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With a fixed clock frequency, they can only take a sample step every 48 or
96KHz and step at that increment. This only simulates fractional delay and
is not true fractional delay. The ear can hear the difference, and so the brain
will perceive this difference as incorrect. Early digital delays like the
Lexicon PCM 41/42 did use a variable clock using a different, now
obsolete, process, but these are the exception not the rule. Most digitally-
controlled analog delay based modulation that use a processor generated
clock do not use steps small enough for true fractional delay, so while they
may use a BBD, the modulation does not sound as natural as some
picosecond step clocks or true analog modulation.
In addition, BBDs have some other “artifacts” that give them their unique
sound. The transfer characteristics, gain, noise, distortion and frequency
response vary depending on the bias voltage and clock frequency making
the sound change as the delay time changes. They also do not put out the
entire signal at the same time. Half the signal is a ½ clock behind the rest
of the signal, yet combined at the same time, making for a strange, smeared
mixing of the signal samples slightly out of time.
Other components in a BBD circuit are also responsible for the signature
sound of analog delays. BBDs require companding and emphasis/de-
emphasis networks to improve the signal to noise ratio, but these add
limiting and an animated “breathing” compression character to the sound.
The design of the filters affects the frequency response, peaking or ringing,
and potential aliasing or ring modulation. There are many more parts of the
analog design often overlooked in digital recreations that affect the sound
as well including the clipping, feedback network, clock stability,
modulation wave shaping/curves (exponential or linear, etc.) that are all
important to get that great analog delay sound.
For more reading about BBDs check out these great websites:
https://www.electrosmash.com/mn3007-bucket-brigade-devices
https://www.premierguitar.com/articles/25035-behind-the-bucket-brigade
https://pedals.thedelimagazine.com/bbd-chips-the-magic-behind-analog-
delay-pedals/