The home theater (of English: Home Cinema ) is used for acoustic and visual reproduction of the cinema atmosphere in private homes. The demand for quality grows with the development of the cinema on the one hand ( room sound process ) and with the development of the technical possibilities of entertainment electronics on the other.
As early as the 1930s there were narrow films from the AGFA company (e.g. Ozaphan ) that could be created with specially designed cameras and shown silently at home with projectors. At that time there were already films for sale, i.e. professionally produced films that could be copied in large numbers and bought in specialist shops.
In the 1960s, Kodak introduced the Super8 narrow film format. This was followed by a real boom in home cinema. Several Hollywood studios opened archives to distribute Super8 versions of films. In Germany, for example, films by the Disney group were sold under the name “Disney Home Cinema Films”. The short film boom ended in 1982 after the introduction of the Video Home System (VHS), but the production of short films continues to this day.
In the 1960s, a simple television was already seen as a home theater (or: slipper cinema ); Devices used today, on the other hand, include Blu-ray Disc players, Dolby Digital / DTS sound systems, and high-quality, large-format televisions or video projectors .
Typical structure and components
The term home cinema describes a room that is specially equipped for showing films, usually with high-quality entertainment electronics. A typical home theater includes:
- A video projector (popularly known as “beamer”) with a corresponding projection surface ( “screen” ) or a large-screen television ( LCD , plasma screen or rear projection ). These devices usually offer resolutions in HDTV - or even Ultra HD quality.
- One or more audio-video sources. High- resolution sources, mostly Blu-ray discs or high-definition television signals, are preferred these days . Sources according to the older PAL standard with a lower image resolution, on the other hand, are used less and less, only DVDs and laser discs as well as digital television receivers are still more widespread, while analog sources such as video cassettes or analog television have largely been pushed out of home theaters due to their limited picture quality. Hard disk recorders , home theater PCs ( HTPCs ) or special playback devices for playing back films that are stored on digital storage media (especially hard drives, network storage devices and USB sticks) or that are also transmitted directly over the Internet ( streaming ) serve as additional players . The WMV-HD -DVD and HD DVD formats are still used in isolated cases, but no new films are published in them.
- A multi-channel audio system. 5.1 and 7.1 systems with five to seven full-range or satellite speakers and a subwoofer with LFE (Low Frequency Effects) support are typical , but impressive surround sound results can be achieved with four speakers or more . These are controlled by a suitable AV receiver - an amplifier that can decode and play back the various, mostly digitally fed audio formats, but also convert and switch the fed video sources such as HDMI , composite video , S-video , component video and usually also contains an FM radio receiver.
- Comfortable seating that is aligned with the screen or screen and ideally in the so-called sweet spot of the surround sound speakers.
In some cases, the room is completely rebuilt and modeled on a real cinema. This is partly from a technical point of view (to improve the room acoustics and, for example, to increase the image contrast through a dark and non-light-reflecting device, especially when using a video projector), but partly only to create an atmosphere that is as similar as possible to a commercial cinema. Attention is paid not only to the choice of cinema seats, but also to the appropriate room structure with a separate chamber for the equipment to reduce noise. Some home theaters are also set up with automatic curtains in front of the screen, dimmable lights and even popcorn machines . Occasionally, system automation is also implemented, which automatically runs a wide variety of programmed scenarios (lighting, curtain, motorized screen) at the touch of a button.
Recently, more and more home theaters are being equipped with home theater PCs, so-called HTPCs . These versatile systems can now be easily integrated into the existing home cinema thanks to standardized connections (especially HDMI) and individually adapted to suit your own requirements. With the right configuration, picture and sound quality can be achieved that not only compete with those of high quality equipment at a comparable price, but can also surpass them. These multi-media PCs can either be operated with normal operating systems such as Windows, Linux or similar, or with specially developed software packages such as OpenELEC or Kodi or VDR . Some manufacturers have specialized in the construction of such machines and the number of providers of home theater-optimized PCs is steadily increasing.
In general, as in the past in the hi-fi scene, there are often heated and controversial discussions about the quality and usefulness of individual home theater components.
Surround sound systems
Over time, the cinema sound standard continued to develop. In the beginning it was mainly HiFi stereo video recorders and the less common laser disc that offered the possibility of improved sound. In home cinema, the development went from mono (one sound channel) and stereo to multi- channel sound systems such as Dolby Surround or Dolby Surround ProLogic, in which a stereo signal was converted into a rear effects channel that was inaudible without a decoder , which for example also resulted in sound effects moving from back to front or more realistic ambient noises became possible. In addition, a central signal between them can be generated from the two front channels.
From 1992, the digital systems Dolby Digital and DTS were introduced first in the big cinemas and later also on laser discs , with which for the first time a maximum of six discrete (independently transmitted) channels plus one subwoofer channel (LFE) for the bass effects could be reproduced. With the introduction of DVD , this technology was also widely used in home theaters. While these systems can only be transmitted via digital signal paths, the advanced analog systems Dolby Pro Logic II and Dolby Pro Logic IIx, in which the surround sound information is decoded with up to 7 channels plus subwoofer channel from two analog audio channels , deliver a similarly high quality. Further developments of the digital surround sound formats with even better sound quality are Dolby Digital Plus , Dolby TrueHD , DTS-HD and Dolby Atmos .
