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Reader SurveyDigital Definitions: The Best 6mm Digital Format Depends on Your Priorities
Mar 1, 1998 12:00 PM, Steve Mullen
It seemed there was every reason to be optimistic when, in September 1993, the Digital VCR Conference, colloquially named "DVC," announced a digital format had been adopted by 55 manufacturers. These manufactures included Sony, Matsushita (Panasonic), JVC, Philips, Toshiba, Mitsubishi, Hitachi, Sharp, Thomson, and Sanyo. The specification-simply called DV-could have become a single, worldwide video standard. Four years later, there are six extant digital formats rather than a single standard. How did this come about?
Some of the optimism was likely naive. There was probably no way a single standard could serve both consumer and professional video markets. But it's equally likely that there is no technical need for the number of formats which have been unleashed into the market. Nevertheless, six formats are what we've got-so far. The best we can do now is to understand these formats and what advantages each offers.
The DV format, currently represented by consumer and prosumer products from Sony, Panasonic, JVC, and Canon truly is an industry format. It's likely other manufacturers will offer DV products. For professional use, Sony offers two additional digital formats-DVCAM and Beta SX. Panasonic also has two professional formats: one that is shipping, DVCPRO, and one that has been announced, DVCPRO-50. JVC has a single professional format, Digital-S.
Despite the fact there are significant incompatibilities between these formats, there is also an amazing amount of commonalty. With the exception of Beta SX, all the other formats share DVC-developed technology. To keep focus, Beta SX and Digital-S will not be covered in this article, since both use 1/2-inch tape. And, to keep things simple, DVCPRO-50 will be discussed last. So let's begin with the DVC technology that underlies DV, DVCAM, and DVCPRO.
Video Encoding Whether the DVC codec is implemented in a chipset or software running on your computer's CPU, it performs a defined series of operations. During recording, NTSC video luminance signal (Y) is sampled at 13.5 MHz, which provides a 5.75-MHz luminance bandwidth. The R-Y (U) and B-Y (V) color difference signals are digitized at 3.375 MHz, which provides a 1.5-MHz bandwidth for each chroma component. The result is 4:1:1 YUV digital video.
Digitized video is stored in a 720 x 480 pixel frame-buffer before compression is carried out. Logic measures the correlation between the two fields stored in the buffer. If the correlation is low, indicating too much inter-field motion, then the two fields are compressed individually. Normally, however, the entire frame is compressed. In either case, Discrete Cosine Transform (DCT)-based compression is used. The DVC compression algorithm is neither motionJPEG, as is the case with most NLE systems, nor MPEG. (DVC compression does, however, have much in common with "I" frame-only MPEG.)
The DVC formats use a proprietary compression algorithm that can be edited because it is an intra-frame technique. Blocks of 8 x 8 pixels are organized into Discrete Cosine Transform (DCT) blocks. Macroblocks are then organized from four DCT blocks. Each Macroblock has its own Q-table (quantization table). Q-tables are what really perform signal compression. Because both DVC and MPEG compression algorithms have multiple Q-tables per frame, they offer dynamic intra-frame compression. MotionJPEG has only a single Q-table for the entire frame. Therefore, at an equivalent frame compression ratio, DVC compression will look better than motionJPEG.
Digital Audio All 6mm audio is recorded digitally. There are two recording modes: 16-bit and 12-bit. The 16-bit mode uses an A/D sampling frequency of 48 kHz (the same as DAT). Using this rate, the format has a frequency response of up to 24 kHz. DVCPRO offers only 16-bit operation, whereas DV and DVCAM offer both 16-bit and 12-bit sampling.
The 12-bit mode operates at a sampling rate of 32 kHz, which yields a frequency response of up to l6 kHz. A second track of 12-bit audio can be added during editing on a DV or DVCAM VCR. This track can carry audio such as background music or narration. To provide the same 96db offered by the 16-bit mode, the 12-bit mode employs dynamic audio compression during recording. During playback, the audio signal is appropriately expanded.
The DV format uses "unlocked" audio while both DVCAM and DVCPRO use "locked" audio. With the DV format's unlocked sound, also called Non-Synchronous audio, audio sampling is not synchronized on a frame-by-frame basis to the video. (That doesn't mean audio and video are not, over time, synchronized.) Perfect audio and video alignment is not, therefore, guaranteed to exit. Thus an edit could have an audio glitch which could sound like a click. Because audio is in perfect alignment with video-and other audio tracks-professional equipment avoids the potential glitch because it can perform a very rapid (10 millisecond) cross-fade at each cut.
Error Correction All 6mm formats share a potential dropout problem. With an entire frame of data held in area the width of a human hair, even a small dust particle could cause the loss of a frame. The 6mm tape is passed over a drum mechanism only 21.7mm inches in diameter which has read/write heads that rotate at 9,000 rpm!
Moreover, despite advanced tape technology, there is a reasonable probability of a tape defect. For this reason, these formats include a powerful Reed-Solomon error correction scheme.
In very simplified terms, error correction works as follows: Digital information (i.e., "1s" and "0s") is organized into matrices. An error correcting code is then attached to each row and column. The code, likewise a "1" or "0," ensures that an even number of "1s" exist in each row and column. During playback the error correction unit reassembles the information, including the error codes, into matrices. Logic then looks for an uneven number of "1s" in the rows and in the columns.
If an error is found, the row and column addresses point to the cell where a bit is in error. The state of the bit in this cell is then flipped to correct the error. If the error correction effort fails, there are several alternatives. If the error is confined to a Macroblock, the previous good block can be used. If the errors are large, the previous field or frame can be substituted.
