How DV Works 1of 2

Copyright© 1995, 1996, 1997, Roger Jennings, all rights reserved. Revised 1/30/97. A version of this document will be included in the author's Special Edition Using Desktop Video to be published by Que Books, an imprint of Macmillan Computer Publishing, in Fall 1997. Permission to publish this paper has been granted to Adaptec, Inc. by the author. Opinions expressed herein are those of the author and not necessarily of Adaptec, Inc. This document may be copied and/or distributed for non-commercial purposes only. Please send comments and corrections to the author at 70233.2161@compuserve.com.

 

Consumer and Professional Digital Video Recording and Data Formats

Sony Corporation's September 1995 announcement of two digital video (DV) camcorders sparked a flurry of articles in consumer-oriented video magazines. The first "affordable" digital camcorders with digital inputs and outputs had arrived about six months earlier than projected by most industry pundits. Sony's 3-CCD DCR-VX1000 (U.S. MSRP $4,199) and, to a lesser extent, the single-CCD Sony DCR-VX700 (MSRP $2,999) captured the imagination of both professional and amateur videographers. Direct digital output to non-linear editing (NLE) systems and lossless multigenerational editing using conventional linear methods seemed about to become a reality. Although Matsushita beat Sony to the punch by a couple of weeks with a DV camcorder announcement, most reviewers considered the Panasonic model's lack of an IEEE-1394 connector to be a critical omission. The Sony and Panasonic camcorders claimed 500 TV lines of horizontal resolution achieved by a luminance sampling rate of 13.5 MHz and chrominance sampling rate of 6.75 MHz (4:1:1 YCrCb format for NTSC), which is a substantial improvement over the bandwidth of Hi8 and S-VHS formats that offer about 400-line resolution.

JVC and Sony released in 1996 two highly-miniaturized DV camcorders to the U.S. market. The $3,000+ Sony DCR-PC7 sports a 2.5-inch fold-out LCD monitor, in addition to a color viewfinder, and offers an IEEE-1394 connector. In January 1997, Matsushita released details of two miniature DV camcorders, one of which has a 4-inch LCD monitor. Both of the new Panasonic camcorders have IEEE-1394 connectors. Sharp's 1996 DV entry featured a large LCD display, but neither the JVC or Sharp models include IEEE-1394 capability. Sony's DHR-1000 DVCR is scheduled for widespread U.S. availability in Spring 1997.

Three "professional" variants of the consumer DV format have set off a new skirmish in the video "format war." Panasonic took the wraps off its nascent DVCPRO product line at the National Association of Broadcaster's (NAB) show and convention in April 1995. DVCPRO uses the same basic video and audio encoding format as the consumer DV format, but most DVCPRO gear is priced substantially above beyond even affluent prosumers' pocketbooks ($10,000 and up.) Sony unveiled its DVCAM format, also based on the DV standard, at NAB '96. Panasonic announced at Amsterdam's International Broadcast Conference (IBC) in September 1996, its intent to deliver a DVCPRO variant with 3.3:1 compression in a 4:2:2 YCrCb format with a 50-Mbps data rate, twice that of the consumer DV format.

This paper describes the technology behind consumer DV camcorders and DV tape decks (consumer DVCRs and professional DVTRs), including recording formats, basic electronic architecture for DV, and the data format for communication between consumer and professional DV devices. For the most part, this paper is based on the Digital Interface for Consumer Electronic Audio/Video Equipment, Draft 2 (DI Draft 2), jointly presented by Philips, Matsushita, Thomson Multimedia, and Sony to the October 1995 meeting of the (IEEE) 1394 Trade Association. Additional details are derived from General Specifications for Consumer-Use Digital Interface (Specification), a proposed International Electrotechnical Commission (IEC) standard for transmitting digital audio/video data over the IEEE-1394-1995 High Performance Data Bus, "The Development of Audio and Video Signal Processing LSI for SD-DVC" (IEEE Transactions on Consumer Electronics, Vol. 41, No. 3, August 1995), and The Digital Video Tape Recorder by John Watkinson (1994, Focal Press, ISBN 0-240-51373-8.) Information about Panasonic's DVCPRO product line is based on a paper entitled "DVCPRO: A comprehensive Format Overview," presented at the SMPTE meeting in New Orleans (Fall 1995), and Panasonic product bulletins. Details of Sony's DVCAM format are based on published Sony specification sheets.

Cassette Dimensions and Interchangability

Part 1 of the Specifications defines the dimensions for standard and small DV cassettes, and recording geometry. The standard (4.9 x 3 x 0.57 inch) cassette is designed for use in DV videocassette recorders (DVCRs) for both home recording and playing prerecorded cassettes up to 4.5 hours in length. Figure 1 shows the bottom view and insertion end view of the standard (4.5-hour) DV cassette, which is smaller than an audio Compact Cassette. The small (2 x 2.2 x 0.5 inch) DV cassette is intended for use in DV camcorders. Figure 2 shows the bottom view and insertion end view of the small (30-minutes or 1-hour) DV cassette.

