FireWire camera

from Wikipedia, the free encyclopedia

FireWire cameras use the IEEE 1394 bus standard to transmit audio , video and control data. FireWire is Apple's brand name for the IEEE 1394 standard.

FireWire cameras come in the form of photo cameras and video cameras that provide image and audio data. A special form of video camera is used in areas such as industry , medicine , astronomy , microscopy, and science . These special cameras do not provide any audio data.

Different forms of FireWire cameras

construction

The basic structure of FireWire cameras is based on the following six modules:

optics

The structure of FireWire cameras

FireWire cameras are based on CCD or CMOS chips. Their light-sensitive area and the individual pixels are small. In the case of cameras with integrated optics , it can be assumed that the optics are matched to these chips.

Exchangeable lenses are often used, especially in the field of professional and semi-professional photography and the field of special cameras . In these cases, a system specialist has to coordinate optics, chips and applications with one another (see system integration ). In addition to normal lenses , interchangeable optics also include microscopes , endoscopes , telescopes, etc. With the exception of the C-Mount and CS-Mount standards , the connections for interchangeable optics are company-specific.

Signal recording

Since the function of FireWire cameras is based on electrical signals, the “signal recording” module converts the incident light and the incident sound into electrons . In the case of light, this task is performed by a CCD or CMOS chip. The conversion of the sound is the task of a microphone .

digitalization

The first step in digitizing the image results from the construction of the CCD and CMOS chips. They break the image down into pixels . If a pixel has collected many photons , it generates a high voltage ; if there are only a few photons, the voltage is low. The voltage has analog values. Therefore, in the second digitization step, an A / D converter converts it into a digital value. The raw, digital image is then available.

The microphone converts sound into tension. An A / D converter converts these analog values ​​into digital ones.

Signal conditioning

There is a color filter in front of the CCD or CMOS chip to generate the colors . It changes red, green or blue from pixel to pixel. The filter is therefore called the mosaic filter or, after its inventor, the Bayer sensor . From this raw digital image, the “signal processing” module creates an image that meets aesthetic requirements. The same goes for the audio data.

Finally, the module compresses the image and audio data and makes them available to video cameras as a DV data stream. In the case of photo cameras , it makes the individual images and any voice comments available as files.

The application areas of industry , medicine , astronomy , microscopy and science often use monochrome special cameras without a microphone . They do without any signal processing, i.e. they output the raw digital image.

Certain variants of color cameras also only output the raw digital image. They are called ColorRAW or Bayer cameras. They are often used in industry , medicine , astronomy , microscopy and science . As a photo camera, they are used by professional photographers. Semi-professional cameras usually offer a RAW mode as an option.

The processing of the raw digital images takes place outside the camera on a computer and can therefore be adapted by the user to the respective task.

interface

The first three modules are part of every digital camera. The interface is the module that characterizes the FireWire camera. It is based on the IEEE 1394 standard of the "Institute of Electrical and Electronics Engineers" organization. This standard describes a bus

  1. for the transmission of time-critical data such as B. Videos as well
  2. for the transmission of data whose integrity is paramount (e.g. parameters or files).

It allows the simultaneous operation of up to 63 different devices (cameras, scanners , video recorders , hard drives , DVD burners, etc.).

Other standards describe the behavior of these devices. It's called a protocol . FireWire cameras mostly use one of the following protocols:

AV / C
AV / C stands for "Audio Video Control" and defines the behavior of DV devices such as B. Video cameras and video recorders. It is a standard from the 1394 Trade Association. The audio / video working group is responsible.
DCAM
DCAM stands for "1394-based Digital Camera Specification" and defines the behavior of cameras that output uncompressed image data without audio. It is a standard from the 1394 Trade Association. The IIDC (Instrumentation and Industrial Control Working Group) is responsible.
IIDC
IIDC is often used synonymously with DCAM.
SBP-2
SBP-2 stands for "Serial Bus Protocol" and defines the behavior of mass storage devices such as B. Hard drives. It is an ANSI standard. The NCITS ( National Committee for Information Technology Standardization ) is responsible.

Devices that use the same protocol can communicate directly with each other. A typical example is connecting a video camera to a video recorder. In contrast to the USB bus, a higher-level computer is not necessary. If a computer is used, it must be compatible with the protocols of the devices with which it is to communicate (see data exchange with computers ).

control

The control coordinates the interaction of the individual modules with one another. The user can influence the control via

  1. Switch directly on the camera,
  2. the FireWire bus using application software or
  3. Mixed forms of the first two cases

Photo cameras

Professional and semi-professional photo cameras and especially digital camera backs often offer FireWire interfaces for transferring the image files and for controlling the camera.

The SBP-2 protocol is used to transfer image files . In this mode, the camera behaves like an external hard drive and thus enables image files to be exchanged easily with a computer (see data exchange with computers ).

To increase work efficiency in the photo studio , cameras or camera back panels can also be controlled via the FireWire bus. Usually the camera manufacturers do not publish the protocols used in this mode. The camera control therefore requires special software from the camera manufacturer, which is mostly available for Macintosh and Windows computers.

Video cameras

While the FireWire bus is only widespread in higher quality cameras, it can already be found in the field of video cameras in amateur devices. Video cameras are mainly based on the AV / C protocol . It defines both the flow of audio and video data and the control of the camera.

The majority of video cameras only allow audio and video data to be output via the FireWire bus (DVout). Some video cameras also offer the recording of audio and video data (DVout / DVin). Video cameras exchange their data with video recorders and / or computers.

