To automatically maintain the balance of the two signals feeding the subtraction
stage under dynamically changing signal conditions, a feedback "servo" circuit
compares the levels of these two signals after full-wave rectification (FWR), and
adjusts the VCA control to force them to be equal (see Figure 4).
The C output is created by adding (instead of subtracting) the same two
balanced VCA output signals.
Since the L and R signals are being adjusted by the VCAs, this is known as the
decoder's Left-Right axis. There is also a Front-Back axis in the decoder,
operating orthogonally to the Left-Right axis, which uses the same basic
technique to servo the L+R (front) signal versus the L-R (back) signal to reduce
crosstalk in the L and R outputs.
By creating a feedback system around the logic steering process, certain benefits
can be attained:
The antiphase signals feeding the output matrix are able to be matched to
the unwanted crosstalk signal levels with high precision and low circuit
complexity, thereby ensuring high channel separation.
The steering logic control time constants are within a feedback loop, so as
the loop gain changes, so does the response time. Dynamically, there is a
greater "speed to smoothness ratio" than is possible in a feed-forward
system, leading to an ideal balance between fast action and stable
operation while using relatively simple circuitry.
In Pro Logic, the Front-Back and Left-Right axes are controlled by a single
slow/fast circuit. If either axis wants to steer fast, they both go fast. Only when
both want to go slow can the logic decoder switch to the slow mode. In Pro Logic
II, each axis operates independently, so they decide how fast to go based on
their individual conditions. Also, the logic speed range is continuously variable in
Pro Logic II, rather than having two fixed rates as in Pro Logic.
These are just some of the more obvious aspects of the new decoder design
principles. The end result is that the Pro Logic II decoder can process any kind of
source material without changing the core logic steering parameters. Since
movies often contain music to a significant degree, a decoder needs to handle
dialogue, music, and effects equally well without creating side-effects.
