Synchronisation and SMPTE Timecode (Time Code)

Authored by Phil Rees

This technical overview explores the SMPTE/EBU timecode synchronization system. The primary focus is on the Bi-Phase Mark LTC (Longitudinal Time Code) sync tone, which is designed to be recorded onto audio tapes or video audio tracks.

Document Objectives

• Explain the technical nature of SMPTE sync.
• Discuss variants like VITC.
• Provide context on field sequences and color TV standards.
• Detail the Binary Coded Decimal (BCD) data word structure.

Personal Motivation: I originally dove into the technicalities of this system to create an affordable MIDI tape sync device. This was a common request from clients and a natural fit for our catalog of electronic music tools. This resulted in the TS1, a highly regarded unit that supported MIDI Time Code and SMPTE (including the Drop Frame variant) for over a decade.

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Understanding Synchronisation Systems

When coordinating various pieces of hardware—such as a sequencer and a tape recorder—you need a method to ensure they operate in perfect unison. This process is known as synchronisation (often abbreviated as sync or synch).

Key Terminology

• Track: The specific area of magnetic tape scanned by a recording head. While usually used for analogue audio, it can also store sync data.
• Sync Tone: An audio-compatible signal used to encode timing information.
• Stripe: A tape track that has had a sync tone recorded onto it.
• Chasing: The ability of a slave device to lock onto the sync tone and align itself regardless of where the playback started.

Alternative Sync Methods

For gear that isn't SMPTE-compatible, "tempo-relative" stripes are used. These include:

1. Standard FSK (Frequency Shift Key).
2. The proprietary FSKplus format developed by Philip Rees.

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The SMPTE/EBU Standard

Before electronic video, film relied on mechanical synchronisation via sprockets (perforated holes in the film and tape). Timing was adjusted by physically slipping these holes. Because video tape lacks these physical perforations, an electronic substitute was required.

In 1967, the Society of Motion Picture and Television Engineers (SMPTE) introduced what we now call SMPTE time code.

The audio-based version is known as LTC (Longitudinal Time Code). Other versions exist for MIDI connections or embedded video signals. As noted on the former tvhandbook.com:

"1969: SMPTE timecode established to end the chaos of incompatible time codes for various editing machines."

The Timecode Structure

SMPTE uses a 24-hour clock represented by eight digits. The format follows this logic: $\text{Hours (0-23)} : \text{Minutes (0-59)} : \text{Seconds (0-59)} : \text{Frames (Variable)}$

The frame-rate is the frequency at which a picture is refreshed to create the illusion of motion.

Frame-Rate Formats

The "Drop Frame" Logic

Since $29.97\text{ fps}$ is not an integer, SMPTE uses an approximation. It assumes $30\text{ fps}$ but skips (drops) two frame counts at the start of every minute.

• Exception: Frames are not dropped during the 10th minute of every hour.
• Safe Minutes: $1, 11, 21, 31, 41, 51$.

In audio post-production, SMPTE serves as a reference for tape position, though the specific frame rate chosen is often arbitrary.

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The Modulation Scheme of SMPTE LTC

In digital logic, a bit is a binary digit ($0$ or $1$), typically represented by an electronic switch being "on" or "off." However, analogue audio channels cannot reliably distinguish these phase states. To solve this, SMPTE LTC encodes data into the frequency of state transitions.

// Conceptual representation of a bit transition
if (bit == 0) {
    execute_single_transition(at_boundary);
} else if (bit == 1) {
    execute_double_transition(at_boundary_and_midpoint);
}

The binary value 0 is represented by a single transition occurring at the start (or "bounda...