While receiving a stream of framed data, frame synchronization is the process by which incoming frame alignment signals, i.e., distinctive bit sequences (a syncword), are identified, i.e., distinguished from data bits, permitting the data bits within the frame to be extracted for decoding or retransmission. This is sometimes referred to as "framing".
If the transmission is temporarily interrupted, or a bit slip event occurs, the receiver must re-synchronize.
The transmitter and the receiver must agree ahead of time on which frame synchronization scheme they will use. The most common frame synchronization schemes are:
- Framing bit. A common practice in telecommunications, for example in T-carrier, is to insert, in a dedicated time slot within the frame, a noninformation bit or framing bit that is used for synchronization of the incoming data with the receiver. In a bit stream, framing bits indicate the beginning or end of a frame. They occur at specified positions in the frame, do not carry information, and are usually repetitive.
- Syncword framing. Some systems use a special syncword at the beginning of every frame.
A frame synchronizer is a device used in live television production to match the timing of an incoming video source to the timing of an existing video system. They are often used to "time in" consumer video equipment to a professional system but can be used to stabilize any video. The frame synchronizer essentially takes a picture of each frame of incoming video and then outputs it immediately with the correct synchronization signals to match an existing video system.
In telemetry applications, a frame synchronizer is used to frame align a serial PCM (Pulse Code Modulated)binary stream.
The Frame Synchronizer immediately follows the Bit Synchronizer in most telemetry applications. Without frame synchronization decommutation is impossible.
The Frame Sync Pattern is a known binary pattern which repeats at a regular interval within the PCM stream. The frame synchronizer recognizes this pattern and aligns the data into Minor Frames or Sub Frames. Typically the frame sync pattern is followed by a counter (Sub-Frame ID) which dictates which minor or sub frame in the series is being transmitted. This becomes increasingly important in the decommutation stage where all data is deciphered as to what attribute was sampled. Different commutations require a constant awareness of which section of the major frame is being decoded.
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