Theory of operation - RCA output
Batteries are used in the examples below to simplify the theory of operation, in reality these are a
programmable linear power supplies as shown in block diagram above.
We have: two batteries (yellow), a RCA output (black), two equal resistors R (orange), and two
electronic switches. Batteries are a short circuit for AC signals (audio signals). Resistors attenuate the
battery voltage in order to generate an analogue output signal.
A) Switches in opposite position (+ & -) currents cancel, zero volts on the RCA output.
B) Switches are both positive (+ & +) positive currents add, maximum + voltage on RCA output.
C) Switches are both negative (- & -) negative currents add, maximum - voltage on RCA output.
In all 3 cases we have two resistors with value R in parallel and batteries are short circuit for AC. So
output impedance in all 3 cases remains the same, 31.25 R.
By changing the electronic switch settings we can have no voltage, positive voltage or negative
voltage on RCA output.
But we need to program the voltage much more precisely for generating audio signals. Therefore we
use 22 electronic switches and resistors with different values (resistor array). Now we can program
262,144 different voltage levels and achieve 18 bit accuracy.
For 18 bits we would only require 18 switches, but because of the low output impedance (31.25 Ohm)
and component limitations this is not going to work in a practical circuit.
That's why we had to add more switches that represent the 3 highest and most critical bits. We
require total of 7 switches for this. The remaining less critical lower bits require 15 extra switches so
we end up with 7 + 15 = 22 switches in total.
This way we can generate highly accurate and consistent audio signals on the RCA output without
requiring any active conversion, amplifier or buffer circuit.