Track Pitch Track pitch is another way that the consumer DV format differs from the professional formats. The DV format's 10-micron track pitch has the potential to limit both video and audio insert editing capabilities. Both Panasonic and Sony claim that the 18-micron and 15-micron pitch employed by DVCPRO and DVCAM formats are necessary to perform perfect insert editing. If, when performing a video and/or audio insert, the inserted segment does not start and end precisely between two complete video frames, a glitch may occur.
A video glitch occurs during playback because the visual information in a frame is drawn from two partial frames. One partial frame comes from the original contents on the tape and the other from the inserted material. Professional equipment is guaranteed to make totally clean inserts. Note that clean edits are not synonymous with frame-accurate edits.
The DV Format The DV format is a specification for a digital video format primarily intended for prosumer-and eventually consumer-applications. The DV format has two cassette sizes: Standard four-hour (125mm x 78mm x 14.6mm thick) and the Mini one-hour (66mm x 48mm x 12.2mm thick).
One of the most innovative aspects of Sony's DV (and DVCAM) cassettes is the built-in read/write memory chip. DVCAM-and some DV-cassettes have 16 kilobits of storage. The primary use of the memory chip is to store the date and time of Photo Mode images-as well as the time of the first video recording made on any date.
The DVCAM Format DVCAM is identical to prosumer DV in all aspects but one-its track pitch is 15 microns. Sony's prosumer DV and DVCAM "large" tape transports will play both DV (10 micron track pitch) and DVCAM (15 micron track pitch) tapes. New Sony Mini DV transports will also play both formats. Recordings made in prosumer equipment will always have a 10microns track pitch while DVCAM equipment always records at a 15microns track pitch. The DVCAM VCR transport will play as well as record (DVCAM format only) on the Mini DV cassettes without the need for an adapter.
The DVCAM format offers two cassette sizes: Standard (125mm x 78mm x 14.6mm thick) and Mini (66mm x 48mm x 12.2mm thick). Sony's DVCAM ME tape is available in 12-, 22-, 32-, and 40-minute Mini DV cassettes. The Standard cassette provides recording durations of 64, 94, 124, and 184 minutes. DVCAM ME tape technology increases carrier-to-noise (C/N) ratio by +6db over the MP tape used by DVCPRO.
The DVCPRO Format Whereas Sony chose to make only one deviation (a track pitch of 15µm) from the DVC standard-Panasonic has made five alterations: a track pitch of 18µm, the "M" field cassette (larger than the Mini DV size), a linear CTL track for editing accuracy, a linear track for cue audio, and the use of MP rather than ME tape. The choice of MP was dictated by the need to record the linear tracks.
DVCPRO's dual-channel, 16-bit digital-audio signal is recorded at a sampling rate of 48 kHz. Panasonic's DVCPRO MP tape is available in two cassette sizes. The "L" general-use cassette is the same size as the 125mm x 78mm x 14.6mm large DV Standard cassette. This cassette provides recording durations of 64, 94, and 123 minutes. The "M" cassette (97.5mm x 64.5mm x 14.6mm) provides a recording duration of 6, 12, 23, 33, and 63 minutes. Panasonic's VCRs will play, but not record to, Mini DV cassettes with an adapter.
The DVCPRO-50 Format DVCPRO-50 differs from DVCPRO in two specifications: It uses 4:2:2 YUV digital sampling and mild 3.3:1 compression. This is an extremely high-quality specification where data flows at 50 MBit/sec.
DV/DVCAM/DVCPRO and Betacam SX offer data rates of only 25 MBit/sec. and 18 MBit/sec., respectively. (JVC's Digital-S shares the same specification as DVCPRO-50.)
The DVCPRO-50 codec uses 4:2:2 sampled component digital video. Luminance video is sampled at 13.5 MHz, and the R-Y and B-Y components are sampled at 6.75 MHz. Some argue that 4:2:2 sampling is necessary for high-quality images-especially computer-generated pictures. This is because 4:2:2 sampling offers 2X greater spatial chroma resolution than does 4:1:1 sampling. It is generally understood that Panasonic (and JVC's Digital-S format) employs a pair of DV codecs operating in conjunction with each other.
So Which is Best? The answer to this question will not be easily answered by looking and listening to video playback. In most situations, casual inspection will disclose little difference from DV all the way to DVCPRO-50. However, over time it is possible differences could emerge.
The DVC formats allow any manufacturer to design its own encoder. The only rule is that the encoder's output be able to be decoded by a DVC format decoder. Sony is free to improve its DV and DVCAM encoders while Panasonic can enhance its DV, DVCPRO, and DVCPRO-50 encoders. Thus, it is possible one of the professional formats could emerge as offering "the best" image. Nevertheless, it seems improbable because encoder design involves trade-offs that make it very difficult to create one which is-in all image aspects-better than another.
If the 6mm formats can't be ranked by quality, what differentiates them? Panasonic clearly believes there are major reliability and cost advantages to the use of MP tape. Sony is confident that ME delivers a more robust signal that increases reliability. Sony also believes the degree of compatibility they have created between DV and DVCAM is important in today's market.
Sony and Panasonic have, unfortunately, each created their own standard for the transfer of compressed, digital video (QSDI and CSDI, respectively). Thankfully, both companies also support SDI uncompressed, digital transfer.
I believe users will select amongst these formats based upon five criteria: If acquisition with subsequent transfer to a NLE system is the primary task for the format-and low cost and/or small camcorders are a high priority-then DV is an obvious choice; if low tape costs are mandatory, then DVCPRO has a clear advantage; DVCAM offers the twin advantages of compatibility with DV equipment (without adapters) and equipment with IEEE 1394 ports; while DVCPRO-50 combines the all the size and weight benefits of a 6mm format with the quality of the much bulkier 4:2:2 formats; and lastly, brand loyalty will have a strong influence.
