"Look Ahead" analog limiter?

[mikep]

You consider look-ahead to eliminate the need For infinitely fast attack (diode clip) to ensure no overshoot. A usefull way to think about fast attack time in a limiter is to relate it in terms of a number of periods at a frequency of interest, namely the high frequenncy limit of the band. Perhaps 4 cycles at 20khz for a minimum look ahead of any use? Thats 200us. Perhaps a series of all-pass networks could work.

  The problem is the AUDIO frequency part. If you do the calculation for an RF look ahead limiter the necessary delays can be achieved with a length of transmission line: some coax or a spool of fiber (optic) can create quite a long delay, relative to the period of the waveform at high frequencies

ok how about this: a diode clipper with an adjustable knee that is varied by the instantaneous slew rate. Faster slew would cause a softer knee. 

 

[abbey road d enfer]

http://home.btconnect.com/ssa/whitepaper/whitepaper.htm

I had looked in vain for such an explanation; it's an interesting and fresh approach (somehow not unsimilar to Dolby SR). In stead of "look-ahead", they should say predictive.

 

[JohnRoberts]

http://home.btconnect.com/ssa/whitepaper/whitepaper.htm

Thanks for the link.. well they tell you how to do it (mostly).

it is not that unusual to use one VCA with several parallel side chain circuits to establish a fast attack/release hard limiter, on top of a conventional hard or soft knee compressor, and even add functions like downward expander, de-esser, etc. all from one VCA.

Their rate of change detection (they called it acceleration, but it seems more like velocity to me) with a several dB offset is interesting to make anticipation limiter and general compression occur at the same threshold.. I'd be inclined to make the limiter a last resort with a higher threshold , but I guess the modern schema is to squeeze out every last drop of dynamic range.

I hope it works better for you than standard approaches.

JR

 

[jensenmann]

regarding the initial question: EMT used to delay the signal path with an allpassfilter in their EMT266 transient limiter to gain time for the sidechain to react.

 

[Rob Flinn]

I have an Allen & Heath analogue feed forward limiter.  It feeds the signal through a bucket brigade delay to allow it to catch fast transients.

 

[JohnRoberts]

I used to design BBD delay lines... That sounds like the medicine could be worse than the disease...  8)

I guess if you are already using a VCA, you could add a second one and compand the delay path to reduce the noise floor, but BBDs were not very linear either. 

JR

 

[Conductor]

AudioDesign uses 10 x allpass-array to achive 0,3 ms delay in signal chain.

 

[PRR]

There have been several limiters with short ~~0.1 milliSecond delay in the direct path. This can be implemented with a few coils and caps. The undelayed sidechain can then begin GR before the signal gets out of the delay in the direct path. ""Look Ahead""

No latency? Define latency. This scheme clearly has latency, but arguably too small to bother most users.

> a headphone amp monitor

Performer monitor?? Use any semi-decent limiter. They do not need zero-clip limiting.

 

[Rob Flinn]

I just found a pdf on my hard drive of an AES presentation from 1976 Entitled "a feedforward controlled delay line limiter".  I'm not sure, but I think this describes the Allen & Heath limiter that I have.

If anyone would like a copy send me an email.

 

[strangeandbouncy]

Hi,


I used to have some BBC 6/17 comp/lims which used a mass of coils and caps to achieve this( or so I was led to believe).


  Kindest regards,



        ANdyP

 

[jdiamantis]

http://home.btconnect.com/ssa/whitepaper/whitepaper.htm

mmm....variable clipper threshold driven by limiting depth. Sounds alot like the Orban "smart clipper" design in their 8100A FM processor. Multiple time constant/side chains have been done before by others as well....not that this is a bad thing, but truly nothing revolutionary.

jD

 

[Gold]

The BSB74 Acceleration Limiter in the Neumann SAL74 lathe rack uses a bank of about 20 coils to do look ahead high frequency limiting.