| Federico Vanni wrote:
>
> hi list,
>
> two important question:
>
> 1)
> i know that a cut frequncy of a filter
> corresponds to its -3 dB point. So in a frequency
> responce of a first order filter
> we get - 3 dB point and, after, the rolloff of a -6 dB for each octave.
>
> If we use a second order filter we know that the rolloff of -6 dB
> becomes -12 dB... BUT... does the -3 dB becames -6 dB????
>
>
No. The -3dB point is part of the standard description of a filter (of
whatever kind): the bandwidth of a bandpass filter is defined as the
width (in Hz) of the response between the two -3dB points. As the
resonance (Q) increases (in the case of a recursive filter such as
reson), those points move closer together. Slightly more complex to
describe in relation to something such as a lowpass but the principle is
the same: there may be a resonant peak, but the -3dB point is, by
definition, where the nominally flat passband response (discounting any
ripple) has reduced by that amount. It is not applied only to filters -
the -3dB points are still a standard element of the description of the
overall frequency response of an amplifier.
> 2)
> in the 'reson' opcode, does the iscale value of 2
> means an output signal with the same RMS value
> of the input one?
>
That's the general idea; but how close you get to that depends a lot on
the nature of the input (the documentation says it works as described
when used with a white noise input). I haven't analysed reson that
closely, someone who has will be able to give a more comprehensive answer.
Why -3dB? Partly convention, but it does relate to rms measurements and
such things as summing to unity amplitude (as in a constant-power pan).
With a sinusoid at digital peak amplitude (= 0dBFS), the corresponding
rms level is -3dB (=0.707, or sqrt(2)/2).
Richard Dobson
|