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5-28 Programming On-board Devices and Memories
Analog Video Input Interface
The Bt835 supports composite and S-video sources in NTSC, PAL, and
SECAM video formats. Four composite, or three composite and one
S-video sources are selected through an on-chip 4:1 analog multiplexer.
The Bt835 integrates two flash Analog-to-Digital Converters (ADCs)
which provide high-performance 8x oversampling of the NTSC/PAL color
subcarrier. Oversampling of the analog signal enables simple external anti-
aliasing filters and enhances digital filtering performance on-chip.
An on-chip PLL multiplies a 14.318 MHz reference frequency to 28.64
MHz for NTSC and 35.48 MHz for PAL, for 8x Fsc sampling of the
baseband video signal. The fixed, integral relationship between the
subcarrier frequency (Fsc) and the sample rate optimizes the performance
of the Y/C separation. The Bt835 also incorporates auto-format detect
capability through which it can sense the presence of an NTSC, PAL, or
SECAM video source on the input after power up and configure itself
accordingly.
Ultralock™
Employing Conexant patented Ultralock technique, the Bt835 achieves
both horizontal and subcarrier synchronization to the incoming analog
video signal. It will always generate the required number of pixels per line
from an analog source in which the line length can vary by as much as a
few microseconds. Ultralock‘s digital locking circuitry enables the Bt835
to quickly and accurately lock on to virtually any video signal. Since the
technique is completely digital, Ultralock can recognize unstable signals
caused by VCR headswitches or any other deviation and adapt the locking
mechanism to accommodate the source. Ultralock uses non-linear
techniques that adapt the locking mechanism automatically. This removes
the requirement a conventional genlock circuit to reset the locking range
to accommodate a particular system setup.
Digital Video Output Interface
The video pixels are delivered as a continuous video stream running at a
4x Fsc rate which is synchronous to the ADC sample clock. YCrCb and
YUV 4:2:2 pixels are provided over an 8-bit data bus to the SM731, using
data and clock which operate at 2x the video pixel rate. The Bt835 also
has the capability of embedding the video control signals in the digital
pixel stream in either ViP mode or ByteStream mode. The video
field/frame synchronization signals and others are coded into the pixel
data stream using out-of-range luminance and chrominance data values.