ANNEX I
September 2013
98
ANNEX I ALC / LIMITER detailed explanations
The analog inputs of the Nagra Seven are managed using a completely new concept. Audio limiters, automatic
gain control (ALC) and digital filters are all achieved digitally through a dedicated DSP (digital signal processor).
The principal advantages are to create a better and cleaner ALC, give limiter and filter processing with the
possibility of adjusting more parameters and thus achieving better microphone preamplifiers due to less
electronics in the analog audio signal path.
Below, we explain various different parameters of the ALC/LIMITER to give a clearer understanding of the
various adjustments.
Ganging:
This setting is used to lock the ALC/lim of both channels together for a stereo pair. When an ALC/LIM process is
active on a particular channel due to a loud sound, the other channel gain will be limited by the same amount,
keeping the gain of the stereo pair correlated.
Preset:
This is a quick way to select predetermined parameters for a standard limiter or a standard ALC, avoiding the
necessity to understand all the parameters of the ALC/LIM programming process. The preset drop-down will
be cleared if any of the Attack, Delay, Release and Ration parameters have been altered, indicating that non-
standard parameter has been chosen.
Headroom:
The ALC/LIM processing uses a principle called “scaling”: where the analog gain of the input is decreased by a
specified factor while the input gain is increase digitally by the same amount, resulting in a signal of the same
level; this scaling factor is called headroom. The DSP uses this headroom to handle the ALC/LIM processing, by
reducing the gain up to headroom factor.
The headroom of the ALC/LIM and the headroom of the filters are same; the Nagra Seven simply uses the larger
of the two. In other words, if the headroom for the limiter is set to 18 dB, and a filter is subsequently selected
with a headroom of 24 dB, then the limiter will be active at 24 dB rather than 18 dB.
The ALC system and limiters use the same process, they differ only in the parameters used to set them (open
speed factor, close speed factor, delay): a limiter uses very fast speed factor and an ALC uses a slower factor.
Beware not to specify an unnecessarily large headroom, the disadvantage of the scaling is to increase to
noise floor level by a factor equal to the headroom; larger headroom can noticeably increase the noise floor.
Target:
The target setting specifies the level (dBFS) not to be overshot. With a flat ratio the ALC/LIM process will not
limit the gain while the level is below this value. With a non-flat ratio the ALC/LIM process will already start
limiting the gain before this value is reached.
With the Attack parameter set to “immediate”, this limit will never be overshot except if the gain limitation is
greater than the headroom.
Input attenuation:
As the ALC/LIM processing is done digitally, the analog input gain is still controlled by the user, but physically
the gain is decrease by the amount of the headroom; with an input attenuation of -12 dB and a headroom of
24 dB the microphone preamplifier gain is physically -36 dB (-12dB – 24dB). This is true as long as the
microphone preamplifier gain does not reach its maximum attenuation of -45 dB; when reached the
microphone preamplifier gain will stay at -45 dB and the “rest” of the attenuation will limit the digital gain that
the DSP can use, as a result reducing the headroom and the output level; with an input attenuation of -30 dB
and a headroom of 24 dB the microphone preamplifier gain is physically -45 dB (max reached) and not -54 dB
(-30 dB – 24 dB), the “rest” of the attenuation is -9 dB (-54 dB - -45 dB), limiting the headroom at -15 dB (-24
dB - -9 dB).
To Next Page
