Some advantages of using the CT3680 instead of a BBD

• No input bias required or adjustments needed
• No filters needed for clock noise or aliasing
• No compander needed for good S/N ratio
• No external clock is required
• No aliasing distortion at any delay time
• Device can be switched between different numbers and length of delay lines
• Multiple independent delay lines in a single device
• Multi-tap delay lines and be configured with independent and continuously adjustable tap points
• Higher S/N ratio and lower distortion than any BBD can achieve with no filtering or compression
• Longer delay times than any BBD with no sacrifice in signal quality
• Opportunity for easy control of delay settings and parameters by a microcontroller

Some disadvantage of using the CT3680 instead of a BBD

• For existing designs, the CT3680 cannot directly replace a BBD chip
• The CT3680 is a physically larger device and has a larger PCB footprint
• Single part cost of the CT3680 is higher than current-production BBD chips, although cost per msec of delay time is lower. For example, to create a delay of 680ms would require (3) MN3005 chips at a higher cost than a single CT3680 module. Some NOS obsolete BBDs are sold in the gray market for 2X or more the cost of the CT3680.

In general this is not a 'drop in' replacement for any BBD chip. From a circuit design point of view the most significant difference is that BBD delay time is set by the frequency of a clock (the higher the frequency, the shorter the delay time). Delay time for the CT3680 is set by a voltage (the higher the voltage the longer the delay time). With a frequency-to-voltage converter it might be possible to adapt an existing BBD circuit to work with the CT3680. Many BBD circuits also include filters and compression to minimize BBD noise, which are not necessary with the CT3680.

From a mechanical viewpoint the CT3680 has a completely different PCB footprint and is physically larger than any BBD chip.

In general when creating an effect with a DSP like the FV-1, the entire effect system is created with the DSP. It takes in the (dry) audio as input, and outputs the final effect (chorus, reverb, etc). The effect is defined by the DSP program code including any modulation, filtering, etc. There is little, if any, external analog signal processing other than buffering.

The CT3680 is not, in general, a complete effects solution. It provides only the delay function for the effect (much like a BBD provides only a delay function). External (analog) circuitry must provide feedback paths, filtering, modulation, and other analog signal processing to create a final effect. For example, if some modulation of the delay time is desired, some external circuit must create a CV (control voltage) signal that varies over time to control the CT3680 delay time. In a pure DSP solution, that modulation would be coded into the DSP program.

From a hardware point of view, an FV-1 type solution can be simpler to build as there are fewer components and the circuit is a simpler design. But that solution requires good expertise in DSP programming unless you are satisfied with the pre-programmed effects. It moves effects creation from analog circuitry to an exercise in programming (with a very good understanding of audio signal processing). Such an implementation is subject to the limitation of the device (e.g. limited program size, limited memory, limited number of control inputs, etc).

The CT3680 on the other hand, delegates all signal processing except for the delay function to traditional analog circuitry. No DSP programming or digital design is required, and all audio inputs and outputs are simple analog signals. The effects design techniques are very familiar to anyone versed in BBD circuits. Conceptually the CT3680 just replaces the BBD, albeit with much better performance, capability, and flexibility than any BBD chip. Analog circuits provide the feedback paths, filtering, delay modulation, and whatever other signal processing the effect requires. There is no limit to the complexity and control parameters of those analog circuits.

It depends very much on how you define 'analog'. From a system design point of view, the CT3680 is an analog-in, analog-out device. Internally the audio signal is sampled (made discrete in the time domain) and quantized (digitized in the amplitude domain). A BBD device also does sampling and is discrete in the time domain, but does not digitize the amplitude. Compare that to a pure analog circuit which is continuous in both time and amplitude. So neither BBDs nor the CT3680 are pure analog in both domains.

Although the CT3680 requires no programming or digital logic, it can be easily controlled with a microcontroller such as an ATtiny, Raspberry Pi, etc. Such controllers typically have multiple D/A converters to generate control voltages that can be used to drive CT3680 CV delay inputs. A controller can also be used to select CT3680 programs, set OPTION values, etc.

The minimum absolute delay for the device is 0.3ms, however it is possible to achieve relative delays down to zero. Very short delays are useful in building some types of effects such a phasers.

To create a very short delay, the CT3680 is configured for multiple independent channels (such as program number 3). The same audio signal is supplied to 2 of the channels. The outputs of the two channels are mixed. Consider one to be the "dry" signal, the other "wet" (the original input is not mixed into the output at all). When the delay time of both channels are set to the same (e.g. their CV delay input voltages are equal) the two signals will have a zero relative delay and will be summed in the mix without effect. By changing the delay of the "wet" channel slightly, the two outputs will become out of phase and produce the "phaser" audio effect.

It is helpful to use the global MIN/MAX scalars to restrict the overall delay time to something small (say 3ms). Then the individual channel delays can use their full-scale 0.0V to 3.3V range to make fine resolution changes in delay times. Varying the "wet" channel over it's full scale will produce a relative delay from 0 to 3ms, a range that is useful for these types of effects.

Yes, multiple CT3680 modules can be "chained" to create longer delay lines of up to several seconds. The chaining mechanism is digital so there is no signal degradation when modules are linked together. Note that V2 of the module also supports lower sampling rates (below 48kHz) which results in longer maximum delay times.

Version 2 of the hardware includes several significant new features including:

• High density surface-mount "castellated" edge pins for direct soldering, no edge connector required
• Selectable sampling rates of 48kHz (default), 32kHz, 24kHz, and 12kHz
• Maximum delay time is increased from 0.682sec to 2.7sec using lower sampling rates
• Updatable firmware
• Smaller physical size

Firmware updates are not common, but if necessary it is possible to field-update a CT3680 with new firmware. Such an update requires a bit of hardware to interface the module to a computer USB port. An update program can then be downloaded from our website which will connect to the module via the USB port and update the embedded firmware to the latest version. The update hardware is available on our website, and may also be available for loan, contact us for details.