“Word clock is a master signal.... Address is also a master signal.”

 

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TUTORIAL JULY 28, 2000
Zen and the Art of Troubleshooting [page 2 of 2]

Software manufacturers do not expect the user to understand every detail of each component. The reality is for most of us, in the first stages of discovering these components, the details can just be too overwhelming. The goal in the beginning is to simply know what they are and basically what they do. For example, know that Performer installs FreeMIDI or Studio Vision installs OMS. FreeMIDI and OMS are MIDI operating components that manage MIDI hardware drivers and MIDI information to and from your synths and the software. Another example, DAE (Digidesign's audio engine) is a digital audio signal-operating component that speaks between your audio hardware and your sequencing software. These types of components maintain settings and routings for MIDI and audio playback. It is also important to know a little about the computer's hardware and operating software (OS). The type of computer, processor, bus speed hard drive, mother board and PCI/ISA expansion capability can give you insight on the powers and limitations of the software components.

Signal flow is the communication to and from your MIDI and audio hardware and the computer software. The purpose of a sequencer is to trigger your determined MIDI and audio events consistently within a time grid. There are two properties of a sequencer's signal flow, a master signal and a MIDI trigger signal. The foundation of a sequencer is the master signal flow of timebase and address. Timebase is the rate of the sequence playback and address is the location of events. MIDI trigger signals are bi-directional and simultaneously flow to and from a source and target. MIDI trigger signals are recorded within the timebase and address framework.

In a MIDI and digital audio workstation, timebase is word clock. Digital audio samples run on a clock rate for output signal, just as a tape is sped across a tape head. This rate is measured in Kilohertz rates like 44.1kHz or 48kHz. A MIDI and digital audio sequencer will derive the rate of playback from the audio hardware's word clock.

Word clock is a master signal. You simply have to establish a master and configure the slave devices to follow the signal chain. The simplest example of this is connecting two DAT (Digital Audio Tape) machines to copy digital audio to another tape.  To use common digital audio terminology, the first playback DAT machine is the source or master and the recording machine is the target or slave machine. The master DAT machine sends the sample data via word clock to the second target or slave DAT. You must set the source or master DAT to generate the word clock rate. For the target or slave DAT, you must have it's digital input button or similar setting engaged. In a MIDI and Digital workstation, the concept is the same. The hardware that determines the rate of your sequence can be the master or a slave.

Address is also a master signal. It determines where MIDI and audio play back within the time line of your sequence. In a MIDI and digital audio workstation, address is usually MIDI Timecode and is measured in SMPTE time—hours, minutes, seconds, and frames. As well, the placement of your hours, minutes, seconds, and frames to be determined by the rate of the timebase, so that a sample's placement has an address in time just as MIDI events do. This is why timebase and address will generally need to be referenced from the same master source if you wish your audio and MIDI to playback at a consistent point in time together, in synchronization.

The bidirectional signal of a workstation is MIDI recording and playback. The most common example is recording your keyboard performance into a MIDI track. You play a keyboard; it records your performance into the sequencer, which allows you to play the keyboard's sounds back to you. When recording MIDI into a sequencer, a note on and off signal is sent to the MIDI out. It then travels through your MIDI interface and is translated as an event in the software.  This signal in turn may be patched through the software, back out through the MIDI interface and to a MIDI instrument for playback. Though MIDI is serial, meaning these MIDI data travels a series, one event after the other, the result will appear simultaneous. Differing from master signals, MIDI trigger signals can be routed to many different destinations, from many different sources simultaneously, like a phone switchboard.

So this is a MIDI and Digital Audio workstation in a nutshell. I purposely did not cover details about your workstation in this writing. These words are an attempt to help you see the forest for the trees, a way to focus your thought process so you may discover the details on your own. Obviously, a basic understanding of your components and their signal flow will not always enlighten a solution. It will give you the power to pinpoint simple problems. For more complex issues, it's a tool that helps you communicate with troubleshooting professionals.  Most important of all, it instills a sense of control over the situation and the confidence to work towards a solution.

Part 2 of this two-part series will appear next Friday, Aug. 4. Daniel Cates is currently a tech support technician for Mark of the Unicorn.

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