Fig. 1. Standard (4.5-Hour) DV Cassette for VCRs

Fig. 2. Small (30-Minute or 1-Hour) DV Cassette for Camcorders

Tape width is 6.35 mm (1/4-inch) and tape speed in normal play/record mode is 18.81 mm/sec (0.75 inch/sec.) Thus a one-hour mini-DV cassette has a total tape length of about 65 m and the standard cassette contains more than 250 m of tape. Consumer DVCRs accommodate by means of moving reel motors both standard and small cassettes without the need for an adapter. One of the interesting features of both cassette sizes is Memory in Cassette (MIC); cassettes include non-volatile RAM that the specifications allow to store up to 16MB (Megabytes). MIC can store a variety of information in a combination of fixed and optional data areas. Sony consumer DV cassettes have 512 bytes of MIC RAM that hold information on the tape type, cassette grade, plus date and time of multiple recording segments. MIC data is used internally by the VCR or camcorder, and is transmitted over the digital interface. Only Sony DV tapes currently offer MIC; MIC is not required for DV cassette compatibility.

Sony DV tape consists of the following five layers:

Hi8 metal-evaporated (ME) tapes have a history of problems with dropouts, both in new and heavily-used tapes. Thus many Hi8 shooters use a metal particle tape, such as Fuji's ME-221, designed for the flux density and having the coercivity of metal-evaporated tape. Initial reports indicate that neither Sony or Panasonic ME tapes for DV camcorders exhibit dropout problems, even with repeated reuse. One user of Sony tapes recently reported logging 24 hours of footage without a single dropout. One-hour mini-DV cassettes have a street price from about $12 (Panasonic, no MIC) to $20 (Sony with MIC).

Tape Track Geometry and Recording Sectors

DV recording is an extraordinary example of miniaturization of electronic devices. DV uses helical azimuth recording (+/- 10 degrees), which requires a minimum of two heads on a drum rotating at approximately 9,000 rpm for NTSC video (exactly 9,000 rpm for PAL and SECAM.) The track width is 10 microns (millionths of a meter), compared with Hi8's 20.5 microns and VHS's 58 microns. (A human hair is about 100 microns in diameter.) The track slant angle is about 9 degrees, resulting in a nominal track length of 35 mm and an active track length of 33 mm. DV does not provide a conventional control track to maintain head tracking; instead, pilot tones are embedded in the data tracks. The speed of the capstan varies in accordance with the relative intensity of low- and high-frequency pilot tones created by "flipping" unused bits in the data. In addition to the helical tracks, the Specification shows two optional linear tracks; DVCPRO uses these two linear tracks for linear timecode (LTC) and audio cuing during shuttle. DVCPRO also provides an internally-generated vertical interval time code (VITC) for backward compatibility with existing editing systems. Figure 3 shows the track geometry and division of the helical (slant) track into standard sectors for the DV format.

Note: The basic track geometry shown in figure 4 can be used for a variety of video and audio applications, in addition to DV. As an example, the audio and video sectors could be combined into a single sector containing MPEG-2 video and audio data for timeshift recording of direct-to-home satellite broadcasts from programmers such as DirecTV and USSB. An application code (APT) in the ITI sector specifies the number and location of track sectors, as well as their purpose. The version of the Specification discussed here is limited to the DV format in which the APT value is 000. The Panasonic DVCPRO format uses optional linear track 1 as an audio cue track and track 2 as a control track providing linear timecode (LTC). The track width of the DVCPRO format is 18 microns with a tape speed of 33.8539/1.001 (33.82 mm/sec.) The increased track width and the concomitant increase in tape speed reduces recording density to allow the use of metal particle tape, rather than DV's metal-evaporated tape. DVCPRO uses a pair of separate erase, record, and playback heads, for a total of 6 heads on a 21.7 mm drum.

Fig. 3. Track Recording Geometry (Dimensions in millimeters)

To maintain maximum commonality between 525/60 (NTSC) and European 625/50 video formats, the DV format changes the number of tracks per frame. NTSC uses 10 tracks per frame (at 29.97 fps +/- 1 percent) and PAL requires 12 tracks per frame (at 25 fps). Figure 4 shows the frame sequence for NTSC systems.

Fig. 4. Frame Sequence for Recording (NTSC)

To provide for video-over-sound (VOS) insert editing, audio dubbing, and timecode during shuttle, the helical track is divided into the following four track sectors:

Note: The NTSC versions of DV camcorders and VCRs use SMPTE dropframe timecode (29.97 fps), rather than the non-dropframe timecode (based on 30 fps) used by consumer and prosumer Hi8 VCRs (Sony Rewritable Consumer Timecode, RCTC).

