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Audio and Video Synchronisation

The main problem of sound in the digital age.

One of the most infuriating problems with sound in the digital age is incorrectly timed lip sync. Home receivers, such as TVs and amplifiers, can add latencies that also contribute to certain digital reproduction effects (see Table 1).

Pictures and sound are synchronous when the sound is reproduced at the same time as the corresponding visual component, many people think of the clapper board as the best example of a single point in time when sound should match the pictures. However it’s not only in the acquisition and post-production phases that sound and pictures can drift from being in sync. Audio delay can occur after sound and picture processing or during the broadcast transmission. Broadcast chain errors can create a picture and sound offset, which must be compensated by the use of delay devices to either delay the picture or the sound to keep them in sync. For video a frame store is used, and for audio, an audio delay process.


Table 1: Typical latency (practical values)

While working with picture-related sound: it’s vital that both sound and pictures are synchronous at the point of transfer! Whenever video, stereo sound, and surround sound leave a work area (for example, OB, car, studio, transmitter), all elements (essences) must be synchronised!
The tolerance range of the sound-image offset depends on the respective task or particular production step. The values in Table 2 are the maximum values for the entire process! If for example when transferring a football match from the OB Van live into the studio and further processing takes place there for a broadcast, a sound offset of 5ms BEFORE the picture and 15ms AFTER the picture may not be exceeded at the last point of the production chain (see table 2).


Table 2: Maximum permissible deviation of sound - image


While preparing for television transmissions  a line test is often carried out. The test is often uses and electronic clapper board to check the synchronisation.
A practical solution is the KS 1017 Lip Checker from Kamesan. Here, a signal generator emits flashes of light with synchronised sound pulses, which can either be recorded by the camera and microphone or transmitted directly via audio and video connections. The time offset between sound and image is then measured and displayed on the receiver side. A similar solution is provided by sync generators, such as Tektronix TG700, which generates an AV delay test sequence, which is automatically evaluated by a suitable analyser (such as the Tektronix WFM7120). A possible time offset can then be compensated by video frame store, video delay or audio delay. The recommendations according to Table 2 apply.

Most common audio delays are NOT bit-transparent, resulting in the degradation of quality of encoded stereo or multichannel sound. To delay Dolby E or Dolby Digital, use of Dolby Frame Synchronisers are required, which also synchronise clocked signals to the local house clock.

The BLITS test is used for the fast testing of stereo and multi-channel transmission paths. Black & Lane's Identity for Surround (BLITS) was introduced in 2004 and is standardised in EBU Tech 3304, with audio level meters, such as DK-Audio and RTW, offering BLITS as an option. The BLITS test allows the automatic measurement of phase and time shifts between the channels as well as the measurement of channel synchronisation (level equalization). The correct channel assignment is also checked automatically.


BLITS Test


For all productions, where sound and image are recorded on separate media or later transferred and edited separately, synchronicity and the management of the sound-picture offset is a must. In general, synchronisation techniques are used to synchronise timecode to SMPTE / EBU, which is distributed by a common source to all recording devices (to cameras, remote field recorders, audio recorders) by cable or radio. This timecode information is then recorded and embedded in the files but he recording device. 
For location productions and feature films, an electronic clapper board is still used to mark the beginning of each take (electronic clapper boards usually have an integrated timecode display) at the shooting location. This provides a visual and auditory marker in the recording. If the recorded timecode is incorrect or not available in the camera, the correct time reference can still be achieved using the clapper board during post-production.

Other options are available for mobile clock and time code generators, e.g. The LockIt Sync Boxes from Ambient Recording or the Denecke Sync Box. They are used when direct cable or radio connections for timecode are not possible.
The sync boxes are initially synchronised to a common time code and then connected to the remote recording devices. Thanks to high-precision internal oscillators, they are able to keep the timecode synchronized for more than 24 hours.

Author: Karl M. Slavik, Arte Cast Vienna