MPEG Compression

MPEG uses an asymmetric compression method. Compression under MPEG is far more complicated than decompression, making MPEG a good choice for applications that need to write data only once, but need to read it many times. An example of such an application is an archiving system.

MPEG uses two types of compression methods to encode video data: interframe and intraframe encoding.

Interframe encoding is based upon both predictive coding and interpolative coding techniques, as described below.

When capturing frames at a rapid rate (typically 30 frames/second for real time video) there will be a lot of identical data contained in any two or more adjacent frames. If a motion compression method is aware of this temporal redundancy, then it need not encode the entire frame of data, as is done via intraframe encoding. Instead, only the differences (deltas) in information between the frames is encoded. This results in greater compression ratios, with far less data needing to be encoded. This type of interframe encoding is called predictive encoding.

A further reduction in data size may be achieved by the use of bi-directional prediction. Differential predictive encoding encodes only the differences between the current frame and the previous frame. Bi-directional prediction encodes the current frame based on the differences between the current, previous, and next frame of the video data. This type of interframe encoding is called motion-compensated interpolative encoding.

To support both interframe and intraframe encoding, an MPEG data stream contains three types of coded frames:

* I-frames (intraframe encoded) * P-frames (predictive encoded) * B-frames (bi-directional encoded)
An I-frame contains a single frame of video data that does not rely on the information in any other frame to be encoded or decoded. Each MPEG data stream starts with an I-frame.

A P-frame is constructed by predicting the difference between the current frame and closest preceding I- or P-frame. A B-frame is constructed from the two closest I- or P-frames. The B-frame must be positioned between these I- or P-frames.

A typical sequence of frames in an MPEG stream might look like this:

IBBPBBPBBPBBIBBPBBPBBPBBI

In theory, the number of B-frames that may occur between any two I- and P-frames is unlimited. In practice, however, there are typically twelve P- and B-frames occurring between each I-frame. One I-frame will occur approximately every 0.4 seconds of video run time.

Remember that the MPEG data is not decoded and displayed in the order that the frames appear within the stream. Because B-frames rely on two reference frames for prediction, both reference frames need to be decoded first from the bit stream, even though the display order may have a B-frame in between the two reference frames.

In the previous example, the I-frame is decoded first. But, before the two B-frames can be decoded, the P-frame must be decoded, and stored in memory with the I-frame. Only then may the two B-frames be decoded from the information found in the decoded I- and P-frames. Assume, in this example, that you are at the start of the MPEG data stream. The first ten frames are stored in the sequence IBBPBBPBBP (0123456789), but are decoded in the sequence:

IPBBPBBPBB (0312645978)

and finally are displayed in the sequence:

IBBPBBPBBP (0123456789)

Once an I-, P-, or B-frame is constructed, it is compressed using a DCT compression method similar to JPEG. Where interframe encoding reduces temporal redundancy (data identical over time), the DCT-encoding reduces spatial redundancy (data correlated within a given space). Both the temporal and the spatial encoding information are stored within the MPEG data stream.

By combining spatial and temporal sub-sampling, the overall bandwidth reduction achieved by MPEG can be considered to be upwards of 200:1. However, with respect to the final input source format, the useful compression ratio tends to be between 16:1 and 40:1. The ratio depends upon what the encoding application deems as "acceptable" image quality (higher quality video results in poorer compression ratios). Beyond these figures, the MPEG method becomes inappropriate for an application.

In practice, the sizes of the frames tend to be 150 Kbits for I-frames, around 50 Kbits for P-frames, and 20 Kbits for B-frames. The video data rate is typically constrained to 1.15 Mbits/second, the standard for DATs and CD-ROMs.

The MPEG standard does not mandate the use of P- and B-frames. Many MPEG encoders avoid the extra overhead of B- and P-frames by encoding I-frames. Each video frame is captured, compressed, and stored in its entirety, in a similar way to Motion JPEG. I-frames are very similar to JPEG-encoded frames. In fact, the JPEG Committee has plans to add MPEG I-frame methods to an enhanced version of JPEG, possibly to be known as JPEG-II.

With no delta comparisons to be made, encoding may be performed quickly; with a little hardware assistance, encoding can occur in real time (30 frames/second). Also, random access of the encoded data stream is very fast because I-frames are not as complex and time-consuming to decode as P- and B-frames. Any reference frame needs to be decoded before it can be used as a reference by another frame.

There are also some disadvantages to this scheme. The compression ratio of an I-frame-only MPEG file will be lower than the same MPEG file using motion compensation. A one-minute file consisting of 1800 frames would be approximately 2.5Mb in size. The same file encoded using B- and P-frames would be considerably smaller, depending upon the content of the video data. Also, this scheme of MPEG encoding might decompress more slowly on applications that allocate an insufficient amount of buffer space to handle a constant stream of I-frame data.


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