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AM-10HD / Aug 2008
A M - 1 0 H D G U I
There are three controls associated with the HF
Protect algorithm that need explaining.
These controls operate in concert with each other
and are “tied” together inside the algorithm in (some
non‑intuitive) ways that result in the best possible
audio quality over the widest possible variety of
program material. These controls are:
HF Protect
When the HF Protect checkbox is selected the
HF Protect algorithm is enabled.
The HF Protect algorithm is adjustable from
3kHz to 16kHz in 1kHz steps, and the HF Protect
frequency control sets the frequency where program
controlled HF Protection begins. Higher settings allow the HF Protect algorithm
to “barely touch” high frequency material while lower settings cause protection to
occur at ever lower frequencies.
Ceiling
Adjustable from 0dB to ‑48dB, this control sets the dynamic audio threshold
where HF Protect operation begins. Higher settings cause the HF Protect algorithm
to be less aggressive while lower settings cause it to be more so.
Recovery
When a high frequency “splash event” has been detected (via proprietary looka‑
head means) and is subsequently prevented the HF Protect algorithm must recover
to the response that has been set by the Bandwidth control (explained above). The
Recovery control adjusts how fast this action takes place. Faster settings cause opera‑
tion to be restored more quickly while slower settings do the reverse. Settings that
are too slow can cause the audio to seem to dull when it shouldn’t be. The setting
is adjustable from 33.0 mSec to 330.0 mSec.
HyperMono
Occasionally the incoming audio material is either stereo with weak L‑R levels,
is “mono” but with slightly offset left and right channel levels, or is mono with
poor left to right channel phase behavior. When encoded by a perceptual codec, this
material is treated as a stereo source and coding bits are expended trying to code it
as a stereo signal when it would sound better at the decoder if it were simply coded
as a pure mono signal. In fact, poor or improper L‑R content is one of the primary
reasons for the “swimming” sound heard at low bitrates when the coded is operat‑
ing in a stereo mode.
The HyperMono algorithm analyzes the incoming program audio to determine
when such audio program conditions exist, and when they do, it creates a pure mono
signal. This aids the codec’s ability to more accurately encode the signal and the
result is a drastically cleaner and less fatiguing sound. HyperMono does not affect
normal stereo material, nor does it inadvertently create mono from a source that is
truly stereo.