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Digital video basics

Jan 1, 2006 12:00 PM, BY MICHAEL ROBIN

Analog composite signals, such as PAL, NTSC and SECAM, are subject to cumulative distortions and noise that affect the quality of the reproduced picture. Separate distortions of the luminance and chrominance components, as well as intermodulation between them, are likely to occur. Such distortions can be reduced, but not completely eliminated, by performing all or at least a major part of production and post-production operations using component analog video signals.

The cumulative composite or component analog video signal impairments and their effect on the reproduced picture can be reduced considerably by using a digital representation of the video signal and effecting the distribution, processing and recording in the digital domain. The A/D and D/A conversions introduce some impairments.

By a proper selection of two parameters, namely the sampling frequency and the quantizing accuracy, these impairments can be reduced to low, visually imperceptible values. As long as the digitized signals are distributed, processed and recorded in the digital domain, these impairments are limited to those introduced by a single-pass A/D and D/A processing.

Sampling

Figure 1. Sampling spectrum of 4:2:2 SDTV signals. Click here to see an enlarged diagram.

The sampling of the video signal is essentially a pulse amplitude modulation process. It consists of checking the signal amplitude at periodic intervals (T). (See Figure 1.) The sampling frequency (F_S=1/T) has to meet two requirements:

It has to be higher than twice the maximum baseband frequency of the analog video signal (F_B), as stipulated by Nyquist. This is required in order to avoid aliasing. Aliasing is visible as spurious picture elements associated with fine details (high frequencies) in the picture. The only way to avoid aliasing is to use an anti-aliasing filter ahead of the A/D converter. The task of this filter is to reduce the bandwidth of the sampled baseband to less than F_S/2.
It has to be coherent with and related to an easily identifiable and constant video frequency.

An early approach, 3F_SC, sampled the composite video signal at three times the color subcarrier frequency. This resulted in F_S = 3 × 3.58MHz = 10.7MHz in NTSC and F_S = 3 × 4.43MHz = 13.29MHz in PAL. A later approach, 4F_SC, sampled the composite video signal at four times the color subcarrier frequency, or 17.7MHz in PAL and 14.3MHz in NTSC.

While sampling at a multiple of F_SC works well in PAL and NTSC, it doesn't work at all in SECAM. This is due to the inherent nature of SECAM, which uses two separate line-sequential frequency-modulated color subcarriers carrying, respectively, the D_B and D_R color-difference signals.

It appeared evident in the 1970s that a digital video system in which the luminance and chrominance are individually coded would ease the program interchange between the PAL and SECAM countries. This resulted in the component digital concept, which is at the core of all contemporary digital video systems.

The component digital concept uses three separate A/D converters, one each for the E'_Y, E'_CB and E'_CR component video signals. The sampling frequencies are a multiple of the horizontal scanning frequency F_H. The most pervasive SDTV sampling strategy, the 4:2:2, samples the luminance signal at 13.5MHz and each of the two color-difference signals at 6.75MHz. The luminance signal is low-pass filtered starting at 5.75MHz, and the color difference signals are low-pass filtered starting at 2.75MHz, resulting in a comfortable guard-band with respect to the Nyquist frequency and an alias-free sampling. The sampling frequencies are the same in the 525/59.94 and the 625/50 standard, resulting in an equal number of samples, 720 luminance samples and 360 each color-difference samples, during the active line in the two formats. Similar sampling strategies are used with the HDTV formats.

Quantizing

The pulse amplitude modulation results in a sequence of pulses, spaced at T=1/F_S intervals, whose amplitude is proportional to the amplitude of the sampled analog signal at the sampling instant. There are an infinite number of shades of gray — ranging from black (lowest video signal amplitude) to white (highest video signal amplitude) — that the analog video signal can represent.

The instantaneous sampling pulse amplitudes can be represented in the digital domain by only a limited number of binary values, resulting in quantizing errors. The possible number of shades of gray is equal to 2ⁿ, where n is the number of bits per sample.

Experiments have shown that when less than eight bits per sample are used, the quantizing errors appear as contouring. With eight bits per sample or more, the quantizing errors appear, in general, as random noise (quantizing noise) in the picture. In practical applications, in order to avoid clipping, the signal occupies less than 2ⁿ steps, resulting in a specified quantizing range.

Figure 2. Relationship between analog component signals and 10-bit Y, CB and CR digital sample values. Click here to see an enlarged diagram.

Figure 2 shows the relationship between the E'_Y, E'_CB and E'_CR analog component signal levels corresponding to a 100/0/100/0 color bars signal and the 10-bit Y, C_B and C_R digital sample values, as specified in ITU-R BT.601. In a 10-bit system, there are 1024 digital levels (2¹⁰), ranging from 0 to 1023 (000 to 3FF hex). Levels 000, 001, 002, 003 and 3FC, 3FD, 3FE, 3FF are reserved to indicate timing references. Note that the sync is not sampled. This leaves a maximum quantizing range of 1016 digital levels, ranging from four to 1019 to represent the signal levels.

The normalized (700mV p-p) Y signal levels are assigned a range extending from 64 to 940, a total of 877 quantizing levels. This leaves a small upper headroom (940 to 1019) and lower headroom (four to 64).

The normalized (700mV p-p) C_B and C_R signal levels are assigned a range extending from 64 to 960, a total of 897 quantizing levels. This leaves a small upper headroom (960 to 1019) and lower headroom (four to 64). An eight-bit system would have 220 quantizing levels for the Y component and 225 quantizing levels for the C_B and C_R components.

Advantages and disadvantages

The advantages of digital video are:

Single-pass, analog-type impairments are non-cumulative if the signal stays digital. However, a concatenation of digital black boxes using analog interfaces leads to cumulative analog signal degradations and should be avoided.
There is a reduced sensitivity to noise and interference.
Digital equipment efficiently and economically performs tasks that are difficult or impossible to perform using analog technology.
It is amenable to the application of techniques for efficient retention of essential information such as compression.

The disadvantages of digital video are:

Analog-type of distortions, as well unique digital distortions related to sampling and quantizing, result in a variety of visible impairments.
Wide bandwidth requirements for recording, distribution and transmission necessitate sophisticated bit-rate reduction and compression schemes to achieve manageable bandwidths.
Unlike analog signals, the digital signals do not degrade gracefully and are subjected to a cliff effect.

Michael Robin, a fellow of the SMPTE and former engineer with the Canadian Broadcasting's engineering headquarters, is an independent broadcast consultant located in Montreal. He is co-author of “Digital Television Fundamentals,” published by McGraw-Hill.

Send questions and comments to: michael_robin@primediabusiness.com