The Bob System

Analogue Modular Audio-Visual Synthesizer

by Jesus Himself


The Bob System is a prototype Analogue Modular Synthesizer, capable of producing audio, video and control signals in real-time. It is based on control voltage principles, thus being infinitely expandable. It consists of a number of prototype modules designed and built by myself, as well as several other devices that were modified to accept or produce control voltages. It is a tool geared towards live performance, but is equally useful in a studio environment. This project started in 2005 at 52 Studios in Thessaloniki, Greece and reached its current state in 2007 in Sheffield, UK.

1. Historical Background

1.1 Electronic Sound

Electronic sound synthesis owes to C.G. Page of Salem, Massachusetts, for being the first person to produce electronically generated sound in 1837. He called his discovery “Galvanic Music”. At that time, this phenomenon was not pursued as a musical instrument, until Elisha Grey introduced the Musical Telegraph in 1874 which can be considered the first electronic musical instrument. In 1987 Thaddeus Cahill introduced the Telharmonium, a massive electro-mechanical musical instrument played with a keyboard. However, the aforementioned instruments were still significantly different from the way the popularised synthesizers of the 60's operated.

Hugh LeCaine invented the Electronic Sackbut in 1948, which was the first instrument to make use of control voltage. This was a landmark in Electronic Music. Analogue synthesizers adopted control voltage and soon they started being built on control voltage principles. During the early 1960's two very influential figures, Robert Moog in Trumansburg, New York, and Donald Buchla, in Berkeley, California, introduced their musical innovations to the world. Robert Moog started by building Theremins and designing voltage controlled oscillators and filters. Donald Buchla was affiliated with Morton Subotnick and Ramon Sender, from the San Francisco Tape Music Center, who were interested in an electronic musical instrument for real-time performance. By the end of the decade, Moog and Buchla became established names in the musical instrument market and with them came the synthesizer's rise in popularity as a new musical instrument.

1.2 Synthetic Image

The first form of video synthesis is considered to be the Piano Optophonique in 1912, by Vladimir Baranoff Rossine, a futurist pianist from Russia. It consisted of an instrument that was very similar to the piano, with a mechanism fitted in it that projected light while the pianist was playing. In 1916, Vladimir Baranoff Rossine added a mechanism that rotated glass discs that he would paint, resulting to patterns from the glasses being projected on the wall behind the instrument.

Electronic Video Synthesizers were developed in the 1960's, in several universities across the USA, with the prominent institutions probably being the University of Illinois, in Chicago and the University of California, in Berkeley. One of the very first electronic image synthesizers was the Animac, introduced in 1962 by Lee Harrison III, which produced animation images. Dan Slater has created several video synthesizers from 1966 and on, some of which even found commercial success in the film industry. Bill Hearn created the Vidium in 1969, which was an analogue synthesizer useful as a colour pattern generator.

Max Mathews introduced the GROOVE in 1970, which was one of the very first instruments capable of producing both audio and video. It was dismantled in the mid-70's due to changes at the Bell Labs where it was situated. Laurie Spiegel built it back together, modifying it in the process and naming her creation VAMPIRE.

More video synthesis instruments followed, such as Eric Siegel's Electronic Video Synthesizer, Steven Beck's Direct Video Synthesizer, Dan Sandin's Image Processor, the Rutt-Etra Video Synthesizer, the Paik-Abe Video Synthesizer, the EMS Spectre and many more until eventually the field was dominated by computers due to their much lower price and smaller size.

1.3 Synthesis

Synthesis comes from the Ancient Greek word “Σύνθεσις”, pronounced the same as “synthesis”. This word is a combination of the Greek words “Σύν” (noun, plus/add [+]) and “Θέτω” (verb, to place). In the Greek language it is synonymous with composition. According to the American Heritage Dictionary of the English Language, Synthesis is 'the combining of separate elements to form a coherent whole'. I particularly like this simplistic definition as it can be applied to sound and video. Sound synthesis would be the combining of separate sonic elements to form a coherent sound, while video synthesis would be the combining of separate visual elements to form a coherent image. According to this definition and the etymology of “Synthesis”, the result of sound synthesis would be a new sound that would have been created by “composing” the characteristics of sound, such as the harmonic content, the fundamental frequency and hence the pitch, the duration, the amplitude and envelope of the sound.

