100600B
Conexant
2-1
2
2.0 Functional Description
2.1 UltraLock
TM
Functionality
2.1.1 The Challenge
The line length (the interval between the midpoints of the falling edges of
succeeding horizontal sync pulses) of analog video sources is not constant. For a
stable source such as studio quality source or test signal generators, this variation
is very small:
±
2 ns. However, for an unstable source such as a VCR, laser disk
player, or TV tuner, line length variation is as much as a few microseconds.
Digital display systems require a fixed number of pixels per line despite these
variations. The Fusion 878A employs a technique known as UltraLock to
implement locking the horizontal sync and the subcarrier of the incoming analog
video signal, generating the required number of pixels per line.
2.1.2 Operating Principles of UltraLock
UltraLock is based on sampling using a fixed-frequency, stable clock. Since the
video line length will vary, the number of samples generated using a
fixed-frequency sample clock will also vary from line to line. If the number of
generated samples per line is always greater than the number of samples per line
required by the particular video format, the number of acquired samples can be
reduced to fit the required number of pixels per line.
The Fusion 878A requires an 8 × Fsc (28.63636 MHz for NTSC and 35.46895
MHz for PAL) reference time source. The 8 × Fsc clock signal, or CLK x 2, is
divided down to CLK x 1 internally (14.31818 MHz for NTSC and 17.73 MHz
for PAL). CLK x 2 and CLK x 1 are internal signals and are not made available to
the system. UltraLock operates at CLK x 1 although the input waveform is
sampled at CLK x 2 then low-pass filtered and decimated to CLK x 1 sample rate.
At a 4 × Fsc (CLK x 1) sample rate there are 910 pixels for NTSC and 1,135
pixels for PAL/SECAM within a nominal line time interval (63.5
µ
s for NTSC
and 64
µ
s for PAL/SECAM). For square pixel NTSC and PAL/SECAM formats,
there should only be 780 and 944 pixels per video line, respectively. This is
because the square pixel clock rates are slower than a 4 × Fsc clock rate; for
example, 12.27 MHz for NTSC and 14.75 MHz for PAL.
UltraLock accommodates line length variations from nominal in the incoming
video by always acquiring more samples, at an effective 4 × Fsc rate, than are
required by the particular video format and outputting the correct number of
Содержание Fusion 878A
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