263
1
2
20
264
282
283
Vertical Blanking
First Field
Active Video
First Field
Vertical Blanking
Second Field
Active Video
Second Field
Line
HSYNC
VSYNC
VSYNC transitioning at the
same time as HSYNC going
active means the following
lines represent the first field
VSYNC transitioning half a line
before HSYNC going active
means the following lines
represent the second field
Derived
FID
Internal Modules
154
SPRUHI7A – December 2012 – Revised June 2016
Copyright © 2012–2016, Texas Instruments Incorporated
High-Definition Video Processing Subsystem (HDVPSS)
1.2.8.2.5.8 Rationale for FID Determination By VSYNC Skew
FID determination by VSYNC skew is a method for field ID determination derived from the analog NTSC
and PAL interlaced specifications. Under this method, the sending device will not be providing a FID
signal. NTSC has 525 total lines split between two fields. PAL has 625 total lines split between two fields.
Each of these interlaced standards support an odd number of lines.
Let’s consider just the 525 active line NTSC signal. For the sake of consistency, let’s call Line 1 the first
line of the 2-field pair and Line 525 the last line of the 2-field pair.
Figure 1-103. Example of 525-line FID Determination By VSYNC Skew
A waveform is shown in
. VSYNC is defined to go active at the same time as HSYNC for the
first line of the first field in a two-field picture pair. For this first field, VSYNC will go inactive after Line 20.
For the second field, VSYNC will go active in the middle of Line 263 to signal that Line 264 is the start of a
vertical blanking interval. When HSYNC for Line 264 arrives, coinciding with the vertical blanking interval
for the beginning of the second field, VSYNC has already been active for half of Line 263. For the second
field, VSYNC will go inactive midway through Line 282 to indicate that Line 283 is active video. When
HSYNC for Line 283 appears, VSYNC has already been inactive for half a line.
By seeing whether VSYNC transitions at the beginning of a line or whether it transitions at the midway
point of a line, one can determine whether the upcoming group of lines represents the first field or the
second field. The derived FID is shown in dashed lines.
The analog NTSC specification defines the field ID changing part way into the vertical blanking. That is,
the first few lines of vertical blanking belong to the previous field and the next several lines of vertical
blanking belong to the upcoming field. The VIP Parser saves one channel of the entire vertical blanking
interval between two active video fields into a single buffer. The hardware does not discriminate between
whether the vertical blanking lines belong to the bottom of the previous field or those belonging to the start
of the next field. This usage model is consistent with vertical ancillary data capture for embedded sync
mode of operation.
Obviously, FID Determination by VSYNC skew cannot be used when framing does not use the VSYNC
signal but rather relies on the ACTVID signal instead.
1.2.8.2.5.9 ACTVID Framing
Instead of an HSYNC signal, the VIP Parser can use ACTVID framing as described in
. Under
ACTVID Framing, VSYNC is used to separate vertical blanking lines from active video lines.
FID determination by VSYNC Skew is not allowed for ACTVID framing because there is no HSYNC input
signal in this mode. Also, the VSYNC transition window is not employed. VSYNC is captured at the first
pixel of each ACTVID grouping of pixels. Lines are separated by ACTVID transitioning inactive.
