SAA7108AE Datasheet, PDF (69/208 Pages) NXP Semiconductors – HD-CODEC

NXP Semiconductors
SAA7108AE; SAA7109AE
HD-CODEC
9.3.2.1 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 the sharpness
and aliasing effects.
The FIR prefilter stage implements different low-pass characteristics to reduce the
anti-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 24.
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.
The equation for the XPSC[5:0] calculation is: XPSC[5:0] = lower integer of -_N-^N---_p--^p-_i--^ix--^x-_{_}--^_--_o--ⁱ-_u-ⁿ---_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.
SAA7108AE_SAA7109AE_3
Product data sheet
Rev. 03 — 6 February 2007
© NXP B.V. 2007. All rights reserved.
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