View attachment 21294
I *THINK* this is so that line level inputs are dc-offsetted appropriately so it can be read by an ADC within the range of 0 to 1.2v. i.e. adding 0.6v DC offset? am I correct? If I am, what exactly is that RC circuit doing? How were the values chosen? What do I search on google to learn more about this??
Yes, there's a virtual ground at 0.6V comprising the 10k, 2k2, 10uF to ground. The capacitor makes this
this virtual ground referenced to real ground at audio frequencies (ie it suppresses all the digital hash
on the 3.3V rail).
The other series capacitor (only needs to be about 470nF, 10µF is overkill), is a DC blocker.
The cutoff freq of 10µF into 47k load is 10µ * 47k = 0.47s, corresponding to 0.3Hz. 470nF gives
a more rational 22ms and 7Hz.
The input 47k can be much larger (1M), it serves only to discharge the series capacitor when
nothing's plugged in, so there isn't an almighty full-scale *CRACK* when you do plug something in.
The values weren't really chosen particularly carefully, but they will work. Normally audio input
impedance is 47k (although in modern equipment is often less, such as 10k, since valve circuitry
isn't used any more(!)).
and one more question,
I hear that opamps can achieve DC offsetted output too. Is there any advantage of doing this with either the RC circuit or opamp that i'm missing? I'm asking because I do have some opamps lying around, but not those exact resistors and capacitors.
Opamps can buffer (lower the impedance) and amplify (raise the signal level). Passive networks can't amplify, and can
only lower the impedance through attenuation (which raises the noise floor).
You'll need the resistors and capacitors with an opamp too though.