Philips Semiconductors
Product specification
Multimedia bridge, high performance
Scaler and PCI circuit (SPCI)
SAA7146A
The ‘Chroma Signal Key’ generates an alpha signal used
in several RGB formats. Therefore, the processed UV data
amplitudes are compared with thresholds. A logic 1 is
generated, if the amplitude is within the specified
amplitude range. Otherwise a logic 0 is generated. Keying
can be switched off by setting the lower limit higher than
the upper limit!
7.9.3
HORIZONTAL PHASE SCALING
In the phase correct Horizontal Phase Scaling (HPS) the
pixels are calculated for the geometrically correct,
orthogonal output pattern, down to 1⁄4 of the prescaled
pattern. A horizontal zooming feature is also supported.
The maximum zooming factor is at least 2, even more
dependent on input pattern and prescaling settings.
The phase scaling consists of a filter and an arithmetic
structure which is able to generate a phase correct new
pixel value almost without phase or amplitude artefacts.
The required sample phase information is generated by a
sample phase calculator, with an accuracy of 1⁄64 of the
pixel distance. The up/downscaling with this circuitry is
controlled by the scaler register parameters XSCI and XP.
As the fine scaling is restricted to downscales >(1⁄4 of the
fine scalers input pixel count), XSCI is also a function of
the prescaling parameter XPSC.
7.10 Binary Ratio Scaler (BRS)
7.10.1 GENERAL DESCRIPTION
The BRS is the second scaler in the SAA7146A. The BRS
is supposed to support different encoder applications while
the HPS is processing video data. The BRS does not
support clipping.
The mainstream application of the BRS is to read data via
PCI, e.g. a QCIF-formatted video data to proceed with
horizontal and vertical upscaling to CIF-format and place it
at the encoder’s disposal (normal playback mode).
With NIP = Number of Input Pixel/line (at DD1 input) and
NOP = Number of desired Output Pixels/line, XSCI is
defined to:
To support CCIR encoder and square pixel encoder, an
active video window as input for the BRS can be defined.
It will prevent black pixels being displayed at the end of the
line or at the bottom of the field.
XSCI = INT [(NIP/NOP) × 1024/(XPSC + 1)]
The maximum value of XSCI = 4095. Zooming is
performed for XSCI values less than 1024. The number of
disqualified clock cycles between consecutive pixel
qualifiers (at the fine scalers input) defines the maximum
possible zoom factor. Consequently, zooming may also be
a function of XPSC. It should be noted that if the zooming
factor is greater than 2, some artefacts may occur at the
end of the zoomed line.
The BRS supports only the YUV 4 : 2 : 2 video data format
(see Section 7.11.2). The used DD1 I/O data format is
8-bit. The BRS uses video DMA Channel 3 (FIFO 3) which
is only available, if the HPS is not in planar mode or writes
back clip information.
Vertical upscaling is supported by means of repeated
reading of the same line via PCI. Vertical downscaling is
achieved by line dropping.
7.9.4
COLOUR SPACE MATRIX (CSM), DITHER AND
Horizontal downscaling is performed by an accumulating
FIR filter. The downscaling is available for the inbound
mode and the upscaling is available for the outbound
mode (see Figs 23 and 24).
• Vertical ratios: 4, 2, 1, 1⁄2 and 1⁄4; select with BRS_V
• Horizontal ratios: 8, 4, 2, 1, 1⁄2, 1⁄4 and 1⁄8; select with
γ-CORRECTION
The scaled YUV output data is converted after
interpolation into RGB data according to CCIR 601
recommendations. The CSM is bypassed in all YUV
formats or monochrome modes.
The matrix equations considering the digital quantization
are:
BRS_H.
If the data is sent from DD1 to PCI, the processing window
for the BRS scaling unit is defined in the acquisition control
(see Section 7.8.7).
R = Y + 1.375V
G = Y − 0.703125V − 0.34375U
B = Y + 1.734375U.
A dither algorithm is implemented for error diffusion. ROM
tables are implemented at the matrix output to provide
anti-gamma correction of the RGB data. A curve for a
gamma of 1.4 is implemented. The tables can be used to
compensate gamma correction for linear data
representation of RGB output data.
1998 Apr 09
76
NXP [ NXP ]
