Video formats 73 hh:mm:ss:ff frame count 00 - 29 or 00 - 24 second count 00 - 59 minutes count 00 - 59 hours count 00 - 23 Figure 4.9 Time code format. have a standard used throughout the television industry that counts each frame in frames, seconds, minutes, and hours. Drop-frame time code The NTSC standard further complicates the problem, with relation to program durations. The line and frame frequencies are derived from color subcarriers rather than from the arbitrary line and field rate values of PAL. The new line scanning frequency was defined as 2/455 times the subcarrier (3.579545 MHz), or 15,734.26 Hz, and the frame rate was defined as 1/525 times the line frequency, or 29.97 Hz. With this frame rate of 29.97 Hz (rather than exactly 30 Hz), 30 frames last 1.001 seconds rather than precisely 1 second. This means the time code registering the frame count is not the same as the studio clock (probably locked to GPS). If the duration of a clip is calculated from the time codes, over a period of time this error will accumulate. This can be ignored for short clips like commercial spots, but for a two-hour movie the error amounts to a seven-second overrun. This may not sound like much, but with the tight scheduling of television commercials, seven seconds is potential revenue. To get around this, the SMPTE standard specifies an algorithm for dropping certain time code values. This drops the first two frames (00 and 01) of every minute, except every tenth minute. This amounts to dropping 108 frames per hour. This is 3.6 seconds, which effectively corrects the error. Most editing equipment has configuration settings for time code, to select drop-frame (29.97 Hz) or non-drop-frame (30 Hz). Note that this setting is not required for PAL (625/50) because the frame rate is exactly 25 Hz.
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The compressed formats often can be copied between devices at higher than real-time, typically four times. This can be very useful for cutting down the capture time from tape to disk storage systems. DV and MPEG I-frame MPEG-2 at main profile/main level is ideal for program distribution, satellite DTH, and DVB transmissions. But the long group of pictures does not lend itself to editing. The ideal for editing is to be able to cut to the frame. DV does not use interframe Coding, so it meets the requirements for editing. The alternative for MPEG-2 is to adopt a profile that does not use interframe coding, or I-frame only. It does not use the B or P frames (bidirectional and predictive). The drawback of the avoidance of interframe coding is that for a given video quality the data rate is much higher than regular MPEG-2. ENG users have adopted 24 or 25 Mbit/s; 50 Mbit/s is for studio use. Compare this with the maximum rate used by MP@ML of 15 Mbit/s. So DV and MPEG-2, I-frame only, deliver a similar level of video quality for a given bit rate, yet allow freedom to edit at random frames. What format of VTR deck should I purchase? If you are setting up an encoding station from scratch, the question of which format to use always comes up. There is no straight answer to this. A video dubbing facility may well have at least one of each format, but that is their core business. Of course the choice may be restricted suppose you have decided to use a DV-based desktop editing system. It would make sense to opt for a DV format. Alternatively you may have a large archive of Betacam tapes. In that case one of the Sony 1/2-inch decks would be a better choice. The later models can play both analog and digital tapes. It is always useful to have a deck that can play VHS and S-VHS a huge amount of corporate material is available in these old formats. You could standardize on one format and outsource interformat dubbing to a video facilities house, but remember that this will add to turnaround time, which could cause problems with rush jobs. Time code Computers usually count time in x millisecond increments from a certain date. Television uses a system more suited to reading by humans. Since program tapes rarely exceed three hours in length, the time code needs to resolve only up to 1 day or 24 hours. The resolution needs to be to one frame. The SMPTE 72 The Technology of Video and Audio Streaming
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Video formats 71 Table 4.6 Digital Tape Formats Format Tape width Max. playing Composition Compression Sampling time (minutes) Digital composite D-2 3/4 in 22 Metal uncompressed 4 fsc D-3 1/2 in uncompressed 4 fsc Digital component D-1 3/4 in 76 uncompressed 4:2:2 D-5 1/2 in uncompressed 4:2:2 Digital Betacam 1/2 in 124 Metal DCT 2.3:1 4:2:2 Betacam IMX 1/2 in MPEG-2 4:2:2P@ML 4:2:2 Digital-S (D-9) 1/2 in 104 Metal Particle DCT 3.3:1 4:2:2 Digital 8 Metal Evaporated M-JPEG variant or Particle DV/DVC 1/4 in Mini 60 Metal Evaporated DV 5:1 4:1:1/4:2:0 standard 300 DVCPRO 1/4 in 120 Metal Particle DV 5:1 4:1:1/4:2:0 DVCAM 1/4 in 180 Metal Evaporated DV 4:1:1/4:2:0 DVPRO-50 1/4 in 60 mins Metal particle DV 3.3:1 4:2:2
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versions and uses the more reliable metal particle tape. This improves performance in editing applications and lowers the bit error rate. It is also available in a 50 Mbit/s version. DVCAM This is the Sony professional format based on the DV cassette. Like DVCPRO it uses a higher linear tape speed and it uses metal particle tape. There are many other formats; a few of the more common ones are: D-5 Uses a similar transport to the D-3 digital composite recorder, but offers 10-bit 4:2:2 component recording. The video signal is not compressed so it is popular for high-end uses in postproduction. A development of the D-5 formats allows high definition video to be recoded, albeit with mild compression. The HD-D5 has become a popular format for program interchange. D-9 (Digital-S) Uses a metal particle tape in a VHS footprint cassette. D-9 decks potentially can play back VHS, S-VHS, and Digital-S tapes. 70 The Technology of Video and Audio Streaming Table 4.5 Analog Tape Formats Format Tape width Max. playing time Composition Analog composite C-format 1 in 94 mins Ferric oxide U-Matic 3/4 in 60 mins Ferric oxide U-Matic SP 3/4 in 60 mins Metal particle Analog component Betacam 1/2 in 300 mins Ferro-chrome Betacam SP 1/2 in 90 mins Metal particle MII 1/2 in 90 mins Metal Video compression The compression used in Digital Betacam is a closed system, optimized for recording on tape. The DV format is more general-purpose, and can be interchanged between units in the native format this avoids unnecessary decompression and recompression stages that would degrade the signal. The IMX format uses MPEG-2 compression.
