CXD1968AR
COFDM Signal Acquisition
There are three stages in acquiring and locking to a COFDM signal. First, the COFDM demodulator must
acquire the signal; then the FEC must synchronize to the Viterbi decoder output; finally, the FEC must
synchronize to the transport stream.
The CXD1968AR contains a new auto-recover algorithm. This is a hardware implementation which
automatically reacquires an interrupted signal without intervention from the host controller. After the initial
setup this minimizes the load on the host processor.
The recommended method of operating the demodulator is to;
1. Configure the clocks and registers as described in the application note EAN-0066 and explained above.
2. Enable the core which will result in an acquired channel if present.
3. Set the auto-recover enable bit for automatic acquisition on any channel change.
This uses the default acquisition mode and will acquire the channel regardless of the DVB-T mode.
Default Acquisition
In this mode, nothing need be known about the transmission parameters (except the channel bandwidth).
The COFDM demodulator tries all the guard intervals in the two modes (2K and 8K) and then waits until it
has received a whole super-frame before decoding the TPS data and configuring itself to receive that
particular COFDM signal. Signaled TPS data in the stream is valid for the NEXT super-frame so the
decoder may have to wait for 5 frames – 1 symbol (= 339 symbols) before it can use the received
transmission parameters.
Typical acquisition times for this mode: 125ms (2K) / 500ms (8K).
Settings for Faster Acquisition
There are several short cuts that can be taken to speed up acquisition. These depend on how the control
registers are programed. It is not generally recommended that the user overrides the default settings
unless the host processor has sufficient resources available.
Transport Stream Locking
FEC lock occurs at the input of the Reed-Solomon decoder when the number of sync bytes (47h) measured
exceeds a certain value in the SET_SYNC_DETECT register. Transport Stream (TS) lock occurs at the
output of the Reed-Solomon decoder, and uses the same mechanism as FEC lock.
The SET_SYNC_DETECT register can be used to adjust the criteria for gaining/losing lock. The higher the
number of bytes required to gain lock, the harder it is to acquire lock, but, once acquired, the chip should
be less likely to lose lock. Lock can be lost either by counting the number of missed sync bytes (Decrement
mode) or one missed sync byte can lose lock straight away (Reset mode). Reset and decrement modes
are independently settable for TS lock and FEC lock.
Example: 0xD8 0x86 0x67
; recommended configuration
Reacquisition
It may be necessary to reacquire a COFDM signal, for example if the user were to select a different RF
channel, because of reception problems with a poorly sited set-top aerial or to simplify channel scanning.
It is recommended to use the auto-recovery algorithm and default acquisition settings to minimize host
processor overhead.
The auto-recovery block has two modes of operation, scanning and zapping.
Scanning mode will tell the auto-recovery state machine to attempt only 1 acquisition when enabled, after
which it falls back to an idle state. The status can read by the host processor, ie. TPS and/or TS lock at the
end of acquisition. Note that ready bits will indicate if the status readings are valid, ie. the process has
completed.
Zapping or channel change mode, also to be used during static operation to automatically recover a lost
signal. In this mode the auto-recovery state machine will continuously monitor the demodulator status and
reacquire when TPS/TS lock is lost. It is not necessary to read the status as it will continue to operate until
disabled.
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SONY [ SONY CORPORATION ]
