Boots – shoes – and leggings
Patent
1994-08-26
1997-02-11
Trammell, James P.
Boots, shoes, and leggings
341 61, G06F 1700
Patent
active
056027621
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PLT/GB92/01846 filed Oct. 9, 1992.
This invention relates to the sampling of video or other signals.
When converting video signals to digital form, there are conflicting criteria in selecting the sampling frequency. For historical reasons and because of certain regulatory standards, much existing hardware operates at arbitrary fixed frequencies. A commonly used frequency is 13.50 MHz. For other reasons, it is often preferred to use a sampling rate which is an integral multiple of the colour sub-carrier frequency. There are particular advantages in sampling at four times the sub-carrier frequency, these advantages including a simple demodulation procedure and a convenient phase relationship between pixels in neighbouring lines. On this theoretical basis, a sampling frequency of 17.73 MHz might for example, be selected for PAL.
A technique already exists for converting a digital video signal sampled at, say, 17.73 MHz to a digital signal at 13.50 MHz. Generally, the signal is converted to analogue, using of course the original sampling frequency; low pass filtered and--usually--amplified. The analogue signal is then reconverted to digital at the desired sampling frequency. The frequency response of the low pass filter is required to be of high quality and is generally specified in the regulatory standards such as CCIR 601.
Whilst analogue filtering techniques are available to meet the frequency response standards, it is inconvenient, and often, extremely so, to add digital-to-analogue and analogue-to-digital conversions in an otherwise digital environment, for the purpose only of rate conversion. Also, the analogue stage is of course a possible source of error. There is, accordingly, considerable advantage to be gained if rate conversion could be achieved digitally.
There have hitherto been considerable difficulties in the use of digital filters in this application. This can be explained as follows.
The order or quality of frequency response of a digital filter can be related to the number of samples employed. It will be recognised that very close parallels exist in this context between digital filtering and the interpolation between digital signal values. In deriving an estimated value, away from a sample point, by interpolating from neighbouring sample points, accuracy is of course increased as the number of samples included within the interpolation, is increased. By the same token, the frequency response of a digital filter will improve as the number of samples (or in filter terminology, the number of taps) is increased. It can be estimated, for example, that to achieve the quality of frequency response laid down in CCIR 601, a digital filter would require to have at least 32 samples or taps.
In deriving a number of sample values about a specific point in a video signal, a filter will apply an aperture function--often sin x/x--centered about the point in question. To produce 32 samples, the filter therefore requires 32 coefficient values calculated in accordance with the aperture function. It will be recognised, however, that the aperture function will generally not remain fixed in position relative to the sample points of the original signal, so that the values of the 32 coefficients are not fixed. Indeed, with relatively close frequencies, the value of each coefficient is likely to change significantly from one sample to the next.
There are of course enormous problems in signal processing terms of handling 32 or more variable multiplicands at megahertz frequencies. This is the reason why, despite the disadvantages mentioned, analogue rate conversion persists in digital environments.
It is an object of this invention to provide improved apparatus and processes for digital conversion of signal sample rates, which enable high frequency response standards to be met without imposing excessive demands on signal processing hardware.
Accordingly, the present invention consists, in one aspect, in a process for digitally converting the sample rate of a signal comprising a first digita
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Ingerman Jeffrey H.
Snell & Wilcox Limited
Trammell James P.
LandOfFree
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