveform, hence the detection accuracy of the amplitude level is quite important. The amplitude level of the reproduced waveform is determined at a time position of point A and the like in the reproduced waveform shown in
FIG. 21
, and the time position is determined by the phase of the sampling clock signal, hence the detection accuracy of the amplitude level depends largely on the phase of the sampling clock.
For example, when the phase of the sampling clock signal is delayed, there is a case where the amplitude level at point E exceeds the threshold “1”. In this case, the data at point E will be erroneously detected as “1”, and the data at point F will be erroneously detected as “−1”.
Thus, the phase adjustment of the sampling clock is quite important for enhancing the detection accuracy of the amplitude level in the reproduced waveform.
In the conventional apparatus, the phase adjustment of the sampling clock signal is performed by an operator on the production stage, and it takes time for adjusting the phase. Moreover, since the phase of the sampling clock signal is fixed to a value adjusted on the production stage, when tapes having different properties are reproduced due to the property difference between tape manufacturers or the difference between lots, the optimum phase point is shifted, causing a problem that the detection accuracy of the amplitude level in the reproduced waveform is decreased.
There is known a method in which even if kinds of magnetic tapes and properties of magnetic heads change, the optimum point of the clock phase is automatically searched using a mean value in the error rate of the reproduction data (Japanese Patent Application Laid-open Hei 2 No. 259891), as a conventional method for adjusting the phase of the sampling clock signal to the optimum point.
However, when the error rate is 10
−5
, for example, an error occurs at a rate of once for every 100,000, hence it is necessary to read at least 1,000,000 data in order to accurately count the error rate. When the error rate becomes 10
−6
, an error occurs at a rate of once for every 1,000,000, hence it is necessary to obtain at least 10,000,000 data.
Therefore, according to the conventional method, it is necessary to read much more data in order to adjust the sampling clock phase to the appropriate point. However, since the error rate becomes low as the sampling clock phase approaches the appropriate value, the data quantity to be read or the calculation time of the error rate increases exponentially. Accordingly, there has been such a problem that as the optimum condition is aimed, much time is required.
SUMMARY OF THE INVENTION
With a view to solving the above problems, it is an object of the present invention to provide a digital recording/reproduction apparatus which can quickly adjust the phase of the sampling clock signal for quantizing the reproduced waveform of the video signal to an optimum value.
In order to attain the above objectives, the present invention has the following construction.
The digital recording/reproduction apparatus according to the invention as set forth in the first aspect is a digital recording/reproduction apparatus which quantizes and decodes the information signal reproduced from a recording medium based on a sampling clock, and is characterized in that it includes detection means for detecting the frequency of occurrence of data contained in the information signal, and phase control means for controlling the phase of the sampling clock based on the frequency of occurrence of data detected by the detection means.
The digital recording/reproduction apparatus according to the invention as set forth in the second aspect has a structure of the digital recording/reproduction apparatus according to the first aspect, wherein the detection means includes means for comparing the information signal with the threshold and converting it into a binary format, and threshold control means for controlling the threshold based on the frequency of occurrence of either one of the two theoretical values contained in the output of the means for converting into a binary format.
The digital recording/reproduction apparatus according to the invention as set forth in the third aspect has a structure of the digital recording/reproduction apparatus according to the first aspect, wherein the phase control means controls the phase of the sampling clock based on the detection results of the detection means, so that the frequency of occurrence of data contained in the signal obtained by quantizing the information signal based on the positive phase signal of the sampling clock is made maximum, or the frequency of occurrence of data contained in the signal obtained by quantizing the information signal based on the negative phase signal of the sampling clock is made minimum.
The digital recording/reproduction apparatus according to the invention as set forth in the fourth aspect has a structure of the digital recording/reproduct
Katayama Hironobu
Kawamura Takeshi
Nguyen Huy
Sharp Kabushiki Kaisha
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