ڤيديو رقمي

عودة للموسوعة

ڤيديورقمي

ڤيديورقمي Digital video هوتمثيل للصور المرئية المتحركة في صيغة بيانات رقمية مكوّدة. وهي النقيض للـanalog video، الذي يمثل الصور المرئية المتحركة بإشارات analog.

المصطلحات كاميرا، كاميرا ڤيديو وكامكوردر تُستخدم بدلاً من بعضها البعض في هذا الموضوع.

استعراص الصفات الأساسية

Digital video comprises a series of orthogonal bitmap digital images displayed in rapid succession at a constant rate. In the context of video these images are called frames. We measure the rate at which frames are displayed in frames per second (FPS).

Since every frame is an orthogonal bitmap digital image it comprises a raster of pixels. If it has a width of W pixels and a height of H pixels we say that the frame size is WxH.

Pixels have only one property, their color. The color of a pixel is represented by a fixed number of bits. The more bits the more subtle variations of colors can be reproduced. This is called the color depth (CD) of the video.

An example video can have a duration (T) of 1 hour (3600sec), a frame size of 640x480 (WxH) at a color depth of 24bits and a frame rate of 25fps. This example video has the following properties:

  • pixels per frame = 640 * 480 = 307,200
  • bits per frame = 307,200 * 24 = 7,372,800 = 7.37Mbits
  • bit rate (BR) = 7.37 * 25 = 184.25Mbits/sec
  • video size (VS) = 184Mbits/sec * 3600sec = 662,400Mbits = 82,800Mbytes = 82.8Gbytes

The most important properties are bit rate and video size. The formulas relating those two with all other properties are:

BR = W * H * CD * FPS
VS = BR * T = W * H * CD * FPS * T
(units are: BR in bit/s, W and H in pixels, CD in bits, VS in bits, T in seconds)

while some secondary formulas are:

pixels_per_frame = W * H
pixels_per_second = W * H * FPS
bits_per_frame = W * H * CD


Regarding Interlacing

In interlaced video each frame is composed of two halves of an image. The first half contains only the odd-numbered lines of a full frame. The second half contains only the even-numbered lines. Those halves are referred to individually as fields. Two consecutive fields compose a full frame. If an interlaced video has a frame rate of 15 frames per second the field rate is 30 fields per second. All the properties and formulas discussed here apply equally to interlaced video but one should be careful not to confuse the fields per second rate with the frames per second rate.

خصائص الڤيديوالمضغوط

The above are accurate for uncompressed video. Because of the relatively high bit rate of uncompressed video, video compression is extensively used. In the case of compressed video each frame requires a small percentage of the original bits. Assuming a compression algorithm that shrinks the input data by a factor of CF, the bit rate and video size would equal to:

BR = W * H * CD * FPS / CF
VS = BR * T / CF

Please note that it is not necessary that all frames are equally compressed by a factor of CF. In practice they are not, so CF is the average factor of compression for all the frames taken together.

The above equation for the bit rate can be rewritten by combining the compression factor and the color depth like this:

BR = W * H * ( CD / CF ) * FPS

The value (CD / CF) represents the average bits per pixel (BPP). As an example, if we have a color depth of 12bits/pixel and an algorithm that compresses at 40x, then BPP equals 0.3 (12/40). So in the case of compressed video the formula for bit rate is:

BR = W * H * BPP * FPS

In fact the same formula is valid for uncompressed video because in that case one can assume that the "compression" factor is 1 and that the average bits per pixel equal the color depth.

المزيد حول bit rate وBPP

As is obvious by its definition bit rate is a measure of the rate of information content of the digital video stream. In the case of uncompressed video, bit rate corresponds directly to the quality of the video (remember that bit rate is proportional to every property that affects the video quality). Bit rate is an important property when transmitting video because the transmission link must be capable of supporting that bit rate. Bit rate is also important when dealing with the storage of video because, as shown above, the video size is proportional to the bit rate and the duration. Bit rate of uncompressed video is too high for most practical applications. Video compression is used to greatly reduce the bit rate.

