Appendix E HDTV Output Mode
CX25870/871
E.11 Recommended Output Filters for HDTV & SDTV
Flicker-Free Video Encoder with Ultrascale Technology
E.11 Recommended Output Filters for HDTV &
SDTV
According to the SMPTE274M standard that governs the 1080i ATSC resolution,
“the Y signal for Component HD video output shall have a bandwidth nominally
of 30 MHz. In addition, the PR and PB signals shall have the same bandwidth as
that of the associated Y signal at the analog originating equipment.” Other filter
criteria found in the various SMPTE standards include the amplitude limit for
ripple tolerance in the passband of +/- 0.5 dB relative to insertion loss at 100 kHz.
For Group-Delay, SMPTE states that the ”group delay in the filters (should be)
sufficiently tight to produce good performance while allowing the practical
implementation of the filters themselves" (from SMPTE 274M and
SMPTE293M(720p) standards).
As result of these criteria, during High Definition Output Mode, each
CX25870 DAC output requires a low pass filter with a passband from DC to
30 MHz. while adhering to the aforementioned group delay and passband ripple
tolerances. Unfortunately, the filtering requirements for standard definition
television are quite different in several areas than filtering for HDTV. The most
important difference is that standard definition standards such as NTSC, PAL,
and SECAM require a much lower 8 MHz. passband starting at DC than HDTV.
This bandwidth difference coupled with the differing voltage amplitudes of
the signals themselves forced Conexant to design a new low pass filter with a
wider passband to accommodate the HD outputs. After extensive testing and
cost/benefit trade-offs, the company recommends that any customer using the
encoder for both its standard definition and high definition capabilities design-in
the low pass filter found in Figure E-4. This filter has been shown to exhibit many
of the desired roll off, ripple, and passband characteristics defined in the
aforementioned SMPTE standards.
E-18
Conexant
100381B
CONEXANT [ CONEXANT SYSTEMS, INC ]
