LMH0031 Datasheet, PDF (11/31 Pages) National Semiconductor (TI) – SMPTE 292M/259M Digital Video Deserializer Descrambler with Video and Ancillary Data FIFOs

Device Operation (Continued)
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FIGURE 1. Optional Input Biasing Scheme
The SMPTE descrambler receives NRZI serial data, con-
verts it to NRZ, then decodes it to either 10-bit standard
definition or 20-bit high definition parallel video data using
the reverse polynomial X⁹ + X⁴ + 1 as specified in the
respective standard: SMPTE 259M, SMPTE 344M (pro-
posed) or SMPTE 292M. The data reception bit order is
LSB-first. All data processing is done at the parallel rate.
The LMH0031 incorporates circuitry that implements a
method for handling data that has been subjected to LSB
dithering. When so enabled, data from the de-scrambler is
routed for de-dithering. The De-Dither Enable bit in the
VIDEO INFO 0 control register enables this function. De-
dithering of data present in the vertical blanking interval can
be selectively enabled by use of the V De-Dither Enable bit
in the VIDEO INFO 0 control register. The initial condition of
De-Dither Enable and V De-Dither Enable is OFF.
The descrambler supplies signals to theTRS character de-
tector which identifies the presence of the valid video data.
The TRS character detector processes the timing refer-
ence signals which control raster framing. TRS (sync) char-
acters are detected and the video is aligned on word bound-
aries. Data is re-synchronized with the parallel word-rate
clock. Interraction and operation of the character alignment
control signals and indicators Framing Mode, Framing En-
able and NSP (New Sync Position) is described later in this
datasheet.
The LMH0031 implements TRS character LSB-clipping as
prescribed in ITU-R BT.601. LSB-clipping causes all TRS
characters with a value between 000h and 003h to be forced
to 000h and all TRS characters with a value between 3FCh
and 3FFh to be forced to 3FFh. Clipping is done after de-
scrambling and de-dithering.
Once the PLL attains lock, the video format detector pro-
cesses the received data to determine the raster character-
istics (video data format) and configure the LMH0031 to
handle it. This assures that the parallel output data will be
properly formatted, that the correct data rate is selected and
that Ancillary Data and CRC/EDH data are correctly de-
tected and checked. Supported parallel data formats or sub-
formats may belong to any one of several component stan-
dards: SMPTE 125M, SMPTE 267M, SMPTE 260M, 274M,
295M or 296M. Refer to Table 4 for the supported formats.
(See also the Section Application Information section for
handling of other raster formats or format extensions devel-
oped after this device was designed). The detected video
standard information is passed to the device control system
and saved in the control registers from whence it may be
read by the user.
The LMH0031 may be configured to operate in a single
video format by loading the appropriate FORMAT SET[4:0]
control data into the FORMAT 0 control register. Also, the
LMH0031 may be configured to handle only the standard-
definition data formats by setting the SD ONLY bit or only the
high-definition data formats by setting the HD ONLY bit in the
FORMAT 0 control register. When both bits are reset, the
default condition, the part automatically detects the data rate
and range.
Aligned and de-processed parallel data passes into a
variable-depth video FIFO prior to output. Video FIFO depth
from 0 to 4 registers is set by a 3-bit word written into the
VIDEO FIFO Depth[2:0] bits in the ANC 0 control register.
The video FIFO permits adjustment of the parallel video data
output timing or delay at a parallel word rate. The occurence
of corresponding TRS indicator bits, EAV, SAV and NSP, in
the control register corresponds to the input register position
of the FIFO. This positioning permits a look-ahead function
in which the alignment status of the video data can be
determined up to four parallel clock periods prior to the
appearance of that data at the parallel data output.
The parallel video data is output on DV[19:0]. The 20-bit
parallel video data is organized so that for HDTV data, the
upper-order 10 bits DV[19:10] are luminance (luma) infor-
mation and the lower 10 bits DV[9:0] are colour difference
(chroma) information. SDTV data use the lower-order 10-bits
DV[9:0] for both luma and chroma information. (The SDTV
parallel data is also duplicated on DV[19:10]). V_CLK is the
parallel output word rate clock signal. The frequency of V_CLK
is appropriate to either the HD or SD data being processed.
Data is valid between the falling edges of a V_CLK cycle. Data
may be clocked into external devices on the rising-edge of
V_CLK. The DV[19:0] and V_CLK signals are LVCMOS-
compatible.
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