BPP is a measure of the efficiency of compression. A true-color video with no compression at all may have a BPP of 24 bits/pixel. Chroma subsampling can reduce the BPP to 16 or 12 bits/pixel. Applying jpeg compression on every frame can reduce the BPP toثمانية or even 1 bits/pixel. Applying video compression algorithms like MPEG1, MPEG2 or MPEG4 allows for fractional BPP values.

Constant bit rate versus variable bit rate

As noted above BPP represents the average bits per pixel. There are compression algorithms that keep the BPP almost constant throughout the entire duration of the video. In this case we also get video output with a constant bit rate (CBR). This CBR video is suitable for real-time, non-buffered, fixed bandwidth video streaming (e.g. in videoconferencing).

Noting that not all frames can be compressed at the same level because quality is more severely impacted for scenes of high complexity some algorithms try to constantly adjust the BPP. They keep it high while compressing complex scenes and low for less demanding scenes. This way one gets the best quality at the smallest average bit rate (and the smallest file size accordingly). Of course when using this method the bit rate is variable because it tracks the variations of the BPP.

استعراض تقني

Standard film stocks such as 16 mm and 35 mm record at 24 frames per second. For video, there are two frame rate standards: NTSC, which shoot at 30/1.001 (about 29.97) frames per second or 59.94 fields per second, and PAL, 25 frames per second or 50 fields per second.

Digital video cameras come in two different image capture formats: interlaced and deinterlaced / progressive scan.

Interlaced cameras record the image in alternating sets of lines: the odd-numbered lines are scanned, and then the even-numbered lines are scanned, then the odd-numbered lines are scanned again, and so on. One set of odd or even lines is referred to as a "field", and a consecutive pairing of two fields of opposite parity is called a frame. Deinterlaced cameras records each frame as distinct, with all scan lines being captured at the same moment in time. Thus, interlaced video captures samples the scene motion twice as often as progressive video does, for the same number of frames per second. Progressive-scan camcorders generally produce a slightly sharper image. However, motion may not be as smooth as interlaced video which uses 50 or 59.94 fields per second, particularly if they employ the 24 frames per second standard of film.

Digital video can be copied with no degradation in quality. No matter how many generations of a digital source is copied, it will still be as clear as the original first generation of digital footage. However a change in parameters like frame size as well as a change of the digital format can decrease the quality of the video due to new calculations that have to be made. Digital video can be manipulated and edited to follow an order or sequence on an NLE, or non-linear editing workstation, a computer-based device intended to edit video and audio. More and more, videos are edited on readily available, increasingly affordable consumer-grade computer hardware and software. However, such editing systems require ample disk space for video footage. The many video formats and parameters to be set make it quite impossible to come up with a specific number for how many minutes need how much time.

Digital video has a significantly lower cost than 35 mm film. In comparison to the high cost of film stock, the tape stock (or other electronic media used for digital video recording, such as flash memory or hard disk drive) used for recording digital video is very inexpensive. Digital video also allows footage to be viewed on location without the expensive chemical processing required by film. Also physical deliveries of tapes and broadcasts do not apply anymore. Digital television (including higher quality HDTV) started to spread in most developed countries in early 2000s. Digital video is also used in modern mobile phones and video conferencing systems. Digital video is also used for Internet distribution of media, including streaming video and peer-to-peer movie distribution. However even within Europe are lots of TV-Stations not broadcasting in HD, due to restricted budgets for new equipment for processing HD.

Many types of video compression exist for serving digital video over the internet and on optical disks. The file sizes of digital video used for professional editing are generally not practical for these purposes, and the video requires further compression with codecs such as Sorenson, H.264 and more recently Apple ProRes especially for HD. Probably the most widely used formats for delivering video over the internet are MPEG4, Quicktime, Flash and Windows Media, while MPEG2 is used almost exclusively for DVDs, providing an exceptional image in minimal size but resulting in a high level of CPU consumption to decompress.