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Digital Betacam Digital Betacam is a general-purpose, high-quality digital recorder intended as a replacement for the Betacam SP analog format. It uses modest DCT compression, and can be considered essentially transparent. It is very popular for all forms of teleproduction, from field shooting to studio recording and editing. IMX IMX uses MPEG-2 recording at rates up to 50 Mbit/s. The coding is the studio profile with 4:2:2 sampling and I-frame only compression. Note that the digital Betacam is essentially a closed system with regular video interconnections. IMX can exchange video as the compressed data files, either to other VTRs or to disk recorders. DV DV is a group of different professional and consumer camcorder formats, all based on the DV 1/4-inch format cassette. The format specifies two tape cassette sizes, the standard for VTRs and the miniDV for camcorders. The compact size lends itself to the implementation of very small camcorders, the so-called palmcorders. The DV format samples video at 4:1:1 (525/60) or 4:2:0 (625/50), then uses DCT intraframe coding with a 5:1 reduction in data; this gives a final data rate of 25 Mbit/s for standard definition pictures. It often is referred to as DV25 compression. It has two digital stereo audio channels. The normal tape stock is metal evaporated. When broadcasters expressed interest in these compact and lightweight DV camcorders, Panasonic and Sony developed two new formats based on the DV tape, and achieved a better performance (the DVCPRO and the DVCAM, respectively). Panasonic also offered enhanced professional formats, including 50 Mbit/s recording (DVCPRO-50). This uses 4:2:2 sampling and milder compression (3.3:1). Most manufacturers have adopted the IEEE 1394 interface as the general port on their products to allow users to download the DV compressed files to the video editing system in the native format, as an alternative to regular video interconnects. DV comes in several flavors from different manufacturers: DV The original consumer format, also known as DVC. DVCPRO (SMPTE D-7) A high-quality format from Panasonic, much liked for professional field use. Uses nearly twice the linear tape speed of the consumer Video formats 69
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channel). Broadcasters rapidly adopted this for general production recording as well as ENG use. The SP version offered improved performance. MII Another 1/2-inch format, this time from Matsushita, MII used very similar design principles to Betacam. It replaced the largely unsuccessful M-format. Several major networks around the world adopted MII, which probably had the best performance of any analog format. Digital formats Digital formats split into composite and component. Digital composite D-2 was developed as a replacement for aging robotic cart machines that for 20 years played out television commercials 24 hours a day. Being a cassette format, it was easily handled by machinery rather than a human tape jockey. The digital format meant that tapes could be cloned as repeated plays caused tape wear. It was also used as a general-purpose replacement for the C-format open reel machine. Although a digital format, the recorder usually was used with analog plant. D-3 was a Panasonic product, using 1/2-inch tape. It was used in a similar way to D-2, with analog composite interconnections. Digital composite has largely been replaced by component VTRs, giving the many advantages of component processing. Digital component The first digital component VTR was D-1. This was the pioneering digital component format, much loved for film transfer and high-end commercial production. It does not use compression, and was designed to use the new BT601 sampling format. It uses 19-mm tape with 8-bit words per color channel. Panasonic later developed the D-5 format, using 1/2-inch tape, similar to the D-3 machines. It offered 10-bit recording of 270 Mbit/s video, or 8-bit for the littleused 16:9 360 Mbit/s format. Most component formats now use 1/2- or 1/4-inch tape. The basic choice is between the Betacam derivates (1/2 inch) or the DV format (1/4 inch), although there are several other formats. Sony manufactures two 1/2-inch formats, Digital Betacam and IMX. 68 The Technology of Video and Audio Streaming
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Analog formats One-inch C-format This was the most successful of the open-reel 1-inch formats, and was the workhorse of the edit bay until the production-quality cassette formats were developed. A consequence of its success is that much archive material recorded on this format is still lying in vaults. To use this for streaming content it is probably best to have it dubbed to a digital format and then encode from the digital copy. U-matic A consortium of JVC, Matsushita, and Sony developed the U-matic format in the late 1960s, originally as a domestic format. It soon was adopted for industrial and professional use, and also by television networks for early electronic newsgathering trials. Sony only ceased manufacture in 2000, so there is a lot of material out there in this format, especially corporate and industrial