poynton@vector.Sun.COM (Charles A. Poynton) (08/08/89)
TN33--Overview of Digital Video Interfaces 89/08/07 11:34 Charles A. Poynton, Sun Microsystems, Inc. This document presents an overview of interfaces for digital video: component (525-line and 625-line), composite (NTSC and PAL), and high definition television (HDTV, 1125 line). CCIR REC. 601-1 COMPONENT DIGITAL VIDEO STANDARD There is now a firmly established standard for digital video interface at the studio according to the abstract parameters of CCIR Rec. 601-1. This international standard specifies that luminance is sampled at 13.5 MHz, that U and V colour difference components are sampled at 6.75 MHz, and that there are 720 active samples per total line. This interface is referred to as 4:2:2, since luminance is sampled at four times 3.375 MHz, and each of the U and V components at twice 3.375 MHz (that is, the colour difference signals are horizontally subsampled by a factor of 2:1 with respect to luminance). The significance of the sampling frequency 13.5 MHz is that the same frequency results in an integer number of samples per line in both 525/59.94 systems (e.g. NTSC, 858 samples per total line) and 625/50 systems (e.g. PAL, 864 samples per total line). Component digital video tape recorders are now widely available for both 525-line and 625-line systems, and have been standardized with the designation D-1. CCIR-Rec. 601-1 specifies luminance coding which places black at code 16 and white at code 235. Colour differences are coded in two's complement, centred on code 128, with the negative peak at code 16 and the positive peak at code 240. CCIR REC. 656 PARALLEL INTERFACE CCIR Rec. 656 defines a parallel interface which conforms to the parameters of CCIR-Rec. 601-1. This interface has been adopted by SMPTE for 525-line systems as SMPTE RP-125, and by the EBU for 625-line systems as EBU Tech. 3246. This interface carries eight (or optionally nine or ten) data pairs at ECL levels, and an ECL clock pair carrying a 27 MHz clock signal, on a DB-25 connector. Synchronization is achieved through a timing reference sequence of four words, including the codes 0 and 255 which are protected from appearing elsewhere in the data stream. The interface defines a method of conveying ancillary data packets. CCIR REC. 656 SERIAL INTERFACE CCIR Rec. 656 also specifies a serial interface for CCIR-Rec. 601 signals. The interface specifies an eight-bit/nine-bit (8B9B) coding, where each 8-bit word of the CCIR-Rec. 601 stream is mapped through a table to a 9-bit code. The output codes are such that each word has either four zeros and five ones, or five zeros and four ones; this coding guarantees sufficient clock energy at the receiver to assure reliable bit-clock recovery, and also makes it unnecessary for the channel to convey energy at DC or low frequencies. The total bit rate across the interface is 243 Mb/s. The disadvantages of 8B9B mapped coding are that only eight data bits can be accommodated, not ten, and that lookup tables (of size about 4 Kb) must be provided at every transmitter and every receiver. The lookup tables force IC implementations to be fairly complex. The interface uses 75 ohm transmission, BNC connectors, and ECL levels. SONY SERIAL INTERFACE During the standardization of the serial form of CCIR Rec. 656, Sony Corporation indicated that it found the proposed CCIR serial interface impractical for a number of reasons. Sony has adopted a set of interfaces based on scrambled coding, where the data stream is serialized directly but then passed through a shift register arrangement with exclusive-OR taps. Such scrambling techniques have been well-known for some time, but the Sony scrambler has a second-stage scrambler (whose characteristic function is x+1), which Sony claims has improved performance over a conventional single-stage scrambler. The data rate for Sony interfaces is just the word rate times the number of bits per word. This would be 216 Mb/s for the case of eight-bit data at 27 MB/s, but Sony have indicated that their interface will be specified with a minimum of ten bit data, so their basic serial component video interface has a rate of 270 Mb/s. Quantel have endorsed the Sony system. Sony intends to use exactly the same interface (and exactly the same ASICs) for both component and composite interfaces. Sony have introduced an interface device (SIF-1000), packaged in a 1RU box, which will serialize and deserialize either composite or component video. COMPOSITE DIGITAL VIDEO INTERFACES The earliest digital video equipment processed signals in composite form, that is, with the colour component of the signal modulated onto a colour subcarrier and summed with luminance. Processing of composite signals is simplified if the sampling frequency is an integer multiple of the colour subcarrier frequency; typically, a multiple of four is used. For NTSC colour systems (e.g. in North America), the conventional sampling frequency has therefore been about 14.318182 MHz. For PAL colour, as in most of Europe, the conventional sampling frequency has been 17.734475 MHz. Composite digital processing was necessary in the early days of digital video, but many processed (such as resampling for picture size reduction) cannot be carried out in the composite domain. However, the data rate of a composite signal is only about half that of a comparable component 4:2:2 signal, and the four-times-subcarrier composite digital coding has been resurrected for use in a composite NTSC VTR format, D-2, first introduced by Ampex and now endorsed by Sony. Standardization of a dub connector interface for D-2 is now underway. It appears that this interface will conform roughly to the component video interface described in the next section, but with composite coding and therefore roughly half the sampling rate of a component system. There is currently discussion within the standards community regarding the exact coding and dynamic range parameters of this interface. It appears that the signal will be sampled on the I and Q axes, and synchronization codes incompatible to those of CCIR 601 will be adopted. The dynamic range of the coding must encompass the sync tip (-40 IEEE) and peak composite video (+133 IEEE), and so for an "eight bit" system there are almost exactly seven bits of amplitude resolution between black and white. Sony offers a serial interface for their D-2 VTR. The serial interface transmits ten-bit data at 143 MB/s. FIBRE OPTIC INTERFACES Fibre optic interfaces for digital video are not currently commercially available, but their introduction is expected soon. Both the CCIR and Sony serial interfaces were designed with coding appropriate for use with fibre optic transmission, and standardization of these interfaces for fibre optic media is awaiting the specification of appropriate mechanical components (fibre optic cable, connectors). HIGH DEFINITION TELEVISION The process of standardizing digital HDTV interfaces has only just begun. However it is reasonably certain that the luminance sampling frequency will be between 70 and 75 MHz, and that early digital processing equipment will process eight bit data with colour difference signals conveyed at half the luminance sampling frequency. The sampling rate used by the Sony demonstration equipment is 74.25 MHz, which corresponds to a serial data rate of 1.188 Gb/s. Sony have demonstrated a digital HDTV VTR operating at this data rate, using 1 inch open reel tape and having a recording time of one hour. A number of manufacturers have demonstrated equipment with parallel interfaces compatible with the CCIR 656 interface, but with a clock rate of about 70 MHz and two connectors, one for the luminance signal and a second for the multiplexed colour difference signal. The total bit rate for digital HDTV is well outside the range of fibre optic transmitter and receiver components which are currently economical. ----- Charles A. Poynton Sun Microsystems Inc. <poynton@sun.com> 2550 Garcia Avenue, MS 8-04 415-336-7846 Mountain View, CA 94043 "Japan has no laws against damage to its flag, but it has strict laws forbidding the burning of foreign flags lest this give offense to the country in question." -- The Economist, July 1, 1989, p. 19. -----