Alesis ALESIS M20 owner manual Smpte Time Code Rates and Types, 16.4A Drop Frame, 16-7

Chapter 16, SMPTE Synchronization Overview

16.4SMPTE TIME CODE RATES AND TYPES

SMPTE time code is the universal standard for transmitting location reference in audio and video systems. SMPTE is primarily a location reference, though it may also be used as a reference clock if necessary, because its signal is sent on a pulse-width modulated audio tone. It transmits time as hours, minutes, seconds, and frames; by looking at the tone it may be divided into subframes. The M20 can read and generate SMPTE time code in any one of several formats.

The Society of Motion Picture and Television Engineers agreed on several code rates to accommodate the needs of film and television work all around the world, which already had different television standards. The frame rate of television in the United States was originally 30 frames per second (fps) for black-and-white monochrome transmission, which slowed for technical reasons to 29.97 fps at the advent of color television (the NTSC standard). The standard television frame rate in Europe and many other parts of the world using the PAL/SECAM standard is 25 fps, and the standard film rate worldwide is 24 fps. When transferring between American television and film (the telecine process), a slight 0.1% variation on the frame rate is necessary in some situations. This is called pull-up and pull- down.

16.4A DROP FRAME

In addition to the different SMPTE rates, there is a need to number or count the frames differently in NTSC systems. The two slightly different speeds (29.97 vs. 30) leads to a problem: if the SMPTE time code numbers 29.97 frames as if there are 30 per second, a progressive error occurs. After half an hour, the SMPTE reader will show “00:29:58:06” instead of “00:30:00:00”– almost two seconds short of real time. By using the “drop frame” frame counting feature of SMPTE time code, certain numbers in the frame sequence are skipped so that real time and SMPTE time agree. Note that this is only a numbering convention: frames are not actually lost, they are just counted differently. An analogy is when we add February 29 to our calendars during leap years to keep our counting of days in sync with the seasons of the year.

SMPTE time code carries a bit identifying the code as drop or non-drop code, so all receivers will know the counting method being used. If you are trying to locate to a particular frame (i.e., the 00 or 01 frames at the start of most minutes) and it “doesn’t exist”, it’s because the code is drop frame.

Tip: Drop-frame code is typically used with the 29.97 fps rate, and non-drop with the 30 fps rate. Most music projects use 30 non-drop if they’re not syncing to NTSC television. Use non-standard codes (29.97 non-drop, and 30 drop frame) under special circumstances. Check with the other people on your project to find out the best rate to use.

16.4B AUTO-DETECTION OF SMPTE RATES

When the M20 is set to synchronize to an external SMPTE source, it will automatically detect if the incoming time code rate is 24 fps or 25 fps. It will also automatically detect if the incoming code is drop frame or not.

ALESIS M20 REFERENCE MANUAL 1.06 16-7