The sample rate node outputs the current sample rate that Audulus is running (the number of samples per second) For Audulus the sample rate is typically controlled by the operating system although in the case of the AU the sample rate is set in the host DAW. It is used as a convenience when building filters etc. where the sample rate is one of the inputs to the circuit. For macOS it is typically 41000 samples per second but this depends on the interface in use as well as the audio settings. Audulus will run at higher sample rates if necessary but this increases CPU load considerably.
The mapper node is similar in concept to the spline node. It maps a 0-1 input to an output using a curve that you specify by moving the control points. the x axis is the input and the y axis is the output. The leftmost point represents the output for a zero input and the rightmost represents the output for a 1 input.
The pitch bend node outputs the current value of the MIDI pitch bend message from an external controller and is used to link an external pitch bend control to the oscillator frequency. It is similar in concept to the keyboard node.
The pitch shift node uses an algorithm to shift the pitch of an audio signal applied to the input. It can be used to create octave shifts etc.
The phasor node is a ramp oscillator that outputs from 0 to 2 PI (or 360 degrees). By combining it with a sin() function you can generate a sine output but it can also be used to create a wide range of waveforms. The Oscillator node in Audulus is anti-aliased which means that the output is band limited to prevent aliasing. This means that a ramp or square wave is not a mathematically pure waveform, but has had its higher frequency components removed. This is desirable for audio but not necessarily for other applications. Because it is simple to generate multiple waveforms from a single phasor, it is often used to create a variety of waveforms that are all in sync. It is also very CPU friendly. It is the oscillator of choice when you need a periodic signal rather than an audio waveform.
All feedback circuits in Audulus have a delay at some point in the circuit. In order to calculate the feedback, the output must already be known, so the feedback is necessarily delayed. Ordinarily Audulus processes signals in frames (blocks of about 250 samples) to minimize CPU loads. Audulus will automatically insert a one frame delay in any feedback circuit you create. In some cases it is necessary to have the delay occur at a specific point in the circuit in order to get the correct behavior. The feedback delay node is a mechanism for specifying exactly where in a feedback circuit the delay occurs. The unit delay is similar but instead of delaying the signal by a whole frame, it delays the signal by only one sample. This is necessary for many digital filter designs, but causes the circuit to be evaluated for each sample instead of one frame at a time which results in significantly increased CPU overhead. Additionally the unit delay will work in a feed-forward configuration whereas the feedback delay node does nothing if it is not in a feedback loop.
The high pass and low-pass nodes are an implementation of a moving average 6dB per octave filter whereas the filter node is a 12 dB per octave resonant design. The low-pass node is useful for creating thing like slew limiters as well as other applications where a gradual roll-off is desired. It is also a bit more CPU friendly than the resonant filter.
Hope that helps. Let me know if any of this doesn’t make sense and I’ll try to do a better job explaining.