Soft Limit Function And Calibration - Apogee AD-1000 Operating Manual

Soft Limit™ Function
Evolved from the legendary AD500E, this unique Apogee feature makes your mixes jump out in the highly com-
petitive market for CD airplay. Short transients can clip analog to digital converters and can produce unwanted
harmonics. Typical peak limiters used to remove short transients are very abrupt and actually spread unwanted
harmonics. If these short transients are removed without hard clipping, the average recording level can be
raised by several dB with little or no sonic penalty (ie your ear doesn't miss the short transients if you remove
them cleanly and don't overdo it).
Soft Limit does not make a hard, sharp-
edge clip as you would get if the con-
verter was over–driven. Instead, once
you pass the threshold, it rounds off any
peaks in a manner that makes their
removal difficult to hear. These sharp
peaks do not usually affect the sound
quality if they are cleanly removed. Also,
you get none of the typical limiter/com-
pressor byproducts such as pumping and
breathing. Depending on the material
being recorded, the threshold of audibili-
ty will vary. This is best found by experi-
mentation. Just a few dB increase in aver-
age level can result in more powerful,
hotter sounding CD's without fear of
leaving a trail of 'overs'.
The Soft Limit feature of the AD-1000, unlike conventional peak limiters, does not have a precise kick-in point.
It is, in fact a logarithmic curve whose steepness increases as you approach the "threshold" (therefore "thresh-
old" is a misnomer.) Determining the amount of limiting or increased effective headroom you wish to attain is
really best done by ear. However, you may find the following procedure a good reference point, or a way to
check left/right symmetry once you've found a pleasing threshold by ear.
Two different methods of calibration are described. One is for calibration with a recording console and the other
is for calibration with test bench gear.
Soft Limit Calibration Procedure using a recording console
1. Calibrate the AD-1000 inputs to your optimum headroom setting using an oscillator via your mix desk's stereo
output, or any metered source.
2. Patch the oscillator through a mult (parallel) into the analog inputs of the AD-1000.
3. Patch from the same mult into a line input of your mixing desk.
4. With Soft Limit turned off, set the oscillator to 1 kHz, increase the oscillator level to where the "over" leds
light up, then reduce the level until they just go out.
5. Making sure there is no signal processing (dynamics, EQ, effects, etc.) dialed into the mixing desk's signal
path, push up the line fader until the desk's stereo output meter reads the threshold you wish to attain using
the Soft Limit feature. The factory setting is –4, so to check that it had been set correctly you would make your
stereo meter read –4. If you wish to decrease the threshold to –6, (2 dB more limiting, effectively 2 dB more
headroom) make the meter read –6. Alternatively, if you wish to increase the threshold to –2, (2 dB less limit-
ing) your meter should read –2.
AD-1000 Operating Manual
Short transients that would normally cause overs
are controlled with a gentle rounding with Soft Limit™
'OVER'
Signals under –2dB are not
affected by Soft Limit
Page 23
PEAK WITH NO
SOFT LIMIT
SAME PEAK
WITH
SOFT LIMIT
EXPANDED PEAK
CONVERTER CLIP
TRANSIENT
8–10dB
OVER MAX
INPUT
LEVEL
THE EFFECTS OF SOFT LIMIT
LAST 2dB
SOFT LIMIT OFF
(in bold)
LAST 2dB