Program Clock Reference
The MPEG Standard defines a Program Clock Reference (PCR) to be included in a transport stream to allow a decoder to lock to the
27 MHz clock being used by the encoder.
The DVB standards require a PCR to be present in a transport stream every 40 ms or less, (but it is possible to configure the unit to
generate PCRs less frequently).
The PCR can be embedded in a component PID, (i.e. video or audio), or it can be carried on its own PID.
When multiple services are carried in a single transports stream, each service may have its own PCR, or they may all reference the
same PCR, (
single PCR Operation
).
Single PCR Operation
If it is desired to use a single PCR for multiple services then the video and audio encoders must all be locked to the same 27 MHz
clock. This can be achieved in a number of ways:
The Mux SCR Source can be set to internal. Then all option cards in the unit will receive and use the internally generated 27 MHz
clock and the related PCR value. However this does not ensure that the output of other units are synchronised.
If an External Sync card is fitted, the Mux SCR Source can be set to ‘Ext Sync’. The unit’s internal 27 MHz reference is then locked to
the external sync reference input. So if all units are fed with the same sync reference signal they will all be locked to the same 27
MHz clock. The units will however all have different PCR values, but as they are all locked to the same 27 MHz clock they will all
have a fixed delta between them. The downstream multiplexer can calculate the deltas between the streams and re-stamp the
PCRs, PTSs and DTSs in the output stream to all have a common reference.
Another way of configuring a single PCR system is to lock all the video and audio sources to the common reference 27 MHz clock
before they are sent to the encoders. Then the units can all have MUX SCR Source set to video, i.e. each encoder will lock to the 27
MHz of its video input. Therefore all the outputs will be locked to the same 27 MHz, but without the need for an Ext Sync card. The
rest of the system operates in the same way with the Multiplexer re-stamping the time references to a common reference.
Real Time Clock
Overview
The Host Controller card has a real time clock, which is battery powered when the chassis is unpowered, so that the time and date is
maintained. This clock is designed to be accurate to ±1 minute per month. The battery is capable of powering the clock for over two
years.
User Configurable Parameters
The user configurable parameters associated with the clock are defined below
SNTP Server
An SNTP server can be configured to update the clock and calendar or else the user can update the time and date. Any changes to
Parameter
Values
Description
Local Time
hh:mm:ss
Current local time in hours minutes and
seconds
Local Date
dd:mm:yyyy
Current date in day month and year
© Ericsson AB 2011. All rights reserved
© Ericsson AB 2011. All rights reserved
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