OPTIMOD OPERATION
3-15
The dual-band limiter can produce a maximum of 25dB of limiting. This is more than
adequate for protection limiting. It is important not to overdrive the limiter past 25dB
gain reduction; the sound will rapidly become highly distorted.
Because the gain control section of the Protection Limiter structure is not peak-sensing,
its output contains peak overshoots that must be eliminated by further processing. The
look-ahead peak limiter does this. Its attack time is essentially instantaneous and its re-
lease time is very fast, so it acts more like a clipper than a traditional peak limiter. How-
ever, compared to a simple clipper the bandwidth of the modulation distortion that it
produces is much lower. Hence the modulation distortion is more likely to be psycho-
acoustically masked by the desired program material.
Modulation of Sine Waves
There is an important and sometimes confusing consequence of this system design: The
system will not permit sine waves to reach 100% peak modulation. It will restrain sine
wave modulation to a lower level — typically 7dB below 100% (45% modulation).
Therefore, in its normal
OPERATE
mode the Protection Limiter structure will not pass
an externally-generated line-up tone at 100% modulation; it will produce limiting that
constrains the tone to approximately 45% modulation.
This is a direct consequence of the level detection’s being power-sensing. For a given
peak level, sine waves have very high average power by comparison to program mate-
rial. To preserve natural sound, the processing must reduce their peak level below the
peak level of program material to preserve consistent average power at the limiter’s out-
put. This is a characteristic of any limiter that achieves natural-sounding dynamic per-
formance and that does not modulate program loudness according to the peak-to-average
ratio of the input signal.
Almost all program material will produce frequent peaks at 100% modulation at the
6200’s output. Program material that does not produce such peaks has an unusually low
peak-to-average ratio and will sound naturally balanced when applied to the transmission
system below 100% peak modulation.
To Next Page
Loading...
