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GS9001 - DETAILED DEVICE DESCRIPTION.
The GS9001 contains all functional blocks required to implement
Error Detection and Handling according to SMPTE RP165. It
also provides Field, Vertical, and Horizontal timing information
as well as Ancillary Data and TRS-ID indication. The device
offers standard independent operation and an I
2
C serial
communications interface to allow reading/writing of error
flags, device configuration and video standards format. The
device can also be operated in stand alone mode without the
I
2
C interface with error flags available on dedicated output
pins. In all modes, the device latency is four clock cycles.
Automatic Standards Detection
This block analyses the incoming 8 or 10 bit data to determine
whether it is component or composite. In total, six standards
are automatically detected. For composite data conforming to
SMPTE 259M, the Timing Reference Signal and Identification
(TRS-ID) packet contains line and field information used to
detect the format. For component data conforming to SMPTE
125M, the TRS-ID packets for End of Active Video (EAV)and
Start of Active Video (SAV) are used to determine the format.
The TRS information is then used to determine whether the
composite signal is NTSC or PAL, or whether the component
signal has 13.5 MHz or 18 MHz luminance samples.
Noise immunity has also been included, to ensure that
momentary signal interruption does not affect the auto-
standards detection function. This built in noise immunity
results in delayed switching time between standards. Delays
range from as little as eight lines when switching between
component standards to as much as four frames when switching
between PAL and NTSC composite standards. The latter
delay is due to the method used to differentiate PAL and NTSC,
which counts the number of lines per frame and requires four
sequential frames before switching standards. Manual override
of the auto-standard feature is provided via the I
2
C interface,
for applications where the standards recognition delay is
intolerable. Standards indication is provided on multiplexed
output pins or via the I
2
C interface.
Control Logic
The control logic coordinates operation and extracts timing
signals such as vertical blanking, horizontal sync, field ID,
ancillary data indication and TRS-ID indication.
The vertical blanking interval signal is active during the digital
vertical blanking period for all signal formats. The horizontal
sync signal is provided as a pulse with a duration of one clock
period for every TRS-ID occurrence in composite video. For
component video, the horizontal sync is a positive going pulse
which starts at EAV and ends at SAV. Three field ID bits (pins
40, 41, 42) indicate the two fields for component video standards,
the four colour fields for composite NTSC or eight colour fields
for composite PAL.
The ancillary data indication allows external circuitry to identify
ancillary data in the data stream for extraction or masking.
The presence of ancillary data is indicated by a logic high that
extends from the Data ID word to the Checksum word of each
ancillary packet. These timing signals are available on
dedicated output pins and through the I
2
C communications
interface.
The control logic also verifies incoming data validity by checking
the occurrence of consecutive TRS-IDs. If the absence of
seven consecutive TRS-IDs is detected, a “NO TRS” flag is
output on pin 34. This flag is reset once seven consecutive
TRS-IDs occur.
CRC Calculation
A cyclic redundancy check (CRC) is calculated for each video
field according to the CRC-CCITT polynomial X
16
+X
12
+X
5
+1.
Separate CRCs are calculated for active picture and full field
to provide an indication that active video is still intact despite
possible full field errors. This allows the user to distinguish
between different classes of data errors, which yields the best
compromise in error detection for all types of equipment. In
order to provide compatibility between 8 bit and 10 bit systems,
all data words with values between 3FC
H
and 3FF
H
inclusive,
are recoded as 3FF
H
at the input of the polynomial generator.
Start and end points for the CRC calculation are as defined in
RP165 and depend on the standard and check field being
calculated. Calculated CRC words can be read through the
I
2
C interface.
CRC Comparison
The GS9001 can be configured for transmit or receive mode.
In receive mode, the calculated CRC is checked against the
incoming CRC embedded in the error data packet. Any
mismatch will generate status error flags to indicate transmission
related error flags in either active picture, full field or both. The
error flags resulting from CRC mismatch are full field error
detected here (
EDH
) and active picture
EDH
.
Ancillary Checksum Verification
The ancillary data checksums are also verified to ensure data
integrity. Ancillary data is preceded by the Data Header, Data
ID, Block Number and Data Count. The Data Count shows the
number of ancillary words contained in each ancillary data
packet. A checksum is calculated for each incoming ancillary
data packet and compared with the transmitted checksum.
Any difference is reported as an error via the ancillary
EDH
error flag. A separate
ANC EXT
error flag is also provided to
indicate corruption of the EDH data packet.
Error Flags and Formatting
This block performs the functions of error flag reporting and
recoding, EDH data packet construction, programmable
interrupt generation and interface with the I
2
C communication
block.
