Akai GX-65 MKII

DOLBY HX PRO Headroom extension
system
The Dathy Ly DOP at.
The Dolby HX PRO system (HX stands for Headroom eX-
tension) is a system designed to improve the saturation level
of cassette tapes by controlling the bias current. This im-
proved saturation level means clearer sound when recording
classical music, or other music that contains many instru-
ments. Although it is a Dolby system the HX PRO headroom
extension system should not be confused with Dolby NR as
it is not a noise reduction system.
The Dolby HX PRO headroom extension system is basically
a circuit used in cassette decks that makes use of the fact
that the amount of high frequency energy that the tape can
hold varies according to the amount of bias current present.
Less bias current will result in more headroom for the high
frequencies, but at an increase in low-frequency distortion.
More bias helps to lower distortion, but results in loss of high
frequency “headroom”.
During recording, the Dolby HX PRO headroom extension
circuit monitors the recording signal. When high frequency
peaks that are too high are detected, the bias is reduced
momentarily so that those peaks can be accomodated. This
is especially helpful for signals which have high levels at
high frequencies. When these peaks are no longer present,
the bias is restored to the normal level.
Even when using a normal position tape during recording,
the Dolby HX PRO headroom extension system works to im-
prove its saturation level to almost that of a metal position
tape. Because the system is built into the recording circuits,
it is working whenever recording is taking place. Cassette
tapes recorded with the Dolby HX PRO headroom extension
system can be played back without any problems on any
cassette deck.
Block diagram of the DOLBY HX PRO headroom extension system
input O
Signal
Re rr errr ne a re
Control
signal
Bias
+
Signal
Bias + Signal
* Dolby noise reduction and HX Pro headroom extension
manufactured under license from Dolby Laboratories Li-
censing Corporation. HX Pro originated.by Bang & Olufsen.
z VCA: Voltage Controlled Amplifier
“DOLBY”, the double-D symbol OO and “HX PRO” are
trademarks of Dolby Laboratories Licensing Corporation.