Pemnr
between signal bandwidth (sharpness impression) and alias.
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Filename:
SAA7115_Datasheet.fm
Confidential - NDA required
page 55
Last edited by H. Lambers
Philips Semiconductors
CS-PD Hamburg
CVIP2
Datasheet
SAA7115
Date:
10/23/01
Version:
0.67
8.3.2
H
ORIZONTAL SCALING
The overall horizontal required scaling factor has to be split into a binary and a rational value according to the equation:
With
there is
where the parameter of prescaler
and the parameter of VPD phase interpolation
For example,
1
3.5
is to split in
1
4
×
1.14286. The binary factor is processed by the prescaler, the arbitrary non-integer
ratio is achieved via the variable phase delay VPD circuitry, called horizontal fine scaling. The latter calculates
horizontally interpolated new samples with a 6-bit phase accuracy, which relates to less than 1 ns jitter for regular
sampling scheme. Prescaler and fine scaler create the horizontal scaler of the SAA71157115.
XPSC[5:0]
XSCY[12:0]
= 1 to 63
= 300 to 8191 (0 to 299 are only theoretical values).
Using the accumulation length function of the prescaler (XACL[5:0] A1H[5:0]), application and destination dependent
(e.g. scale for display or for a compression machine), a compromise between visible bandwidth and alias suppression
can be determined.
8.3.2.1
Horizontal prescaler (subaddresses A0H to A7H and D0H to D7H)
The prescaling function consists of an FIR anti-alias filter stage and an integer prescaler, which creates an adaptive
prescale dependent low-pass filter to balance sharpness and aliasing effects.
The FIR prefilter stage implements different low-pass characteristics to reduce alias for downscales in the range of
1 to
1
2
. A CIF optimized filter is built-in, which reduces artefacts for CIF output formats (to be used in combination with
the prescaler set to
1
2
scale); see Table 13.
Fade-in and fade-out of the filters is achieved by copying an original source sample each as first and last pixel after
prescaling.
Figs 25 and 26 show the frequency characteristics of the selectable FIR filters.
Table 13
FIR prefilter functions
The function of the prescaler is defined by:
An integer prescaling ratio XPSC[5:0] A0H[5:0] (equals 1 to 63), which covers the integer downscale range 1 to
1
63
An averaging sequence length XACL[5:0] A1H[5:0] (equals 0 to 63); range 1 to 64
A DC gain renormalization XDCG[2:0] A2H[2:0]; 1 down to
1
128
The bit XC2_1[A2H[3]], which defines the weighting of the incoming pixels during the averaging process:
– XC2_1 = 0
1 + 1...+ 1 +1
– XC2_1 = 1
1 + 2...+ 2 +1
The prescaler creates a prescale dependent FIR low-pass, with up to (64 + 7) filter taps. The parameter XACL[5:0] can
be used to vary the low-pass characteristic for a given integer prescale of
1
XPSC[5:0]
. The user can therewith decide
PFUV[1:0] A2H[7:6]
PFY[1:0] A2H[5:4]
LUMINANCE FILTER COEFFICIENTS
CHROMINANCE COEFFICIENTS
00
01
10
11
bypassed
1 2 1
bypassed
1 2 1
3 8 10 8 3
1 2 2 2 1
1 1 1.75 4.5 1.75 1
1
1 2 2 2 1
H-scale ratio
input pixel
=
H-scale ratio
XPSC[5:0]
XS1024
×
=