Model
8901B
Operation
FM
De-emphasis
FUNCTIONS
Pre-Display,
25ps,50ps,
75ps.
750ps
DESCRIPTION
The de-emphasis networks equalize the pre-emphasis often encoded on broadcast FM signals. This
type of equalization provides a simple means of reducing audible noise on FM communication systems.
Pre-emphasis gives a
6
dB
per octave, high-frequency boost
to
the baseband audio before modulating
the carrier. De-emphasis reverses the boost
after
demodulation at the receiver. The
3
dB
comer of the
pre-emphasis
or
de-emphasis network
is
usually expressed
as
a time constant. The relationship between
the time constant,
r0,
and the 3dJ3 comer frequency,
fo,
is
fo
=
1%
The 3dB comer frequencies
of
the de-emphasis equalizers (typically accurate
to
f3%)are listed in the
following table:
I
FM De-emphasis
I
Time
Constant
(PS)
3
dB
Frequency
(Hz)
25
50
75
750
6366
31 83
21 22
21 2.2
PROCEDURE
To add de-emphasis
to
the demodulated FM, select the key corresponding
to
the desired time constant.
(Unless AUDIO INPUT has been selected, the de-emphasized FM is available at the MODULATION
OUTPUT/AUDIO INPUT.) To also display the de-emphasized FM, select PRE DISPLAY.
HP-IB
In local operation,
to
turn
off
one of the de-emphasis functions, press the lighted key a second time.
In remote operation, turn the equalizers
off
with code
PO.
(HP-IB code PO also turns
off
the
PRE
DISPLAY function.)
FM
De-emphasis
3-159
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