An advantage of digital recording is that a substantial amount of auxiliary data can accompany the digitized audio and video. As an example, the Specification specifies formats for the following types of auxiliary data in subcode packs:

The DV format includes a provision for copy management, although the copy management mechanism remains "to be determined" in the Specification. A combination of high-price and copy management issues led to the demise of DAT as a major consumer audio recording format. Sony explained the delay in U.S. availability of a companion DVCR for the DV camcorders as due to copy management issues. (The Sony DHR1000 PAL DVCR was available in Europe about six months before the U.S. version was announced.) Copy management relates to DVCR owners copying digital data from Digital Video/Versatile Discs (DVDs), not today's movies recorded in VHS format. DV technology creates a duplicate indistinguishable from the original, so US movie makers want the DVD format and DVD players to include an encryption system that prevents copying and to include a geographical playback restriction so, for example, German users can't play back DVDs released in the UK.

Encrypted programming with and without a pre-recorded decryption key is provided for a "restricted audience." A recorded decryption key allows only those who know the key to play a prerecorded tape. Thus youngsters can be thwarted from watching adult fare and children can be protected from overly violent movies and even user-recorded TV programs. This feature assumes a market for pre-recorded DV tapes, a highly-unlikely event. An unrecorded decryption key might be used for DBS timeshift recording in conjunction with a DBS set-top box. (MPEG-2 video compression has a substantial lower data rate than DV, and the DV format provides an MPEG-2 option.) As long as you pay your monthly programming fee, the encryption code is transmitted by satellite, and you can watch your recorded tapes; stop paying and your tapes won't play.

DV Camcorder and DVCR Video Quality and Architecture

The architecture of DV camcorders and DVCRs doesn't differ materially from today's high-end compressed digital video recording systems, such as Digital Betacam (DB). Both DV and DB are component formats, encoding luminance (Y) and two separate chrominance (R-Y and B-Y) signals on tape. DV uses a 13.5-MHz sampling rate (as does DB), but DB uses 4:2:2 encoding to obtain increased chrominance fidelity compared with DV's 4:1:1 sampling. DB also offers 10-bit encoding (versus DV's 8 bits) for improved signal-to-noise ratio (SNR). Digital Betacam uses almost 2:1 spatial compression (with 8-bit sampling), while DV employs nominal 5:1 compression. (The next section discusses relative compressed data rates.) DV gains some of its compression ratio by adding interfield (not interframe) compression of video images that don't have substantial motion. Because interfield compression results in a variable amount of data per frame and DV requires a constant digital data rate, adaptive intraframe spatial compression is necessary. As the amount of motion in a scene increases, the spatial compression increases (and vice-versa).

Most video professionals conclude that the DV format compares favorably with analog Betacam SP. DV's video signal-to-noise ratio of 54 dB is somewhat better than Betacam SP's 51 dB, and DV's 5.75 MHz luminance bandwidth beats Betacam SP's 4.1 MHz by a significant margin. Professional Betacam SP camcorders have substantially better lenses and larger CCDs than consumer DV gear, but cost at least five times as much as the street price of the DCR-VX1000. Most professional-quality interchangeable camcorder lenses (Fujinon, Canon) cost more than the entire DCR-VX1000. Direct comparison of DV vs. Betacam SP image quality requires the use of a professional dockable (digital) camcorder with a DV recorder, such as JVC's BR-DV1U that mates directly with most JVC dockable camcorders. (An optional adapter fits Sony dockable camcorders. Sony offers only a DVCAM dockable recorder.)

Fig. 5. Simplified Block Diagram of DV Camcorder or DVCR in Record Mode

Figure 5 is a simplified block diagram of a DV camcorder or DVCR based on the specifications for the Sony DCR-VX1000. Figure 5 shows record mode; reversing the data flow direction provides an approximation of the playback architecture. The Sony camcorders have digital (IEEE 1394-1995) and analog video (S-video/composite) and single-channel (32-kHz, 12-bit, non-linear stereo) audio outputs, but only a digital input. (European versions of the DCR-VX1000 don't have a digital input because of the much higher tariff for video recorders.) DVCRs, such as the Sony DHR-1000, incorporate full digital and composite/S-video analog and audio I/O, including support for two stereo channels using the 32-kHz, 12-bit format or one stereo 48/44.1-kHz 16-bit linear channel. As shown in figure 5, interfield motion detection is used to determine if a substantial amount of information is duplicated between fields. If such is the case, only the differences between fields are decoded and the quantization threshold is changed to maintain a constant data rate.

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