3.0 PC Board Considerations
CX25870/871
3.6 Applications Information
Flicker-Free Video Encoder with Ultrascale Technology
3.6.5 Clock and Subcarrier Stability
The color subcarrier frequency is derived directly from the XTALIN/XTALOUT
ports when EN_XCLK=0. The color subcarrier frequency is derived directly from
the main clock input, CLKI, when EN_XCLK=1 (slave interface). In either case
any jitter or frequency deviation from 13.500 MHz (XTALIN/XTALOUT) or the
CLKI (slave interface) rate will be transferred directly to the color subcarrier.
Jitter within the valid clock cycle interval will result in hue noise on the color
subcarrier on the order of 0.9–1.6 degrees per nanosecond. Random hue noise can
result in degradation in the AM/PM noise ratio (typically around 40 dB for
consumer media such as Videodiscs and VCRs). Periodic or coherent hue noise
can result in differential phase error (which is limited to 10 degrees by FCC cable
TV standards).
Any frequency deviation of CLKI from the transmitted clock (i.e., CLKO)
will challenge the subcarrier tracking capability of the destination receiver. This
may range from a few parts-per-million (ppm) for broadcast equipment, to 100
ppm for industrial equipment and to >100 ppm for consumer equipment. Greater
subcarrier tracking range generally results in poorer subcarrier decoding dynamic
range. So, receivers that tolerate jitter and wide subcarrier frequency deviation
will introduce more noise in the decoded image. Crystal-based clock sources with
a maximum total deviation of 50 ppm (NTSC) or 25 ppm (PAL, SECAM) across
the temperature range of 0 °C to 70 °C produce the best results for consumer and
industrial applications. In rare cases, temperature-compensated clock sources
with tighter tolerances may be warranted for broadcast or more stringent PAL
(e.g., type I) applications.
Some applications call for maintaining correct Subcarrier-Horizontal (SC-H)
phasing for correct color framing. This requires subcarrier coherence within
specified tolerances over a four-field interval for 525-line systems or 8 fields for
625-line systems. Any clock interruption (even during vertical blanking interval)
which results in mis-registration of the CLKI input or nonstandard pixel counts
per line, can result in SC-H excursions outside the NTSC limit of 40 degrees
(reference EIA RS170A) or the PAL limit of 20 degrees (reference EBU
D23-1984).
In slave interface, any deviation exceeding the 50 ppm (NTSC) or 25 ppm
(PAL, SECAM) limits of the number clock cycles between HSYNC* falling
edges may result in a switch to Master Mode.
A list of recommended crystals and crystal vendors is contained in
Appendix B.
3-28
Conexant
100381B
CONEXANT [ CONEXANT SYSTEMS, INC ]
