Transcribing, in video or audio, is the fact of changing codec (encoding) used to compress or encapsulate audio or video media in a file.

For speaking about transcribing, it is necessary to have the concept of Codage, codec and Média.

Foreword

Per hour of Internet, the diffusion of media, that is audio or video, follow numerical transmission channels whereas by nature the voice and the image (animated or not) are analogical phenomena (Photons, Vibrations).

The difficulty in transporting under good condition of the analogical signals converted into digital information is subjected to physical contraines which are:

For the photograph:

  1. the number of significant points of the photograph on a scale lengths (resolution per square centimeters).
  2. the number of necessary information to code a point (depth of color).
  3. size of the photograph.

For the video one:

  1. the number of significant points of the image on a scale lengths (resolution per square centimeters).

  2. the number of necessary information to code a point (depth of color).
  3. size of the image.
  4. the number of images per seconds.
  5. duration of the recording.

For the audio:

  1. the number of samples a second with which one will numerically cut out a sound wave or an electrical signal.

  2. the resolution given to each sample and physical size that one gives him.
  3. duration of the recording.

Because of compatibility with the existing material, the methods of codings of the various media currently used are not inevitably most powerful; and it is not rare that media undergo several encodings, réencodages and transcribings before arriving at the user.

The need for coding, or for transcribing media comes owing to the fact that the support to use to convey it does not allow in general a catch of load " native" media.

For example: A film DVD of the trade (format STAKE) to a resolution of 720 columns of 580 lines at a frequency of 25 images per seconds (30 for NTSC). what is equivalent, in term of number of information to be managed:

720 X 580 = 345600 point by image. 345600 X 3 Byte for encoder the color and the brightness of the point = 1036800 bytes per image (either 1012,5 KB) For one minute one to thus 60,750 Kilo bytes (or 60Mo) per minutes of information (either 208 Giga bytes per hour)

The commercial DVD being able to contain only 4,7 Giga bytes of information, one are obviously obliged to find a solution which passes by the transcribing of the source.

Transcribing

Transcribing makes it possible to adapt the format of the media to the support on which it is transported, stored or diffused.

Format SVCD for example makes it possible to make contain a film of more than one hour on a support of 650Mo. But it is at the price of a quality of image less large than a DVD because it codec used to arrive to that: one speaks then about codec more destructive than another.

One will thus adapt, that is to say the quality of the media to a quantity of data of information (resolution, smoothness of the details) compatible with the support under consideration with transport, the storage or the diffusion of the media; that is to say its quantity of conveyed information (the number of images per seconds e.g.)

Trancodage and quality.

Rare are the codecs not degrading the image, but some do it more discreetly than others.

With final of a transcribing, the sought-after goal is the restored quality of the media. But it is necessary also that it can be stored on a practical support and little incombrant. The ratio " Compression of the media/Space which can the contenir" is directly bound and will determine the type of transcribing to carry out.

With not neglecting non-plus, possibilities of the final material (on which the media will be read) because the assumption of responsibility different codecs is not ensured according to the type and the generation of the reader of the media. But there too, attention not to be share too minimalist because the possibilities of decoding of the readers of living room, for example, evolve/move unceasingly.

Another attention is to be carried to the choices of the codec used is with respect to the number of calculations to carry out to restore the integrality of media. For example, the posting of a film HD encodé with X264 request a specific resource considerable processor, or integrated circuits which will perhaps not be available on the final machine of the user.

Choice of a Transcribing

The compromises to be carried out at the time of a destructive transcribing must be guided by the finality of the media.

A film will be able to souffir of some defects on the most moving scenes, while a poor sound will cause the disappointment of the spectator.

With opposite, a talk with shémas and formulas drawn on a white table will be able to allow a certain poverty in term of quality of sound; while the image will appear more important.

The transport capacity of information, in term of band-width for the hertzian Diffusion and of flow for the data-processing networks, is decisive if the media must be diffused in Streaming or in Broadcasting.

In theory, one should never take into account the capacities materials of decoding of the current computers like standard. Those evolving/moving so much quickly that what true stays today will not be it inevitably tomorrow. If the media with lost of its integrality because of a too destructive transcribing, it will be impossible to retrogress.

One will thus carry out a very destructive transcribing for the diffusion on Internet (immediate consumption), but one will realize, either a storage such as it is, or a less destructive transcribing me for the original media and his storage (conservation of the source). With this method, one preserves the possibility of réeffectuer later a transcribing; with new codecs with the last style and more powerful.

The transcogage being limited in the facts to decode then with réencoder media, one will refer to the section codec for a selection judicious of the codecs to be used.

Continuation of a transcribing

The audio and video codecs evolving/moving so much quickly, the continuation of the transcribed media can easily be called in question. If the goal is filing, one will make use of codecs tested and largely répendus such as MPEG2, or with strict implementations of standards (H.264 e.g.).

The codecs " exotiques" or owners, although powerful can generate unusable media in ten or twenty years.

Transcoders

A selection of some transcoders.

transcribing and private law

Before any transcribing, please attentively read to read the mention legal on the copyright provided with the media which you plan to transcribe.

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