BT848 ETC, BT848 Datasheet - Page 31

• Manufacturer Part Number: BT848
• Description: Single-Chip Video Capture for PCI
• Manufacturer: ETC

Bt848/848A/849A
Single-Chip Video Capture for PCI
Brooktree
Horizontal and Vertical
Luminance Scaling
®
Scaling
Video Scaling, Cropping, and Temporal Decimation
The Bt848 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.
The Bt848 provides independent and arbitrary horizontal and vertical down scal-
ing. The maximum scaling ratio is 16:1 in both X and Y dimensions. The maxi-
mum vertical scaling ratio is reduced from 16:1 when using frames to 8:1 when
using fields. The different methods utilized for scaling luminance and chromi-
nance are described in the following sections.
The first stage in horizontal luminance scaling is an optional pre-filter which pro-
vides the capability to reduce anti-aliasing artifacts. It is generally desirable to lim-
it 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 shown in Figure 8 reduce the hor-
izontal high-frequency spectrum in the luminance signal. Figure 9 and Figure 10
show the combined results of the optional low-pass filters, the luma notch filter and
the 2x oversampling filter. Figure 12 shows the combined responses of the luma
notch filter and the 2x oversampling filter.
The Bt848 uses 32 different phases to accurately interpolate the value of a pixel.
This provides an effective pixel jitter of less than 6 ns.
duce a step function in the video signal that effectively introduces high-frequency
spectral components. Poly-phase interpolation accurately interpolates to the cor-
rect pixel and line position providing more accurate information. This results in
aesthetically pleasing video as well as higher compression ratios in bandwidth lim-
ited applications.
tering options. The filter characteristics are shown in Figure 11. The Bt848 pro-
vides up to 5-tap filtering to ensure removal of aliasing artifacts.
select 2, 3, 4 or 5 taps. The number of taps must be chosen in conjunction with the
horizontal scale factor in order to ensure the needed data can fit in the internal
FIFO (see the VFILT bits in the VTC register for limitations). As the scaling ratio
is increased, the number of taps available for vertical scaling is increased. In addi-
tion to low-pass filtering, vertical interpolation is also employed to minimize arti-
facts when scaling to non-integer scaling ratios.
The Bt848 implements horizontal scaling through poly-phase interpolation.
In simple pixel- and line-dropping algorithms, non-integer scaling ratios intro-
For vertical scaling, the Bt848 uses a line store to implement four different fil-
The number of taps in the vertical filter is set by the VTC register. The user may
L848A_A
Video Scaling, Cropping, and Temporal Decimation
F
UNCTIONAL
D
ESCRIPTION
21