Nagra PORTABLE ANALOGUE AUDIO TAPE RECORDER Instruction Manual - page 26
can affect the ambient sound and can be a greater nuisance than saturation by a brief sound. To
avoid this phenomenon, it is necessary to reduce the recovery time of the limiter. Detailed
instructions as to how this can be done will be sent on request. The limiter can be switched off by
setting the Main Function selector to position RECORD - NO LIMITER. This may be advantageous
for certain recordings, e.g. gunshots, explosions, where tape saturation is desired.
AUTOMATIC LEVEL CONTROL (ALC)
The Nagra 4.2 is fitted with an Automatic Level Control. It replaces the manual potentiometer when
the selector switch placed on the left of the meter is on the position "Automatic". In the position "1-
Automatic", only microphone No 1 has its sensitivity controlled automatically. Microphone No 2 is
controlled by its respective potentiometer. In "2-Automatic", the ALC circuit controls both
microphone inputs. The operation of the ALC is complex. It is useful to place the meter switch in the
position "Compression" as needle will indicate by how much the sensitivity of the amplifier chain has
been reduced. The extreme left position corresponds to a maximum gain; the extreme right to a
reduction of gain or compression of approximately 30 dB.
When a signal, whose amplitude is sufficient for the maximum recording level to be exceeded,
arrives from the microphone, the Automatic Level Control will instantaneously reduce the gain to
avoid over-modulation (similar to the limiter). If the signal is short, the Automatic Level Control
concludes that it is accidental, and forgets it rapidly, i.e. it returns to its previous sensitivity. On the
other hand, a longer signal is considered useful.
The sensitivity is therefore memorized and maintained. This avoids the increase of background
sound level between words. It can happen, however, that an interfering signal is long. This will
therefore produce a long reduction of sensitivity. To avoid that, the Automatic Level Control has a
circuit of rapid recovery, which enters into action if the level remains less than -10 dB for about 1.5
seconds. Therefore, if a long and loud noise upsets the recording, the recorder can be reset to its
normal sensitivity by keeping quiet for 1½ seconds.
Experience has shown that the Automatic Level Control allows an absolutely automatic recording to
be made for reporting interviews etc. and of a quality, which is difficult to attain with manual control.
MEASUREMENT OF SIGNAL LEVEL
The Nagra 4.2 is equipped with a device for measuring the signal level. Normally, this is a
modulometer, but a v.u. meter is available on special request.
The meter is connected to the "Direct" signal. During recording it measures this level. If the line
amplifier is connected to the "Direct" signal ("Line and Phones" switch in "Direct"), the measurement
will thus be of the signal sent along the line. On the other hand, if the "Line and Phones" switch is in
"Tape", the level sent along the line will be independent of the "Direct" signal and will consequently
not be measured. In any case, the level of this signal cannot be altered. A signal recorded at
nominal level will give a line signal of nominal level.