Category Archives: V

Variable bit rate (VBR) compression

While many video compression schemes are ‘constant bit rate’ – designed to produce fixed data rates irrespective of the complexity of the video, VBR offers the possibility of fixing a constant picture quality by varying the bit-rate of, typically, MPEG-2 or MPEG-4 compressed video according to the needs of the pictures. This allows the images that require little data, like still or slow moving video sequences, to use less data and to use more for those that need it where there is more detail or/and movement; so maintaining a constant quality. This reduces the need for storage on DVDs, while delivering better overall quality, or more efficient allocation of total available bit-rate in a multi-channel broadcast multiplex.

See also: Constant bit rate, DVD, Statistical multiplexing

VariCam

Panasonic range of cameras that offer variable frame rates, typically from 1-60 f/s for video and cinematographic projects. So, if working at a nominal 24 f/s, the system offers x 6 speed up (undercranking) to x 2.5 slow down (overcranking). The system works by continuously recording 60 f/s to tape while the images are captured at the appropriate rate. Then the relevant useful frames are flagged. Editing equipment with a VariCam interface can use the flag to record the right frames and so replay them at the right speed (e.g. with Panasonic and Quantel editing systems).

The range covers SD, HD and 4K UHD models. There is also P2, a solid-state camera recording system that offers long-form recording and high-speed transfers to editing equipment. P2 cards offer up to 64 GB storage and can operate in an editing environment as well as on the camera.

See also: DVCPRO P2, VFR

VC-1

VC-1 is a video codec specification (SMPTE 421M-2006) implemented by Microsoft as Windows Media Video (WMV) 9, and specified in Blu-ray Disc, and many others. It is designed to achieve state-of-the-art compressed video quality at bit rates ranging from very low to very high with low computational complexity for it to run well on PC platforms. The codec can handle 1920 x 1080 at 6 to 30 Mb/s for high-definition video and is capable of higher resolutions such as 2K for digital cinema, and of a maximum bit rate of 135 Mb/s. An example of very low bit rate video would be 160 x 120 pixel at 10 kb/s.

VC-1 uses some similar transforms to H.261 (1990, the first practical digital coding standard) but much more like H.264/AVC. It includes some distinctive innovations and optimizations. These include 16-bit transforms to help to minimize decoder complexity and interlace coding using data from both fields to predict motion compensation. Also fading compensation improves compression efficiency for fades to/from black and a modified de-blocking filter helps handling areas of high detail.

Individual opinions differ but broadly speaking VC-1 offers at least similar performance and efficiency to H.264/AVC; some say it looks better. VC-1 offers a number of profiles for coding features, and levels of quality combinations defining maximum bit rates. These have a wide range from 176 x 144/15P which may be used for mobile phones, to 2K (2048 x 1536/24P) for movie production.

ProfileLevelMax Bit RateResolutions and Frame Rate  
SimpleLow96Kb/s176 x 144 @ 15 Hz (QCIF)
Medium384 Kb/s240 x 176 @ 30 Hz
352 x 288 @ 15 Hz (CIF)
MainLow2 Mb/s320 x 240 @24 Hz (QVGA)
Medium10 Mb/s720 x 480 @ 30 Hz (480p)
720 x 576 @ 25 Hz (576p)
High20 Mb/s1920 x 1080 @30 Hz (1080p)
AdvancedL02 Mb/s352 x 288 @ 30 Hz (CIF)
L110 Mb/s720 x 480 @ 30 Hz (NTSC-SD)
720 x 576 @ 25 Hz (PAL-SD)
L220 Mb/s720 x 480 @60 Hz (480p)
1280 x 720 @ 30 Hz (720p)
L345 Mb/s1920 x 1080 @24 Hz (1080p)
1920 x 1080 @30 Hz (1080i)
1280 x 720 @60 Hz (720p)
L4135 Mbps1920 x 1080 @60 Hz (1080p)
2048 x 1536 @ 24 Hz

See also: MPEG-4

Websites:
www.microsoft.com
www.smpte.org

VC-2

Standardized by SMPTE, VC-2 (also known as Dirac Pro) is a video codec technology developed by the BBC. VC-2 is open source and royalty-free for all to use. It is an intra-frame compression scheme aimed at professional production and post production. Compression ratios are in the range 2:1 to 16:1, and typical VC-2 applications are seen to include desktop production over IP networks, reducing disk storage bandwidth in D-Cinema production and moving HD video over legacy infra-structure. A current application provides near lossless compression to enable the use of HD-SDI to carry 1080/50P and 1080/60P, which would otherwise require new 3G SDI infrastructure.

Website: www.bbc.co.uk/rd/publications/whitepaper238

Vector fonts

Fonts that are stored as vector information – sets of lengths and angles to describe each character. This offers the benefits of using relatively little storage and the type can be cleanly displayed at virtually any size. However it does require that the type is RIPped before it can be used – requiring processing for interactive use when sizing and composing type into a graphic. Quantel’s range of graphics and editing equipment uses vector fonts.

See also: TrueType

Versioning

In today’s multimedia world there is much demand for many version of a finished production. This business has ballooned. Historically versioning involved making copies from the edited and graded master to various videotape formats and, via a standards converter, to other video standards (e.g. NTSC to PAL). Now technical variations involve many more formats being supplied, including Web, mobile, HD and SD TV, DVD and cinema, as well as a variety of display systems including LED, LCD, Plasma and digital cinema. Aside from the technical needs, other requirements such as commercial, language and religious influences are among the many factors that can be causes for more yet versions.