Special cameras

In the fields of industry, medicine, astronomy, microscopy and science, FireWire cameras are often used, which are not used for aesthetic but for analytical purposes. They output uncompressed image data without any audio. These cameras are based either on the standard DCAM (IIDC) protocol or on company-specific protocols.

Due to their area of ​​application, the special cameras differ significantly from photo and video cameras in some points:

  1. The housings are small, mostly made entirely of metal, and do not follow aesthetic, but functional design criteria.
  2. The majority of special cameras do not offer built-in lenses, but a standardized lens connection called C-Mount or CS-Mount . Not only many lenses, but also microscopes, telescopes, endoscopes and other optical devices are based on this standard.
  3. As a result, recording aids such as autofocus or image stabilization are not available.
  4. Special cameras often use monochrome CCD or CMOS chips.
  5. Special cameras often do without infrared cut filters and optical low-pass filters in order to influence the image as little as possible.
  6. Special cameras deliver image data streams and individual images, the recording of which is triggered by an external trigger signal . In this way, these cameras can be easily integrated into industrial processes.
  7. Mass storage devices are not available, as the images must be evaluated promptly in the computer connected to the camera.
  8. The great majority of special cameras are controlled via the application software of a computer. They therefore do not have external switches.
  9. The application software is only available off the shelf in exceptional cases. It usually has to be adapted to the respective task. The manufacturers therefore offer programming tools tailored to their cameras. If a camera uses the standard DCAM (IIDC) protocol , it can also be operated with third-party software. Many industrial computers and embedded systems are compatible with the DCAM (IIDC) protocol (see also data exchange with computers ).

Special cameras per se are therefore more simply constructed compared to photo or video cameras. Using these cameras in isolation is pointless, however. Like other sensors, they are only components of a larger system (see system integration )

Data exchange with computers

FireWire cameras can exchange data with any other FireWire device as long as both devices use the same protocol (see interface ). These data are dependent on the respective camera

  1. Image and audio files (protocol: SBP-2 ),
  2. Image and audio data streams (protocol: AV / C or DCAM (IIDC) ) and
  3. Parameters for controlling the camera (protocol: AV / C or DCAM (IIDC) )
Data exchange between FireWire cameras and computers
Left: company-specific system
Right: open system

If the FireWire camera is to communicate with a computer, it must have a FireWire interface and be able to use the camera's protocol. The early days of FireWire cameras were dominated by company-specific solutions. Some specialists offered interface cards and drivers that the application software could access directly. In this approach, the application software is responsible for the protocol. Since this solution can use the computer resources very efficiently, it can still be found in special industrial projects today. However, this procedure often means that no other FireWire devices such as B. hard drives can be used. Open systems avoid this disadvantage.

Open systems are based on a layer model . The behavior of the individual layers (interface cards, low-level drivers, high-level drivers and API ) follows the specifications of the respective operating system manufacturer . Application software may only access the APIs of the operating system, never any layer below it. In the context of FireWire cameras, the high-level driver is responsible for the protocol. The low-level driver and the interface cards implement the requirements of the IEEE 1394 standard. The advantage of this strategy is the simple implementation of the application software that is independent of the hardware and its manufacturers.

In the areas of photo cameras and special cameras, in particular, there are mixed forms between open and company-specific systems. In these cases, the interface cards and the low-level drivers typically follow the standard, while the layers above are company-specific.

The main characteristic of open systems is that they do not use the hardware manufacturer's APIs, but those of the operating system. For Apple and Microsoft, the subject of image and sound is very important. Their APIs QuickTime and DirectX are well known . In the public perception, however, they are reduced to the reproduction of audio and video. In fact, there are powerful APIs that are also responsible for image acquisition.

Under Linux this API is called video4linux. Since it is less powerful than QuickTime and DirectX, other APIs have been developed parallel to video4linux:

Access to FireWire cameras under Linux
Photo cameras
The Linux infrastructure for mass storage is available for photo cameras. A typical application program is digiKam.
Video cameras
Video cameras are operated via various APIs. The access of the editing program Kino to the API libavc1394 is shown in the picture on the right. Kino accesses other APIs that are not shown in the picture for reasons of clarity.
Special cameras
The most important API for special cameras is libdc1394. The picture on the right shows the access of the Coriander application software to this API. Coriander controls FireWire cameras based on the DCAM (IIDC) protocol and acquires their images.

In order to simplify the use of video4linux and the dedicated APIs, the meta-API unicap was developed. It hides their details with the help of a simple programming model.

System integration

FireWire cameras are often just cogs in a larger system. A system specialist also uses various other components to solve a specific task. There are two basic approaches:

  1. The task is of interest to a group of users. A typical feature of this situation is the availability of off-the-shelf application software. One example is studio photography.
  2. The task is only interesting for one specific application. In these cases, there is typically no off-the-shelf application software available and must therefore be written by a system specialist. One example is the measurement of steel plates.

Many aspects of system integration are independent of the use of a FireWire camera. This applies in particular to the high influence of lighting on the result, be it aesthetically or analytically oriented.

When creating application software, however, there is a typical FireWire feature, namely the availability of standardized protocols such as AV / C , DCAM , IIDC and SBP-2 (see also data exchange with computers ). Using these protocols, the software is written independently of a specific camera from a specific manufacturer.

If you leave the implementation of the protocol to the operating system and use your software exclusively on the APIs of this operating system, the hardware independence is maximum. Uses z. For example, under Linux, application software using the API libdc1394 (see data exchange with computers ) can access all FireWire cameras that use the DCAM (IIDC) protocol. The use of the API unicap also enables access to other image sources such as B. Frame grabber.

See also

Web links

Individual evidence

  1. see en: Mosaic (digital image) in the English language Wikipedia
  2. unicap at sourceforge