------------------------------------------------------------------------ Copyright ⎩ 1997 Roger Jennings, all rights reserved. Written May 20, 1997. Parts of this document will appear in the author's Special Edition Using Desktop Video to be published by Que Books, an imprint of Macmillan Computer Publishing, in Fall 1997. This document may be copied and/or distributed for non-commercial purposes only. DV vs. Betacam SP: 4:1:1 vs. 4:2:2, Artifacts and Other Controversies Sony Corp.'s Betacam SP format is the standard of comparison of video tape recording formats. According to Sony Europe, more than 350,000 Betacam SP devices have been sold world-wide. The majority of broadcast electronic news gathering (ENG) operations currently use Betacam SP camcorders and VTRs. Virtually all broadcast stations require (or at least strongly prefer) Betacam SP source footage. Most clients of professional video production firms specify Betacam SP for industrial shoots and are likely to require videographers to use Sony or Ikegami camcorders. Although the M-II format from Panasonic Broadcast and Digital (formerly Television) Systems Company (PB&DSC) offers about the same performance as Betacam SP, Sony and Betacam SP are untouchable when it comes to brand recognition and status. As a result, all other video recording formats are ranked as "not up to Beta SP," "equal to Betacam SP," or "better than Beta SP." These comparisons, based on the beholders' perception of image quality, are reminiscent of the meaningless "broadcast quality" and "studio quality" bullet points on advertisements for consumer and low-end prosumer video gear. The advent of the Digital Video (DV) format has ignited a controversy among current and prospective users of DV gear. Initially, arguments appeared regarding the "legality" of broadcasting NTSC DV's 480 instead of 483 active lines of video. Obviously, if broadcasting less than 483 active lines was illegal, all U.S. stations transmitting letterboxed movies would have by now lost their licenses. The subsequent controversy, DV's 4:1:1 vs. ITU-R BT.601-4 (formerly CCIR-601) 4:2:2 sampling, has generated thousands of messages in on-line forums, newsgroups, and listservers. This paper represents an attempt to dispel the rumor and innuendo surrounding the 4:1:1 versus 4:2:2 issue, especially as it relates to the "Is DV better (or worse) than Betacam SP?" controversy, and DV compression artifacts. Horizontal Resolution and Bandwidth of Analog Recording Formats Traditionally, resolution of analog video cameras, recording formats, and monitors has been expressed as "TV lines" or "horizontal lines." Originally, TV lines were determined by the ability to clearly distinguish sets of vertical lines in a standard TV test pattern. Subsequently, horizontal resolution became related to the bandwidth of the recording format. The generally-accepted "rule of thumb" is 80 lines per MHz of luminance bandwidth for recording formats. The following table lists the lines of horizontal resolution of various recording formats, based on Sony and other manufacturers' data sheets, with the NTSC broadcast signal added for comparison. Bandwidth is implied from the "rule of thumb." Format Horizontal Lines Implied Bandwidth, MHz VHS and 8-mm VCRs 240 3.0 U-matic (3/4-inch) VCRs 250 3.1 U-matic SP VCRs 330 4.1 NTSC Broadcast Signal 330 4.1 S-VHS and Hi8 VCRs 400 5.0 Laserdisc 425 5.3 DVD Video 500 6.3 DV Formats 500 6.3 * Note: DV formats include consumer DV, DVCPRO (25 Mbps), and DVCAM, all of which use the same video data format for NTSC. Horizontal resolution of professional-grade three-CCD video cameras (often 750 or more) are related to the number of pixels per CCD, the type of CCD (field interline transfer, HyperHAD, etc.), the spatial relationship between the three CCDs, and internal signal processing techniques. Increasing the resolution of the camera section results in improved perceived image quality, regardless of the resolution of the recording format used. Horizontal resolution of monitors is determined by a combination of the aperture mask dot (or grill) pitch and design, as well as the video signal bandwidth. Sony claims its popular PVM-1354Q (0.25-mm pitch) and PVM-1954Q (0.40-mm pitch) monitors deliver 600-line resolution, corresponding to a theoretical luma bandwidth of 7.5 MHz. Another rule of thumb is that your monitor's resolution (or bandwidth) should exceed that of the recording format, preferably by 25 percent or more. Commencing with introduction of the Betacam format, Sony and its competitors ceased publishing horizontal resolution values for professional and broadcast recording formats. The reason for doing so is clear: Using 80 