1.4 Modularity

Modular Synthesis is not an actual technique of sound synthesis, but rather an approach to synthesis systems engineering. Modularity in the design of a system refers to the division of a complex system into smaller sub-systems which are called modules. Each module is capable of operating independently of the full system, carrying out a limited number of functions. Any number of modules are able to be connected together to form a complex system able of carrying out complicated functions.

A fully modular system theoretically has no limit to the number of modules that can be connected together. Such systems can be expanded as necessary and can be used for several different applications, related or not.

Modular systems were not developed for synthesis systems. The most widely known use of modularity in engineering is the old Telephony Switchboards, where operators would manually patch two telephone lines together to connect a call.

Modular systems enable users to understand the function of each module separately and combine them in any way without having to modify the individual modules. It is also valuable in servicing and maintenance of systems, where only parts of it can be removed to be repaired or maintained, while the rest of the system remains functional.

2. Synthesis Techniques on the Bob System

2.1 Audio Synthesis

Audio synthesis is the process of electronically generating sound. There are several techniques for shaping the sound while it is being created, while other techniques rely on processing the audio signal after the sound has been generated.

The most common devices for sound generation are electronic oscillators. These transform their DC power supply to AC current of the desired frequency and a certain periodical waveform. Voltage Controlled Oscillators are capable of accepting a signal that modifies the frequency according to the level. The most usual format for this was the 1 Volt per octave, as used by Moog Synthesizers. If the signal in the input of a voltage controlled oscillator is AC, then the frequency is modulated by the frequency of the control voltage. This produces FM synthesis, widely used to create interesting timbres and sound effects.

Additive synthesis can be as simple as mixing the outputs of two oscillators together to create a more complex sound. Subtractive synthesis is the process of removing a part of a complex signal, most commonly by using a filter. Voltage controlled filters accept control voltages to determine the cut-off frequency of the filter.

AM synthesis is usually performed by means of a Voltage Controlled Amplifier, which accepts a control voltage that determines the amplification factor.

2.2 Video Synthesis

Video synthesis is the process of electronically generating video. Video signals are more complicated since visual frequencies are extremely high. Such high frequencies are not compatible with common electronic circuits, so electronic video signals have to be transposed to a usable range. Video synthesizers attempt to imitate video signals as these are found on magnetic video tapes. This complicates matters even worse, since there are several different video standards in different countries. There are also different kinds of connections between video equipment. Moreover, video monitors use linear scanning to create the images. The composite video signal comprises of horizontal and vertical sync information, luminance and chrominance information. A video synthesizer has to be able to create all the parameters of a video signal individually and combine them together in accordance to the desired video standard.

Other techniques for video synthesis include video feedback. A video feedback setup consists of a camera capturing a video monitor image, while the camera output is fed to that monitor. This results to different shapes with strobing effects, depending on the angle of the camera to the screen and the distance. The output of a video feedback loop is usually routed to video effects such as colorisers and chroma keyers.

3. The Bob System's Engineering

3.1 Modularity

Modularity in systems engineering refers to the practice of dividing a complex system in small, self-sufficient sub-systems. It is the equivalent of problem partitioning in computer programming. This approach has several benefits. Firstly, it is easier to understand. It is easier to maintain as each module can be repaired or serviced individually. It can be reconfigured at any time with any combination of modules depending on the user's needs. But most of all, a fully modular system like the Bob System, is infinitely expandable. There is no limit to the number of modules that can be used together.

3.2 Patch interface

The majority of analogue modular synthesizers used some form of a patch interface, where connections between modules are done externally using patch cords. The Bob System uses banana type sockets and plugs for control signals, ¼' Jack sockets and plugs for audio signals and 3,5mm Jack sockets and plugs for power supplies.

3.3 Original Modules

Most of my own designs were built on hand-drawn and hand-etched copper circuit boards. The components were soldered manually and the controls were routed to the front panel. Good quality components have been used and wherever possible those included vintage Germanium transistors and diodes.

3.4 Modified Devices

Several commercial devices that were not originally intended for use with synthesizers have been heavily modified to produce or accept control voltages. These include digital 8-bit Pulse Code Modulation synthesizers, FM/AM radios, automated switching systems and guitar effects pedals. The techniques involved include circuit-bending and signal transformation.

