Q2 Audio Compex F760X-RS User Manual - page 6
LIMITING & COMPRESSION – Considerations
The usual purpose of Limiting and Compression is to a). Increase loudness and b). to provide overload protection and
generally control level.
LIMITING
implies the use of a level control divide to provide overload protection; its purpose to ‘limit’ the signal level at
some specified point. Transients are the major concern (that is peaks of a short duration) tghat exceed the pre-determined
peak recording level. Control of these will not markedly affect the dynamic range of the input signal since gain reduction
with it does occur, will be momentary and of a relatively low order of magnitude. In most cases a fast release time will be
desired so that transients are punched down without apparently affecting the programme content. A tight compression
ratio greater than 10:1 wold be used.
Such an action enables the engineer to reduce his system ‘headroom’, operate with a higher recording level without fear
of overload. The dynamic range or a recording or transmission system is thus increased.
COMPRESSION
is used to describe conditions of rain reduction that are more or less continuous; thus the original dy
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namics are compressed. The compression ratio selected may be anything from the softest slope (say 2:1) to the tightest
(say 20:1) dependent on the effect desired.
When it is desirable to preserve some sensible relationship to the original dynamics, ratios of 2:1 or 3:1 would be used.
There is some significant increase at the output as the input rises on these slopes. 5:1 to 20:1 are tighter and the output
will not rise by much whatever the input level.
The diagram of the F760 slopes show this clearly; 15dB compression on a slope of 10:1 wold occur on the top 16.5dB
of the signal; the same amount effected at 2:1 is spread and affects the top 30dB of the signal. The 2:1 ratio is therefore
more subtle and the effect of fast release times less apparent.
The advantage of the F760 series over a straight Compressor-Limiter is that the softer slopes can be used to provide
subtle compression combined with overload protection at the top. It is thus safe to use the soft slopes’ and one does not
have to compromise by using a tighter slope that one would ideally like.
RELEASE-TIME
is important in compression since it determines the moment to moment gain change in the system’ and it
is the rate of gain-change that determines loudness. The faster the release and the tighter the slope the more lower level
signal is brought up toward peak level. The problem introduced is that of ‘pumping’ or ‘breathing’ plus the effect of fast
release times on low frequency distortion. With very fast release the low frequency wave forms are being flattened’ this is
naturally most noticeable under conditions of considerable gain reduction coupled with a tight slope and fast attack time.
In practice it is surprising what one can get away with in ‘pop’ applications where these sort of extreme conditions can be
useful. With ‘serious’ programme material slower times will be used. The automatic programme controlled release position
‘A’ produces for a fast release over a range of 5dB whereupon it becomes long. Thus short duration gain change is limited
to a small amount obviating most of the problems described. It can be adjusted to operate as a ‘mean level’ control such
that the quieter sections are subtly lifted. Considerable overall compression can be effected without being apparent, and is
ideal for background music ore reducing dynamics for mediums of very limited dynamic range ( e.g. AM transmissions and
optical track etc.).
ATTACK TIME
will determine the size of the transients allowed to pass through the compressor section (to be handled by
the Peak Limit section in a F760). In a limiter this shojld be very fast in order to control high frequency spikes and prevent
overloading. Some mediums are more critical than others; on tape it may be preferable to let the fastest through to satu
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rate and be clipped in the tape itself’ with optical systems and transmitters it is usually essential to keep overshoot to the
absolute minimum.
In a compressor or limiter the slowing down of the attack time has the effect (dynamically speaking) of reducing the static
ratio, increasingly overshooting. Listening to 15dB compression on a 20:1 slope with a fast attack time, the tightness will
be quite apparent in the sound and this can be softened by reducing the attack time.
COMPRESSION & NOISE: From the point of view of minimal noise, it is preferable for the compression to be effected
on direct microphone signal since the signal-to-noise ratio is better at this stage
(this is true if recording to tape, less so if
recording to modern professional digital audio mediums)
. It will be appreciated that 15dB compression will increase the
source noise by 15dBwhen the compressor fully recovers; off tape this would be more noticeable.
The use of the Expander or Gate section (where fitted) will obviate this when correctly set up.