BT848 ETC, BT848 Datasheet - Page 25

• Manufacturer Part Number: BT848
• Description: Single-Chip Video Capture for PCI
• Manufacturer: ETC

Bt848/848A/849A
Single-Chip Video Capture for PCI
Brooktree
Operation Principles of
®
The Challenge
UltraLock
UltraLock
The line length (the interval between the midpoints of the falling edges of succeed-
ing 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.
variations. The Bt848 employs a technique known as UltraLock to implement
locking to the horizontal sync and the subcarrier of the incoming analog video sig-
nal and generating the required number of pixels per line.
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-frequen-
cy 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 par-
ticular video format, the number of acquired samples can be reduced to fit the re-
quired number of pixels per line.
crystal or oscillator input signal source. The 8*Fsc clock signal, or CLKx2, is di-
vided down to CLKx1 internally (14.32 MHz for NTSC and 17.73 MHz for PAL).
CLKx2 and CLKx1 are internal signals and are not made available to the system.
UltraLock operates at CLKx1 although the input waveform is sampled at CLKx2
then low pass filtered and decimated to CLKx1 sample rate.
for PAL/SECAM within a nominal line time interval (63.5 s for NTSC and 64 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, i.e., 12.27 MHz for
NTSC and 14.75 MHz for PAL.
video by always acquiring more samples, at an effective 4*Fsc rate, than are re-
quired by the particular video format and outputting the correct number of pixels
per line. UltraLock then interpolates the required number of pixels in a way that
maintains the stability of the original image despite variation in the line length of
the incoming analog waveform.
Digital display systems require a fixed number of pixels per line despite these
The Bt848 requires an 8*Fsc (28.64 MHz for NTSC and 35.47 MHz for PAL)
At a 4*Fsc (CLKx1) sample rate there are 910 pixels for NTSC and 1,135 pixels
UltraLock accommodates line length variations from nominal in the incoming
L848A_A
F
UNCTIONAL
D
ESCRIPTION
UltraLock
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