4. The Modules of the Bob System

4.1 Sound and Image Sources

The Bob System uses a wide range of sound and image sources. It features 10 Voltage Controlled Oscillators, line inputs and a noise source for audio use. Video feedback setups are very often used for video synthesis with the Bob System, along with video range oscillators and noise sources.

4.2 Timing

Two square wave pulse generators are used as Low Frequency Oscillators, one of which is capable of frequency division due to a Trigger input. The trigger input can be used with CV/Gate keyboard controllers, to start the timing when a key is pressed and stop once it is released.

4.3 Analogue Signal Processors

Both voltage controlled and envelope controlled filters are being used on the audio of the Bob System. Other audio signal processors include two Ring Modulators, that compute the sum and heterodyne of two input signals, an amplitude modulator for AM synthesis and a Spring Reverb unit. The Spring Reverb module utilises a real dual spring system which provides a distinctive reverberation sound with a metallic feel. This reverberation technique had been widely used in guitar amplification systems during the 1960's, most famously on the Fender Twin Reverb and Ampeg Reverborocket models, shaping the guitar sound of the Psychedelic Rock movement.

4.4 Digital Signal Processors

The video signal processors used in the Bob System are mostly old digital modified devices. These are a Coloriser/Chroma Keyer, a pattern generator, a solid color generator, a pixelator and a video frame delay. Vintage 8-bit Pulse Code Modulation synthesizers have also been modified to accept control voltages for several parameters. The addition of digital PCM synthesis modules creates interesting sounds by combining the analogue and digital worlds.

4.5 Control Modules

There are several different control modules used in the Bob System, that allow the user to program automated functions. The most sophisticated module in this category is an 8-step Voltage Controlled Analogue CV Sequencer. This was based on the circuit of an LED (Light-Emitting Diode) Flashing Light effect. The circuit was modified to route a direct current to eight different outputs sequentially or in various patterns. The level of the control voltage for each step is defined by eight potentiometers. Each step has a separate banana socket output, that allows control voltages to be routed to any device using patch cords. The speed of the sequencing is controlled by a built-in LFO (Low-Frequency Oscillator).

A different type of Control is provided by a Voltage Controlled Sweep Generator. This is a circuit that generates sweeping voltages. This means a rising or a dropping current, in a linear or non-linear mode. The slope shape is voltage controllable and can be either linear if controlled by a direct current or non-linear if controlled by an alternating current. The module used in the Bob System is a triple version of the aforementioned circuit, using three slightly different variations that produce different results.

5. The Bob System compared to other synthesis systems

5.1 Real-time processing and reliability

The Bob System was designed to serve as a live performance instrument. The signal processing is done in real-time, using analogue and digital electronic circuits with reliable operation. This is an advantage for the performing musician, allowing for total control over the parameters of sound during the course of a live performance, in contrast with digital synthesizers and software synthesizers that are often not as reliable as expected. The reliability of a software synthesizer depends on the computer system it is designed to run on. With analogue modular synthesizers, there is no operating system or central processing unit. Each module operates independently, something which means that even in the case of a fault in one of the modules, the rest of the system remains unaffected.

5.2 Influential synthesis systems

Analogue Modular Synthesizers have always been a great influence to my music. I have carefully studied the way such systems work and implemented some of their functions in the design of the Bob System. More specifically, I have always admired the endless possibilities of a patch-cord interface, as used in the Moog Modular 55 system, the Series 100 Modular Systems by Don Buchla, the Roland System 700 and the ARP 2500. I decided to implement such an interface on the Bob System to enjoy the same flexibility users of the aforementioned systems would enjoy.

I was also interested in the unique sound of the Roland TB-303, which was obtained by using square wave oscillators. Among the Bob System's 10 oscillators, 8 can produce square waves and triangle waves while the remaining two can only produce square waves. There are no sine wave oscillators currently on the Bob System. Sine waves have been used excessively in sound synthesis from the very early systems until today. I felt that I had to create something with a distinctive sound.

To achieve that, I used extremely non-linear astable oscillator circuits with no filtering to reduce the harmonics that are produced. The result is a sound rich in harmonics that is in closer resemblance to the sound of a natural musical instrument than the sound of a synthesizer, which is usually characterised by pure sine waves.

