[linux-audio-dev] Paper on dynamic range compression

[Gene Heskett]

On Thursday 05 October 2006 14:09, John Rigg wrote:
>On Thu, Oct 05, 2006 at 09:01:19AM -0500, Andres Cabrera wrote:
>> I was assuming that RMS measruments and windowed integration act as a
>> high pass filter, is this not true?
>
>No, a low pass filter integrates and a high pass filter differentiates.
>
>The audio signal is rectified before it can be filtered and used for
>gain control, so there is no AC signal for the filter to act on,
>just a varying DC level. The time constants for the filter
>(eg. charge and discharge rates on a capacitor in an analog LP filter)
>determine the rate at which the control signal can increase or
>decrease, and hence the attack and decay times.
>
>John

Which is why one of the best compressors of all time, designed by CBS Labs 
back in about 1960, was so good.  It had 2 versions for two different 
broadcast scenarios, one for the 75u-s pre-emphasis of fm, and one that 
was flat for am.  But both had 2 circuits, with the faster circuit having 
microsecond response times to control the peaks while allowing a fairly 
high average.  I'm refering of course to the Volumax and Audimax devices.  
Some of those are still in service, hidden away at transmitters in your 
neighborhood.  The single vacuum tube they used in one of them, a 
'nuvistor' has probably long since been replace by a depletion mode fet 
transistor and the device recalibrated back to factory specs.  And all the 
2 uf electrolytic capacitors used in the original have long since been 
replaced with 2 uf mylars.  At that point, you can literally forget its in 
there, for years at a time.

You couldn't hear it working unless the next audio source switched to was 
-30db from the previous one, then you could hear it slowly raisng the gain 
for several seconds, eventually picking up a peak that opened the high 
speed gate and 100ms later the sound was at the normal level again.  All 
without the sleeping dj ever knowing he wasn't doing his job on the board.

Anything else we attempt to put in there just seems to bring back memories 
of that 5 minutes of heavy breathing from last night, just before you went 
to sleep...

-- 
Cheers, Gene
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