Reference frequency (usually 60 Hz) recorded during shooting — syncs camera and audio recorder. Supplements timecode for automatic alignment in post.
When shooting without a cable connection between the camera and the sound recorder, you need a reliable method to synchronize both devices later. This is where the pilot tone comes in — a constant reference frequency that you record onto the sound carrier throughout the entire take. In practice, the 60 Hz frequency dominates (sometimes 50 Hz in PAL regions), which comes from the power circuit or is generated by a stable oscillator. This frequency is inaudible to the human ear but measurable and reliable in editing.
The crucial advantage: the camera and recorder don't need to be cabled. You can shoot freely and mobile — documentaries, ENG productions, or working with multiple sound recorders — and synchronize everything later at the editing suite. Each reel, each magnetic tape spool is digitized during transfer; the pilot tone frequency serves as an anchor point in the sync process. The editor overlays the video image and the audio recording, aligns the frequencies — and the lips move exactly with the dialogue.
In the analog era, the pilot tone system was almost indispensable. Today, in the age of timecode and wireless radio links, it has lost importance — but not disappeared. Some DoPs and sound recordists still rely on this simple, error-free method today, especially for recordings in environments where radio synchronization is problematic or where absolute backup confidence is required. Hybrid setups are common: timecode as primary sync, pilot tone as a safety net.
A practical tip: the pilot tone must be consistent. Voltage fluctuations, motor fluctuations, or poor oscillator stability lead to drift — the audio will be significantly out of sync by the end of the reel. Professional sound recordists calibrate their equipment before each shoot and check frequency stability with simple test measurements. The clapperboard remains important nonetheless — as a visual reference mark for editing and as a backup if the audio frequency is damaged.