User manual

5 - Dynamic Bank/Channel Switching (DBCS)

5.0 - Additional hardware

The additional hardware which enables Dynamic Bank/Channel Switching consists of:

1 - Two 3.5mm mono Audio/CV Input sockets on the left side of the machine.

2 - Two approximately +1.15 volt detector circuits, which are described below.

3 - Two Red LEDs, which are On when the corresponding detector circuit is

activated.

4 - Two Address Bit Inversion Circuits. Each has two inputs: the 0 or 1 output of a

memory bank address switch, and the output of corresponding detector circuit.

The output is the XOR (Exclusive OR) of the two inputs, as described below.

The output drives the corresponding bank address bit of the memory system.

This description is for the TB-303 Devil Fish, the TR-606 and the two channel version of the

DBCS mods for the TR-808. It is also possible to install a four channel version for the TR-

808.

For the TB-303 Devil Fish with the MIDI In and Out system, there are two additional

hardware functions, neither of which are visible:

1 - The MIDI In and Out firmware can drive the “default contact” (AKA “normally

closed contact”) of each of the two Audio/CV sockets. This enables the firmware

to drive the output of the socket, which goes to the detector circuit, if nothing is

plugged into the socket. (If a lead is only partially inserted, sufficient to touch the

contact, but not to lift it off the default contact, then unless the lead has a strong

load or signal, the signal created by the firmware will still drive the output of the

socket, and it will drive the lead as well.)

2 - The MIDI In and Out firmware can read the output of the Address Bit Inversion

Circuits – the same two bits which are driving memory bank address bits 3 and 2.

These bits can be used to change the MIDI In Channel for receiving Notes, so that

instead of receiving only on the currently selected MIDI In channel, Notes can be

received on this channel, this channel plus 1, this channel plus 2 or this channel

plus 3, with wraparound of 17, 18 and 19 to 1, 2 and 3 respectively.

Each detector circuit senses its input voltage going above approximately +1.15 volts.

The exact threshold voltage will be between 1.05 volts and 1.25 volts, with these extremes

only likely to be reached in very hot or cold conditions respectively. This voltage is chosen

so it can be reliably turned off with a 1.0 volt signal from a MIDI to CV converter. The TB-

303 (including the Devil Fish) produces 1.0 volts from its CV out socket, when playing the

lowest C of the Internal Sequencer’s range.

This threshold and its +/- 10 millivolt tolerance means that the input can be reliably turned

on with 1.25 volts, which a MIDI to CV converter can produce in response to a note event 3

semitones above that which produces the 1.0 volts. The TB-303 (including the Devil Fish)

produces this voltage from its CV Out socket when it is playing the D# in the lowest octave

of the Internal Sequencer’s range. Each semitone is represented by 1/12 of a volt = 0.08333

volts.