5.3 Controllers and Automation

The majority of synthesizers are operated by means of keyboard controllers. However, I have always preferred other forms of controllers that were more unconventional. Don Buchla had used sequencers instead of keyboard controllers in his early creations. The Theremin of Leon Theremin used an antenna around which the user's hands would move to control the pitch. Some Theremin models had two antennas, one to control the pitch and one to control the volume. I decided against building a keyboard controller for the Bob System, focusing on other types of control, such as Variable resistors (Potentiometers) and CV automation. The Voltage Controlled Analogue CV Sequencer is designed to function in a similar way to the Moog and Buchla early Analogue Sequencers. However, the Bob System also offers CV and Trigger inputs that can be used to connect a commercial CV/Gate keyboard controller.

I have never seen, heard or read of a Voltage Controlled Sweep Generator being used on a Music Synthesizer. My design was based on theoretical reading about electronic circuits and personal experimentation. The circuit is based on the obsolete AC152 Germanium Transistor. It is a unique approach as it does not rely on a traditional oscillator circuit to create the slope.

6. Conclusion

6.1 Effectiveness

The Bob System has proved to be a highly effective Analogue Modular Synthesizer, reliable for use in live performance and powerful enough for use in the recording studio. It is a true modular system that can be infinitely expanded. From my personal experience of performing with the Bob System around Europe and recording it in several occasions, I must say I am satisfied with my creation and I look forward to exploring the possibilities for expansion.

6.2 Expandability

The modular design of the Bob System is such, that there is no central processing device amongst the modules. This means that any module can be used in any role, either as a master or as a slave device for the synchronisation of the system. This allows for increased expandability, since even when some of the modules are considered to be “outdated”, new ones can replace them with no changes needed on the rest of the system.

I am planning on investigating implementation of a more diverse range of devices in the future, such as telephone systems, radio frequency transmission and reception and TCP/UDP network communication. I would also be interested in including a MIDI to CV or CV to MIDI module, to allow interfacing with MIDI devices and controllers.


Anagnostopoulos, Ilias

2007 Analogue Modular Synthesis, Seminar at the University of Sheffield.

Crab, Simon

2005 120 Years of Electronic Music, Electronic Musical Instrument 1870-1990 [Online] (Accessed on 22/01/2007)

Chabade, Joel

1997 Electric Sound: The past and promise of electronic music. New Jersey: Prentice-hall

Various Contributors

1996 Vintage Synth Explorer [Online] (Accessed on 22/01/2007)

1999 Video Synthesizers Homepage, Inc. [Online] (Accessed on 22/01/2007)

2000 American Heritage® Dictionary of the English Language, Fourth Edition (New York: [Online] (Accessed on 22/01/2007)

The Bob System

The Bob System is an expandable modular synthesizer based on Control Voltage principles. It is mostly used for producing sound and image but it could also be used to control other parameters where necessary. The concept and majority of the modules were designed and built by Jesus H. Other circuits, not designed by Jesus and originally not designed to be used with synthesizers, were heavily modified to be able to accept or produce control voltages. Several other commercially available devices such as audio/video mixers and signal processors are often used modified or unmodified to enhance the outcome. This project started in 2005 and will probably never be complete, as the possibilities for expansion are endless.

Bob, the heart of the system:

This "mainframe" is built in an aluminum case with a single-piece iron panel. It consists solely of analogue circuits and two power supply units with auxiliary outputs/inputs. The modules that are housed in the main case are Ten Voltage Controlled Oscillators of different designs, a Voltage Controlled Low Frequency Oscillator/Pulse Generator/Clock, two Pre-Amplifiers/Function Attenuators and a Multi-Module for Additive and/or Subtractive Synthesis that can fuction as a Ring Modulator, an Amplitude Modulator, a Voltage Controlled Low-Pass Filter or a Voltage Controlled High-Pass Filter. Eight of the VCO's are astable oscillators capable of producing square and triangle waveforms. Their CV inputs can be DC coupled with the flick of a switch. Their frequency spectrum is divided in three bands, selectable by means of a rotary switch. The other two VCO's are a multiple mode VCO, ranging from subsonic to supersonic frequencies and a very simple square wave oscillator both with a trigger input and a switch to select astable or monostable mode and a CV input that can be disengaged. The VCLFO also has a disengageable trigger and CV input while it is able to function both in astable and monostable modes. The Pre-Amplifiers are based on broad spectrum operational amplifiers and only have a gain control. The Multiplier is capable of performing arithmetical or trigonometrical functions using the inputs and exporting the result in the output. It also features a summing input, which allows simple summing operations to occur. The modes are selected by means of simple toggle switches and patching. The power supply units can provide stabilized +15, -15 and +9 Volts. The 15 Volt ones have a total 1 amp tolerance while the 9 Volt one has a 2 amp tolerance, both with front-panel fuses. The peak level of all signals is approximately 5 Volts. Signal outputs and inputs use 1/4' jack sockets while Control Voltages use 4mm banana sockets. The power supply auxiliary outputs and inputs use 3,5mm jack sockets.