lines/MHz would have positioned oxide Betacam, with it's 4.0 MHz luminance bandwidth, at 320 lines (inferior to U-matic SP.) Even worse, metal-particle Betacam SP would fall below consumer-grade S-VHS and Hi8 TV-line ratings. Both oxide Betacam and Betacam SP produce image quality superior to S-VHS and Hi8 formats, primarily because of Betacam's separate recording tracks for luminance (Y) and chroma (Cr and Cb), rather than the color-under modulation method of S- VHS and Hi8. Thus the two licensees of Thomson's original Betacam technology, Sony and Philips BTS, and their erstwhile competitors (Panasonic and JVC with M-II) began using bandwidth and signal-to-noise ratio (SNR) specifications for the luma and chroma components, instead of specifying TV-lines. The following table lists the published values for luma and chroma bandwidth, plus SNR (in dB) for several Betacam and related VTRs. Product Line Luma Bandwidth Chroma Bandwidth Luma SNR Chroma SNR Sony UVW 30 Hz – 4.0 MHz +1.0/-4.0 dB 30 Hz – 1.5 MHz +1.0/-4.0 dB > 49 dB > 52 dB Sony PVW/PVV 30 Hz – 4.5 MHz +0.5/-4.0 dB 30 Hz – 1.5 MHz +0.5/-3.0 dB > 51 dB > 53 dB BTS SP 2000 30 Hz – 4.5 MHz +0.5/-4.0 dB 30 Hz – 1.5 MHz +0.5/-4.0 dB > 51 dB > 53 dB Panasonic M-II W-Series 30 Hz – 4.5 MHz +1.0/-4.0 dB 30 Hz – 1.5 MHz +0.5/-3.0 dB > 49 dB > 52 dBVideo recording bandwidth is determined by the modulation method, tape particle size, record/reproduce head design, internal electronic circuitry, and a variety of other factors. Maximum luma bandwidth is reported by the manufacturers at the point at which the signal falls to 63 percent of its original value (-4.0 dB). The high frequency content of the image, such as very small objects with sharply-defined outlines (leaves of trees in backgrounds, blades of grass, and the like) is attenuated in the recording process. The visible effect of attenuation is a combination of lower brightness levels and blurring of tiny objects. Maximum chroma bandwidth is reported at the 63 percent or 71 percent (-3.0 dB) loss point. The human eye is less sensitive to variations in color than in brightness, so the reduced chroma bandwidth gives acceptable perceived image quality. Betacam SP and Panasonic M-II represent the upper limit of quality for commercial analog video recording technology. These formats sustain several reproduce-record generations without obvious image degradation. * Note: The formula for determining the dB voltage values used in bandwidth and SNR measurements is dB = 20 log (Vout/Vin) and dB = 20 log (Vnoise/Vmax), respectively. This formula differs from the dB values used for audio power ratio specifications, dB = 10 log (Pmeas/Pref). Signal-to-noise ratio is a small-signal effect; thus SNR is important for low-light scenes shot without camera gain boost. Small differences in SNR dB ratings are deceptive—in linear terms, 51 dB (355:1) is about 1.3 times better than 49 dB (281:1). The luma SNR of professional Hi8 VTRs is > 45 dB (Sony EVO-9720 and EVO-9850) and most pro S-VHS decks claim > 46 dB. Noise levels are cumulative in dubbing, so the better SNR of Betacam SP contributes to its multi-generation capability. Digital Sampling, Analog Bandwidth, and SNR In the uncompressed digital video realm, sampling rate is conceptually analogous to bandwidth. The standard ITU-R BT.601-4 (D-1, 4:2:2) luma sampling rate for NTSC and PAL video is 13.5 MHz and chroma sampling occurs at half the luma rate or 6.75 MHz. There are two chroma signals (Cr and Cb), so the resulting digital bitstream is composed of equal quantities of luma and chroma data. The Nyquist theorem states that the sampling frequency must be at least twice the highest frequency component of the sampled analog signal. Thus the theoretical maximum luma bandwidth of D-1 is 6.75 MHz and the chroma bandwidth of the two components is 3.375 MHz each. Low-pass filtering is required to prevent aliasing in the digitized signals, so the resulting bandwidths are less than the Nyquist limit. Digital filters using high-performance DSPs (digital signal processors) can provide an analog-equivalent maximum luma bandwidth of about 6.0 MHz, +0.5/– 3.0 dB and a maximum chroma bandwidth in the range of 3.0 MHz, +0.5/-3.0 dB. DV samples luma at D-1's 13.5 MHz and chroma at one-half the D-1 rate or 3.375 MHz. DV-NTSC uses 4:1:1 sampling in which the chroma is sampled once for every four horizontal luminance samples. DV-PAL uses 4:2:0 sampling, which uses the average value of the chroma signal between successive horizontal samples and scan lines. According to the Japanese Digital Video Consortium, 4:2:0 sampling gives better perceived image quality with PAL's increased number of vertical scan lines.