Coded data generation or conversion – Analog to or from digital conversion – Multiplex
Reexamination Certificate
2002-06-10
2003-09-16
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Multiplex
C341S143000, C341S144000, C375S240010, C375S240020
Reexamination Certificate
active
06621434
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for selectively and sequentially converting a plurality of signals from one of a digital and analogue form to the other of the digital and analogue form, wherein the signals are of different frequency band widths, so that the signals can be filtered in the same analogue filter in a common analogue path.
BACKGROUND TO THE INVENTION
Digital video data signals are provided in three commonly used formats, namely, standard definition format, progressive scan format and high definition format. In standard definition format they may be provided in accordance with any of the following standards, PAL, NTSC and SECAM standards, in progressive scan format they may be provided in accordance with 525P and 625P standards, and in high definition they may be provided in accordance with 1080i, 720P and 1250i standards. In general, it is desirable that video signal processing units be capable of processing the video data signals in all three formats. Standard definition format video data signals, in general, are typically of band width of 6 MHz. Progressive scan format video data signals are typically of band width of 12 MHz, while high definition format video data signals are typically of band width of 30 MHz. In general, when being converted from digital form to analogue form, standard definition format video data signals are generally sampled at an over-sampling frequency of between 24 MHz and 29.5 MHz, and typically, at approximately 27 MHz, which is approximately twice the nyquist sampling frequency. Progressive scan format video data signals when being converted to analogue form are sampled generally at a sampling frequency of 24 MHz to 38 MHz, and most commonly at a sampling frequency of approximately 27 MHz, which is approximately the nyquist sampling frequency. High definition format video data signals when being converted to analogue form are sampled at a sampling frequency in the range of 70 MHz to 85 MHz and most commonly at a frequency of approximately 74.25 MHz, which is approximately the nyquist sampling frequency.
At these sampling frequencies, in the conversion of video digital signals in digital form to analogue form, in general, separate analogue reconstruction filters are required for filtering out image frequencies from the analogue signals of the different formats. Such image frequencies are the resultant of the zero order hold characteristics of a DAC, and hereinafter are referred to as image frequencies. Image frequencies occur at the sampling frequency and at multiples thereof. Thus, at a sampling frequency of 27 MHz the image frequencies of the standard definition and progressive scan video signals occur at the sampling frequency of 27 MHz and multiples thereof. Since the frequency of 27 MHz is within the band width of the high definition video signals, a separate analogue reconstruction filter would be required to filter the standard definition signals and the progressive scan signals on the one hand, and the high definition signals on the other hand. Additionally, since the video data signals of each format can require up to three channels, up to nine separate analogue reconstruction filters may be required for filtering the analogue forms of the video data signals in the respective three formats. Analogue reconstruction filters are in general provided as discrete components. Thus, in general, where the digital to analogue converting circuitry for converting digital video data signals to analogue video signals is implemented as an integrated circuit, an appropriate number of output pins are required for providing output signals in the various formats to the respective analogue reconstruction filters. This significantly increases the pin count of the integrated circuit. Alternatively, relatively complex analogue reconstruction filters are required in which the band pass of the filters can be varied for accommodating the video signals of different formats. Such filters require active components which are relatively complex and thus expensive.
Similarly, when analogue video data signals are converted to digital form separate analogue anti-aliasing filters are required for filtering the analogue signals in the respective formats prior to conversion.
There is therefore a need for a method for converting digital video data signals of respective different frequency band widths to analogue form so that the number of analogue reconstruction filters required is minimised. There is also a need for a method for converting analogue video signals of respective different band widths to digital form so that the number of analogue anti-aliasing filters required is minimised. Indeed, there is a need for a method for converting a plurality of signals in one of a digital and analogue form to the other of the digital and analogue form where the signals are of different frequency band widths which facilitates filtering of the analogue forms of the signals in the same analogue filter.
The present invention is directed towards providing such a method.
SUMMARY OF THE INVENTION
According to the invention there is provided a method for selectively and sequentially converting a plurality of signals of different frequency band width from one of a digital and analogue form to the other of the digital and analogue form, wherein the signals in the analogue form are passed along a common analogue path through an analogue filter, the method comprising the steps of;
sequentially selecting the signals to be converted,
sequentially passing the selected signals through a converter circuit for converting the signals from the one of the digital and analogue form to the other of the digital and analogue form, and
over-sampling the respective signals in the converter circuit at respective over-sampling frequencies so that the analogue forms of the signals can be filtered in the same analogue filter in the common analogue path.
Preferably, the signal of widest band width is over-sampled at the lowest over sampling rate. Advantageously, the signal of narrowest band width is over-sampled at the highest over-sampling rate. Ideally, the signal of widest band width is over-sampled at a frequency of at least twice the nyquist sampling frequency.
In one embodiment of the invention the signals are digital signals, and the converter circuit is a digital to analogue converter circuit for converting the signals from digital form to analogue form and the analogue filter is a reconstruction filter.
In another embodiment of the invention the over-sampling frequencies for the respective signals are selected such that image frequencies in the analogue signal are sufficiently displaced from the widest frequency band width signal that the image frequencies can be removed from each of the analogue signals by the same analogue filter without affecting the signals. Preferably, the over-sampling frequencies for the respective signals are selected such that the image frequency, the lowest frequency of which is closest to the widest band width signal is displaced from the widest band signal to a frequency at least twice the band width of the widest band width signal.
In another embodiment of the invention the over-sampling frequencies for the respective signals are selected such that the image frequency, the lowest frequency of which is closest to the widest band width signal is displaced from the widest band signal to a frequency at least three times the band width of the widest and width signal.
In a further embodiment of the invention the signals are in analogue form and are converted to digital form, the analogue filter being an anti-aliasing filter.
Preferably, the over-sampling frequencies at which the respective signals are over-sampled are selected such that the analogue anti-aliasing filter can be such as to preserve the signal of widest band width and reject signals of frequencies above the lowest sampling frequency used less the widest band width of the signal sampled at the lowest sampling frequency. Advantageously, unwanted frequencies between the band wi
Barry Joseph Michael
Cotter Martin Gerard
Analog Devices Inc.
Iandioro & Teska
Mai Lam
Tokar Michael
LandOfFree
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