As we know, LTC (Linear Timecode) is a digital signal transmitted over an audio interface using balanced audio cables. The principle is very similar to that of dial-up modems. We all remember the electronic sound when a modem connected to a server. While home internet technology has evolved, LTC has largely remained the same. However, transmitting a digital LTC signal over an analog audio channel presents several technical challenges.

Unlike an audio signal that is sinusoidal, the SMPTE LTC waveform is square, because it is encoded in a binary format. Over short distances, this difference doesn’t cause much trouble. But over long audio cable runs, factors like cable capacitance and inductance come into play. The result? The sharp edges of the square wave begin to soften, transforming the waveform into something more sinusoidal. This also shifts the phase of the rising edge of the signal.

This distortion can be critical — the receiving device may fail to decode the altered SMPTE code or decode it with instability, leading to dropped frames and synchronization errors.

The same distortion can happen when the LTC signal is passed through an audio console or audio splitter. Many so-called “experts” fail to understand the fundamental difference between audio signals and LTC. It is naive to assume that SMPTE LTC can be handled like a standard analog audio signal. This is a serious mistake, because LTC is a pulse-based digital signal with completely different characteristics.

Because LTC is a digital signal, its volume level is critical. The analog output volume directly determines the amplitude difference between logical 0 and 1 in the waveform. That’s why the LTC output level from the sound card must be monitored carefully.

The optimal LTC signal level ranges from 0 dBu to +4dBu. If the signal is significantly lower, many devices won’t detect it or will interpret it inconsistently.

• Nominal level:

LTC is typically output at 0 dBu, which corresponds to 0.775 V RMS (about 1.1 V peak-to-peak if the signal were sinusoidal).

However, LTC is not a sine wave — it is closer to a square wave, with a rectangular shape and sharper edges.

• Typical range:

Professional LTC-capable equipment such as studio recorders and video servers often accepts levels up to +4 dBu, which is approximately 1.23 V RMS or around 3.5 V peak.

LTC (Linear Timecode) is an audio signal encoded as a series of square wave pulses, where each edge transition carries timecode information. Accurate decoding relies on the decoder’s ability to clearly detect these edges (logic transitions between 0 and 1).

When the signal level drops too low:

1. Amplitude of edges weakens

The rectangular waveform loses its definition. After passing through analog circuits (filters, preamps, ADCs), the edges become less sharp, increasing the risk of misinterpretation.

2. Noise competes with the signal

All audio systems have inherent background noise (thermal noise, electrical interference, hum, etc.). When the LTC signal level is too low, it becomes comparable to the noise floor, and the decoder may:

• fail to detect edges properly,

• misinterpret noise as signal transitions (false triggering),

• or completely lose synchronization (dropouts).

SNR (Signal-to-Noise Ratio) represents the ratio between the desired signal level and the noise floor, expressed in decibels (dB).

• A high SNR means a clean signal, well above the noise.

• At very low LTC levels (e.g., -30 dBu), SNR can drop to 10–20 dB, which is critically low — making it difficult for decoders to reliably distinguish between signal and noise.

Since LTC can be played as an audio track (historically, it was recorded and played from magnetic tape), playback speed affects the timecode’s integrity. If the playback speed is even slightly faster or slower than the original, the data flow rate changes. This may seem minor, but it disrupts synchronization.

Some devices can compensate for slight speed fluctuations. But if the playback speed drifts too far outside the acceptable range, frames are lost or interpreted incorrectly, breaking the timecode sync.

Previously, we mentioned the harmful effects of using analog audio splitters with LTC. There are rare exceptions: some audio splitters may work without damaging the LTC waveform, but this must be verified experimentally using an oscilloscope and a known-good timecode generator.

That said, this is the exception, not the rule, as no audio equipment manufacturer designs their gear specifically to handle square LTC signals.

The same applies to digital audio consoles. While this may seem to contradict earlier warnings, digital consoles are fundamentally different from analog ones. Most digital mixers have no analog audio path and do not introduce the same waveform distortion. Furthermore, they offer much higher dynamic range, which can benefit LTC transmission.

But caution is still needed — some budget digital consoles “enhance” or modify the signal in ways that distort the square wave. Only through practical testing with an oscilloscope can we determine whether a specific model is truly LTC-safe.

• Low signal levels

The LTC waveform depends on correct output amplitude. The recommended range is 0 dBu to +4dBu.

• Routing through audio equipment

Avoid using analog mixers and splitters. They distort the square waveform, making it unreadable.

• Playback speed differences

If playback speed doesn’t match the original recording, the LTC rate changes — causing sync issues.

You might wonder: if so many common tools distort LTC, what should we use to split or amplify it?

Despite its age, LTC has been well supported by hardware manufacturers. There is dedicated equipment built specifically for timecode generation, distribution, and amplification. These devices preserve waveform integrity and provide proper signal buffering.

We’ll explore these tools and best practices in the next chapter, focusing on professional LTC infrastructure and robust timecode systems.

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