Sync signal

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A synchronizing signal is a technical aid in communications engineering for signaling and synchronizing two or more processes. Synchronous signals are usually generated deliberately and usually have a high periodicity as well as recognizability and technical legibility. Modulation with one or more tones or tone sequences was a common method of coding before the breakthrough of digital technology.

Timing technology

By broadcasting time signals, e.g. B. the DCF77 time signal transmitter of the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, through the hourly signal tones of various radio stations, through the teletext signal and the high-precision TV image change signals, but also through the network frequency (of 50 Hz in Europe) it is possible for every receiver to synchronize their timers with these sources. The Internet also offers the option of calling up time signals using time synchronization protocols (e.g. according to the RFC standard). The accuracies that can be achieved are very different. Usually, unknown signal propagation times result in an offset and, if the transmission paths change, so-called jitter can also occur .

The tariff times for switching between day and night electricity are transmitted to the consumption meters by signals modulated onto the power grid.

Radio technology

For wireless networks in time-division multiplexing, the synchronicity of the participants authorized to send is of great importance. Usually a master is used, which specifies a synchronous signal through its transmissions and also ensures that each participant is allocated certain time windows for their own transmissions. There are also networks without a special master, in which each of the subscribers with quasi equal rights is involved in the synchronization with their transmissions.

Audio technology

Classic answering machines with tape or compact cassette use a so-called index track or an embedded index signal, to which an identifier in the form of a signal tone was recorded for the start and end of each recording. During playback, this signal was usually audible and informed the user about the limits of the respective meaning unit. Depending on the model, it was also possible for the device to control itself.

For slide presentations there is the possibility of integrating similar index signals on a presentation tape and thus controlling the switching of the connected projectors. In the meantime, more and more computer-based digital controls have established themselves, especially in the professional sector.

In classical studio technology, multi-track tape systems have long been used for music recordings, which always contain a track for the sync signal. This is usually written to the tape first and contains a continuous, dense stream of individual signals that are used to monitor and control the tape speed . Although the techniques used there are already of very high quality, for reasons such as temperature, humidity, tape tension and the like, there can still be a variation in the recording and playback speeds, which could lead to a deterioration in quality.

Video technology

The so-called HSYNC and VSYNC signals are used in common analog video and TV technology. The HSYNC is used for the horizontal synchronization of the image data reproduced in one line. The VSYNC, on the other hand, coordinates the start of the first line in the vertical direction. See also under composite video and television signal (or BAS signal ). Digital versions of these signals were specified, for example, in the BT.868 standard.

In the computer sector, the so-called VSync interrupt is an important component that ensures that the image changes exactly when new data is available and at the same time the display of the previous data is completely completed. If the VSync is deactivated as a criterion, dirt effects such as "tearing" can be observed, but also measurements of the peak performance of the system can be carried out. In some older systems (e.g. C64) an HSync interrupt is also available, but this has become obsolete as a technical concept in view of the constantly increasing performance of graphics processors .

Multimedia technology

In multimedia systems, the different processing units of the incoming raw data often lead to unattractive time delays. B. Image and sound are no longer sufficiently synchronized with the presentation. By indexing the source data and planning buffers at the output, synchronization can be restored by triggering the display of buffer elements with the same index at almost the same time.

Data technology

In the context of every digital transmission, it is necessary to determine the validity time of a date on a channel. There are various techniques for this which are based on clock signals, recovered clocks, on edges and levels, as well as on various select and strobe signals. In addition, bit and word boundaries in serial transmission are often identified by start and stop bits and header signatures. Parts of this are also mapped in the OSI model .

When data is recorded on magnetic media, synchronization pulses are written in addition to the pure data bits so that the reading electronics can later synchronously track the data rate. There are different methods such as B. MFM and GCR as well as principles such as NRZ .


  • R. Theile: Television technology. Volume 1 Basics, Springer Verlag, Berlin / Heidelberg 1993, ISBN 978-3-540-06209-7 .
  • Ulrich Freyer: News transmission technology. Basics, components, processes and systems of telecommunications technology . 1st edition. Carl Hanser Verlag, Munich 2009, ISBN 978-3-446-41462-4 .
  • Reiner Schmid: Industrial image processing. From visual perception to problem solving, Friedrich Vieweg & Sohn Verlag, Wiesbaden 1995, ISBN 978-3-528-04945-4 .

See also

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