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Audio synchronizer
An audio synchronizer is a variable audio delay used to correct or maintain audio video sync or timing also known as lip sync error. See for example the specification for audio to video timing given in ATSC Document IS-191. Modern television systems use large amounts of video signal processing such as MPEG preprocessing, encoding and decoding, video synchronization and resolution conversion in pixelated displays. This video processing can cause delays in the video signal ranging from a few microseconds to tens of seconds. If the television program is displayed to the viewer with this video delay the audio video synchronization is wrong, and the video will appear to the viewer after the sound is heard. This effect is commonly referred to as A/V sync or lip sync error and can cause serious problems related to the viewer's enjoyment of the program.
==Error correction==
To correct audio video sync problems, the video processing circuitry outputs a DDO (digital delay output) signal, which carries information about the amount of delay the video signal experiences due to the video processing. The audio synchronizer receives the DDO signal and in response delays the audio by an equivalent amount, thereby maintaining proper audio video sync. Modern audio synchronizers operate by digitizing and writing the audio signal into a ring memory, which is most commonly a RAM-based memory having independent read and write ability. At the appropriate delay time (as conveyed by the DDO) after an audio sample (or group of samples) are written into the memory the previously stored audio sample is read from the ring memory. The storage and reading of the audio samples takes place continuously in response to respective memory write and read addresses, which increment by 1 count for every write or read operation. For example, an audio sample would be written at address 1, a different sample read from (previously written) address 5, another sample written at address 2, yet another read from 6, write at 3, read from 7 and so on. The delay between writing and reading a particular sample is 4 addresses which, when multiplied by the amount of time it takes to change from one address to the next, gives the total audio delay.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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