We've sent a verification link by email
Didn't receive the email? Check your Spam folder, it may have been caught by a filter. If you still don't see it, you can resend the verification email.
Started April 30th, 2009 · 4 replies · Latest reply by strangely_gnarled 15 years, 6 months ago
I am just wondering, if you want to do a first order high or low pass on a digital waveform, do you want 3dB/octave or 6dB/octave rolloff? I know in analog electronics it's -6dB voltage per octave, or -3dB power per octave, hence I'm not sure which it would be in the digital world. Thanks. Normally I'd just use which sounds best but I'm working on some various amplifier simulators so the accuracy is pretty important.
As in analogue, 6dB per octave is the standard single pole figure to use. "First order" is always the same as "single pole", second order = double pole etc..
The "Power" dB reference is seldom used these days as most signal interfaces have a very low output impedance (typ. less than 100 ohms) and signal inputs have high impedance, (typ. greater than 10k, or ten thousand ohms) so the signal is not attenuated(very much) as it passes from one bit of kit/circuit to the next. In essence the voltage passes from one stage to the next virtually unchanged, and the amount of power (volts times amps) is pretty irrelevant.
The "Power" dB was important in the early days of the telephone, before amplifier stages were inserted, because every transfer of signal could potentially lose around half it's strength, or power. Power dBs are mainly used today at radio frequencies where impedance matching is more complicated because of "transmission line" type effects of reflecting power back, like a mirror, every time the signal meets an interface.
Hope this helps.
postscript to above.
I suppose I should further add that in the digital domain of course signals are passed as noughts and ones (digits) and aren't attenuated at all.
Hahaa... Ironically, by and large, the algorithms used in digital processing are in fact "analogues" of the way analogue audio systems works which makes digital processing more analogue than analogue.
A further potentially confusing point is that, at the corner frequency, the actual gain of a single pole/first order filter will be -3db. The slope, or "rate of attenuation" will be 6dB per octave however. That is the signal level will halve for every octave you move away from the pass band.