Blacet Research, Guerneville CA, 95446 707-869-9164 www.blacet.com
The Blacet MIDI-Sync IC decodes sequencer timing information from the Musical Instrument Digital Interface (MIDI) serial data stream. The chip provides normal and inverted versions of the 24 pulse per quarter note (ppqn) clock, the run/stop command and the start or reset pulse. These outputs may be used to run devices which are desired to synchronize with MIDI sequences but which lack a MIDI input. The use of a dedicated IC for the sequencer function assures the highly accurate timing necessary for the proper feel or groove.
See Figure 2. Since MIDI is an optically isolated interface, the main extra components are for this function. Crystal control at the rated frequency is necessary to remain in sync with MIDI data. The output waveforms to the right of the chip include 5 µS wide clock and start pulses.
Figure 3 shows a basic MIDI to DIN sync converter. DIN sync is a pre-MIDI method of linking sequencers. DIN clock pulses are generally 24 ppqn, but at least one brand uses 48 ppqn. Other manufacturers have different functions for some connector pins. The configuration shown merely adds a pulse stretcher to convert the 5 µS clock pulses to 1 mS. Most equipment should properly clock and start/stop with this circuit.
Figure 4 shows how to divide the 24 ppqn for use with analog sequencers, which typically use one pulse per beat. Assuming 4/4 time, 24 ppqn equals 96 beats per measure (bpm). We would really like to see 16 or 32 bpm maximum, so this would be a divide by 3 or 6. The 4018 will divide by 6 with a little feedback, as shown. (For divide by 3, AND pins 5 and 4 to pin 1. The output is pin 4.) Now that we have 16 bpm, we can use a dual hex counter to provide even more outputs from 8 bpm down to 0.0625 bpm or 16 measures per beat (mpb). This sort of circuit can obviously act as a master clock for a number of analog sequencers, sequential switches and other event generators.
Figure 5 shows a basic 8 step analog sequencer configured around a 10 step HC4017 sequential output IC. The sequencer could obviously go up to 10 steps. The feedback from the 8 output to RST forces the device back to 0, resulting in 8 steps. Note the use of the high output current HC version of the 4017. This allows directly driving low current or high brightness LEDs without seriously lowering the available output voltage. This is a 5V maximum part.
Two typical output circuits are shown on top of the 4017. On the right side is a variable voltage output controlled by 8 pots. The 8 voltages are summed in the first op amp, which should be run off +/-15V to allow the mix voltage to be scaled higher than 5V if desired. Ra, Rb and the LED must be duplicated for each of the outputs, which are mixed at the op amp via Rc, Rd, etc.
The output circuit to the left shows a gate output sequencer with two busses, A and B. Toggle or slide switches route each step out to the A or B outputs or to Off in the center position. The 4538 dual monstable provides a variable gate time on each output, the B output being a duplication of the A circuitry. Two stages of the output for each step are shown, with 6 more diodes, LEDs, resistors and switches required to complete the sequencer.
A little thought will show how to add both the voltage and gate outputs to the same 4017. For 16 step sequencers, the 4017 is typically replaced by a four bit hex counter such as the 4516. A 4514 decodes the 4 lines into 16 outputs. Many additional features such as glide or slide, up/down counting, random steps, etc. may be added as desired.
Although analog sequencers have been largely replaced by digital and computer program types, their inherent simplicity, ease of programming, ability to step outside the equally tempered scale, and ability to control new and old modular synthesizers has insured their continued popularity.