This is a brief rundown of the ARP 2600 Sample and hold section.
It's a very useful module often used for creating random or stepped sequences of notes, as well as for adding subtle variations to existing signals. It can be triggered by an internal clock, an external signal, or manually.
The stepped levels produced in this manner are useful for controlling things like oscillator and
filter frequencies and VCA gain.
Firstly, what is a sample and hold circuit?
It's a circuit that samples an incoming voltage signal and holds it at a constant level until the next sample is taken. It consists of two main parts: the sample circuit and the hold circuit.
The sample circuit takes an incoming signal and converts it to a voltage that is stored in a capacitor. The rate at which the capacitor is charged determines how quickly the circuit responds to changes in the input signal. The faster the charging rate, the more responsive the circuit will be to changes in the input signal.
The hold circuit is responsible for holding the voltage level once it has been sampled. It uses a switch to disconnect the capacitor from the sample circuit, allowing the voltage level to remain constant until the next sample is taken.
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There are 2 sliders:
Rate & Level
The automatic triggering of the envelopes is controlled from here.
When it is disconnected from the S/H circuit (by plugging a cable into
the external clock in), the rate control will have no effect on the operation of the S/H circuit
Level : The level control attenuates the input signal before it is fed to the S/H circuit.
There is a small switch above the jack.
In the switch's upper position, the ARP's keyboard trigger & gate signals are connected to the EG and will fire them when a key is pressed.
In the lower position the connection to the keyboard will be broken and instead, the EG will be connected to the ARP's internal clock (a square wave), not the sample & hold.
So the Envelopes are triggered by the internal clock.
You can also trigger these envelopes with an external sequencer.
They respond to +10V triggers & gates (pulse waves are nice).
The Sample and hold has 2 inputs and 1 output.
Inputs: a trigger input - Clock in
Sample in (the waveform to be sampled),
Output: S/H out
The trigger input (ext clock in) is defaulted from the internal clock,
but any square or pulse wave, or the keyboard gate or trigger signals, will work.
The default Sample input is from the Noise Generator.
Thus, as the default signal being sampled is random noise, the output voltages are correspondingly unpredictable.
So an infinite variety of cyclical output patterns may be obtained,
You can of course sample periodic waveforms rather than random noise.
The alternative sample input is rather confusing labelled: Noise Gen/Sample & Hold
Here you can plug something like a LFO or a VCO to be sampled instead of noise.
The electronic switch is Bi-directional and consists of 3 jacks
labelled A, B & C.
Ie it can switch switch between two inputs (A/B) & one output (C)
OR
1 input (C) & 2 outputs (A/B).
The electronic switch can be made to alternate between A & B.
The switching speed is controlled by the internal clock & the rate slider.
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Some patch examples
It's sending random CVs to VCO1 -- thus the random notes.
Its similar to a Buchla Source of Uncertainity.
Audio from VCO1 is passed through to the filter.
The filter is modulated by the ADSR which in turn is triggered by the internal clock.
By playing with the two Noise siders & two S/H sliders you can create lots of variation.
(all audio is from the 2600's speakers)
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Patch 2
A single patch cable from the ramp waveform output of VCO2.
Its behaving like a LFO
This is the voltage which will be sampled instead of random noise.
The audio is from VCO 1 only
Remember that the three VCOs have a LF (low frequency)
setting so can be used as LFOs
In the video above I'm using the VCO in the audio setting.
It sounds very random as you increase the frequency.
But if you use the VCO in the LF setting, the voltage is sampled
more slowly and you'll get arpeggios.
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Using the Lag processor.
Its kind of like portamento
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Analog bit-crushing (down sampling)
We need to give the S/H a really fast clock
We can swap out the internal clock with an external clock.
Setup:
I'll use VCO 1 as the clock. (square wave output)
VCO 2 is the sampled audio source
Plug the output of the S/H into the VCO 2 input of the filter
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Use the clock out as a extra LFO
the Clock out is a square wave.
So this 1st photo is of a square wave LFO
This second photo takes the same idea but uses the lag processor to smooth out the square wave
and shape the LFO.
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PWM
Of the ARP 2600 three VCOs, only VCO 2 has a pulse width modulation input.
This patch is a neat way to add some PWM without loosing another VCO
As in the patch above, I'm using the lag processor to convert the square pulse to a triangle LFO
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panning effect
Links
+ ARP Index
+ 17. Sample & Hold ... build notes for the TTSH
+ 18. Electronic Switch .... build notes for the TTSH+ 16. Internal Clock ...build notes for the TTSH
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