اعتبارا من 2011, the highest resolution demonstrated for digital video generation is 35 megapixels (8192 x 4320). The highest speed is attained in industrial and scientific high speed cameras that are capable of filming 1024x1024 video at up to 1 million frames per second for brief periods of recording.


Interfaces والكابلات

Many interfaces have been designed specifically to handle the requirements of uncompressed digital video (from roughly 400 Mbit/s toعشرة Gbit/s):

  • High-Definition Multimedia Interface
  • Digital Visual Interface
  • Serial Digital Interface
  • DisplayPort
  • Digital component video
  • Unified Display Interface
  • FireWire
  • USB

The following interface has been designed for carrying MPEG-Transport compressed video:

  • DVB-ASI

Compressed video is also carried using UDP-IP over Ethernet. Two approaches exist for this:

  • Using RTP as a wrapper for video packets
  • 1-7 MPEG Transport Packets are placed directly in the UDP packet

صيغ التخزين

Encoding

All current formats, which are listed below, are PCM based.

  • CCIR 601 used for broadcast stations
  • MPEG-4 good for online distribution of large videos and video recorded to flash memory
  • MPEG-2 used for DVDs, Super-VCDs, and many broadcast television formats
  • MPEG-1 used for video CDs
  • H.261
  • H.263
  • H.264 also known as MPEG-4 Part 10, or as AVC, used for Blu-ray Discs and some broadcast television formats
  • ثيورا Theora used for video on Wikipedia

الشرائط

  • Betacam SX, Betacam IMX, Digital Betacam, or DigiBeta — Commercial video systems by Sony, based on original Betamax technology
  • D-VHS — MPEG-2 format data recorded on a tape similar to S-VHS
  • D1, D2, D3, D5, D9 (also known as Digital-S) — various SMPTE commercial digital video standards
  • Digital8 — DV-format data recorded on Hi8-compatible cassettes; largely a consumer format
  • DV, MiniDV — used in most of today's videotape-based consumer camcorders; designed for high quality and easy editing; can also record high-definition data (HDV) in MPEG-2 format
  • DVCAM, DVCPRO — used in professional broadcast operations; similar to DV but generally considered more robust; though DV-compatible, these formats have better audio handling.
  • DVCPRO50, DVCPROHD support higher bandwidths as compared to Panasonic's DVCPRO.
  • HDCAM was introduced by Sony as a high-definition alternative to DigiBeta.
  • MicroMV — MPEG-2-format data recorded on a very small, matchbook-sized cassette; obsolete
  • ProHD — name used by JVC for its MPEG-2-based professional camcorders

الأقراص

  • Blu-ray Disc
  • DVD
  • VCD

انظر أيضاً

  • Digital audio
  • Digital cinematography
  • List of video topics
  • Video coding
  • Video editing software
  • Video hosting service
  • Webcam


الهامش

  1. ^ In fact the still images correspond to frames only in the case of progressive scan video. In interlaced video they correspond to fields. See section about interlacing for clarification
  2. ^ we use the term video size instead of just size in order to avoid confusion with the frame size
  3. ^ http://animationants.com/explainer-videos-the-perfect-marketing-tool-for-all-businesses/

وصلات خارجية

  • The DV, DVCAM, & DVCPRO Formats -- tech details, FAQ, and links
  • Standard digital TV and video formats.

نطقب:Video storage formats

تاريخ النشر: 2020-06-08 09:53:23
التصنيفات: مقالات فيها عبارات متقادمة منذ 2011, مقالات تستعمل قوالب صيانة غير مؤرخة, جميع المقالات التي فيها عبارات متقادمة, تكنولوجيا الأفلام والفيديو, Video signal, مصطلحات التلفزيون, 1986 introductions, مصطلحات السينما والڤيديو

مقالات أخرى من الموسوعة

سحابة الكلمات المفتاحية، مما يبحث عنه الزوار في كشاف:

آخر الأخبار حول العالم

وفاة الكاتب الصحفى الكبير صلاح منتصر

المصدر: اليوم السابع - مصر التصنيف: غير مصنف
تاريخ الخبر: 2022-05-15 18:21:56
مستوى الصحة: 42% الأهمية: 43%

محمد الننى يتحدث عن تجديد عقده مع أرسنال وسر تألقه نهاية الموسم

المصدر: اليوم السابع - مصر التصنيف: غير مصنف
تاريخ الخبر: 2022-05-15 18:22:01
مستوى الصحة: 43% الأهمية: 36%

وزير الشباب والرياضة يشهد تكريم المنتخبات القومية لكرة اليد

المصدر: وطنى - مصر التصنيف: غير مصنف
تاريخ الخبر: 2022-05-15 18:21:39
مستوى الصحة: 54% الأهمية: 67%

عبد المحسن سلامة ينعي الكاتب الصحفي الكبير صلاح منتصر

المصدر: وطنى - مصر التصنيف: غير مصنف
تاريخ الخبر: 2022-05-15 18:21:38
مستوى الصحة: 57% الأهمية: 51%

«البنك الدولى» يناقش التقدم المحرز نحو تحقيق المساواة بين الجنسين

المصدر: موقع الدستور - مصر التصنيف: سياسة
تاريخ الخبر: 2022-05-15 18:21:23
مستوى الصحة: 49% الأهمية: 61%

رئيس وزراء روسيا الأسبق: بوتين بدأ يدرك أنه يخسر الحرب

المصدر: اليوم السابع - مصر التصنيف: غير مصنف
تاريخ الخبر: 2022-05-15 18:21:58
مستوى الصحة: 40% الأهمية: 43%

لجمع الشمل.. جولات لـ أسامة الأزهرى مع علماء أفارقة فى زنجبار (صور)

المصدر: موقع الدستور - مصر التصنيف: سياسة
تاريخ الخبر: 2022-05-15 18:21:30
مستوى الصحة: 50% الأهمية: 61%

«شعير»: الحكومة تواجه الأزمة العالمية بتحفيزات جديدة

المصدر: موقع الدستور - مصر التصنيف: سياسة
تاريخ الخبر: 2022-05-15 18:21:23
مستوى الصحة: 46% الأهمية: 52%

بطريرك الكنيسة الكلدانية يزور السفير البابوي بمصر

المصدر: وطنى - مصر التصنيف: غير مصنف
تاريخ الخبر: 2022-05-15 18:21:35
مستوى الصحة: 55% الأهمية: 52%

سحر شعبان رئيس حي غرب تعقد لقاء جماهيريا لحل شكاوى المواطنين

المصدر: وطنى - مصر التصنيف: غير مصنف
تاريخ الخبر: 2022-05-15 18:21:41
مستوى الصحة: 45% الأهمية: 55%

أسرار ديوان «إنه الإنسان».. اكتشف بعد وفاة نجيب سرور بربع قرن

المصدر: موقع الدستور - مصر التصنيف: سياسة
تاريخ الخبر: 2022-05-15 18:21:27
مستوى الصحة: 60% الأهمية: 67%

البدء في أعمال تطوير منطقة عين بشارة

المصدر: وطنى - مصر التصنيف: غير مصنف
تاريخ الخبر: 2022-05-15 18:21:37
مستوى الصحة: 58% الأهمية: 60%

«ن».. كتاب خواطر لـ«هند جاد» عن دار «سما» للنشر

المصدر: موقع الدستور - مصر التصنيف: سياسة
تاريخ الخبر: 2022-05-15 18:21:26
مستوى الصحة: 54% الأهمية: 63%

تنمية وتكافل واستثمار.. أرقام مهمة فى مؤتمر رئيس الوزراء (إنفوجراف)

المصدر: اليوم السابع - مصر التصنيف: غير مصنف
تاريخ الخبر: 2022-05-15 18:22:02
مستوى الصحة: 35% الأهمية: 38%

تحميل تطبيق المنصة العربية