Digital Signal Processing Reference
In-Depth Information
The clocked-video output component latches the underflow
indication
the underflow sticky-bit is set when an underflow
occurs. You can use the debug master or software on an
embedded processor to check this bit. As with the overflow bit in
the clocked video master, if embedded software is monitoring
and resetting the bit then reading it from the debugger will not be
reliable.
The video trace monitor can also indicate when there are
problems with underflow. Normally the stream going in to the
clocked-video output is controlled by “ready”, but if there is
a problem with underflow then “ready” will not be asserted
during the re-synchronization process. The resulting lack of
backpressure is visible in the captured video packet summaries.
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21.8 Free-running Streaming Video Interfaces
The clock rate within a flow-controlled video system is nor-
mally set to sufficient bandwidth on the streaming ports for
a picture of maximum resolution to be transmitted (with a small
amount of overhead to allow for jitter).
The flow control signals
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“ready” near the video output or
“valid” near the video input
ensure that processing does not
run faster than the incoming video stream. If processing runs
too far ahead then frames will be missed and the picture will be
jerky.
This can happen if the design has instantiated multiple triple-
buffer components. Triple buffers do not flow-control their
inputs or their outputs (except temporarily when waiting for
memory accesses). A video pipeline between two triple buffers
will run at the processing clock speed rather than staying in sync
with the video frames.
If part of the video pipeline is allowed to free run then this will
waste memory bandwidth. It can also reduce picture quality as
the input triple buffer will duplicate frames to keep its output
busy while the output triple buffer will delete frames to match the
frame rate on the output. The overall effect will be that some
frames are output multiple times while other frames are not
output at all.
The solution to the free-running problem is to replace all but
one of the triple buffers with a double-buffer component. The
double buffer does no frame rate conversion so will not allow its
input and output to run more than one frame apart. This will
provide flow control to the central part of the system.
The video trace monitor can also be used to detect free-
running streaming video components. Examining the flow-
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