[2347] BBDs are not analog

Date: January 5th, 2018 | Comments : [2] | Categories: DIY.

A bucket-brigade-device (BBD) is not analog. It is a sampling circuit that takes small slices of the incoming voltage just like an analog-to-digital-converter (ADC), and the main difference between the two methods is in how the samples (slices) are stored. The BBD samples are stored as voltage levels on tiny capacitors in the BBD chip, while the digital system converts the voltage levels to a digital measurement and stores the binary word(s) in memory.

Both systems have common problems of Nyquist sampling rates, aliasing and bandwidth limits.

After a time delay, the binary words of a digital system can be retrieved from memory, then sent to a digital-to-analog-converter (DAC) and restored to an analog voltage that is exactly the same as when it was first sampled. However, the BBD generates delay by passing the sampled voltage through a series of capacitors and mosfet switches before it goes to the output. There are a thousand or more of these capacitor/transistor cells in series inside a BBD, and small losses are incurred while the voltage is being held, and as it is being transferred down the chain of capacitors.

If the capacitors and transistors of a BBD were perfectly uniform, the output voltage would be equal to the input level, exactly as with a digital capture. But they are are not perfect and the small voltage losses from the tiny capacitors result in distortion of the reconstructed output voltage in the BBD.

The MN3007 1024-stage BBD chip has 0.5% distortion from the chip alone. It is also limited to a signal-to-noise ratio (SNR) of less than 80db, which is about equivalent to 13 bits of digital conversion. Approximate SNR = 6.02*n where n is the number of bits. (Math!!!) The larger BBD MN3005 4096-stage chip has 1.0% distortion typically, though it could be as high as 2.5%, and its SNR is less than 75db (slightly over 12-bit equivalent). Real world measurements yield lower SNR for BBDs than the datasheets claim.

Conversely, undithered 16-bit digital captures will have quantization error distortion spikes, but they are lower than -110db and certainly not audible in guitar pedals. Furthermore, the software in the digital signal processor can dither the output signal and eliminate the distortion spikes.

The nearly infinite resolution of the analog sample capture by the BBD will make no difference. In comparison, a 16-bit capture of a 1 volt full-scale signal represents only 0.00001526 volt for each bit! No one can hear the difference that might occur between an analog signal at 0.50001234 volt and a rounded digital conversion at 0.50000000 volt, especially after the effect of the reconstruction filter.

The higher distortion and lower SNR of BBDs are the main differences in sound. It would seem that adding some mild distortion (0.5% to 2.5%) and reducing the bit depth to 12 or 13 bits would be the key to getting an equivalent BBD sound with a digital pedal. It is a start but not the whole story. There are clock feedthrough, nonlinearities, aliasing distortion and other effects to be considered too, including the compression/expansion that is used in many BBD-based delay circuits.

There is definitely a difference between the digital and BBD circuits, but neither is strictly analog since they are both working with voltage samples rather than the continuously varying analog voltage.


2 Responses to “BBDs are not analog”

[782140] xolotl studio Says: 1:14 pm, January 12th, 2018

according to common electronic definitions found in almost all textbooks,BBD are analog since they do not quantize amplitude, but they are not continuous-time device, so they are subjected to aliasing problems. some confusion may arise since normally we are used to deal with digital devices, which are both discrete-time and digital (that is: they quantize amplitude) and continuous time analog devices, like an analog synth.

[782142] admin Says: 3:52 pm, January 12th, 2018

Analog audio is a continuously varying voltage. When you start to slice it up into samples, it is no longer continuous. It is discrete time slices used as a sample of the original signal, and that is not analog.

The chunks are stored in memory for the time shifting but whether it is stored in silicon memory or in a capacitor is immaterial. It is still a voltage sample that it delayed in time and then reconstructed at the output.

Best regards, Jack

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