In a digital video signal, a large amount of transmission bandwidth and space is needed to send it safely without any loss of data so here it is important to use video compression technique. The redundant data in videos are removed or reduced to store on a computer disk or send it over the internet or wireless networks with slight or without having any bad effects on video quality.
Types Of Video Compression
There are two types of video compression:
a. lossless compression
b. lossy compression
· Lossless Compression:
Few methods are lossless compression and original data is recovered 100%. It is used where it should be no loss of information like test or executable files. To decrease the redundancy the statistical information is used frequently by the compression algorithms. Examples include Run Length Encoding and Huffman-Coding.
· Lossy Compression:
Most of the methods are lossy compression because original data is not recovered 100%. Data has high redundancies and is not sensitive to lose like image sound or a video. Compression ratio is high in lossy compression as compared to lossless compression.
2. Error correction techniques:
There are other different kinds of error correction techniques like Forward Error Correction (FEC) and Automatic Repeat Request (ARQ).
a. Forward Error Correction (FEC):
In this technique, the channel coding is used which means that the redundant packets are transmitted which allow the recovery of lost packets at the receiver side. It is also called Erasure Recovery. It would be enough to compensate if the loss rate is known. But practically it is time-varying and the amount of losses is unknown.
· Advantages:
· Advantages:
It is very difficult to use because as a result errors are either less or too high than expected.
i. In the first case when errors are less, the losses can be recovered but the capacity will be wasted and results in a guaranteed and unaffected decoded quality of the video which is possible if the designed system is considered in the worst case.
ii. In the second case when losses are very high, the chances of recovery are increased and losses will not be retrieved if the redundancy is not enough, as a result, the decoding quality will be very bad and will have cliff effect. Considering it the worst case the amount of redundancy is increased by the broadcasters. The final decode quality will be low and the video will be compressed heavily.
· Disadvantages:i. In the first case when errors are less, the losses can be recovered but the capacity will be wasted and results in a guaranteed and unaffected decoded quality of the video which is possible if the designed system is considered in the worst case.
ii. In the second case when losses are very high, the chances of recovery are increased and losses will not be retrieved if the redundancy is not enough, as a result, the decoding quality will be very bad and will have cliff effect. Considering it the worst case the amount of redundancy is increased by the broadcasters. The final decode quality will be low and the video will be compressed heavily.
i. Not only memory but also a lot of computational work is needed when the redundant packets are encoded or decoded.
ii. Added delay is significant because efficient schemes in error correction need processing of a large number of video packets.
a. Automatic Repeat Request (ARQ):
In Automatic Repeat Request (ARQ) the lost packets are retransmitted to enhance the reliability. It is considered an effective technique. Packets are sent successfully once if the losses are not frequent but if they are frequent then retransmission occur which will cause loss rate and congestion.
· Advantage:
· Advantage:
In point-to-point communication with a feedback channel, the retransmission is beneficial.
· Disadvantage:
· Disadvantage:
It will be difficult for the broadcaster to handle the individual retransmission requests while broadcasting it to too many receivers.
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