GS2971A Datasheet, PDF (36/152 Pages) Gennum Corporation – Integrated audio clock generator

The device first determines if a valid serial digital input signal has been presented to the
device. If no valid serial data stream has been detected, the serial data into the device is
considered invalid, and the LOCKED signal is LOW.
Once a valid input signal has been detected, the asynchronous lock algorithm enters a
“hunt” phase, in which the device attempts to detect the presence of either TRS words or
DVB-ASI sync words.
By default, the device powers up in auto mode (the AUTO/MAN bit in the host interface
is set HIGH). In this mode, the device operating frequency toggles between 3G, HD and
SD rates as it attempts to lock to the incoming data rate. The PCLK output continues to
operate, and the frequency may switch between 148.5MHz, 74.25MHz, 27MHz and
13.5MHz.
When the device is operating in manual mode (AUTO/MAN bit in the host interface is
LOW), the operating frequency needs to be set through the host interface using the
RATE_DET[1:0] bits. In this mode, the asynchronous lock algorithm does not toggle the
operating rate of the device and attempts to lock within a single standard. Lock is
achieved within three lines of the selected standard.
4.7.2 Signal Interruption
The device tolerates a signal interruption of up to 10μs without unlocking, as long as no
TRS words are deleted by this interruption. If a signal interruption of greater than 10μs
is detected, the lock detection algorithm may lose the current data rate, and LOCKED
will de-assert until the data rate is re-acquired by the lock detection block.
4.8 SMPTE Functionality
4.8.1 Descrambling and Word Alignment
The GS2971A performs NRZI to NRZ decoding and data descrambling according to
SMPTE 424M/SMPTE 292/SMPTE 259M-C and word aligns the data to TRS sync words.
When operating in manual mode (AUTO/MAN = LOW), the device only carries out
SMPTE decoding, descrambling and word alignment when the SMPTE_BYPASS pin is set
HIGH and the DVB_ASI pin is set LOW.
When operating in Auto mode (AUTO/MAN = HIGH), the GS2971A carries out
descrambling and word alignment to enable the detection of TRS sync words. When two
consecutive valid TRS words (SAV and EAV), with the same bit alignment have been
detected, the device word-aligns the data to the TRS ID words.
TRS ID word detection is a continuous process. The device remains in SMPTE mode until
TRS ID words fail to be detected.
NOTE 1: Both 8-bit and 10-bit TRS headers are identified by the device.
NOTE 2: In 3G Level B mode, the device only supports Data Stream 1 and Data Stream
2 having the same bit width (i.e. both data streams contain 8-bit data, or both data
streams contain 10-bit data). If the bit widths between the two data streams are different,
the GS2971A cannot word align the input stream, and switches in Data-Through mode.
GS2971A 3Gb/s, HD, SD SDI Receiver
Data Sheet
54311 - 2
September 2012
36 of 152