Audio synchronizer
From Wikipedia, the free encyclopedia
An audio synchronizer is a variable audio delay utilized to correct or maintain audio video sync timing. Television systems utilize large amounts of video signal processing such as MPEG preprocessing, encoding and decoding, video synchronization and resolution conversion in pixelated displays 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.
In order to correct audio video sync problems the video processing circuitry outputs a DDO (digital delay output) signal which carries the amount of delay the video signal experiences. The audio synchronizer receives the DDO 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 after the audio samples are written into the memory(as conveyed by the DDO) the previously stored audio samples are read from the ring memory. The storage and reading of the audio samples takes place continuously in response to respective write and read addresses which are incremented by 1 count for every write or read operation. For example the audio samples would be written at address 1, read from (previously written) address 5, written at 2, 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.
Unfortunately, the video delays frequently make quick and large changes, for example a jump in delay time from 2 seconds to 6 seconds is possible. In order to maintain proper audio video sync, the audio delay needs to track these video delay changes. In order to change the audio delay the difference between the write address and the read address must be changed. This change can be accomplished by causing either the write or read address to jump forward or backward, however this jump causes some audio samples to be repeated or lost resulting in an unwanted and annoying pop, gap or click in the audio signal. Some audio synchronizers operate by making repeated, very small jumps which causes unwanted (but less annoying) distortion and noise in the audio signal, rather than pops, gaps and clicks. Other audio synchroners change delay by changing the speed of the reading of audio from the ring memory. If audio samples are read out of the memory more slowly than they are written, the delay increases. If audio samples are read out faster than they are written the delay decreases. Using variable speed reading prevents pops, clicks, gaps, distortion and noise from being introduced into the audio, but does create unwanted and annoying pitch errors. For example reading faster than writing causes the pitch to increase and reading slower than writing causes the pitch to decrease.
Audio synchronizers which utilize variable speed reading are generally preferred in professional applications. Pitch errors in lower performance devices are uncompensated and are kept to a level that is generally not perceived by the average viewer, by limiting the amount of change of reading speed. Typically the change limit is in the order of .2%. Unfortunately this limits the rate of delay change and can cause the audio video sync to be off for several seconds or minutes. Additionally, critical listners such as people who have musical training can hear even the small pitch error.
In higher performance audio synchronizers, the rate of delay change is allowed to be much faster, generally in the order of 25%, and the resulting pitch error is corrected with a pitch correction circuit. The pitch correction circuitry is frequently a proprietary design due to the difficulty in performing correction which is imperceptible to the critical listner.
