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Composite digital video

Jun 1, 2004 12:00 PM, BY MICHAEL ROBIN

Table 1. Summary of coding parameters for 4fSC NTSC composite digital signals Click here to see an enlarged diagram.

Along period of concept, product and electronic component development resulted in a large number of application-specific digital black boxes operating at incompatible sample rates, number of bits per sample and quantizing ranges. These products were developed to fulfill specific production needs and were designed for analog composite video interconnection compatible with the all-analog composite video production studios.

Figure 1. Spectrum of a 4fSC-sampled NTSC signal. Click here to see an enlarged diagram.

The composite digital video format constitutes a stepping stone toward the all-digital video teleproduction studio. In North America, there was an initial interest in composite digital videotape recorders. This had to do with the need to replace the obsolescent analog composite videotape recorders with digital videotape recorders featuring analog input/output ports.

Figure 2. Phase diagram showing the relationship between the chrominance vector projections on the B-Y/R-Y axis system and the I/Q axis system. Click here to see an enlarged diagram.

A number of manufacturers developed such products identified as D2 (Sony and Ampex) and D3 (Panasonic) digital videotape recorders. A wide range of compatible composite digital video studio-type production equipment appeared on the market subsequently. The SMPTE 244M standard defines the characteristics of the 4f_SC NTSC composite digital signals as well as the bit-parallel interconnect characteristics. The digital signal aspects defined by the standard are summarized in Table 1.

The sampling structure

Figure 3. 4fSC sampling instants of an NTSC composite analog signal. Click here to see an enlarged diagram.

The sampling frequency is equal to four times the subcarrier frequency or 14.3181MHz (14.32MHz nominal). The sampling clock is derived from the color burst of the analog signal. Figure 1 shows the sampling spectrum of 4f_SC NTSC.

There is a significant gap between 4.2MHz (the maximum nominal NTSC baseband frequency) and 7.16MHz (the Nyquist frequency). The standard does not specify the characteristics of the anti-aliasing and reconstruction filters. The manufacturer has the choice of developing complex and costly wideband brick-wall ripple-free filters, resulting in an extended frequency response, or moderate-cost 4.2MHz low-pass filters with a gradual roll-off.

As a result, various 4f_SC products have different analog bandwidths. Note that a digitally generated signal fed directly to a digital 4f_SC unit will have an equivalent analog bandwidth equal to f_SC/2 = 7.16MHz. Severe overshoot and ringing of the derived analog composite signal may result unless special precautions are taken to ensure that digital blanking edges and rise times, compatible with the analog waveforms, are included as an integral part of the digital signal.

Figure 4. 4fSC NTSC sample numbering and horizontal sync relationship. Click here to see an enlarged diagram.

The SMPTE 244M standard was developed with reference to the original NTSC specifications that used I/Q encoding instead of B-Y/R-Y encoding, as is the current practice. Figure 2 shows that any chrominance vector can be represented by I/Q or B-Y/R-Y vectors. The original intent of the NTSC standard was to assign different bandwidths to the I signal (1.2MHz) and to the Q signal (0.6MHz), thus allowing for a better resolution for the orange visual information.

The I/Q-encoded NTSC signal can be decoded along the I/Q axis, with equal or unequal bandwidths, or the B-Y/R-Y axis with equal (equiband) bandwidths. Because the transmitter video frequency cutoff occurs at 4.2MHz, the wider-bandwidth I signal is transmitted with unequal lower (-1.2MHz) and upper (+0.6MHz) sidebands (vestigial upper sideband), unlike the narrowband Q signal, which is transmitted with equal lower (-0.6MHz) and upper (+0.6MHz) sidebands. Few I/Q decoding monitors and receivers were built because of decoding circuit complications resulting in no visible picture improvements.

Figure 5. 4fSC NTSC digital horizontal blanking interval showing the location of some significant samples. Click here to see an enlarged diagram.

