Q2 Audio Compex F760X-RS User Manual - page 7
MODULATION:
If more than about 6dB of compression is used it is desirable to treat individual instruments or groups of
instruments, separately in order to avoid modulation effects when a dominant instrument or sound modulates the rest of
the signal. It is impossible to limit satisfactorily low frequency instruments on a final balance. Try limiting the low frequency
section of an organ recording or well modulated tympani in an orchestral recording; the effect on the remaining signal and
particularly the change in ambience is most apparent. By separately compressing groups of similar instruments consider
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able compression may be effected on direct signal without difficulty.
The F760X-RS re-issue has been fitted with a compression side-chain insert to allow an EQ to be inserted at the input to
the side-chain. A common use would be to reduce the low frequencies reaching the side-chain to greatly reduce modula
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tion due to low frequency content.
EXPANDER-GATE
– Considerations
The diagram of slopes attached indicates the difference between Expansion and Gate action. The Gate will attenuate its
full range for a very small change in input level at the threshold point; whilst the 2:1 expansion slope needs a greater input
change to effect a comparable amount of low level attenuation.
The Expander section with its 2:1 slope needs a change of 10dBat the input (below threshold) to obtain a 20dB attenua
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tion range. The Gate section is 20:1 and theoretically needs only a change of 1dB below threshold for full 20dB attenua
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tion.
The most common use for both systems is that of eliminating unwanted noise during pauses in programme signal; used in
the manner the threshold will normally be set just under the wanted signal level.
The Expansion slope being softer, is less critical to set-up and easier to use provided the signal-to-noise level is reason
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able in the first place. If one is to avoid actually expanding some of the low level signal, the noise must lie 10dB below the
wanted signal at least, in order to get a further 10dB improvement. If it’s only 5dB below the expander can only improve
it by a further 5dB where it is then held open by the noise itself. In that case the threshold can be lifted so that part of
the lower level signal is being expanded, but there is then the danger of modulation effects and ‘hunting’ unless slower
release (attenuate rate) times are used. This does rather defeat the idea of getting rid of the noise fast before the ear can
detect it; it does have its uses however where the noise is so poor that it actually merges with the wanted signal. In such a
case, as with an older orchestral recording, the attenuation range can be reduced to about 8dB maximum (to avoid great
noise contrasts) and with slower attack and release parameters the lower part of the signal can be expanded.
The Gate is effectively an audio switch with an attenuation rate controlled by the release time; it will effect the full range
of low level attenuation for a small change of level on the input. In the circumstances already outlined it can be invaluable
and more effective than the expander. The liability is that it is more critical to set.
Where one has reasonable signal-to-noise ratio, the expander is the easiest system to work with; but there will be occa
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sions of poor separation such as studio cross-talk between mics where the Gate section will work miracles.
The basic purpose of the Expander/Gate sections in the F760 unit is to counter increased noise to due compression, so
that it is now possible to compress off tape without worsening noise. With the low level attenuation range set to 20dB up to
20dB of compression can be applied without deterioration in the overall noise level. The use of less than 20dB compres
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sion would result in a net gain over the original. This is of course most important in a multi-track work and use in reduction
sessions.
When used at the same time as compression, it is important that the Expander/Gate threshold is set just above the noise
with fastest release time so that the noise is attenuated before becoming audible. As the Compressor releases the meter
will indicate a fall toward 0 gain reduction but will then move quickly left towards 20dB as the Expander operates on the
noise. The green light will come on until the signal is present and compressing again.
With certain signals such as ones composed predominantly of low frequency fundamentals; the high frequency noise is
not masked even when the instrument is modulated to peak recording level. It will be found preferable to compress these
prior to going onto tape (i.e. on the direct signal) and using expansion coming off tape.