SAA7118 Datasheet, PDF (51/169 Pages) NXP Semiconductors – Multistandard video decoder with adaptive comb filter and component video input

Philips Semiconductors
Multistandard video decoder with adaptive
comb filter and component video input
Preliminary specification
SAA7118
8.4.2 HORIZONTAL SCALING
The overall horizontal required scaling factor has to be split
into a binary and a rational value according to the
equation:
H-scale ratio = ^{o--i--nu---pt--p-u--u--t--t-p--p-i--xi--x-e--e-l--l}
H-scale ratio = _{X-----P----S----C1----[--5---:--0---]} × _{X-----S----C-1---Y0---2-[--1-4--2---:--0---]}
where the parameter of prescaler XPSC[5:0] = 1 to 63 and
the parameter of VPD phase interpolation
XSCY[12:0] = 300 to 8191 (0 to 299 are only theoretical
values). For example, ¹â„_3.5 is to split in ¹â„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 SAA7118.
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.
8.4.2.1 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 ¹â„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 ¹â„₂ scale); see Table 11.
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 ¹â„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 ¹â„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 ¹â„_XPSC[5:0]. The user can therefore
decide between signal bandwidth (sharpness impression)
and alias.
Equation for XPSC[5:0] calculation is:
XPSC[5:0] = lower integer ofN--N---p--p-i--xi-x--_-_--o--i-nu----t
where,
the range is 1 to 63 (value 0 is not allowed);
Npix_in = number of input pixel, and
Npix_out = number of desired output pixel over the
complete horizontal scaler.
The use of the prescaler results in a XACL[5:0] and
XC2_1 dependent gain amplification. The amplification
can be calculated according to the equation:
DC gain = [(XACL[5:0] − XC2_1) + 1] × (XC2_1 + 1)
It is recommended to use sequence lengths and weights,
which results in a 2^N DC gain amplification, as these
amplitudes can be renormalized by the XDCG[2:0]
controlled _2--1--N-- shifter of the prescaler.
The renormalization range of XDCG[2:0] is 1, ¹â„₂... down
to ¹â„₁₂₈.
Other amplifications have to be normalized by using the
following BCS control circuitry. In these cases the
prescaler has to be set to an overall gain of ≤1, e.g. for an
accumulation sequence of ‘1 + 1 + 1’ (XACL[5:0] = 2 and
XC2_1 = 0), XDCG[2:0] must be set to ‘010’, this equals ¹â„₄
and the BCS has to amplify the signal to ⁴⁄₃ (SATN[7:0]
and CONT[7:0] value = lower integer of ⁴⁄₃ × 64).
The use of XACL[5:0] is XPSC[5:0] dependent.
XACL[5:0] must be <2 × XPSC[5:0].
XACL[5:0] can be used to find a compromise between
bandwidth (sharpness) and alias effects.
2001 May 30
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