5.2.6.13.1 Video Scaler Description
The scaling operation is based on a vertical and horizontal resampling algorithm. The sampling rate of the original image is increased when the video is upscaled, and decreased when the video is downscaled. A vertical resampler is used to perform vertical interpolation by a factor of vI, and a decimation by a factor of vD. A horizontal resampler is used to perform vertical interpolation by a factor of hI, and a decimation by a factor of hD. The horizontal and vertical low pass filters are both designed to minimize the aliasing effect. The frequency response of the low pass filter has the following characteristics:
Taking into account the linear phase condition and anticipating the filter length M, the desired frequency response is modified.
The impulse response of the defined low pass filter is:
Or, for the filter of length M:
This ideal filter is non-causal and cannot be realized. The unit sample response h(n) is infinite in duration and must be truncated depending on the expected length M of the filter. This truncation is equivalent to the multiplication of the impulse response by a window function w(n).
Name of Window Function | Time Domain Sequence w(n) |
---|---|
Barlett | |
Blackman | |
Hamming | |
Hanning |
The horizontal resampler includes a 16-phase 4-tap filter equivalent to a 64-tap FIR illustrated in Horizontal Resampler Filter Architecture below.
The vertical resampler includes a 16-phase 4-tap filter equivalent to a 64-tap FIR illustrated in Vertical Resampler Filter Architecture below.