Digital Signal Processing Reference
In-Depth Information
IMAGE
Bursty video stream due to
downscaling!
Might cause unstable output!
background
Alpha
Blending
Mixer
foreground
Scaler
Valid video signal
IMAGE
IMAGE
Invalid video signal
Figure 20.3. A down-scaler output connected an alpha blender.
Another example is shown in
Figure 20.3
. This shows what
happens when a down-scaler output is connected to a function,
such as an alpha blender, which blends two or more video
streams. A down-scaler also produces fewer pixels than it takes in.
Therefore the output of this function is bursty
valid pixels fol-
lowed by not valid data. A down-scaler is also called a decimation
filter and as a rule these filters will produce bursty video data.
As before, we should insert a double-buffering frame buffer
between the scaler and the alpha blending mixer to minimize the
chance of starving all the downstream modules and creating
unstable output at the alpha blending mixer.
As a matter of fact, 2-D FIR filters, 2-D Median filters and
deinterlacers (with Bob algorithm) all produce bursty video
streams. Depending on the application requirement, a double-
buffering frame buffer might need to be connected after each of
these functions.
e
20.2 Calculating External Memory Bandwidth
Required
In video-signal chains the two functions that require lots of
external memory transactions are the frame buffer and the dein-
terlacer. The frame-buffer memory transactions are described
