Nagra NAGRA 4.2 Instruction Manual - page 21
Limiter
The Nagra 4.2 is equipped with a safety limiter which instantaneously reduces the gain of the recording ampli-
fier when the signal exceeds the maximum level providing that the "RECORD" with limiter position of the Main
Function Selector has been selected. This limiting evidently causes distortion, but it is less objectionable than
that caused by saturation of the tape. Normally, the limiter will only be brought into action when accidents occur.
It is possible that, in the middle of a dialogue, for example, a short but high power sound exceeding the maxi-
mum level can occur. Under these conditions it is preferable to saturate rather than to limit because the satu-
ration will be practically unnoticeable due to the short duration of the sound. Moreover, the limiter requires a
certain recovery time before the normal sensitivity of the chain is re–established. This sensitivity variation can
allect the ambient sound and can be a greater nuisance than saturation by a brief sound. To avoid this phe-
nomenon, 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 poten-
tiometer. In "2–AUTOMATIC", both microphone inputs are controlled by the ALC circuit. The operation of the ALC
is complex. It is useful to place the meter switch in the position "COMPRESSION" as the needle will indicate by
how much the sensitivity of the amplifier chain has been reduced. The extreme left position corresponds to a max-
imum 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 and thus results in a long sensitivity
reduction. To avoid this, the Automatic Level Control has a circuit of rapid recovery which enters into action if
the level remains at less than –10 dB for about 1 1/2 seconds. Therefore, if a long and loud noise upsets the
recording, the recorder can be reset to its normal sensitivity by keeping quiet for 1 1/2 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
VU–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 be that 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.