Time to Move
Fig. 1 — In this pin diagram for a Matsushita MN3005 BBD chip, pin 7 is where the signal enters the chip, pins 3 and 4 are the output, and 6 and 2 are the tick and tock pins.
Just as a movie projector and tape recorder require a motor to pour out their content, a BBD requires something to move the samples along. That something is a clock that puts out a complementarytickandtockto pins on the BBD chip at varying speeds, which in turn help dictate the fidelity and bandwidth of the signal. Internally, the chips themselves are essentially divided up into two parallel sets of stages—two lines of little people passing buckets, if you will. And just as with a real bucket brigade made up of people passing pails of water, each pin in the BBD can hand off or take a bucket—but it can’t do both at once. Consequently, the two parallel sets of stages take turns. When the clockticks, one set of stages is passing off a sample to the next stage in line, and the other set of stages is receiving a sample. When the clock goes tock, the reverse happens. The two paths work in complementary manner so that there is always one sample available in interleaved fashion, each arriving at a separate output pin.Fig. 1is a pin-out diagram of a Matsushita MN3005, the chip used in many analog delays from the late ’70s to the early ’80s. As you can see, there is only one input pin (7) but two output pins (3 and 4). Thetickandtockpins are 6 and 2.
When these alternating interleaved samples reach the output of their respective path, they are combined to provide a continuous, seamless stream of samples (see Fig. 2). Some pedals (such as the Boss BF-2 Flanger) simply link the two output pins together directly, while others (like Electro-Harmonix’s Deluxe Electric Mistress) use a pair of equal-value resistors to electronically mix the two paths together. Still others (like the old A/DA Flanger) use a trimpot to perfectly balance the two outputs. In the latter example, not only does the more precise balancing yield a more faithful representation of the input signal, but it also serves to better cancel out artifactual noise from the clock—kind of like a humbucker in a chip!
Fig. 2 — The input signal is sampled, with a tick and tock from the BBD’s clock assigning each sample to its respective path. The alternating samples are interpolated and “stitched” together at the output.
How Much Time You Got?
Many tube-amp fans can calculate roughly how much power an amp will deliver based on the number and types of output tubes it uses. For example, we don’t expect more than 5 to 10 watts from a single-ended amp using one power tube, 12 to 25 watts from a pair of 6V6GT tubes, and so on. Similarly, we wouldn’t expect a builder to design a 100-watt amp based on a dozen EL84s, because a quartet of EL34 will yield that kind of power in a smaller, more affordable, easier-to-maintain package.
Similarly, BBDs come in different capacities, as indicated by the number of stages they have. Just as with power tubes, stages can be coaxed to yield a little more or a little less, depending on the rest of the circuit. For audio purposes, BBDs begin at 256 stages, with each unit higher in the series generally doubling in capacity: 512, 1,024, 2,048, and 4,096 stages. The latter is the largest capacity available on a single chip.
A vintage ad for SWTPC’s Ambience Synthesizer, which used three 1,024-stage BBDs in series.
The maximum time delay available from any single BBD is a joint function of the number of stages, the clock (i.e., sampling rate), and the desired bandwidth. So, a 1,024-stage chip can provide twice as much delay time as a 512-stage chip if it runs at the same clock rate, and the same amount of delay if clocked twice as fast in order to give the signal a little more bandwidth and fidelity. It works the same in reverse: An effect designer could squeeze a full second out of a 4,096-stage BBD chip (which is normally not used for more than maybe 350–400 milliseconds), but doing so would require a very slow clock rate—which would seriously limit bandwidth and fidelity. (It would sound like something happening in the apartment next door, and lose considerable fidelity after two or three repeats through the chip.) The old Boss DM-1 used a now out-of-production Reticon chip with 2,000 stages and claimed a maximum delay time of 500 ms, but at that delay setting there was a steep roll-off of the signal above 1 kHz. The next Boss analog delay, the DM-2, used a 4,096-stage chip to generate a maximum delay time of 300 ms at greater bandwidth.
Cascading multiple BBDs yields the sum of their individual delays—although each handoff between BBDsdoes pose small possible risks to audio quality. But careful circuit design can mitigate these risks. There have been some interesting products over the years that used this cascading BBD approach. Before 4,096-stage units became available, the earliest EHX Deluxe Memory Man pedals, as well as the MXR Analog Delay and the Southwest Technology Products Corporation (SWTPC) Ambience Synthesizer, used a trio of 1,024-stage chips in series. More recently (but still before 4,096-stage chips were back in production), Electro-Harmonix used four 2,048-stage Panasonic MN3008 chips for its Deluxe Memory Man reissue.