content. The format used 19-mm tape, with color-under recording. It used a 110-mm diameter head drum rotating at 1,800 rpm. Contrast this with the DV recorder, which has a drum diameter of 21.7 mm turning at 9,000 rpm. There were several implementations, including hi-band and SP. VHS Dating from 1976, this has been the most successful tape format to date. It was developed for the domestic market for single-generation recordings. It does not offer acceptable multigeneration performance that is required for professional editing applications. As an analog format it does exhibit visible picture noise, an artifact that impairs the potential quality of streaming video. It is best avoided for capture of new material; you should opt for one of the DV formats. S-VHS This development of VHS offered improved record quality. It is noted for the adoption of the S-video interconnection, where the modulated chroma is kept separate from the luminance channel as opposed to being combined as is the case with composite video (NTSC, PAL). Betacam and Beta SP This format used compressed time division multiplexing of the R-Y and B-Y chroma channels, making it a true component recorder as opposed to the Y-C recorders previously available (U-matic records a single modulated chroma Video formats 67
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66 The Technology of Video and Audio Streaming 1970 1980 1990 2000 Year of introduction Tape width Video format U-matic D-1 3/4 in D-2 19 mm analog composite analog composite digital composite digital component 1/2 in 12.5 mm 1/4 in 6.35 mm VHS S-VHS Betacam analog component Digital Betacam Beta SX digital component Beta SP DVCPRO DVCAM digital component DV Figure 4.8 Evolution of videotape formats.
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from a square by about 10 percent; 525/60 is higher than wide by 11/10, 625/50 wider than high by 59/54. Later versions of Adobe Photoshop introduced the facility to use nonsquare pixels, so art can be exported directly to video applications. Does it matter? Usually the scaling algorithms in the video codecs should convert from one aspect to another, so you should not have to worry about it unless a circle is no longer circular by the time it is displayed on the player. Video formats 65 square pixel 1 x 1 625/50 525/60 1.1 1.09 1 2 3 4 5 6 1 2 3 4 5 6 7 625/50 525/60 Figure 4.7 Pixel aspect ratios. Videotape formats The newcomer to video recording might well ask Why are there so many formats, and which should I use? Videotape formats seem to grow with a geometric progression. They were developed originally as all-encompassing formats, but now are optimized for specific applications. Formats may be developed for the highest quality for studio recording and high-definition work or for the low-cost, low-weight demands of newsgathering and consumer camcorders. There have been about 100 formats developed over the last 50 years, but less than 20 have been successful in the marketplace. The SMPTE has standardized many of the tape formats, using letters for analog formats (type C) and numbers for the digital formats (D-1). Many formats have evolved to give backward compatibility with legacy formats. This allows broadcasters to use the same deck to play back new digital recordings and archive analog recordings, saving cost and space (that several decks would occupy). The enabling development for videotape recording was the flying head. This gave a fast enough writing speed to record the high frequencies of the video waveform, while retaining a low linear tape speed. Initially released in the transverse- scan quadruplex format by Ampex, it soon evolved into the helical scan system that video recorders use today.
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Scanning resolutions There are two picture dimensions for standard definition television: the 525-line system used in North America and the 625-line system used in Europe and Asia. 64 The Technology of Video and Audio Streaming Table 4.3 Digital Picture Resolutions Line/field Active picture area Total including sync Frame rate rates (pixels) interval (pixels) Width Height Width Height 525/60 720 486 864 525 29.96 Hz 625/60 720 576 864 625 25 Hz Table 4.4 Computer Monitor Resolutions Designation Resolution VGA 640 480 Video graphics array SVGA 800 600 Super video graphics array XVGA, XGA 1024 768 Extended video graphics array SXGA 1280 1024 Super XGA UXGA 1600 1024 Ultra XGA Computer displays use a plethora of scanning rates and screen resolutions. The frame rates vary from 60 Hz up to 120 Hz. There are higher resolution displays, 1920 1200 for HDTV, plus specialist workstation displays with resolutions of 3840 2400 for demanding applications. Square and rectangular pixels Most computer paint programs use square pixels. That means a circle 10 pixels high is also 10 pixels wide. Digital television does not use square pixels. The horizontal resolution is chosen to have a sensible mathematical relationship to sampling frequencies, line rate, and frame rate. Simple geometry was not an overriding concern for the original analog systems. The television pixels depart
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