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08.04.2003
Originally featured on BroadcastEngineering.com
Sigma Electronics proposes synching video to time of day

In the infancy of digital television, maintaining the relative timing between audio and video signals can be difficult. The lack of relative time information in the audio signal prevents comparison (synchronization) with video time to allow accurate delay compensation. This can cause lip-sync errors and signal being out of phase.



All OctaStream modules come equipped with a proprietary TDM interconnect that allows OctaStream modules to share their inputs with each other. This feature, called the OctaBus, is a proprietary TDM interconnect that allows OctaStream modules to share their inputs with each other. With OctaBus, each module can see 64 inputs, including its own.

Sigma Electronics of East Petersburg, PA, has come up with a solution it calls Digital Audio Time Code (DATC). They describe it as a method of inserting accurate signal timing information within the user bits of an AES digital audio signal. Joe Wellman, marketing manager at the company, said Sigma's president Nigel Spratling, came up with the idea after mulling over the details for several months. Wellman said many in the industry have been left scratching their heads and saying "It's so simple, why didn't someone else think of it before?"

Wellman also said they plan to demonstrate the new time code for "several broadcast networks in New York towards the end of [September.]"

DATC is formatted: HH:MM:SS:BBB or Hours:Minutes:Seconds:Blocks, where "Blocks" are AES data blocks (192 samples). DATC can only be utilized for AES signals of fixed sample rates at 48kHz, 96kHz and 192kHz, which result in block counts per second of 0-249, 0-499 and 0-999 respectively.

DATC was designed to provide an accurate method of comparing the origination time of video and audio signals so that individual path delays could be calculated and corrected. Without a technology such as DATC, if video and audio signal coincidence is lost, there is no simple way to provide recovery. In the past, audio delays of fixed length have been used to compensate for identifiable video path delays.

Today, path delays have become longer and in some cases dynamically variable, making accurate delay compensation practically impossible using traditional methods. With DATC, users can now extract timing data from video and audio signals and make comparisons to accurately determine each signal's relative position.



DATC is a method of inserting accurate signal timing information within the user bits of an AES digital audio signal.

Most methods for compressing audio retain AES user bit data. Mezzanine transport schemes such as Dolby E and APT pass these bits without interference. This means that DATC can be reliably extracted from compressed audio. However, some video compression schemes do not carry SMPTE time code data, but instead may recode it as a time stamp within the compressed data. In this type of situation, time stamping must be recovered and converted back to the SMPTE time code format in order for comparison to be made.

The existence of DATC in an audio signal makes it possible to accurately control relative time and phase multiple in AES channels. In transmission schemes where separate AES channels are sent as a surround sound mix, interchannel phasing can be recovered easily.

DATC is a simple method of providing path latency control with an accuracy of >1/125th of a second. It relies on time code being present in both signals and that intermediate processing and compression equipment retain the timing data.

DATC is video system independent as it is based on empirical units of time and not video frame rates. The combination of DATC and GPS referencing systems provides a method of eliminating A/V and inter-site timing difficulties.

Sigma Electronics has introduced two DATC products: an AES reference generator that can read SMPTE time code and generate DATC, and an AES delay compensator that can be used to insert as well as compare DATC data to adjust delay times. These products are designed to function together, via Sigma's OctaBus interface, for DATC insertion or comparison and automatic delay compensation.

For more information, visit www.sigmaelectronics.com.

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