The 24dB FILTER is a 4 pole-filter with a slope of 24dB. Each pole is phase shifted by 45 degrees at the cutoff frequency giving you a phase shifted signal of 180 degrees. If the signal is then phase inverted once more you gain another 180 degree and when fed back into the input of the filter the signal is phase shifted by a total of 360 degrees. At this point a time delay occurs between input and output and at the point of the cutoff frequency there appears a very strong resonant peak. The first 24dB analogue filter was originally invented by Dr. Bob Moog. With the 24dB FILTER you can also create band-pass, low-pass and high-pass filters that use phase cancellation to shape the cut-off frequency. Using two oscillators you can spectrally crossfade through the frequency spectrum. Filter distortion, or so called ‘tube distortion’ is achieved by using FM feedback.
This chapter demonstrates the connection of an external line level signal to the DUAL FADER. The line signal can manipulate various modules of the synthesiser, for example, fluctuate the pitch of an oscillator. There are 3 points to connect an external line level devices to the system. They are two secondary inputs on the DUAL FADER A2 and B2 and on the left of the MATRIX in the NODE PROC.
Routing Functions of the DUAL FADER
Panner: connect only the left input and use both outputs.
Crossfader: connect to both inputs and one output.
Ducker: connect to both inputs and both outputs.
The DUAL FADER applies a RMS curve to crossfading and panning. This is demonstrated by connecting an oscillator to input one and sending output to either the left channel, or right channel. The mid-point of fader is 3bB louder because mixer curves are not linear, they are RMS (Root Mean Squared) curves, or equal loudness curves. The DUAL FADER utilises a RMS curve when used as a VCA. In the Hordijk system, there are several VCAs exponential curves in the filters, VCA’s in NODE PROC’s and 2 RMS curves in the DUAL FADER.
This chapter discusses the implementation and benefits of using linear FM synthesis in the analogue domain. By combining and manipulating a sine/cosine oscillator with a linear FM input to create rich analogue waveforms consisting of both odd and even harmonics. To implement linear FM synthesis on the Hordijk system use a crossfade mixer to crossfade between a sine and cosine oscillator with incremental Chebyshev functions of first order and second order, etc. The output then goes into a modulation index correction and the output is feed back into the input. By modulating the levels of each component you can shape any spectrum, without detuning, and the waveform stays constant over a very wide frequency range. Using parallel processing in the analogue domain is an economical way of working with sound without mathematical limitations found in digital systems. The limitation of bandwidth in the digital domain is caused by the Nyquist theorem.