The Cabintech CT3680 is a hybrid module that implements from 1 to 4 flexible, configurable audio delay lines. This module can be used by system designers to create many different delay-based effects (reverb, echo, chorus, flanger, etc.) using traditional analog feedback paths, filters, and modulation techniques. Although the CT3680 is digital at its core, all inputs and outputs are analog -- no programming or digital logic design is required and it integrates easily into an analog signal flow. The CT3680 uses a fixed sampling rate that is independent of delay time, producing consistently high quality audio at any delay setting.

The CT3680 is designed to be easy to use with simple analog audio inputs and outputs, control voltages to set parameter values (such as delay time), and a single +5V power supply. The small module size (22x38mm, 0.9”x1.5”), surface mount edge pins, and through-hole adapter options provide flexibility for mechanical fitment into small spaces and optimize the use of PCB board space.

Audio Inputs/Outputs

The CT3680 has 4 analog audio inputs, and 4 analog audio outputs. The relationship between the inputs and outputs and delay times is controlled by the selected configuration settings. All audio inputs and outputs are line level (2.5V peak), single ended AC coupled for easy integration into an analog signal chain. Traditional analog effects circuits with filtering, modulation, and gain control can be used to create feedback paths for various types of effects.

Configuration

The configuration options control the number of independent delays lines (“channels”) and the maximum delay times they have. Each channel has 1 audio input and 1 or more outputs (“taps”). Each tap is an audio output with an independent delay control. For example, a channel with 2 taps would have one input and two outputs. The outputs are both delays of the same input, but with different delay times, each controlled with a separate CV (control voltage). There are many possible configurations of channels and taps. For example, one of the configurations is 2 channels with 2 taps each:

In this configuration, output 1 will be a (variable) delay of input 1. The amount of delay at output 1 is set by a control voltage (CV-1). Output 2 is also a variable delay of input 1, with its own control voltage (CV-2) which sets the delay of output 2. The delays at outputs 1 and 2 are both of the same signal at input 1 but are independently variable. Likewise, outputs 3 and 4 are independent delays of the signal at input 2, controlled by CV-3 and CV-4 respectively. A configuration like this might be used in a true-stereo effect that requires (at least) 2 separate channels of delay.

Another configuration consists of 3 independent delay lines, 2 of which have a single delayed output, and 1 has two delayed outputs (taps):

The different configurations can be useful for constructing different types of effects including stereo and effects that utilize multiple delay lines.

Configuration is done with 4 input pins that choose between different “programs”, each of which defines a specific configuration. In addition, 5 “option” input pins control additional features depending on the selected program. See Program Selection on page 12 for a description of all the available programs.

Additional inputs select the audio sampling rate (48kHz, 32kHz, 24kHz, 12kHz). Lower sampling rates allow for longer delay times. The sampling rate remains fixed at the selected rate (e.g. sampling rate is not used to vary the delay time as in traditional BBD devices). (See Sampling Rate Selection on page 11)

Delay Control

The selection of configuration and sampling rate defines the minimum and maximum delay of each channel in the configuration. The taps (outputs) for that channel can have any delay time between the min and max, and the delay time can vary dynamically by use of control voltages (CVs).

A delay CV is an input voltage in the range of 0.0V to 3.3V. When the CV is at 0.0V then the corresponding output is at the minimum delay. When the CV is at the maximum of 3.3V then the output is at the maximum delay. Voltage levels between 0.0 and 3.3 form a linear scaling of delay time between min and max. For example, the following channel configuration has 4 taps controlled by CV-1 through CV-4. With a sampling rate of 48kHz this channel has a max delay of 682ms and a min delay of 0.3ms.

With the control voltage values shown, the outputs have the delays shown[1]. Note that the lines as drawn in the diagram are conceptual and not drawn in proportion to the actual delay time. E.g. the line for output 1 is drawn to the right of channel 2 on the green channel bar (closer to Max delay), but actually has a shorter delay time. The CV for any output can select any delay time in the range from min to max, independent of all other outputs.

Also see Setting Delay Times on page 27.

Additional Features

Some programs (configurations) support additional features. These features are optional and enabled using the 5 OPTION input pins. See the individual program descriptions to see what features are supported in which programs.

· Simulated aliasing: Some programs support the ability to simulate sampling-rate aliasing similar to the aliasing distortion that occurs in BBD devices when run at lower clock (sampling) rates. In some effects this distortion is desirable and is part of the overall sonic design. The amount of aliasing distortion can be manually controlled by CV or automatically determined by the delay settings. Aliasing distortion is disabled by default. (See Aliasing Feature on page 25).

· Mono-to-stereo effects: Some programs perform mono-input to stereo-output conversion using a Haas delay effect. The left-right spread of the stereo effect can be varied and is controlled by a CV input.

· Chaining: Multiple CT3680 modules can be “chained” together to achieve very long delay times and more inputs and outputs. Audio signals are passed digitally through the chain so there is no loss in fidelity no matter how many modules are linked together. (See Chaining Multiple Modules on page 27).

