Bucket brigade device (BBD) is an analog discrete time delay line using capacitors that has been replaced in most digital signal processing applications. But it continues to be used in some special applications such as guitar and audio sound effects and some types of sensors. It consists of a series of capacitance sections where a stored analog signal is moved from section to section step by step during each clock cycle. (Figure 1). Conceptually, it is similar to a brigade of people passing buckets of water from one to another, hence the name of the brigade device.
This frequently asked question examines the broader implications of the technology behind BBD, examines the main BBD architectures and practical considerations for implementation, and concludes by presenting how BBD is used in guitar sound effects.
As well as being useful in audio effects, BBD is interesting because of its connection to other well-known electronic devices. For example, the concept behind BBD led to the development of charged devices (CCDs) used in imaging devices. The use of capacitors to maintain voltage is associated with dynamic random access memory (DRAM) designs, where charges are refreshed instead of propagated. Outside of audio special effects, BBDs have been largely replaced by digital signal processing technologies.
BBDs are used as an analog beamformer for phased arrays with piezoelectric ultrasonic sensors. And CCD-based pixel arrays work like BBDs, with the light-generated charge from each pixel passing through the entire array of pixels to the corner of the chip, where it is amplified and recorded.
BBD IC first appeared around 1980. The most common BBD uses an antiphase clock configuration (Figure 2). Designs range from 512 degrees to over 4,000 degrees and can include a single delay line or a double delay line. Various BBD implementations support clocks from 1.5 kHz to 1.5 MHz.
BBD ICs have limited dynamic range due to inefficiency of charge transfer and nonlinearity. The input signal level must be a small part of the supply voltage to achieve low total harmonic distortion (THD). BBD IC has about 60 dB signal-to-noise ratio (SNR) and variable insertion gain up to about 2 dB for optimal input levels. Insertion gain and SNR become more problematic as the number of stages increases and they are also sensitive to clock frequency. As a result, compensation schemes are essential in applications such as echo effects with a large number of stages and variable clock frequencies. Compensation circuits are not required in applications such as choirs, flangers, phasers and vibrato with less variable clock frequencies and shorter delay times. Examples of BBD IC applications include:
- Echo chains (Figure 3)
- Thousands of stages
- Highly variable clock frequency
- Filters for smoothing and reconstruction were used
- It can perform compression or expansion
- Duplicate echo feedback may be included
- Choir, flanger, phasers and vibrato
- About 1000 stages
- Low frequency oscillator used to change the clock frequency
- Filters for smoothing and reconstruction were used
- Feedback is used in some designs
What is the effect of a choir?
The chorus effect simulates subtle differences in timing and pitch when multiple musicians or vocalists play the same note but differ slightly in timing and pitch. The chorus sound is described as making the sound look “bigger”. It is based on a multiplier effect by creating a copy or multiple copies of sound and changing the time or pitch using a low frequency oscillator (LFO) with BBD, then mixing the modulated copies with the original sound. Instead of using a pitch scheme to modulate the pitch, the chorus modulates the delay time of the sound. An ever-changing phase shift is created between the duplicate sound (s) and the original sound. Horus effects typically use delay times between 15 and 35 ms. Slower delays produce a finer chorus effect, while higher speeds cause more phase shift and height modulation.
How do flangers work?
Flangers produce a variation of the chorus effect by creating a copy of the sound and using the LFO to modulate the delay time via BBD. Flangers use shorter delay times (usually 1 to 5 ms) and only one copy of the original sound. Because choirs use differences in pitch and time, the frequencies of the original sound and the copies are not the same, which minimizes interference. Flanders experience significant interference as the copied signal is the same as the original. Instead of creating a “bigger” sound like a chorus effect, the flanger “blurs” the sound. Because the two sounds are shifted in phase, the waveforms move from completely out of phase, causing structural disturbances, to completely out of phase, causing destructive interference. LFO is used to modulate the delay time of the copied sound. As a result, the flanger sound has a resonance that travels at the speed of the LFO. Flangers also use the feedback of the copied signal to further blur the sound, resulting in harsh metallic tones.
How do phasers and vibrato work?
Unlike choirs and flangers, phasers do not modulate the delay time. Phasers use LFO to move copied sound against the original sound, but instead of slowing down the sound, it is sent through a filter for all passes. The all-pass filter is used to change the phase ratios of the different copied and original audio frequencies. As the copied sound passes through the filter for all passes, certain frequencies are shifted in phase and the result is combined with the original sound, forming frequency slots where the circuit for all passes creates phase shifts.
Vibrato is a specific form of fiberboard. In fiberboard, identical capacitor values are used in the phase shift stages. In contrast, the vibrato uses striking capacitor values to create a quick, slight variation in pitch and a stronger or richer tone.
Summary
BBDs are mostly outdated technology that continues to be used in niche applications such as guitar special effects and some sensor systems. The technology behind BBD can be found in more modern devices such as CCD images and DRAM memories. Like Vinal Records, BBD has resisted the advent of digital signal processing in audio systems.
References
An effective model of bucket-based audio circuitsColin Raffle
Bucket-brigade deviceWikipedia
Brigade bucket devices: MN3007Electrosmash
Ppractical modeling of bucket-brigade device circuitsIEEE International Conference on Digital Audio Effects
Understanding of chorus, flangers and phasers in audio productioniZotope
