| A video format describes how
one device sends a video pictures to another device: a DVD
player to a television, or a computer to a monitor. It also
describes the sequence and structure of frames that create
the moving video image.
Video Formats are a sum of two parts: the video container
and the codec used inside the container.
Video Container
The video container describes the structure of the file:
where the video data is stored, how data is interleaved, and
which codecs are used by which data.
In most cases there will also be an audio codec.
Common Container Formats
- AVI [.avi Audio Video Interlaced'] -
One of the oldest formats, created by Microsoft. Mostly
contains M-JPEG [digital cameras] or DivX [movies]. Can
contain nearly any format [except Sorenson]. 'Fourcc' refers
to a four-character code [such as "divx" or "mjpg"]
inside the AVI container which specifies the video codec
being used. Manufacturers adapted the open AVI format according
to their own requirements. AVI is difficult to work with
at higher but is still used in semi-professional video editing
cards. Many TV cards and graphic boards with a video input
also use the AVI format to grab video clips at low resolutions
[mostly 320 x 240 pixels].
- Quicktime - mainly used for Apple Sorenson
codec, or Cinepak [free]. Can also hold other codecs such
as mjpeg.
- WMV [.wmv] - Mainly MPEG4; but can contain
nearly any codec, including Microsoft derivatives of MPEG-4
which vary in their openness and licensing requirements.
- ASF [.asf] - Advanced Streaming Format
is a subset of wmv, intended primarily for streaming.
Introduced by Microsoft as an early MPEG4 codec.
Comparison
of Container Formats
Codec
A codec [coder/decoder] is a way of encoding audio or video
into a stream of data [bytes].
Common Codecs
- MPEG [Moving Pictures Expert Group]
- three video formats: MPEG 1, 2, and 4. The most popular
standard used.
- MPEG-1 - 1993, supported by everything
up to 352x240; a reasonably efficient and good format for
use online. Provides acceptable frame rates, image quality
and sound signals for low bandwidth [1 - 1.5 MBit/s]. Adequate
for most home movies and business applications [image videos,
documentation].
- MPEG-2 - An improved version of MPEG-1,
with better compression. 720x480. Used in HDTV, DVD, SVCD
and professional video studios. Supports data rates up to
100 MBit/s. MPEG-2 allows the scaling of resolution and
data rate over a wide range. Needs more memory space than
MPEG-1, hence only suitable for playback in the home environment.
Video quality is much better than MPEG-1 for data rates
approx. 4 MBit/s.
- MPEG-4 - A family of codecs; some opensource;
others Microsoft proprietary. Provides highest video quality
possible at very low data rates [between 10 kBit/s and 1
MBit/s]. Data integrity and loss-free data transmission
is essential for mobile communications. MPEG-4 organizes
image contents into independent objects to address or process
them individually. MPEG-4 is ideal for video transmission
over the Internet and mobile phones.
- MPEG-7 - most recent MPEG standard to
describe multimedia data. Can be used independently of other
MPEG standards.
- MPEG Derivatives - mp3 [music] and VideoCD.
- MJPEG [Motion JPEG] - an intermediate
step between a still image and video format, an MJPEG clip
is a sequence of JPEG images. MJPEG is a compression method
that is applied to every image. Common in video from digital
cameras. Good format for editing videos, but does not compress
well, so not suited for use online. Video editing cards
[such as Fast's AV Master or Miro's DC50 or Matrox Marvel
product series] reduce the data stream of a standard TV
signal from approx. 30 MB/s down to a 6 MB/s MJPEG file.
A compression ratio of 5:1. Since there is no MJPEG standard
for the synchronization of audio and video data during recording;
manufacturers of video editing cards have created their
own implementations.
- DV [Digital Video] - Usually used for
video grabbed via firewire off a video camera. Fixed at
720x480 @ 29.97FPS, or 720x576 @ 25 FPS. Not very highly
compressed.
- WMV [Windows Media Video] - A collection
of Microsoft proprietary video codecs. Since version 7,
uses a special version of MPEG4.
- RM [Real Media] - a closed codec developed
by Real Networks for streaming video and audio.
- DivX - early versions were an ASF codec
inside an AVI container. DivX 4+ is a full MPEG-4 codec.
No resolution limit. Requires more processing power to edit
than mpeg1, but less than mpeg2. Hard to find mac and windows
players.
- Sorenson 3 - Apple's proprietary codec,
commonly used for distributing movie trailers [inside a
quicktime container].
- Quicktime 6 - Apple's implementation
of an MPEG4 codec.
- RP9 - a very efficient streaming proprietary
codec from Real [not MPEG4].
- WMV9 - a proprietary, non-MPEG4 codec
from Microsoft.
- Ogg Theora - open format from Xiph.org.
- Dirac - open format under development
by the BBC.
Comparison
of Video Codecs
Comparison
of Audio Codecs
Containers + Codec
Certain video formats, such as "mpeg-4", describe
both a codec and a containe.
Format Examples
A video movie may be encoded with an mpeg-4 codec inside
an avi container
A movie may be encoded with the Sorenson codec inside an
mpeg-4 container.
To Find Out The Format
To find out the container format of a video file, use either:
- The Linux file program
- Mencoder - [part of mplayer] - this will
identify both the container and video codec of a file.
- Mpginfo - [part of the mpgtx package]
to identify the audio codec of mpeg files .
For other formats, go to :
mplayer > identify> frames 0 filename | grep ID_
H.261 And H.263 Protocol
H.261 - designed for videoconferences and
video telephony via an ISDN network. Enables the image quality
to be adapted to the bandwidth of the transmission line. Entire
images from a sequence can be omitted during playback to improve
image quality. Transmission can occur at a bit rate of 64
kBit/s or 128 kBit/s.
H.263 - a higher precision successor standard
for motion compensation in comparison to H.261. Other image
formats are supported for different application such as gate
monitoring systems and wide screen videoconferences.
NEXT: Video Compression
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