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IQBRK00/01
B12
IQBRK00OPS 05/04/07
www.snellwilcox.com
Version 1 Issue 6
B12.77
Appendix
1
The
Firewall
What is the function of a firewall?
The firewall protects a digital signal output against
propagation of errors or disruptions in the input
signal. It ensures that the signal stream at that
output is continuously valid no matter what
happens at the input. If the input suffers any
dropout, loss of data, or break or discontinuity in
the carrier then this will not be reflected in the
output. Default valid data will be used to pad the
output stream such as video black, a freeze, or
audio silence.
The firewall does not ensure the pictures or the
audio is what you want to see or hear of course, a
corrupted signal can lead to loss of original
content. It will ensure that whatever valid content
you have it will get through however. In short it
ensures the integrity, not the content, of the digital
stream.
How a firewall equipped product behaves
The essential behavior of a system containing a
firewall is that:
The input to the unit containing the firewall can be
removed and later replaced with another signal
and during this sequence the unit following the one
with the firewall will see no disturbance at its input.
Why use a firewall?
Any equipment downstream of a Snell & Wilcox
product with firewall protection can expect to be
fed with a continuous input stream. Dropouts in the
signal, disturbances due to switching and errors in
the composition of the signal will not penetrate the
firewall and will therefore not corrupt operations
downstream. The examples below illustrate where
firewalls are of most benefit.
At the beginning of a chain
They are excellent for use at the beginning of a
chain. Traditionally a damaged signal would be
passed by all equipment in a chain. This meant
that each unit would lose lock on the digital signal
it was passing. The damage in the signal would
immediately appear at the input to every device all
the way through to the last point in the chain.
When the signal was restored, each unit in the
chain might take a few seconds to recover. The
next unit would not start recovery until the previous
unit had locked to the signal again. The effect
could be a huge delay in restoring the signal by the
end of the chain as each unit recovered one by
one. A minor flaw in the input signal such as a tiny
break in the carrier could lead to several seconds
of disruption on the output.
To protect MPEG encoders
Many people have found to their regret that MPEG
encoders do not survive a break in the integrity of
their input signal. They often corrupt their output
and take a long time to recover. Video
synchronizers can help but they only protect the
video. Loss of the audio would prove terminal even
though the integrity of the video was maintained.
For transmission encoders this could mean a
break in the output. For recording encoders such
as those in a video server this often meant the
recording was lost. Placing a Snell & Wilcox
firewall product in front of any such encoders will
ensure that they never get disrupted in this way
even though there may be a break in the incoming
content.
How a firewall is tested
A variety of equipment is used to test the firewall
behavior. The test consists of analyzing the data
downstream of the firewall product looking for
discontinuities in the signal stream. While it is
intended that all products fed by a firewall
equipped
unit
will
accept
the
signal
as
uninterrupted, it is accepted that there is a wide
variety of real-world performances. Therefore the
reference product used to test whether the output
streams are continuous is the Snell & Wilcox
IQMUX01 for both AES and SDI signals.
Performance of firewall equipped products
versus genlock mode
The tables below summarize the behavior of
products for different genlock modes:
Synchronizer Mode
Referenced Input locked
Free run
SDI
Video
Firewall
No firewall
Firewall
AES
Firewall
Firewall
Firewall