Electrical computers: arithmetic processing and calculating – Electrical digital calculating computer – Particular function performed
Reexamination Certificate
1999-07-19
2002-08-27
Mai, Tan V. (Department: 2121)
Electrical computers: arithmetic processing and calculating
Electrical digital calculating computer
Particular function performed
C708S319000
Reexamination Certificate
active
06442580
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a resampling method and a resampler circuit for use in a digital filter, particularly in a so-called interpolation filter, which is installed in a digital decoder or the like and capable of effectuating a timing recovery function.
2. Description of the Related Art
In accordance with the remarkable development in the field of semiconductor technique, conventional analog signal processing employing analog electronic circuit elements such as resistors, capacitors, coils, OP amplifiers and so on is gradually shifting to signal processing in a digital fashion which is the so called digital signal processing, in various fields including the field of communications, audio and/or visual field, as well as the fields of measurement, control and so on.
It is required to transmit digital data or discrete data between systems in which the digital signal processing is essential. However, in a case that digital data is transmitted (for communication), it is normally transmitted to a remote place in terms of distance or time, and due to this, the data transmission side and the data reception side cannot share the same time, whatever the medium for the data transmission is.
FIG. 7
shows an exemplary view for explaining the relationship between transmitted data and a received waveform in a digital transmission system, although it is applicable to many fields other than the field where this digital transmission system is employed.
As shown in the figure, the discrete digital data is transmitted from a transmission side to a reception side at predetermined time intervals. Digital data transmitted by way of a transmission line (such as a cable, a telephone line, wireless communication line and so on) between the transmission side and the reception side is deteriorated during transmission due to the transmission line characteristics, so that the original shape of the digital data is somewhat transformed before it is finally received as an analog waveform data at the reception side. This received data normally contains some noise due to interference possibly caused during data transmission.
A digital signal processing device at the reception side requires a function to recover the data of an analog waveform to the original digital data transmitted from the transmission side, and this recovering process is called a data recovery. On the other hand, in an analog signal processing system, there is no occurrence of data drop or the like from an analog waveform, as the received data of analog waveform can be processed just as it is.
However, in the digital signal processing system, since it is necessary to perform sampling of digital data from the received analog waveform, the data which has not been sampled is regarded as being lost. Therefore, in order to correctly recover the received data at the reception side, the transmission side has to send digital data in accordance with a predetermined protocol.
In short, transmission of digital data from the transmission side to the reception side is equivalent to a transmission of square pulses. Although the square pulse requires a wide-band transmission line as it has a wide frequency spectrum characteristic, the transmission line used for this purpose has to be limited to the frequency band in which the data can be correctly transmitted, as there is a limitation in the frequency resources.
FIG. 8
is an exemplary view showing the transformation of digital data when it is transmitted by way of a low pass filter. For example, when the wide frequency band component of digital data is eliminated by use of a low pass filter, the waveform of the square pulse is transformed to a gentle taper-like waveform as shown in the right-side portion of
FIG. 8
, resulting thereby in an expansion of the pulse width. In a case that digital data is transmitted by use of a T-width square pulse train, the pulse width is expanded due to the limitation of band width, and as a result, the adjacent pulses are superimposed with each other, causing thereby an interference (intersymbol interference) between the transmission waveforms. In a case that a digital data which is pulse information is transmitted at time intervals of T, if there is no such superimposition of adjacent transmission waves at sampling points T, correct pulse information can be taken out at the reception side even if a distortion has occurred in the transmission waveform.
The Nyquist's first standard is widely known as a condition under which a band limitation is made possible even without generating an intersymbol interference. According to this Nyquist's first standard, the required minimum frequency band that does not cause any intersymbol interference at sampling points of every time interval T is T/2.
FIG. 9
is an exemplary view of a filter characteristic that does not cause the intersymbol interference. In the figure, when a pulse signal is input at timing intervals of T by use of an ideal low pass filter through which only the signal within the frequency band between the frequency f=0 and the frequency f=1/2T (hereinafter referred to just as Nyquist frequency) is allowed to pass, or by use of a low pass filter having an odd-symmetrical transmission characteristic about the Nyquist frequency 1/2T, then a response, namely an impulse response (refer to
FIG. 2
) becomes amplitude zero at the time point t=±nT, so that even when impulse responses are superimposed with each other, there will be no intersymbol interference generated as long as the data sampling is performed at t=±nT.
There is a roll-off filter as a practical filter that satisfies the Nyquist's first standard. The characteristic of this roll-off filter is a characteristic shown in
FIG. 9
, and its impulse response is shown as a waveform
1
that can be obtained by the SINC function shown in FIG.
2
. However, even when a digital data is transmitted from the transmission side in a state that there is no intersymbol interference as shown above, the data transmission interval T from the transmission side and the sampling interval T of the thus transmitted data at the reception side are not always the same. Generally, in a digital signal processing system, this data sampling interval is generated by a crystal oscillator. However, even if the same crystal oscillator is used at the transmission side and the data reception side, the operation thereof will not be exactly the same.
For example, in a case that there is caused an error of 1 ppm between the crystal oscillator at the transmission side and that at the reception side, if the system is operating at 10 MHz or so, 10 data at every one second can be lagged therebetween. Further, even if the frequency at the both sides are exactly the same, if there is a time lag between the transmitting interval T at the data transmission side and the sampling point at the data reception side, there will be no way to obtain the correct data forever. Further, in the digital transmission system, there is a problem that the sampling points cannot be transmitted from the data transmission side to the data reception side.
Considering the above, the data reception side needs to recover the frequency of the transmission side (data transmission interval) and the sampling point from the received data. The processing operated at the reception side is called a timing recovery, and a block that implements this processing is called a timing recovery circuit.
Conventionally, it has been common to implement this timing recovery circuit by combination of digital processing and analog processing. However, in recent days, there are many types of timing recovery circuits that perform all the operations by use of digital processing only, and this kind of circuit is called a digital timing recovery circuit, or a resampler circuit.
The operation of the conventional digital timing recovery circuit or resampler circuit is now explained below.
FIG. 10
is an exemplary view showing the method of ca
Burns Doane Swecker & Mathis
Mai Tan V.
Mitsubishi Denki & Kabushiki Kaisha
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