Fusion 878A
2.0 Functional Description
PCI Video Decoder
2.4 Video Scaling, Cropping, and Temporal Decimation
100600B
Conexant
2-7
2.4 Video Scaling, Cropping, and Temporal
Decimation
The Fusion 878A provides three mechanisms to reduce the amount of video pixel
data in its output stream: down-scaling, cropping, and temporal decimation. All
three can be controlled independently.
2.4.1 Down-Scaling
2.4.1.1 Horizontal and
Vertical Scaling
The Fusion 878A provides independent and arbitrary horizontal and vertical
down- scaling. The maximum scaling ratio is 16:1 in both X and Y dimensions.
The maximum vertical scaling ratio is reduced from 16:1 (when using frames) to
8:1 (when using fields). The different methods used for scaling luminance and
chrominance are described in the following sections.
2.4.1.2 Field Aligned
Vertical Scaling
If Common Interchange Format (CIF) resolution video is viewed at 60/50 Hz
rates, then the video fields must be field-aligned for proper overlay (sequenced on
top of each other successively). This could be done in interlaced Vertical Scaling
mode (INT set) which group delays (filters) only one field by one line. The two
fields are vertically aligned for overlay, but the two fields have different
frequency responses. One has not been filtered, while the other has been
line-averaged. A new option exists to filter both fields in a similar manner yet
maintain proper field alignment. This mode is selected by setting VSFLDALIGN
and resetting the INT bit to non-interlaced Vertical Scaling mode.
2.4.1.3 Luminance
Scaling
Horizontal Scaling
The first stage in horizontal luminance scaling is an optional pre-filter which
provides the capability to reduce anti-aliasing artifacts. It is generally desirable to
limit the bandwidth of the luminance spectrum prior to performing horizontal
scaling because the scaling of high-frequency components may create image
artifacts in the resized image. The optional low pass filters illustrated in
reduce the horizontal high-frequency spectrum in the luminance
and
illustrates the combined results of the optional
low-pass filters, the luma notch filter and the 2x oversampling filter.
illustrates the combined responses of the luma notch filter and the 2x
oversampling filter.
The Fusion 878A implements horizontal scaling through poly-phase
interpolation. The Fusion 878A uses 32 different phases to accurately interpolate
the value of a pixel. This provides an effective pixel jitter of less than 6 ns.
In simple pixel- and line-dropping algorithms, non-integer scaling ratios
introduce a step function in the video signal that effectively introduces
high-frequency spectral components. Poly-phase interpolation accurately
interpolates to the correct pixel and line position providing more accurate
information. This results in aesthetically pleasing video as well as higher
compression ratios in bandwidth limited applications.