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ever a recording is made using Dolby
B.
The filler blocks fre·
quencies higher Ihan abcut t5 kHz in the signal.
When playing previously recorded tapes, a
very
slight
extension of high·frequency bandwidlh may be obtained by
switching
the MPX filter OFF (button out). But remember to
switch back in when making recordings with Dolby
B.
If
you make any recordings with the Dolby NR switched
OFF, the MPX filter
is optional.
It
is also optional when you
are
using Dolby C
NR,
since the Dolby C circuit contains some
ultrasonic filtering of
its
own.
11. DOLBY
NR.
The recorder
is
equipped with two types
of
Dolby noise reduction
.
Dolby C provides the greatest
quieting of tape hiss, important when taping
"live"
musical
performances or wide· range recordings (digilal records, for
example). Dolby B provides optimum compatibility with older
tape recorders, portable and automobile stereo lape
players,
and should also be used
when
playing most pre· recorded
music
cassettes.
Each type of NR
involves
complementary processes
that are intended
to
match each other in
both
recording and
playback. Thus if a tape
is
recorded with Dolby C noise
reduction, it must also be played back with the Dolby C circuit
operating in order for the benefits of the Dolby
system
to be
obtained. Similarly, if Dolby B is used in recording it should
be used in playback as well. To avoid confusion, it is recom·
mended that
you
label each cassette "B" or
"c"
when it
is
recorded, to
indicate
the
type
of noise reduction used.
Both types of Dolby NR operate by selectively compress·
ing
the dynamic range
of
the middle and high frequencies in
the signal during recording
.
I.e.
the circuit selectively boosts
the level of those middle·to·high frequency sounds that occur
at
a naturally low level, so that these sounds are recorded on
the tape at a level substantially higher than the tape's own
hiss. Then, in
playback,
these signals are expanded back to
their original dynamic range
;
the quiet middle
and
high fre-
quency sounds that were boosted are finalty cut back down
to their original levels,
and
the
tape's
hiss is reduced at the
same
time.
Dolby Band C differ in the amount
of
complementary
boostlcut that they
provide,
and in its frequency
distribution.
Dolby B operates mainly at high frequencies and suppresses
tape hiss by a maximum of 10 dB. Dolby C operates at both
middle and high frequencies and
reduces
hiss by as much as
20 dB. The two Dolby systems are
not
interchangeabl~.
If you
record with Dolby B and play back with Dolby C (or
vice versa)
the tonal balance of Ihe
musical
sound will be
substantially
altered.
Also, any brightening or dulling
of
the
sound
that
is
caused by a mismatch between the recorder and the tape will
produce mistracking in Ihe Dolby circuit and yield an exag·
gerated brightening or dulling of Ihe playback sound. So when
Ihe Dolby circuits are to be
used,
it
is
especially importanllhat
the
TAPE SELECTOR be correctly set to match Ihe tape
(both
:in
recording
and
in
playback)
.
12. TAPE SELECTOR. This
switch
selects the correcl
bias (for recording) and equalization
(for
playback)
,
for each
of
three standard types of cassette
lape.
Whenever
you
record
or playa tape, this switch musl be set to match the
tape.
If you
are not careful aboul
this,
the clarity and lanai balance of your
recordings will vary a great deal, especially when Dolby noise
reduction is
used.
f. Normal. The
largest variety of cassel
Ie
lapes
are
intended
for use wilh Ihe NORMAL setting
of the
TAPE
SELECTOR.
Look al the cassette package for
a statement
of
"normal
bias
"
or
"120
fLsec"
equalization.
In general, tapes
described as
"ferric
oxide,
"
"low noise,
"
or having the Roman
numeral f in their
deSignation,
are intended for
the
NORMAL
selting.
5
It.
Cr02' The
cr0
2
setting
is intended for
tapes
employing
chromium
dioxide or
the
"chrome·equivalenl"
cobalt·doped
high·bias
ferric
oxide.
Usually these are identified
as
requiring
"high bias,
"
"70
fLsec"
equalization, or have
Ihe
Roman
numeral II in their designation.
fV. Metal. The Metal
selling
should be used with metal·
particle tapes, which may be labeled with Ihe Roman
nu·
merallV.
While the
TAPE
SELECTOR switch provides Ihe large
changes in
recording bias and playback equalization that are
needed to accommodate the
Ihree
standard
classes of
tape,
there are significant
brand-la-brand
differences
in sensitivity
and frequency response among tapes
within
each
group.
If
you wish, you can use the 6125 10 make recordings on
any brand and type of
tape,
using any setting of Ihe TAPE
SELECTOR, withoul harming either the recorder or the
tapes.
And by experimenting with
vanous
tapes
you
may
exploit
their
different
tonal
qualities to produce pleasing results-using the
recorder,
in effect, as a
tone
control.
But the basic purpose of a high· fidelity
tape
recorder is
to
provide an exact replica of the original
sound,
capturing and
preserving it for later
playback.
This is what
the
NAD 6125
is designed to do.
If
you want to make the most accurate
recordings, you must use tapes similar to those used
in
the
factory for calibrating the
recorder's
circuits. This is especially
important
when Dolby noise reduction is
used,
because any
departure from accurate high·frequency response in a record·
ing (due to a mismatch between recorder and tape)
will
cause
decoding errors
in
the Dolby
circuit
during playback-and
this,
in
turn,
will
cause
larger
errors
in
frequency
response,
making
the sound excessively bright
or
dull.
For information
on
tapes that are compatible with the NAD
6125, see your NAD
dealer.
13. RECORDfNG LEVEL. This knob controls the strength
of the audio signal fed to the tape when a recording
is
being
made.
The signal
strength
is shown on
the
recording
level
display,
above.
The RECORD LEVEL
control
is actualty
two co·axial
controls
(one for
each
stereo
channel), friction·coupled
so
that
they normally rotate
together.
It
is
possible to alter the stereo
balance by adjusting the two controls differently. The left chan·
nel is controlled by the central part of the knob, while the right
channel is controlled by the outer part (the
sleeve around
the
circumference
of the
knob).
14. RECORDING LEVEL Of
SPLAY.
This dual row of
LEOs displays the peak level of the signal in each channel
during recording
or
playback.
The LEOs ranging from
-
20 dB
to 0 dB
are
green
and
indicate normal recording levels. The
+
2,
+
4,
and
+
6 dB LEOs are red and
indicate
the maximum
permissible levels.
In general, you
should
adjust the recording level so that
the loudest portions
of
the music are recorded at levels be·
tween
-
4 and
+
2 on
the
display.
With
music that spans a
large range
of volume
levels,
such
as a
symphony,
this may
mean that the quiet portions of the music will be recorded at
indicated
levels of
-
20 dB
or
even lower
(with
no LEOs
illuminated);
this
is normal.
To learn how to
sel
optimum recording
levels,
the follow·
ing experiment
is
recommended. Select
a
typical recording of
music and record an excerpt from
it
several times in
succes-
Sion,
setting
the recording level progressively higher each
time.
For example
set the
maximum peak
level
at
about
-
10 dB
the
first
time,
and repeat
it
at about
-
4, at
0,
at
+
4,
and finally at over
+
6
dB.
Then play back
the
recordings and
lislen
for
symptoms of tape
saturation in the
sound.
(Adjust
your amplifier
's
volume control
to compensate
for the
different
loudness levels
of
the
sample recordings,
so that
you can
focus your attention on the quality of the sound without being
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