Peripherals (by casing):

Voltage Controlled analogue 8-step CV Sequencer with an on-board Voltage Controlled Low Frequency Oscillator. With the exception of a 4-bit shift register for the switching, the rest of the circuit is analogue. Since no synthesizer signal goes through the shift register, the signal path remains fully analogue.

Pre-Amplifier/Function Attenuator, Spring Reverb, Ring Modulator. All-analogue vintage circuitry.

Three variations of a Voltage Controlled CV Sweep Generator circuit, the third variation being double. All-analogue extremely vintage-component circuits.

Voltage Controlled TM Tone toy keyboard. This has been modified (yes, circuit-bent) heavily, now featuring switches for short-circuiting or signal injection on the internal clock, a pitch shift pot, a fine tune pot, a body contact and a COM-port (yes, an actual computer COM-port) to connect the toy to a wooden wine box. The external box was fitted with anything that could no longer fit in the original casing, even after the internal speaker was removed to make room for additional circuitry. Initially it was capable of Amplitude Modulation and Frequency Modulation of the original keyboard sound while the switches would trigger aleatoric sequences the tempo of which could be controlled by any of the synthesizer clocks. After a while the keyboard scan was toasted, so the toy could no longer be controlled seperately of the synthesizer system. Nowadays, the Pulse Code Modulation ROM is probably toasted too, so the aleatoric sequences are only limited to Voltage Controlled noises and glitches. Apparently, this is not an analogue circuit. The original toy was a late eighties Chinese design with a monophonic keyboard scan, only one keyboard timbre and a few melody sequences. I am guessing that it falls in the category of the sample-based Pulse Code Modulation circuits, which have two saved samples on a ROM chip, add them or subtract them to create the different timbre variations used in the melodies and change the sample playback speed to affect the pitch by multiplying a different value with the clock speed for each keyboard note.

Voltage Controlled SK3738 toy keyboard. This has been modified as well (yes, circuit-bent), featuring a pitch pot, a speaker disengage switch with a compensation circuit, an external input to the toy's amplifier and an onboard patchbay. The patchbay has two rows of four RCA inputs/outputs. It can be operated by either patching any top row RCA with any bottom row RCA or by connecting a clock pulse or a square wave to any of the bottom row inputs. The keyboard scan is now toasted as well so this is now only useful for aleatoric sequences triggered by the synthesizer, following the LFO tempo. The original toy was quite similar in functions and design to the TM Tone, although probably designed about a decade later, also made in China. The keyboard scan was duophonic and it had five preset timbres.

Now defunct or gone:

Voltage Controlled Function Generator. This was a quartz-synchronised analogue circuit, basically a Voltage Controlled testing Oscillator with a broad range of 0.00001hz to 1Mhz. It was found in Delta Studios and was immediately taken to 52 Studios for further research. Upon the demise of 52 Studios it was unfortunately reclaimed by Delta Studios, where it still remains unused until today.

JVC vintage radio. So vintage and analogue that it doesn't even contain Integrated Circuits. The transistors are finger-sized and the coils were hand-wound. It was modified (yeees, circuit-bent) to allow further modulation of the carrier oscillator using the synthesizer oscillators. Unfortunately the tape transport system broke and the power supply causes a transistor to overheat, so it is currently in pieces waiting to be repaired.

Vintage Ibanez Phaser. This was a half-toasted vintage phaser that was found, repaired, retuned and modified (yeeeeeesss, circuit-bent) to allow CV control of the effect parameters. The circuit was to be removed from the original casing and installed in a suitcase, but it was unfortunately stolen by a former band member before the project could be finished..

The Bob System
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