AC coupling and things

[Magnus Danielson]

>>>>> "TA" == Tony Allgood <oakley at enterprise.net> writes:

 TA> Hi folks,
 TA> Some days ago there was a post about ac coupling an input into a synth's
 TA> internal VCF. There then followed various comments about the value of the
 TA> capacitor to use. However, there was some confusion how the cut-off
 TA> frequency affected the signal. Perhaps, this was just me... but decibels are
 TA> confusing things. So I checked it again....

 TA> The cut-off frequency is the -3dB point, not -6dB. This corresponds to an
 TA> input/output voltage ratio of about 0.7. This gives the half POWER point.
 TA> ie, the output pd produces half the power in a fixed load compared to what
 TA> it did during the pass band. That is, if the input was 10V rms, the output
 TA> of said RC network would be 7.07V rms at the cut-off frequency. The phase
 TA> shift is 45deg at the -3dB point.

 TA> I think... :-)

Ah, you made an correct observation. The -3 dB point (which is really
-3.010299957 dB or there about) comes from the half power case and is
synonymous with a 45 degree phase-shift, they are both derived from
the same geometrical situation.

Filters and dB's can be a bit confusing at times ;)

Cheers,
Magnus