User Manual

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1.4 OPERATIONAL DETAILS
All the sections of the ATSC TRANSMITTER, as showed in the block diagram above, are described in this
chapter in order to give the user a general understanding of the operations of the equipment.
1.4.1 TS INPUT MANAGEMENT
The ATSC TRANSMITTER is equipped with 2 ASI inputs and 2 SMPTE-310M inputs. Each input standard
is available with two inputs in order to enable redundant operation for the transport stream input. You can
decide to operate with SMPTE input or with ASI input depending on your network interfacing needs. In
either case a complete redundancy is guaranteed thanks to a duplicated input.
The ATSC transmission scheme is built over a very stable and accurate data stream clock. The SMPTE-
310M data clock shall have the exact standardized clock frequency. The modulator module takes its
synchronization from this input data stream to be able to recover the same clock frequency. If an external
clock reference is used (10 MHz), the global clock synchronization might be plesiochronous (i.e. not perfect
synchronization between the multiplexer clock and the modulator reference clock). In that case, it is
recommended to enable the "bit rate adaptation" on the ATSC TRANSMITTER.
When this mode of operation is enabled, the input processing module discards or inserts Null Packets in order
to adapt the MPEG-TS input bit rate to the precise bit rate defined by the ATSC recommendation. PCR re-
stamping is then executed accordingly.
The switching is not expected to be seamless. However, in case of feeding the exact data stream on both
inputs (Primary & Secondary), then the ATSC TRANSMITTER performs a seamless switching from one to
the other input. Switching is always performed on a packet basis.
The user may enable or disable the automatic mute when an error on the SMPTE-310M or on the ASI input
is detected (Mute on Input Error feature). The delay for the detection of the lost of SMPTE-310M is 1
second from its occurrence. In case the user disables the automatic mute, null packets are inserted in order to
feed the modulator section with the exact ATSC bit rate.
The user can enable the PRBS test mode (see Test Modes paragraph for more informations) in order to
ignore the real input streams and use a pseudo-random bit sequence. If PRBS test mode is active, the RF
output is not muted even if the “Mute on Input Error” feature is enabled and no valid stream is present at the
input.
- Redundacy management
When set in Automatic Switching mode (default), the input management unit automatically switches to the
other input if no valid MPEG trasport stream is detected on the active input. If the user chooses to operate in
Manual Switching mode, the selected input remains active even if it isn't valid.
The periodic occurrence of the Sync byte in the MPEG-TS packets is used to decide the validity of the
stream.