The Vibrato Scanner is an electromechanical device that produced vibrato and chorus effects in old Hammond organs. Unlike newer technologies that modulate the signal at their source, the Scanner system modified the sound right on its way from the keyboard to the amplifier.
The hardware consists of two main elements: a phase shift line enclosing a series of passive all-pass filter stages and a scanner that is a single-pole 16-throw air-dielectric capacitor switch, connecting taps on the delay line to the output.
Each filter stage of the delay line is shifted in phase relative to the previous one, resulting in an increasing time delay between every successive step of the line (about 50μs per filter stage). Nine of these steps are connected to the sixteen inputs of the scanner, allowing it to sample back and forth along the line. For example, labeling each output of the delay line with a number from 1 to 9 would result in the following 16 digit sequence: [1, 2, 3, 4, 5, 6, 7, 8, 9, 8, 7, 6, 5, 4, 3, 2].
As the scanner gradually transitions between the taps, it alternates the phase shift applied to the sound, causing slight variations to the pitch and resulting in a vibrato effect. The depth of the vibrato effect relies on the width of the frequency shift fed into the scanner, which means that scanning about one-third of the delay line would produce a lighter vibrato effect while scanning the whole line would significantly increase its depth. The chorus effect is achieved by simply mixing the dry input signal with multiple outputs on the delay line.
In effect, scanning back and forth along the delay line is like moving toward and away from a sound source. This causes a change in frequency due to Doppler shift.
(Vorkoetter, 2009)
Originally, the delay line is part of the Hammond Vibrato Scanner unit and it incorporates a series of second-order audio filter stages where each stage is shifted in phase relative to the previous one. This results in an increasing time delay between every successive step of the line (about 50μs per filter stage). This is then fed into a scanner, that is a single-pole 16-throw air-dielectric capacitor switch connecting nine selected taps from the line to the output. As the scanner gradually transitions between these taps, it alternates the phase shift applied to the sound, causing slight variations to the pitch that result in a vibrato effect.
A notable limitation of this original design is the fixed gear ratio tied to the Generator Run Motor, which spins the rotor continuously at about 7Hz. The depth of the vibrato depends on the width of the frequency shift fed into the scanner, which means that scanning about one-third of the delay line would produce a lighter vibrato effect while scanning the whole line would significantly increase its depth. In addition to the vibrato, a chorus effect can also be achieved by mixing the dry input signal with multiple outputs on the delay line, or a chorus-vibrato by mixing the dry signal with the output of the vibrato.
The iconic chorus and vibrato effect is the result of Hammond’s extensive search for expression and emotional intensity. The Scanner Vibrato unit achieved to deliver a unique sonic character by adding depth and movement to the sound of the Hammond Organ. As the popularity of modulation effects were increasingly growing within pop music, so was the Hammond Vibrato unit. Since the 1960s, its unique sound has made its way into a wide range of genres, from blues and rock to hip hop and downtempo. Some notable artists include Booker T. & The MG’s, Yes, Beastie Boys and Portishead.
Sources:
Vorkoetter, S., 2009. Overhauling and Improving the Hammond M-100 Series Vibrato System. [Blog] http://www.stefanv.com, Available at: <http://www.stefanv.com/electronics/hammond_vibrato_mod.html> [Accessed 7 January 2022]. http://www.stefanv.com/electronics/hammond_vibrato_mod.html
Martinic.com. n.d. THE HISTORY OF THE SCANNER VIBRATO. [online] Available at: <https://www.martinic.com/en/products/scanner-vibrato/history> [Accessed 7 January 2022].https://www.martinic.com/en/products/scanner-vibrato/history
Electronic Music Wiki. n.d. Vibrato scanner. [online] Available at: <https://electronicmusic.fandom.com/wiki/Vibrato_scanner> [Accessed 7 January 2022].https://electronicmusic.fandom.com/wiki/Vibrato_scanner
Benton Electronics. n.d. Service Manual – The Hammond Vibrato – Benton Electronics. [online] Available at: <https://bentonelectronics.com/service-manual-the-hammond-vibrato/> [Accessed 7 January 2022].
Modularsynthesis.com. n.d. ModularSynthesis – Hammond Scanner. [online] Available at: <https://modularsynthesis.com/modules/DJB-scanner/scanner.htm> [Accessed 7 January 2022].https://modularsynthesis.com/modules/DJB-scanner/scanner.htm
GIF – https://www.nshos.com/HAMMOND10.htm
