Frame synchronization
Encyclopedia
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:
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 immediately outputs it with the correct synchronization signals to match an existing video system. A genlock
signal is required to provide a means for video synchronizing with the house reference.
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.
Data frame
In computer networking and telecommunication, a frame is a digital data transmission unit or data packet that includes frame synchronization, i.e. a sequence of bits or symbols making it possible for the receiver to detect the beginning and end of the packet in the stream of symbols or bits...
data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
, frame synchronization is the process by which incoming frame alignment signals, i.e., distinctive bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
sequences (a syncword
Syncword
In computer networks, a syncword, sync character or preamble is used to synchronize a transmission by indicating the end of header information and the start of data.-Examples:For example an audio receiver is receiving a bit stream of data...
), 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".
Framing
Bit slip
In digital transmission, bit slip is the loss of a bit or bits, caused by clock drift – variations in the respective clock rates of the transmitting and receiving devices....
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 telecommunicationTelecommunicationTelecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s, for example in T-carrierT-carrierIn telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
, is to insert, in a dedicated time slot within the frame, a noninformation bit or framing bit that is used for synchronizationSynchronizationSynchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....
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 syncwordSyncwordIn computer networks, a syncword, sync character or preamble is used to synchronize a transmission by indicating the end of header information and the start of data.-Examples:For example an audio receiver is receiving a bit stream of data...
at the beginning of every frame.
- CRC-based framing. Some telecommunications hardware uses CRC-based framingCRC-based framingThe concept of CRC-based framing was developed by StrataCom, Inc. in order to improve the efficiency of a pre-standard Asynchronous Transfer Mode link protocol. This technology was ultimately used in the principal link protocols of ATM itself and was one of the most significant developments of...
.
Television
A frame synchronizer is a device used in live televisionLive television
Live television refers to a television production broadcast in real-time, as events happen, in the present. From the early days of television until about 1958, live television was used heavily, except for filmed shows such as I Love Lucy and Gunsmoke. Video tape did not exist until 1957...
production to match the timing of an incoming video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
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
Film frame
In filmmaking, video production, animation, and related fields, a film frame or video frame is one of the many still images which compose the complete moving picture...
of incoming video and then immediately outputs it with the correct synchronization signals to match an existing video system. A genlock
Genlock
Genlock is a common technique where the video output of one source, or a specific reference signal from a signal generator, is used to synchronize other television picture sources together. The aim in video and digital audio applications is to ensure the coincidence of signals in time at a...
signal is required to provide a means for video synchronizing with the house reference.
Telemetry
In telemetryTelemetry
Telemetry is a technology that allows measurements to be made at a distance, usually via radio wave transmission and reception of the information. The word is derived from Greek roots: tele = remote, and metron = measure...
applications, a frame synchronizer is used to frame align a serial PCM (Pulse Code Modulated)binary stream.
See also
- Framing (telecommunication)
- Asynchronous start-stopAsynchronous start-stopAsynchronous serial communication describes an asynchronous, serial transmission protocol in which a start signal is sent prior to each byte, character or code word and a stop signal is sent after each code word...
- Phase synchronizationPhase synchronizationPhase synchronization is the process by which two or more cyclic signals tend to oscillate with a repeating sequence of relative phase angles.Phase synchronisation is usually applied to two waveforms of the same frequency with identical phase angles with each cycle...
- Self-synchronizing codeSelf-synchronizing codeIn telecommunications, a self-synchronizing code is a line code in which the symbol stream formed by a portion of one code word, or by the overlapped portion of any two adjacent code words, is not a valid code word...
- Superframe
- GenlockGenlockGenlock is a common technique where the video output of one source, or a specific reference signal from a signal generator, is used to synchronize other television picture sources together. The aim in video and digital audio applications is to ensure the coincidence of signals in time at a...