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Gear / Technical Help => Post-Processing, Computer / Streaming / Internet Devices & Related Activity => Topic started by: page on September 22, 2009, 10:26:01 PM
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Ok, so if you're using a hardware based HPF and starts at 40hz, and rolls off 6db per octave, then in theory it doesn't touch the 41hz samples. Correct?
Second, whenever I try and start my HPF (The delivered AU type HiPass) in Audacity at 40hz to see if I like the result better, it starts rolling off stuff at around 80 or 90hz, just a bit cause the frequency analysis turns up adjustments above 40 and 60hz.
So how do I simulate in audacity what would occur if I was using a piece of hardware (like the SD boxes)?
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The "cutoff" frequency of a filter, implemented in either hardware or software, is that frequency at which the output power is half (−3 dB) the input power. So a 40 Hz HPF implemented in hardware would touch the 41 Hz sample. I suspect the Audacity digital filter is probably implemented correctly, or a least close enough. If to your ear the Audacity 40 Hz HPF rolls of too much stuff above 40 Hz, lower the cutoff frequency.
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The "cutoff" frequency of a filter, implemented in either hardware or software, is that frequency at which the output power is half (−3 dB) the input power. So a 40 Hz HPF implemented in hardware would touch the 41 Hz sample. I suspect the Audacity digital filter is probably implemented correctly, or a least close enough. If to your ear the Audacity 40 Hz HPF rolls of too much stuff above 40 Hz, lower the cutoff frequency.
Interesting, I appreciate the explanation. I've been trying the -6db at 40hz (or the first pole on the 722) on some of my older recordings as experiments (and LMA downloads of some others for giggles). Will probably try the same for the first number of outings to see what it's like. Thanks!
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Well, I did some research on how the apple unit edition functions, and it has a slope of -6db, so what you control in Audacity by using it is frequency target and resonance effect. Setting it as 40 cycles and a res value of -1.25 comes (and then running HPF twice) comes close to SD's lightest setting of HPF (which is one analog, and one digital). It's not a perfect replica since Audacity either drops further at your cut off freq, or has a bump (up) for the middle of the next octave up which the 722 doesn't do, but gives a good idea.