For the playback of multi-channel sound in the home theater, it is necessary to purchase a special home theater receiver (AV receiver), which forwards the digital multi-channel signals separately to the various loudspeakers. Usual systems consist of three loudspeaker boxes at the front ("front"; left, center, right) and two at the back on the sides (surround left and right), which are often a little higher (according to the THX standard about 60 to 100 cm above ear height) and after are set up rotated inside (in the case of films, it is advisable to let the speakers radiate diffusely, i.e. not directly at the viewer) as well as the subwoofer. If there is no subwoofer, the LFE channel can be redirected to the front speakers, and the center channel can also be distributed to these. Some systems (6.1) also have a rear center speaker (surround back), and 7.1 systems even have two.
Surround sound simulation systems such as Virtual Dolby Surround , which should also enable surround sound with two speakers, do not achieve the quality or richness of sound of real surround sound systems. Relatively good systems here are Dolby Virtual Speakers and Dolby Headphones .
Due to the boom in flat screen televisions, affordable complete packages consisting of amplifier, loudspeaker and integrated DVD or BluRay player are now being marketed as home theater systems.
For a really large picture, a projector, almost always digital in the home cinema, and sometimes analog in the large cinema, is the first requirement. In the early days of the laser disc in the early 1980s, only a very small number of film enthusiasts could afford the very expensive tube projectors available at the time. With the start of digital projection in the 1990s, a process of price decline began that continues today. Today (as of 2015) high-resolution projectors (1920 × 1080 pixels) are available from around € 500, which in the home cinema with the corresponding high-definition source material (Blu-Ray or similar) almost approach the image quality of current cinema halls. Most home theater projectors use lamps from 130 to 180 watts of power. This means that (depending on personal taste) image widths of 200 to 300 cm are possible. In the “high-end” class, however, there are also much more powerful projectors with up to 700 watts of light output, in order to realize image widths beyond 400 cm.
Finally, the brightest digital projectors are already being used in commercial cinemas with lamp outputs of up to 2 × 2000 watts, which enables image widths of up to 20 meters.
Although the term “canvas” is still common, materials such as plastics or glass fibers are often used today instead of fabrics such as linen . Both roll-up screens (some with a motor) and fixed screens are common, although the former can tend to form waves. Mostly pure white is common, some believe that a light gray shade gives the best results. A typical feature of a screen is also the luminance factor , which describes the brightness as a function of the viewing angle. Too high a luminance factor or an unsuitable material can also lead to the formation of a hot spot, i.e. an image area that is too bright and disturbs the image impression.
Typical image widths in the home cinema area are around two meters, usually well above one, but usually below three, as there is a limit due to the performance of the projectors. The usual format is 16: 9 , which is sometimes used with sliding black masking for other image formats.
In addition to ready-to-buy canvases, do-it-yourself solutions are also very common, some of which are quite creative and allow precise adaptation to personal circumstances and preferences. Common components include plastic foils, which are stretched onto a frame together with opaque black material as a background. Other solutions are based on some tensioning - or MDF plates with emulsion paint , plastic coating or stretched films. A black border a few centimeters wide is common practice to improve the contrast.
The artifacts that are most disturbing in practice result from soiling of parts of the projection surface on canvases. Very slight waves, however, are usually much less noticeable. The price of a screen is also far less important as long as it is not dirty, a reasonably reasonable luminance factor has been selected and the formation of waves is kept within limits (with roll-up screens). The cheapest canvases are single-colored room walls, possibly even smoothed and painted with a special color, they are hardly inferior to the frame canvases. In general, the surface should be more matt and less shiny, as otherwise hot spots can form.
Quality of the image sources
Sources suitable for large screen projection
With the large viewing angles required in the cinema or home theater, the quality of the source material is of crucial importance. Really high quality moving pictures for at home are not possible for a long time, if one disregards the laser disc, also known as image disk, which is not widely used in Europe. The color reproduction, freedom from noise and resolution of the various video formats have always been below professional standards, which means that a really good large-scale projection was not possible.
The modest picture only changed with the introduction of digital television and DVD (from 1996). The best format, however, is (since 1992, more widely used in the United States since around 1999) high-definition television HDTV with up to five times the DVD resolution, which has also been available in Europe via Astra since January 1, 2004 . In addition to the high resolution, HDTV has a better color display and almost no compression artifacts. Both are points of criticism that are indeed uncomfortable with DVD material and large home theaters.
In practice, a distinction must be made between large-format displays (currently mostly up to 50 inches (127 cm), sometimes up to around 65 inches - 165 cm diagonal) and screen projection. With an affordable HDTV home cinema projector for around 1000 to 2000 euros, which is often even cheaper than current large-format flat screens, screen widths of 250 centimeters can easily be achieved. With the usual viewing distances in the range of three to four meters, the viewing angle is now so large that the resolution of the European PAL signal (DVD, digital TV) can no longer be sufficient for a satisfactory picture.
