Digital Signal Processing Reference
In-Depth Information
CVO
Clipper
Scaler
Bursty video stream due to
clipper!
Might underflow CVO
Figure 20.2. Clipping a video frame before it is passed it on to a video scaler.
When frame dropping and frame repeating are not allowed,
the frame buffer needs to provide a double-buffering function.
However, if frame dropping and/or frame repeating are allowed,
the frame buffer provides a triple-buffering function and can be
used to perform simple frame-rate conversion.
When triple buffering is in use, three frame buffers are used in
external RAM. As in the case of double buffering, the reader and
writer components are always locking one buffer to respectively
store input pixels tomemory and read output pixels frommemory.
The third frame buffer is a spare buffer that allows the input and
output sides to swap buffers asynchronously. Triple buffering
allows simple frame-rate conversion to be performed when the
input and output are pushing and pulling frames at different rates.
Also, by further controlling the dropping or repeating behavior, the
input and output can be kept synchronized.
Let's look at some examples where you may need to insert
a frame buffer.
Figure 20.1
shows an example where we are
clipping a video frame before passing it on to a video scaler. This
is commonly used to zoom into a portion of an image.
First we clip a corner of the video frame. The rest of the pixels
are now not valid
only the clipped pixels go on to the scaler.
While the frame rate (frames/sec) is the same, the pixel rates that
are input to the clipper and to the scaler are different. If the
clipper output is passed directly on to the scaler, we will have
periods of valid pixel data and periods of not valid pixel data
e
we
e
effectively have a bursty video stream after the clipper.
The bursty video stream would cause the scaler to stall from
time to time
and this will impact downstream processing. To
mitigate that effect, a double-buffering frame buffer should be
inserted between the clipper and the scaler.
e
