The EM Poll

Timing Is Everything

May 1, 2006 12:00 PM, By Brian Smithers

           

Horticulturalist Luther Burbank is credited with defining a weed as any plant that is growing in the wrong place. In that same spirit, one could consider a wrong note as a right note played at an incorrect time. The appeal of such a perspective is that it recognizes that in music, as in life, timing is everything.

As musicians, we gauge our timing with a variety of clocks. We measure the number of beats in a minute, the number of ticks in a quarter note, and the number of frames of film that go by in a second. In this article, I'll discuss musical clocks, how they're used, and how they're connected.

Tempos Fugit

The first musical clock that most of us encounter is the metronome. Originally a mechanical device but now commonly electronic, it creates an audible pulse that is used to give a tempo reference. Tempo refers to a number that relates musical time to clock time, ordinarily cited as beats per minute (bpm). The metronome's function in aiding musicians when they practice scales and arpeggios is often an essential part of their daily regimen.

FIG. 1: Modern sequencers let you set the resolution of their timing signals. This screen from Cakewalk Sonar 5 shows you the options for the source of a timing -signal, the number of ticks per quarter note, and the SMPTE frame rate.

The metronome is also a critical part of any sequencing application, indicating the timing of MIDI data in terms of bars and beats. Each beat is further subdivided into arbitrary nonmusical subdivisions called ticks, and it has become standard to count 960 ticks in a quarter-note beat (see Fig. 1). An 8th note therefore consists of 480 ticks, a 16th note has 240, and so forth. The use of a metronome in sequencing enables grid-based logical operations such as quantizing. Recording to a metronome, or click track, in a DAW (even if no MIDI tracks are used) allows for grid-based editing, beat slicing, loop manipulation, and other common tempo-based operations.

A number of clock messages have been used to synchronize the internal sequencers of electronic instruments. Frequency-shift keying (FSK) encodes a clock pulse in an analog audio signal by alternating between two frequencies at regular intervals. FSK was superseded by DIN Sync, so named because it uses the 5-pin DIN connector subsequently adopted for MIDI. Like FSK, DIN Sync is an analog signal, and it transmits either 24 or 48 clock pulses per quarter note (ppqn), depending on whether the device at hand follows the Roland or the Korg standard, respectively. The Roland DIN Sync standard is also called Sync24.

One of the most recognizable devices to use DIN Sync is the classic Roland TR-808 drum machine. When Roland released the TR-808's successor, the TR-909, MIDI was the hot new thing, and the company designed the 909 so that it spoke DIN Sync and the new MIDI Timing Clock. Popularly known as Beat Clock, MIDI Timing Clock is digital, consisting of a 1-byte message sent at a rate of 24 ppqn.

All of these clocks, from metronomes to MIDI Timing Clock, are considered relative references, because their speed varies in direct relation to tempo. At a tempo of 100 bpm, 40 MIDI Timing Clock messages would be sent in the course of a second (24 × 100/60), whereas at a tempo of 120 bpm, those messages would be sent at the rate of 48 per second.

A Tock of the Chronos

By contrast, an absolute reference does not change with tempo. It ticks away like a clock on a wall, indifferent to musical concerns. The best-known example of an absolute reference is the timecode standardized by SMPTE (the Society of Motion Picture and Television Engineers). SMPTE timecode keeps track of the time using hours, minutes, seconds, and frames of film (HH:MM:SS:FF). Depending on the specific application, there may be as few as 24 frames in a second or as many as 30, so one must always specify the frame rate when using timecode.

You can think of SMPTE timecode as specifying a particular location or address on a reel of film or tape. In the film-production process, those addresses are logged by every camera and audio recorder so multiple camera angles and audio tracks can be aligned in the video and audio postproduction phases. In audio production, timecode allows multiple recorders to be synchronized for extended track count, as when using two 24-track tape machines or combining analog tape with a DAW.

FIG. 2: MOTU’s MIDI Timepiece AV speaks all three varieties of timecode: VITC on BNC, MTC on DIN, and LTC on TRS. The spare BNC connector is for word clock.

There are three common ways of transmitting timecode, and a single device can often manage all three (see Fig. 2). Linear Time Code (LTC) is an analog audio signal, conceptually similar to FSK, that is striped, or recorded, to analog tape. When the signal is played back through a standard audio cable, a synchronizer can listen to the LTC track and keep a second recorder in sync with the first. LTC is customarily recorded to an edge track (track 24 on 2-inch tape) so that it has only one adjacent track that could potentially be affected by crosstalk.

Timecode can also be striped to an audio track of a videotape, or it can be embedded in the video signal as Vertical Interval Time Code (VITC). VITC is literally printed to a blank spot in the video signal — this spot being the point at which the scanning ray blanks so it can reset itself to the top of the screen to begin its next pass. VITC is transmitted on a standard video cable with a BNC connector.

LTC, an audio signal, changes pitch with playback speed and is too low to be reliably tracked at very low playback speeds. VITC, on the other hand, is read by the rotating video head and can be still-framed along with the image. A synchronizer that reads both types of SMPTE can switch between them as needed.

The third way of sending timecode is through MIDI Time Code, a digital implementation of SMPTE. It observes the same HH:MM:SS:FF format but is implemented as a pair of MIDI messages. The 10-byte full-frame message is sent when transport is started and stopped and includes the entire 8-digit address (as well as a transport command). The quarter-frame message is sent four times per frame during playback. Each 2-byte message carries one digit of the SMPTE address, updating the entire address every two frames (eight messages).

Another important video clock is black burst, so called because it is a video signal that has no image. A black burst generator provides a highly accurate and stable timing reference that can be distributed to all devices in a studio to ensure precise synchronization. Black burst, like VITC, is carried on a standard coaxial video cable with a BNC connector.

Take My Word

Once you enter the realm of digital audio, time starts being sliced up into smaller and smaller pieces, down to 1/96,000th of a second or smaller, necessitating clock signals of increasingly fine resolution. Word clock is a sample-by-sample timing reference. Its primary function is to coordinate the timing of a sending and a receiving device so that samples are not dropped. A secondary application of word clock is to slave the A/D and D/A converters of an audio interface to a highly accurate master clock in order to improve their sonic performance. Master clocks such as the Big Ben from Apogee Electronics are highly regarded for such use.

Word clock is ordinarily carried on a 75-ohm coaxial cable with BNC connectors, but it is also embedded in both S/PDIF and AES/EBU digital audio signals. Therefore, when sending signals via either of those protocols, you can slave the receiving device directly to the clock contained within the digital audio signal without the benefit of a separate word-clock connection. Consequently, you can use an AES/EBU connection without any actual audio signal instead of word clock. This technique is called null clock.

Digidesign created a special version of word clock for its Pro Tools systems that ticks off 256 pulses for every sample. Superclock, known generically as 256Fs, is transmitted over the same cable and connector as word clock and was eventually abandoned in favor of word clock in the latest generation of Pro Tools hardware.

Knowing what type of clocks your gear can use is an important part of keeping your studio running smoothly. Put another way, when using multiple devices, everybody better know what time it is.

Brian Smithers teaches audio workstations at Full Sail Real World Education and music technology at Stetson University. He is the author of Sonar 5 Ignite! (Thomson Learning, 2005).