SCART
SCART is a French-originated standard and associated 21-pin connector for connecting audio-visual equipment. The name SCART comes from Syndicat des Constructeurs d'Appareils Radiorécepteurs et Téléviseurs, "Radio and Television Receiver Manufacturers' Association", the French organisation that created the connector in the mid-1970s. The related European standard EN 50049 was refined and published in 1978 by CENELEC, calling it péritelevision, but it is commonly called by the abbreviation péritel in French.
The signals carried by SCART include both composite and RGB video, stereo audio input/output and digital signalling. SCART is also capable of carrying S-Video signals, using the red pins for chroma. A TV can be woken from standby mode and automatically switch to the appropriate AV channel when the SCART attached device is switched on. SCART was also used for high definition signals such as 720p, 1080i, 1080p with YPbPr connection by some manufacturers, but this usage is scarce due to the advent of HDMI.
In Europe, SCART was the most common method of connecting AV equipment and was a standard connector for such devices; it was far less common elsewhere.
The official standard for SCART is CENELEC document number EN 50049–1. SCART is sometimes referred to as the IEC 933-1 standard.
History
Before SCART was introduced, TVs did not offer a standardised way of inputting signals other than RF antenna connectors, and these differed between countries. Assuming other connectors even existed, devices made by various companies could have different and incompatible standards. For example, a domestic VCR could output a composite video signal through a German-originated DIN-style connector, an American-originated RCA connector, an SO239 connector or a BNC connector.The SCART connector first appeared on TVs in 1977. It became compulsory on new TVs sold in France from January 1980, and since 1987 in eastern Europe, such as Poland. The actual French legal decree was adopted on 7 February 1980 and revoked on 3 July 2015.
The standard was subject to several amendments and at least 2 major revisions, approved by CENELEC on 13 November 1988 and 1 July 1997.
Features
The SCART system was intended to simplify connecting AV equipment. To achieve this it gathered all of the analogue signal connections into a single cable with a unique connector, which normally made incorrect connections nearly impossible.The signals carried by SCART include both composite and RGB video, stereo audio input/output and digital signalling. The standard was extended at the end of the 1980s to support the new S-Video signals. A TV can be awakened from standby mode, and it can automatically switch to appropriate AV channel, when the device attached to it through a SCART connector is turned on.
Daisy chaining
SCART is bi-directional regarding standard composite video and analogue audio. A TV will typically send the antenna audio and video signals to the SCART sockets all the time and watch for returned signals, to display and reproduce them. This allows "transparent" set-top boxes, without any tuner, which just "hook" and pre-process the TV signals. This feature is used for analogue pay TV like Canal Plus and was used for decoding teletext.A VCR will often have two SCART sockets, to connect it to the TV, and for video input from a set-top box or other device. When idle or powered off, VCRs will usually forward the signals from the TV to the set-top decoder and send the processed result back to the TV. When a scrambled show is recorded, the VCR will drive the set-top box from its own tuner and send the unscrambled signals to the TV for viewing or simple recording control. Alternatively, the VCR could use the signals from the TV, in which case it would be inadvisable to change channels on the TV during the recording.
The "down" socket can also be used to connect other devices, such as DVD players or game consoles. As long as all devices have at least one "down" and "up" socket, this allows for connecting a virtually unlimited number of devices to a single SCART socket on the TV. While audio and video signals can travel both "up" to the TV and "down" to devices farther away from the TV, this is not true for RGB signals, which can only travel towards the TV.
"Down" and "up" are conventional. Logically, the TV is the last device of the "up" chain-path and the first device in the "down" chain path. Physically, the TV is under the device which sits on its top, hence the name "set-top box" for the device. Moreover, some sockets' relative position may enforce the belief that the TV is physically the last in the down direction.
Logically, the TV is on top and ends the "up" chain-path, translating the electrical info into an image and sound. From the same logical point of view the info stream, wherever it originates, may need processing such as decrypting or adding captioning/subtitles. In this case the info stream is sent logically "down" to dedicated function devices. From the last processing device the info stream is sent logically "up" to the TV, through all the chain-path. Another case is when the info stream is sent "down" and not expected to be sent back "up", for example when sent to a recorder.
