Albalá Ingenieros IPA3000C01, Руководство пользователя

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Albalá Ingenieros | Manual IPA3000C01
An additional reason that a small buffer size is recommended when the IPA3000C01
operates in this mode is that buffer size affects the recovery time before seamless
switching can be performed. The larger the buffer size the longer the seamless
switching recovery time.
In order to perform seamless switching the buffer size must always be greater than
the difference in the delay between the signals being switched - otherwise these
delays cannot be compensated for. The recommended buffer size is equal to twice
the difference in delay between the two signals. Since buffer size is specified in ASI
packets whereas the delay is normally measured in milliseconds the bitrate of the
two signals being switched must be known in order to calculate the proper buffer
size. Several example are listed as follows.
The difference in delay between two signals is 100ms and their bitrate is
20Mbit/s. The minimum buffer size for proper delay compensation between the
signals measured in number of packets is (0.1 s x 20.000.000 bit/s) / (188
bytes/packet x 8 bits/byte) = 1329 packets. The recommended buffer size is twice
this size, or 2659 packets.
The difference in delay between two signals is 100ms and their bitrate is
20Mbit/s. Furthermore, one of the signals suffers from jitter of up to 100ms. In
this case the difference in delay between the two signals can be as high as
200ms. Following the recommendation for a buffer size of twice the maximum
delay yields 2 x (0.2 s x 20.000.000 bit/s) / (188 bytes/packet x 8 bits/byte) = 5319
packets.
If the difference in delay between the two signals is unknown, the information
provided by the IPA3000C01's control software can be used to determine this value.
In order to find the difference in delay the buffer must be set to a sufficiently large
value and then the delay indicators for the two signals at the inputs should be read
while the seamless switching indicators are active to calculate the difference
between them. Then, the steps outlined in the previous examples can be followed in
order to adjust the buffer size. If the input signals are not the same or the buffer is
not large enough to properly perform delay compensation, the delays for the selected
inputs will not settle and the seamless switching indicators will not activate.
When using this operating mode another parameter that is highly important to
configure to ensure that seamless switching can be performed is the recovery time.
This is the time period that must elapse from the moment that the error status of a
given input clears until switching occurs (automatic mode) or becomes possible
(semi-automatic mode). In Generic DVB seamless mode the synchronization time
between inputs depends upon the buffer size and the bitrate of the signals, therefore
the recovery time must be set high enough so that the module can realign the
frames. The maximum time that the module requires in order to realign the transport
streams is twice the size of the buffer. For example:
The difference in delay between two signals is 100ms, their bitrate is 20Mbit/s
and the buffer size has been set to 3000 packets. The minimum recovery time
necessary to assure seamless switching is 2 x (3000 packets x 188 bytes/packet x
8bits/byte) / 20.000.000 bit/s = 0.45 seconds.
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