Alesis ALESIS M20 16.3 REFERENCE COUNTERS VS. CLOCK SOURCES

16.3 REFERENCE COUNTERS VS. CLOCK SOURCES

A wristwatch indicates the passage of time in two ways. One is the face of the watch, which

gives the information of hours, minutes, and seconds. The other is the “ticking” of the watch

several times per second. Conceivably, you could always know what the time is by looking

at the face of the watch one time, then counting how many ticks had elapsed. In video, the

“ticking” is provided by the edge of the video fields (approximately 60 per second), and in

digital audio by the number of samples (48,000 per second).

When locking the M20 to video or another digital recorder, it’s important to understand the

distinction between the location reference, and the reference clock. The row of buttons below

the M20’s main display allow you to set these two clocks independently from each other,

because they have slightly different functions.

Location reference describes the actual position (or address) of each device. This is usually

determined by the SMPTE time code coming from each machine being synchronized. Unlike

reference clock, location reference always knows exactly where it is because the time

reference information (hours/minutes/seconds/frames) is embedded in the code.

The ADAT format used in the M20 has two different location references recorded on tape:

1. ABS time is the sample-accurate time code written on every formatted ADAT tape.

Every ADAT machine can read ABS time, which counts the number of minutes and

seconds from the head of the tape, starting at -00:05 (after 15 seconds of Lead and 2

minutes of Data), continuing to about 40 minutes at the end of a typical ST-120 tape.

There is never a break or interruption in the ABS time of a properly formatted ADAT

tape, and it always starts at zero. It is the ABS time which is transmitted via the ADAT

Sync to all ADATs in a multi-ADAT system so they can sync to the ID1 ADAT.

2. The TC track is a read/write SMPTE time code track recorded in a separate subcode area

from the ABS time. The TC track is only readable in ADAT machines with TC track

capability (currently, the Alesis M20, the Studer V8, and the Fostex RD-8). Unlike ABS

time, the TC track can be blank, have interruptions, start at any

hour/minute/second/frame desired, and/or be discontinuous (i.e., jump in values) if it

was recorded that way. In a typical multi-ADAT system connected via the ADAT Sync,

only the lead (ID1) M20 uses its TC track as a location reference. The ADAT slaves (ID2

or higher) do not know the master’s SMPTE time; they only know the ABS time and

chase to it.

The M20 allows you to select either of these references as a Chase Reference or a Tape

Counter at any time.

Reference clock is the “heartbeat” of your studio. It is a constant, steady “ticking”, without

any other information, connected to all synchronizer systems in your studio. Black burst

video and word clock are both forms of reference clock. Reference clock has no regard for

time code position, but instead is concerned only with making everything travel at exactly

the same speed. In a digital audio-to-video synchronization system, all elements (the SMPTE

time code, the digital sample rate, and the video frame rate) must advance in lock-step with

each other.