Closing a loop on either the "up" or "down" chain-path may not have useful effects and may create instability.
Direct connections
As audio and video use the same pins on "down" and "up" connectors, it is also possible to connect two devices directly to each other without paying attention to the type of the socket.However, this no longer works when S-Video signals are used. As straight links were re-purposed to carry chrominance information, the S-Video pinouts are different for "down" and "up" SCART connectors. Further, they are often not fully implemented.
Paying attention to the type of socket is essential when handling component RGB//S-video. Damage can be caused to devices incorrectly connected as follows:
- connecting SCART 1 from one device to SCART 1 of another device when both SCARTs are configured for RGB//S-video-up. Pins 7, 11 and 15 are outputs.
- connecting SCART 2 from one device to SCART 2 of another device when both SCARTs are configured for S-video-down. Pin 7 is an output.
- connecting SCART 1 from a device configured RGB/, to SCART 2 of another device configured with S-video-down. Pin 7 is an output.
RGB overlays
SCART enables a device to command the TV to very quickly switch between signals, in order to create overlays in the image. In order to implement captioning or subtitles, a SCART set-top box does not have to process and send back a complete new video signal, which would require full decoding and re-encoding of the color information, a signal-degrading and costly process, especially given the presence of different standards in Europe. The box can instead ask the TV to stop displaying the normal signal and display a signal it generates internally for selected image areas, with pixel-level granularity. This can also be driven by the use of a "transparent" color in a teletext page.Device control
SCART allows a connected device to bring it in and out of standby mode or to switch it to the AV channel. A VCR or other playback device will optimally power on when a cassette is inserted, power on the TV and then start playing immediately if the cassette write protection tab is absent. When turned off, the VCR will ask the TV to power off, which it will do if it had been powered on by the VCR's request and if it remained in video mode. Only some TVs will do this—most only implement automatic switching to and from the SCART input.The same signal can be used by a satellite receiver or set-top box to signal a VCR that it is supposed to start and stop recording. This configuration usually requires that the VCR be farther from the TV than the source, so the signal usually travels "down".
Design
Cables
The cables for connecting equipment together have a male plug at each end. Some of the wires such as ground, data, switching and RGB connect to the identical pin number at each end. Others such as audio and video are swapped so that an output signal at one end of the cable connects to an input signal at the other end. The complete list of wires that are swapped are: pins 1 and 2, pins 3 and 6, pins 17 and 18, pins 19 and 20.The original SCART specification provided for different cable types denoted by a key color, but color-coding is rarely used and cables often use different, non-standard configurations.
Maximum SCART cable length is estimated to be about 10 to 15 metres without amplification.
Due to the relatively high signal voltages used in SCART, "hot plugging" is not recommended. Although there is no risk of personal injury, there is the possibility of damaging electronics within the devices if the connector is inserted improperly. Also, since many TVs are Class II rather than earthed, the large exposed shield on the SCART connector will be held at approximately half mains voltage if it is plugged into a powered TV with the other end unplugged. If the cable is then plugged into an earthed device with a metal case, inadvertent contact with the SCART cable shield while the earthed device is touched with the other hand can cause a painful electric shock. For this reason the device end of the cable should always be plugged in first and the TV end plugged in last.
Quality differences exist in SCART cables. While a proper SCART cable uses miniature coaxial cables for the video signals, cheap SCART cables often use plain wires for all signals, resulting in a loss of image quality and greatly reducing the maximum cable length. A common problem on a cheap SCART cable is that a TV outputs a composite video signal from its internal tuner and this is induced or crosstalked onto an incoming video signal due to inadequate or non-existent screening; the result is ghostly images or shimmering superimposed on the incoming signal. To non-destructively verify if a SCART cable uses coaxial cables, unscrew the strain relief at the SCART connector and fold open the plastic shell.
Using higher-quality cables such as those with ribbon cords that have properly shielded coaxial cables inside might help in reducing a 'ghosting' effect, but it does not always eliminate it due to various factors. A more permanent method is to remove pin 19 from the SCART plug that is put into the TV, preventing a signal from being broadcast by the TV into the cable, so it cannot cross-talk with the incoming signal.