As shown in Figure 3, the NTSC 4f_SC standard requires that the sampling instants coincide with peak positive and negative amplitudes of the I and Q subcarrier components. The upper part of the drawing shows that sampling instants provide an adequate 4f_SC representation of the B-Y/R-Y information.

Given a sampling frequency f_S = 14.3181MHz (nominally 14.32MHz) and a horizontal scanning frequency f_H = 15734.25 Hz, the number of samples per total line is equal to f_S/f_H = 910. The digital active line accommodates 768 samples (numbered 0 to 767). The remaining 142 samples (numbered 768 to 909) comprise the digital horizontal blanking interval.

Figure 4 depicts the sample numbering for a nominal NTSC signal. The half amplitude point of the leading (falling) edge of the analog horizontal sync signal falls between samples 784 and 785. The first of the 910 samples represents the first sample of the digital active line and is designated sample 0 for the purpose of reference. The 910 samples per line are, therefore, numbered 0 to 909.

Figure 6. 4fSC NTSC horizontal sync period details showing location of TRS-ID and optional ancillary data. Click here to see an enlarged diagram.

Figure 5 details the digital horizontal blanking interval, showing the location of some significant samples. Note that unlike component digital video, where the horizontal digital blanking interval is not used — with the exception of two four-word timing reference signals (TRS) — the 4f_SC digital signal carries horizontal sync and subcarrier burst signals as well. The standard was designed with bit-parallel distribution in mind. Bit-serial signal distribution, as detailed in SMPTE 259M, requires the reorganization of the horizontal and vertical blanking intervals.

Figure 6 shows the location of the added five-word TRS (samples 790 to 794), as required by SMPTE 259M. This leaves space for 55 ancillary data words (samples 795 to 849), which could be used for embedding four digital audio channels.

Figure 7. Relationship between analog signal levels and digital sample values. Click here to see an enlarged diagram.

The quantizing range

Figure 7 shows the relationship between analog NTSC signal levels and eight-bit and 10-bit sample values of a 100/7.5/100/7.5 color bars signal. The 10-bit approach provides for 1024 digital levels (2¹⁰) expressed in decimal numbers varying from 000 to 3FF. Digital levels 000, 001, 002 and 003 as well as 3FC, 3FD, 3FE and 3FF, are protected and not permitted in the digital stream. This leaves 1016 digital levels, expressed in decimal numbers varying from four to 1019 or in hexadecimal numbers varying from 004 to 3FB, to represent the video signal.

The sync tip is assigned the value 16 decimal or 010 hexadecimal. The highest signal level, corresponding to yellow and cyan, is assigned the value of 972 decimal or 3CC hexadecimal. The standard provides for a small amount of bottom headroom (some call it foot-room), levels four to 16 decimal or 004 to 010 hexadecimal, and top headroom, levels 972 to 1019 decimal or 3CC to 3FB hexadecimal.

The total headroom is on the order of 1dB and allows for mis-adjusted or drifting analog input signal levels. This reduces the S/Q_RMS (signal-to-RMS quantizing error) ratio by the same amount. The theoretical S/Q_RMS of a 4f_SC product featuring analog in/out interfaces is 68.10dB for a 10-bit system and 56.06dB for an eight-bit system. This is considerably higher than any composite analog or component analog VTR.

Conclusion

In most cases, D2/D3 VTRs were used as drop-ins in an NTSC analog composite environment. Their performance figures were superior to older analog composite as well as analog component (BETA-CAM) VTRs, especially if parallel or serial digital (143Mb/s) interfaces were used.

The major handicap of composite digital video was the fact that 4f_SC could not be compressed using highly efficient contemporary transform coding methods typical of MPEG. Consequently, VTRs used a high recorded data bit rate, 127Mb/s, resulting in large videocassettes and no portable camera/VTR gear. The appearance on the market of competitively priced component digital video equipment has tilted the market toward the adoption of component digital video equipment.

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

Send questions and comments to: michael_robin@primediabusiness.com

The Second Edition of Michael Robin's book may be ordered directly from the publisher by calling 800-262-4729. The book is available from several booksellers.