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Topic: Pink Noise Generators
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Aaron Sisemore
Flaming Ribs beat Reeses Peanut Butter Cups any day!
Posts: 3061
From: Rockwall TX USA
Registered: Sep 1999
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posted 07-11-2000 10:48 PM
Dolby's generator is Cat 85C it works with the following processors:CP50
CP55
CP65
CP100
CP200 The CP45,CP500, and I will assume the CP650 have a built-in pink noise generator... Aaron
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Bill Purdy
Expert Film Handler
Posts: 139
From: Seattle, WA
Registered: Oct 1999
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posted 07-14-2000 10:06 AM
Look again---our web master, Brian Long, had to first find the photo, scan it and all that jazz and still keep up with being in charge of production. He assures me that it was there with 15 minutes to spare!OK, what is Red noise? You start with white noise which is what the basic generator creates. Then, because each higher octave in the spectrum has a bigger chunk of it (it's only a change of 100 cycles to go from 100 to 200, but it is a change of 1000 cycles to go from 1000 to 2000), you have to modify the curve so that you can have that straight display you want. This adjustment wants to be at the rate of 3 dB per octave. The easy way to do it would be with one Resistor and one Capacitor, but this combination produces a 6 dB roll off, and for reasons I don't know this is called Red Noise. Therefore, the 3 dB roll off (which in our case takes a 7 pole filter), which is half way, is Pink Noise. Cute huh? The Red Noise is in the generator because in the early days of the THX project there were still a lot of older Altec speakers in use and when the certifying tech was doing his reverb measurements there was the fear that the high frequency driver might be blown. So, use the Red Noise with its lower high end component and adjust the results. For those who like the USL generator, there is a reason. Jack Cashin bought the rights to our basic generator circuit from us. There is a little trick for those who use phase checkers that you might like to know about. If you put the "Noise/Tone" switch in the mid or "Off" position you can feed your phase checker output into the Output test jacks (the Blue one and Gnd) and it will appear at all of the channel selector switches so that you can use them to assign the signal. I wish I could say that it was planned, but it was pure serendipity.
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