ADS Technologies USBAV-708 INSTANT VIDEOMPX, User Manual

Page: 52 / 116

The
Guide
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Here points above the rate-distortion curve evaluate as good decisions, those
below the curve as bad decisions. The best decisions fall towards the upper-left
of the plot (higher quality and lower bit spend).
It follows that the encoder must derive this rate-distortion curve from somewhere.
The curve is approximated by some function inside the encoder accepting a
value that changes the shape of the curve. This value is
Lambda , and you can
manipulate it via the Feedback window.
Begin encoding a video using either
Slow or Slowest performance/quality
modes. From the feedback window set
Show picture to Encoded frame and enable
the Lambda control in the bottom toolbar.
Pause encoding at any suitable frame and
vary the
Lambda control, observing the
effect on image quality and the number of
bits spent for the frame. By altering Lambda
you are manipulating the shape of the rate-
distortion curve.
Increasing Lambda (dragging the slider
to the left) causes a sharper rise in the
rate-distortion curve, meaning larger
improvements are demanded for an
increase in bit spend.
Decreasing Lambda (dragging the slider
to the right) causes the rate-distortion
curve to rise more slowly, meaning
lesser improvements are acceptable for
an increase in bit spend.
If the rate-distortion curve becomes too steep quality will be degraded because
the encoder will not be able to achieve the quality demanded of it for a given rise
in bit spend. If the rate-distortion curve becomes too flat quality will be degraded
because the encoder will waste a lot of bits for very marginal improvements,
reducing the number of free bits available overall.
Deriving the optimal value for Lambda is a very difficult process and under most
circumstances you should not over-ride the encoder default. The encoder will
normally vary Lambda automatically during encoding depending on
circumstances, over-riding instead fixes Lambda for the duration of one pass.
Lambda returns to its default mode of operation at the beginning of each pass.
Qu
ali
ty
Bits
Higher Lambda
Low er Lambda