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VP-8IP / Sep 2013
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P R O C E S S M O D E S
AM-HD Look Ahead Limiter
TheAM-HDoutputisaccuratelypeakcontrolledbyahighlyoversampledfeedforward
limiterwitha0.5millisecond(500microseconds)lookaheadtime.Thecontrolpath(not
theaudiopath)ishighlyoversampledinordertoachieveextremelyprecisepeakcontrol.
Theprimarygoalofoversamplinginthecontrolsidechainisbetterpeakcontrolthrough
finerdefinitionoftheaudiowaveformattheinputtothedetector.Oversamplingofthe
audiosignalitselfisnotonlyunnecessary,butwastesDSPresources.Thegraphicbelow
illustrateswhyoversamplingismoreeffectivewhenthecontrolsignalisoversampled.
Bysamplingtheaudiowaveformwithfinerresolution,peakexcursionsthatwouldbe
missedotherwisebecausetheywouldfall“betweenthesamples”canbereliablydetected.
Note that if the audio and control signals were both oversampled, peaks would be missed
again because their relative time alignment would be identical!
AM-HD Look Ahead Limiter Controls
The graphic on the right shows
the controls available for the look
aheadlimiter.Thefollowingtext
describes each control according
to function.
Look-ahead Limiter Enable
Whenthischeckboxischeckedthelookheadlimiterisenabled.
L/R Linked
Whenthisboxischeckedthelookaheadlimiter’sleftandrightchannelcontrol
signals are common. This prevents unnatural stereo image shifts when one channel needs
toreducethesignalinordertocontrolpeakexcursions.
Lim (Limiter) Drive
This controls the amount of drive to the look ahead limiter. The gain range of this
controlis+12dB/-18dB.Theresolutionofthiscontrolis0.01dB,fineenoughforthe
mostexactingoutputleveladjustments.
Attack
Thiscontroladjuststheattacktimeofthelookaheadlimiter.Becausethelookahead
is0.5mS,anattacktimesettingof0.2mScreatesalimiterthatattacks0.3mSbeforethe
peakarrivesatthelimiter’soutputport.Likewise,asettingof0.5mScreatesalimiter
withzeroattacktime(becausethelookaheadis0.5mS).Allsettingsabove0.5mScreate
AM-HD