Linear Acoustic LA-5448, User Manual

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Linear Acoustic LA-5448 AC-3 Frame Synchronizer/Rate Shaper User Guide
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Chapter 5: Troubleshooting
The Linear Acoustic LA-5448 is a very stable and reliable unit, and most problems can be
traced back either to mis-wiring causing incorrect signals to be applied to the unit or more
than likely to mis-configuration. In an effort to speed troubleshooting, some common
problems and their solutions are described below.
5.1 Problems and Possible Causes
Most problems can be traced back to the input signals applied to the LA-5448. Experience
has shown many early high definition satellite IRDs used for the reception of network sig-
nals to be rather unreliable when presented with changing transport streams. The input sta-
tus menus on the LA-5448 are very useful for tracking down data rate and audio coding
mode discontinuities, as well as for monitoring CRC errors.
5.1.1 Status Lights are Green, but Display Shows Random Data
To recap, the LEDs have three states: Off = no signal applied, Red= AES carrier detected
but AC-3 is not detected, Green = AES signal is present, correct data type marked. So how
is it that with all green LEDs that data can still be wrong? There are multiple places where
the contents of a typical AES signal are tagged with their identities. One is in the AES
header itself (unreliable), one is before the actual data (called the IEC header), and one is
in the data itself, in this case inside the AC-3 packets. Think of the LEDs detecting the first
two: AES presence, and IEC header present, while the display shows the last: AC-3
present.
Once common cause of this is directly attributable to satellite IRDs which will output a
blank AES stream with an IEC header that signifies AC-3, but with either a random or
blank audio payload (i.e. no AC-3 data is actually present). This can be caused by the re-
ceiver being set to an invalid or missing PID, or by the PID disappearing.
5.1.2 Program 4/REF Input LED is Amber
The reference signal applied is PCM and not AC-3. This is a perfectly acceptable condition,
and Amber is simply used to indicate that the input is valid, but is not AC-3 in nature. The
LA-5448 locks to the frequency of the reference input, and the payload is ignored. How-
ever, if the reference input is fed with an AC-3 signal, in addition to it providing the fre-
quency reference for the unit, the signal will also be frame synchronized, have silent frames
inserted if necessary, and be rate shaped if desired.
5.1.3 Unit Produces 448 kbps Output, but it is Silent
Stream Shaper is enabled (and by requirement, so is silent frame), and the input for the
channel in question is either missing, or errored.