Design Requirements

The CT3680 is designed to minimize external component requirements and integrate easily to PCB effects designs. The CT3680 requires no external clock and is powered by +5V. All CV signals are +3.3V maximum (see Specifications and Maximum Ratings on page 7). A +3.3V reference is made available on an output pin to aid in generating CV signals. All audio analog inputs and outputs are single-ended, 2.5V full scale, and AC coupled. Clipping is detected and signaled on an output pin and an onboard LED.

The physical package is a 48 pin surface mount castellated edge pin module. It can easily be hand soldered or soldered in automated systems. A 48-pin standard DIL spacing through-hole adapter is available for through-hole PCB designs.

Features

· Input pin selectable configuration for 1, 2, 3, or 4 independent delay lines in multiple configurations.

· Maximum delay of 2.7 seconds (divided between all delay lines)

· Minimum delay time of 0.3ms

· 6 CV inputs to control delay and other parameters

· Digitally chainable up to 5 modules with total max delay of 13.5 seconds

· +5V 170mA power supply

· Reverse voltage protection

· 4 analog audio inputs

· 4 analog audio outputs

· Internal digital sampling at 48kHz (default), 32kHz, 24kHz, or 12kHz

· No external clock is required

· Clipping-detected output signal and on-board clipping LED

· Optional simulated manual and automatic aliasing distortion

· Optional mono-to-stereo conversion on some programs

· Emulation of any BBD model MN3001 to MN3011 plus SAD512, SAD1024, TDA1022, V3205, V3207, V3208

· Castellated edges for direct soldering to a PCB (through-hole adapter is available)

· Updatable firmware (“programs”)

Pinout

Pin

I/O

Symbol

Description

AUDIO_IN_1

Audio input 1

AUDIO_IN_2

Audio input 2

Out

AUDIO_OUT_1

Audio (tap) output 1

Out

AUDIO_OUT_2

Audio (tap) output 2

N.C.

Leave unconnected

N.C.

Leave unconnected

LINK_1CH

Number of channels linked in a secondary module (internal pullup)
HIGH: One channel is linked
LOW: Two channels are linked

+5V supply

I2C_SCL

I2C connector pins for firmware updating (see Firmware Updates on page 32). Leave unconnected when not used for updates.

I2C_SDA

PGM0

Bit 0 (lsb) of program number (internal pulldown)

PGM1

Bit 1 of program number (internal pulldown)

PGM0

Bit 0 (lsb) of program number (internal pulldown)

PGM3

Bit 3 (msb) of program number (internal pulldown)

Out

REF_3V3

+3.3V output reference (10mA max)

VC_DELAY_4

Set delay time within the global time scale

VC_DELAY_3

Set delay time within the global time scale

VC_DELAY_2

Set delay time within the global time scale

VC_DELAY_1

Set delay time within the global time scale

CV_SCALE_MAX

Global scalar for maximum delay time

CV_SCALE_MIN

Global scalar for minimum delay time

GND

Ground

GND

Ground

RESET

Pull low for 1ms or longer to reset (internal pullup)

RATE_1

Set audio sampling rate, takes effect on next reset or power on (internal pullup)

RATE_1	RATE_0	Sampling Rate
0	0	12kHz
0	1	24kHz
1	0	32kHz
1	1	48kHz

RATE_0

N.C.

Leave unconnected

Out

CLIPPING

Driven high when one or more inputs are at full scale. 5mA max draw. This also drives the on-board clipping LED.

OPTION_5

Digital control inputs (usage depends on the selected program). These inputs have an internal pullups and register as HIGH when disconnected.

OPTION_4

OPTION_3

OPTION_2

OPTION_1

N.C.

Leave unconnected

PRIMARY

Sets this module mode to PRIMARY (high) or SECONDARY (low). See Chaining Multiple Modules on page 24. Changes take effect on next RESET or power on. (internal pullup)

Out

LINK_OUT

N.C on single modules or the last module in a multi-module configuration. Connected to downstream module’s LINK_IN in multi-module configurations.

LINK_IN

N.C. on single modules or the primary module in a multi-module configuration. Connected to the upstream module’s LINK_OUT in multi-module configurations.

In/Out

LINK_BUS_2

N.C. on single modules. Connected to all module’s LINK_BUS2 pins in multi-module configurations.

In/Out

LINK_BUS_1

N.C. on single modules. Connected to all module’s LINK_BUS1 pins in multi-module configurations.

N.C.

Leave unconnected

N.C.

Leave unconnected

N.C.

Leave unconnected

N.C.

Leave unconnected

Out

AUDIO_OUT_4

Audio (tap) output 4

Out

AUDIO_OUT_3

Audio (tap) output 3

AUDIO_IN_3

Audio input 3

AUDIO_IN_4

Audio input 4

Notes:

N.C. pins must be left unconnected.

Unused audio and CV inputs should be tied to ground to minimize noise.

Unused outputs should be left unconnected.