Albalá Ingenieros | Manual
IPA3000C01
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Output buffer:
The IPA3000C01 includes a buffer at the output whose operation depends on the
operation mode of the module:
- DVB-T Mode: In this mode the user sets the network delay, equal to the delay that
the output TS must have with respect to the 1 PPS reference signal received by the
IPA3000C01, and the buffer absorbs the difference between the input and the
output. In order to guarantee seamless switching between inputs the TS output
delay must be 50ms greater than the TS input delays received. This difference is due
to the response time of the frame synchronization error detectors.
When the IPA3000C01 operates in this mode the delay fields in the "General" tab of
the control software display the delay of the transport streams received with respect
to the 1 PPS reference signal. The delay is measured at the input of the output buffer
after IP de-encapsulation.
- Generic DVB Mode: This operating mode is appropriate for work with signals that
suffer from high levels of jitter. In this case the buffer is used to absorb the jitter
introduced by the IP network and the error correction mechanisms. The output
latency depends upon the selected size of the buffer, which ranges from 32 to 14592
TS packets. The latency decreases as the bitrate increases and also decreases as the
size of the buffer decreases, however the smaller the buffer the lower the amount of
jitter that can be absorbed.
The PLL used to generate the output packet frequency of the IPA3000C01 can be
configured to operate with two different bandwidths: one wide and one narrow. The
wide bandwidth is best for transport streams with variable bitrates because it allows
the bitrate to vary without overflowing the buffer. The narrow bandwidth is best for
transport streams with a fixed bitrate because it allows extremely low jitter in the
DVB output frame.
The two most important components of the IP frame jitter consist of the jitter
introduced by the network itself due to the packet routing mechanisms and the jitter
introduced by the IP receiver when recovering dropped packets. This second source of
jitter depends upon the error correction scheme being used. The worst case is a burst
of 20 dropped and recovered IP packets within a transmission of seven TS packets per
IP packet (RTP with FEC and L = 20) which represents 140 TS packets. In order to
absorb the jitter created in such a case a buffer of at least 256 packets is necessary.
When the IPA3000C01 operates in this mode the delay fields in the "General" tab of
the control software display the latency of each input. The latency introduced by this
buffer affects both the DVB-ASI outputs and the DVB over IP outputs.
- Seamless Generic DVB Mode: This operating mode is appropriate for switching
between identical signals with stable bitrates and low jitter. In this case the buffer is
used to compensate for the delay between the two inputs and achieve seamless
switching. When operating in this mode a small buffer size and a narrow bandwidth
filter are recommended in order to minimize the jitter introduced in the output
signal during switching.
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