38.6.9.1 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 a vertical interpolation by a factor of vI, and a decimation by a factor of vD. A Horizontal resampler is used to perform a 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 | 1−2×|n−M−12|M−1 |
| Blackman | 0.42−0.5×cos2πnM−1+0.08×cos4πnM−1 |
| Hamming | 0.54−0.46×cos2πnM−1 |
| Hanning | 0.5−0.5×cos2πnM−1 |
The horizontal resampler includes an 8-phase 5-tap filter equivalent to a 40-tap FIR described in the figure below.
The vertical resampler includes an 8-phase 3-tap filter equivalent to a 24-tap FIR described in the figure below.
