Video compression
Video signals exist in four dimensions: these are the attributes of the pixel, the horizontal and vertical spatial axes and the time axis. Compression can be applied in any or all of those four dimensions. MPEG assumes an eight-bit colour difference signal as the input, requiring rounding if the source is ten-bit. The sampling rate of the colour signals is less than that of the luminance. This is done by downsampling the colour samples horizontally and generally vertically as well. Essentially an MPEG system has three parallel simultaneous channels, one for luminance and two colour difference, which after coding are multiplexed into a single bitstream.
Video signals exist in four dimensions: these are the attributes of the pixel, the horizontal and vertical spatial axes and the time axis. Compression can be applied in any or all of those four dimensions. MPEG assumes an eight-bit colour difference signal as the input, requiring rounding if the source is ten-bit. The sampling rate of the colour signals is less than that of the luminance. This is done by downsampling the colour samples horizontally and generally vertically as well. Essentially an MPEG system has three parallel simultaneous channels, one for luminance and two colour difference, which after coding are multiplexed into a single bitstream.
Figure 1.7(a) shows that when individual pictures are compressed without reference to any other pictures, the time axis does not enter the process which is therefore described as intra-coded (intra = within) compression. The term spatial coding will also be found. It is an advantage of intra-coded video that there is no restriction to the editing which can be carried out on the picture sequence. As a result compressed VTRs such as Digital Betacam, DVC and D-9 use spatial coding. Cut editing may take place on the compressed data directly if necessary. As spatial coding treats each picture independently, it can employ certain techniques developed for the compression of still pictures. The ISO JPEG (Joint Photographic Experts Group) compression standards are in this category. Where a succession of JPEG coded images are used for television, the term ‘Motion JPEG’ will be found.
Greater compression factors can be obtained by taking account of the redundancy from one picture to the next. This involves the time axis, as Figure 1.7(b) shows, and the process is known as inter-coded (inter = between) or temporal compression.
Temporal coding allows a higher compression factor, but has the disadvantage that an individual picture may exist only in terms of the differences from a previous picture. Clearly editing must be undertaken with caution and arbitrary cuts simply cannot be performed on the MPEG bitstream. If a previous picture is removed by an edit, the difference data will then be insufficient to re-create the current picture.
Intra-coded compression
Intra-coding works in three dimensions on the horizontal and vertical spatial axes and on the sample values. Analysis of typical television pictures reveals that whilst there is a high spatial frequency content due to detailed areas of the picture, there is a relatively small amount of energy at such frequencies. Often pictures contain sizeable areas in which the same or similar pixel values exist. This gives rise to low spatial frequencies. The average brightness of the picture results in a substantial zero frequency component. Simply omitting the highfrequency components is unacceptable as this causes an obvious softening of the picture.
A coding gain can be obtained by taking advantage of the fact that the amplitude of the spatial components falls with frequency. It is also possible to take advantage of the eye’s reduced sensitivity to noise in high spatial frequencies. If the spatial frequency spectrum is divided into frequency bands the high-frequency bands can be described by fewer bits not only because their amplitudes are smaller but also because more noise can be tolerated. The wavelet transform (MPEG-4 only) and the discrete cosine transform used in JPEG and MPEG-1, MPEG-2 and MPEG-4 allow two dimensional pictures to be described in the frequency domain.
Greater compression factors can be obtained by taking account of the redundancy from one picture to the next. This involves the time axis, as Figure 1.7(b) shows, and the process is known as inter-coded (inter = between) or temporal compression.
Temporal coding allows a higher compression factor, but has the disadvantage that an individual picture may exist only in terms of the differences from a previous picture. Clearly editing must be undertaken with caution and arbitrary cuts simply cannot be performed on the MPEG bitstream. If a previous picture is removed by an edit, the difference data will then be insufficient to re-create the current picture.
Intra-coded compression
Intra-coding works in three dimensions on the horizontal and vertical spatial axes and on the sample values. Analysis of typical television pictures reveals that whilst there is a high spatial frequency content due to detailed areas of the picture, there is a relatively small amount of energy at such frequencies. Often pictures contain sizeable areas in which the same or similar pixel values exist. This gives rise to low spatial frequencies. The average brightness of the picture results in a substantial zero frequency component. Simply omitting the highfrequency components is unacceptable as this causes an obvious softening of the picture.
A coding gain can be obtained by taking advantage of the fact that the amplitude of the spatial components falls with frequency. It is also possible to take advantage of the eye’s reduced sensitivity to noise in high spatial frequencies. If the spatial frequency spectrum is divided into frequency bands the high-frequency bands can be described by fewer bits not only because their amplitudes are smaller but also because more noise can be tolerated. The wavelet transform (MPEG-4 only) and the discrete cosine transform used in JPEG and MPEG-1, MPEG-2 and MPEG-4 allow two dimensional pictures to be described in the frequency domain.
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