Crest Factor Detection for a Compressor

[ruairioflaherty]

Hi,


 fascinating stuff! It's interesting to hear of that digital sidechain. It makes perfect sense, I guess, at least in terms of processing data . When you say "Low Latency", even at 384kHz, in mastering, especially disc-cutting, any latency must surely be a serious problem, no? I am sure that broadcast applications must be very critical in terms of "instantaneous" limiting. In the analog domain, latency is not a problem, I surmise( with high slew-rates anyway(?))

Latency is a problem in real-time applications only. Mastering is not a real-time application, neither is mixing. Broadcast very often is not real-time, since may broadcasters have a permanent 7 seconds delay-line called profanity delay. Recording and concert sound are real-time applications for which it is impossible to delay the main path in order to leave enough time for the side-chain to do whatever it has to do.

Interestingly George has plans to use the same sidechain in both a plug in and hardware piece.  In the hardware piece a VCA will control the level with the digital sidechain. I use some high latency plugs in my mastering work but the challenge with the digital sidechain controlling an analog piece is obviously that one doesn't want the audio event to have passed before a sidechain can pass control info to the VCA.

 

[abbey road d enfer]

Yes, the whole idea of delaying the main path is to delay it just the right amount in accordance with whatever time the side-chain needs to operate.

 

[strangeandbouncy]

Hi guys,



    with all due respect, and I hope I am not being a pedant, but I understood that disc-cutting is very much real time . . . . innit? surely there is NO margin for overshoot . . .



  I am well acquainted with the fab old BBC AM6/17 comp/lims with their delay to the signal path to catch those tricky little peaks!



      Hope I am not digressing too far off piste . . .



    Kindest regards,



    ANdyP

 

[strangeandbouncy]

HI,



  not thinking straight - too early/no coffee . . . . It won't matter that the signal is a tad latent at all will it?  ;D



    Kindest regards,



      ANdyP

 

[ruairioflaherty]

Hi Andy,

I don't think Massenburg is worried about the vinyl cutting market - I doubt there's money to be made there (not that he is motivated by money only by any means).  In the plug in version he will obviously delay the audio path to account for sidechain delays, in the hardware piece I'm not sure.  AFAIK there is no free lunch when it comes to delaying a signal in the analogue domain, and only very small delays are possible.  I suppose it depends what you are trying to do, absolute peak limiting is still probably best done in digi land.

I'm struggling to find a typical latency for an AD/DA at 384khz.....

Paul - sorry for the thread derail.

Cheers,
Ruairi

 

[Gold]

Paul - sorry for the thread derail.

No problem. The derails have had lots of useful information. If the A/D has latency in the micro seconds I doubt it will be a problem. For peak limiting maybe, but not for most compression.

 

[jdbakker]

I'm struggling to find a typical latency for an AD/DA at 384khz.....

If the A/D has latency in the micro seconds I doubt it will be a problem.

Most sigma/delta audio AD/DAs have a latency between a few hundred microseconds to a few milliseconds, depending on sampling rate and digital filter implementation.

JDB.
[but then, there's no law that says you need an audio AD/DA for your sidechain]

 

[JohnRoberts]

Riding this thread veer further. Latency in a digital side chain for an analog processor includes both the A/D capture, a few clock ticks for decision making, and D/A output gain control.

The A/D can be lower resolution than needed to pass full range audio since noise floor of side chain is pretty inconsequential for most dynamics applications. This lower bit A/D can be quite fast. The D/A to control the gain will likely already be smoothed to reduce control voltage feedthrough or other artifacts of too fast gain modulations. Even pure analog comps can't reduce transients instantly. If desired a fast and slow A/D could be combined to provide speed and resolution.

Using a digital pot as a gain element, gain could be written to and updated in microseconds, probably faster than you would ever want to make a major gain change step in a VCA.


JR