CX25870/871
1.0 Functional Description
Flicker-Free Video Encoder with Ultrascale Technology
1.3 Device Description
1.3.28 Burst Generation
The subcarrier burst generation is a function of the video standard (e.g., NTSC,
PAL, or SECAM), the subcarrier frequency increment (MSC[31:0]), and the burst
horizontal begin and end register settings (HBURST_BEGIN[7:0] and
HBURST_END[7:0]). To calculate the value of HBURST_END[7:0] subtract
128 from the desired location in clock cycles. The burst will automatically be
blanked during the horizontal sync preventing invalid sync pulses from being
generated. Burst blanking is automatically controlled by the selected video
format. Burst rise and fall times are automatically generated by the device. The
burst amplitude is programmed by the BST_AMP[5:0] field.
1.3.29 Video Amplitude Scaling and SINX/X Compensation
Both the luminance and chrominance video amplitudes can be scaled by the
MCR, MCB, and MY registers. This allows various colormetry standards to be
achieved, and can also be used to boost the chroma to compensate for the sin x/x
loss of the DACs. Appendix A show the range of values achievable and values for
various video formats.
The DAC output response is a typical sinx/x response. For the composite
video output, this results in a slightly lower than desired burst and chroma
amplitude value. This is compensated for, to some extent, by choosing an output
filter that boosts the frequency response slightly. Another method which can be
used effectively, and is used by default in the auto configuration modes, is to
boost the burst and chroma gain as programmed by the BST_AMP and
MCR/MCB register values by a factor of(x/sinx). The amount of sinx/x amplitude
reduction is calculated by:
sinx/x = sin (π * Fsc/Fclk) / (π * Fsc/Fclk)
Fsc = desired subcarrier burst frequency
Fclk = present input clock frequency
1.3.30 Chrominance Disable
The chrominance subcarrier can be turned off by setting the DCHROMA bit to a
logical 1. This disables the subcarrier burst as well, providing luminance-only
signals on the CVBS output and a static blank level on the chrominance output.
100381B
Conexant
1-53
CONEXANT [ CONEXANT SYSTEMS, INC ]
