MPEG

Question 1

Explain what I-frames

Answer 1

1. Intra-coded frames
2. Coded as single frames as in JPEG without reference to any other frames.

Question 2

Explain P-frames

Answer 2

1. Predictive coded frames.
2. Coded as the difference from a motion compensated prediction frame, generated from I or P frame.

Question 3

Explain B-frames

Answer 3

1. Bi-directional coded frames
2. Coded as the difference from a bi-directionally interpolated frame, generated from earlier and later I or P frames in the sequence.

Question 4

What is motion-compensated predictive coding

Answer 4

1. A technique that has found to be the most successful for exploiting interframe correlations.
2. Idea -> encode the difference between the current image and the motion compensated previous image.

Question 5

What is the problem with motion-compensated predictive coding.

Answer 5

There are quantisation errors when calculating the encoded DFD.

Quantised to +-0.5 pixel accuracy.

Question 6

How big are macroblocks?

Answer 6

16x16

Question 7

How are motion vectors encoded?

Answer 7

1. Difference coding

Question 8

In any multimedia transmission system that involves compression the following issues become important:

Explain the consideration of: data type.

Answer 8

1. Multimedia sources can be video/images/text
2. Different compression techniques are needed for each data type.
3. Each piece of data has to be identified with unique codewords for transmission.

Question 9

In any multimedia transmission system that involves compression the following issues become important:

Explain the consideration of: sequencing.

Answer 9

1. Compressed data from each source scanned into a sequence of bits.
2. Sequence is then packetised for transport.
3. Problem here is to identify each different part of the bitstream uniquely to the decoder (e.g. header information, DCT coefficient information)

Question 10

In any multimedia transmission system that involves compression the following issues become important:

Explain the consideration of: multiplexing.

Answer 10

1. Audio and video data have to be decoded at the same time to create a coherent signal at receiver.
2. Challenge -> identify different parts of the multiplexed stream and insert information about the timings of each elementary data stream.

Question 11

In any multimedia transmission system that involves compression the following issues become important:

Explain the consideration of: media.

Answer 11

1. Compressed and multiplexed data has to be stored on some digital storage and then later broadcasts to receivers across air or other links.
2. Access to different media channels is constrained.

Question 12

In any multimedia transmission system that involves compression the following issues become important:

Explain the consideration of: error.

Answer 12

Receiver must cope with errors such that system is robust or degrades in a graceful way.

Question 13

In any multimedia transmission system that involves compression the following issues become important:

Explain the consideration of: bandwidth.

Answer 13

1. BW available for multimedia transmission is limited.
2. Transmission system must ensure that the bandwidth of the bitstream does not exceed these limits.

Question 14

In any multimedia transmission system that involves compression the following issues become important:

Explain the consideration of: multiplatform.

Answer 14

1. Different devices have different processor speeds and storage resources.
2. Need construction of a suitable bitstream format; Scalability.

Question 15

How is the multiplexing problem solved?

Answer 15

1. Synchronisation between packets
2. Decoding time stamp flag tells the decoder when to decode a packet.
3. The presentation time stamp flag tells the decoder when to pass the decoded frame to the output device for display.