Versioning is big business, as the number of versions can run to many tens and involve much more than simply making copies of the master. For example, work may involve re-grading to suit different viewing conditions, re-insertion of text or images to suit different regions or countries, pricing (for commercials) adding or removing shots or scenes for censoring, etc. Generally, for this to be done efficiently and effectively requires nonlinear editing in an uncommitted environment; where original footage and all the post processes that produced the master are available for recall and allow direct access to further adjustments, to re-make the result in a short time.

VFR

Variable Frame Rate shooting used to be only possible with film cameras; all electronic cameras worked at fixed frame rates. Panasonic’s HD Varicam was the first to offer variable speeds, originally with frame rates from 4 to 60 f/s in one-frame increments. Sony’s XDCAM HD offered the same range. There are also specialized digital cameras and solid-state recorders able to capture video at frame rates up to 1000 f/s, or more. Instant replay shows an otherwise unseen world of extreme slow motion.

See also: Digital cinematography, Varicam

Website:
www.nacinc.eu
www.panasonic.com/pbds

Video projection

Video projector technology can now show up to 4K images (4096 x 2160) on large cinema screens. Such technology is a major part of Digital Cinema development. There are two major technologies used for large-scale projection, D-ILA and DLP Cinema.

See D-ILA, DLP cinema

Viewing Distance

Distance of a viewer from a screen or picture. If viewers are too far away they will not be able to see the full detail of the images; if too near they will see the limitations of the detail in the images and possibly not see the picture’s edges. The ideal viewing distance, where you clearly see all the available detail, depends on the size of the video image, SD, HD, etc, and the size of the screen.

There is a rule-of-thumb to work out the ideal viewing distance, where you can see all the detail – provided you have a matching HD or 4K (or maybe 8K) capable screen. This is calculated by taking into account the acuity of the human eye, which is about one second of arc (1/60 of a degree). This means to see all the detail of SD television pictures on a 16 x 9 screen, we need to be no more than 6 screen heights (H) away. 1080-line HD slightly more than doubles the horizontal and vertical detail so we need to half the distance to 3H to see all that HD is offering. With 4K (twice the HD vertical and horizontal lines and pixels) we need to halve the distance again – to 1.5H. And if you are looking at 8K, it’s just 0.75H. If you don’t want to get so close to the screen the other way around is to get much a bigger one that is able to display all the pixels of your biggest chosen format.

Fortunately TV screen manufacturers have risen to the challenge… or have they? If viewing 1920 x 1080 HD on a 40-inch screen a viewer would have to sit no more than 60 inches (1.5m) away to see all the detail. For 4K UHD that should be reduced to 30 inches (0.72m). Then for 8K the distance would be just 15 inches (38cm). Perhaps that feels too close, so the alternative is to have a bigger screen. Assuming viewing at 1.5m is comfortable, then 4K viewing would require an 80-inch screen and for 8K, the rarely observed 160-inch screen!

The same principals apply in other digital viewing environments, such as cinema. Typically to appreciate all the detail of the DCI 4K (4096 x 2160) pictures, you would probably have to be in the front third of the cinema. For 8K (not a DCI standard), keep in the front few rows.

With this in mind, the design and shooting of scenes should allow the audience to see a great deal of detail, and have most, if not all, of their vision filled with the screen’s images. This has implications for scenery, make-up and wardrobe as well as lighting.

Visually lossless

A non-technical term commonly used to describe the quality of video after it has been through a process or processes: often compression / decompression. Strictly speaking, it can only mean it looks the same, but that does not mean it is the same. So, if the input quality was good, then the output should look the same and so fine for an audience looking at a similar screen. However, if the material is required for a video process, such as pulling a key, or color correction, visually lossless may not be good enough, as the process can be more demanding tasks, such as creating a key single.

VITC

Vertical Interval Timecode (pronounced ‘vitsy’). Timecode information in digital form, added into the vertical blanking of a TV signal. This can be read by the video heads from videotape at any time pictures are displayed, even during jogging and freeze but not during spooling. This effectively complements LTC ensuring timecode can be read at any time.

See also: LTC

VSB

Vestigial Sideband modulation – an established modulation technique which is used in the RF (radio frequency) transmission subsystem of the ATSC(1) Digital Television Standard. The 8-VSB system has eight discrete amplitude levels supporting a payload data rate of 19.28 Mb/s in a 6 MHz TV channel. There is also a high data rate mode – 16 VSB – designed for CATV (cable television) and supporting a payload of 38.57 Mb/s.

Things move on; E-VSB, Enhanced-VSB, was approved by ATSC in 2004 as an amendment to the A/53C DTV Standard as an optional transmission mode with additional forward error correction coding layers to help reception under weaker signal conditions. This was responding to the wishes broadcasters for more flexibility in DTV. E-VSB allows broadcasters to trade-off data rate for a lower carrier-to-noise threshold for some services, e.g. “fall back” audio, and targeted at receivers with indoor antennas, non-realtime transmissions of file-based information, and more.

Website: www.atsc.org