Alesis ALESIS M20 M20 SMPTE SYNCHRONIZATION OVERVIEW

M20 SMPTESYNCHRONIZATION OVERVIEW

This chapter is an overview of the M20’s SMPTE time code features, in addition to the basics

covered in Chapter 11. Before you design a synchronization system, and particularly if you

have not used time code before, please read this chapter carefully. It is impossible for any

manual to cover all the possible combinations of synchronizers, decks, automated mixing

consoles, sequencers, and video editors; but a solid understanding of synchronization

principles will help you be successful no matter what equipment you plan to combine with

the M20.

When machines are synchronized, they move together at exactly the same speed, passing

through corresponding locations. To do this, there must be some kind of time marker

recorded continuously on each machine, along with the audio and video tracks. One

machine must be designated the master or controlling machine, and others are considered

slaves. Each slave machine must have the intelligence to compare the incoming time signal

from the master machine to its own location, and speed up, slow down, or locate to match

the master’s position. When it’s doing this, this is called chase mode.

The M20 may be used as master or slave because of its built-in time code synchronizer.

When it’s a master, you command the entire recording system by using the transport

controls on the M20 (or CADI). The M20’s SMPTE OUT jack will issue SMPTE time code

(depending on the parameters you set) to video or other audio decks. If they are also

equipped with synchronizers and are in chase mode, they will follow the M20 system’s lead.

When the M20 is used as a slave, it receives time code from the master (a video deck, hard

disc recorder, or edit controller) and moves to match it, depending on the M20’s time code

and SMPTE Chase parameters. On a slightly higher level, it may also receive direct

commands (play, rewind, record etc.) from a controller via MIDI Machine Control or (when

2.0 software becomes available) via the RS-422 port.

In addition to SMPTE time code synchronization, digital audio recording has its own

synchronization needs. When recording from a digital source (either ADAT Optical or the

optional EC-1 AES/EBU card), the M20’s speed must match the flow of digits as they come

in. This can lead to certain problems that analog recorders did not face.

For example, suppose you have the M20 in SMPTE Chase mode, following time code and

video clock from a VCR. You want to record a sound effect from a CD player digitally into

the AES/EBU card at a certain cue. But at the cue, when you press PLAY on the CD player,

you notice clicks and pops in the audio. What happened?