If, however, an experience like in the cinema is to be created, this inevitably leads to a large screen in combination with a projector. Frequently encountered viewing distances are often in the range of around 1.5 times (sometimes down to a factor of 1, maximum 2) image width, common viewing angles in the range of around 30 to 35 degrees (sometimes even more).
VHS video, a small hobby camcorder and analog TV reception with artefacts are only partially or not suitable for such purposes. Very good PAL signals, such as those offered by some DVDs or with 16: 9 broadcasts with a high bit rate from a few TV stations on digital television ( ZDF is known for this ) are sufficient in many scenes, but leave out scenes in the Missing total image sharpness. Here HDTV is now considered indispensable, at least if an impression comparable to a really good 35 millimeter copy is to be created. An improvement is also possible with an HDTV-capable projector and good scaling (for example to 720p for the current models, possibly with an external scaler ). In addition to a DVD player and digital satellite receivers, home theater PCs, HDV camcorders, and HD DVD and Blu-ray devices are also suitable.
If the term “home cinema” also means installations with significantly smaller viewing angles (around 10 °), as is the case with the various types of flat screens with picture widths of usually less than one meter, in this case playback via DVD or digital TV with good PAL is sufficient -Signal mostly still goes off without any problems.
35-millimeter movie compared to home theater
In theory, depending on the program content, 35-millimeter cinema film can still be superior to an HDTV projection in a well-coordinated home cinema, in practice, however, it happens again and again that the film copies shown in cinemas look worse than HD videos from Blu -ray disc.
In movie theaters, 35-millimeter film is used, a film format that was developed in the 1890s and was also used as photo film in the late 1920s, but there, horizontally, with twice the image size.
An original version (mostly without sound, but already with all digital effects) is cut from the original film rolls from the recording cameras, usually from several hundred individual "takes". This serves as a template for the so-called “ answer print ”, a light-corrected film copy of the entire work with dialog sound including music and sound effects. This first copy of the film is used for test demonstrations with producers, directors and, if necessary, a test audience, and in Germany forms the basis for ancillary copyright under Section 94 of the Copyright Act. Another light-corrected “correction copy” is made of this, which is used as the basis for further positives. These are then projected onto the screen in the cinemas using analog technology, although the quality can vary considerably depending on the care taken by the copier and the care taken with the copy.
In the past (until the 1980s), this procedure was purely analogous, so only scissors and film putty or adhesive tape were used. Today all film rolls are digitally scanned and stored in a computer system for editing in most films. Only here are the following steps, such as color and brightness corrections or digital effects, added. The currently (2006) most efficient process is called " digital intermediate " and is used in almost all large productions. The best image quality is now available in digital form on a disk cluster with sometimes well over 100 terabytes as a digital master. Unfortunately, the transfer in the 35-millimeter form for the cinema copies via laser imagesetter is only possible for the master tape for reasons of cost. Only this master tape and a few other “premiere copies” also convey almost the full image quality. From now on, the image quality steadily decreases with further analogue copying, for example the common Hollywood productions often come to the cinemas on 35-millimeter film copies, which are already the fourth or fifth generation.
Since the method of distribution usually loses a lot of image sharpness and color brilliance, in today's practice the 35-millimeter format in the cinema can actually never deliver its maximum possible quality. Due to the small film area, there are often insufficient reserves for the necessary duplication in the high-speed copier lines. This is the main reason for the often significantly better overall impression with digital HDTV projection, whether in the cinema with digital projection or at home. Only the premier copies that are not intended for the mass market create a similar or better impression.
This is also the reason why a 16-mm blow-up , i.e. copying from 16-millimeter film (or even DV ) to 35-millimeter cinema copies, does not necessarily have to be negative.
The fact that the image quality is significantly better with IMAX versions is due on the one hand to the larger film format (a special 70 mm format) and the quality requirements.
Since most films are processed digitally and some are already digitally recorded, data is already available. The transfer to 35-millimeter film is only done in order to be compatible with the traditional technology available in the cinemas.
It makes it much easier for film distributors to distribute films digitally, the image impression can be better in practice, but in larger cinemas it can also happen, depending on the image resolution of the source material and projector, that the digital projection is noticeable due to artifacts (pixel stairs).
At the moment, digital distribution is taking place on hard drives, which take up much less space than rolls of film. Transmission via data line or satellite is also conceivable later.
For home cinema use, the films are supplied on Blu-ray discs, which have a much smaller capacity than the hard drives intended for cinema use. This means that only considerably lower data rates are possible, but they are still considerably better than with DVD.
Enjoying a movie in a home cinema can in principle be more attractive than going to a movie theater, where a number of factors can spoil the enjoyment of a movie that the viewer has no influence on. If one disregards the factors that have to do with the presence of other viewers and that would in principle also exist during visits to the theater, there is in principle too much residual light due to poorly placed emergency exit signs and factors that have to do with the nature of the playback system and / or the Program source, such as projection that is too dark, poor image sharpness, poor sound and, depending on the method used, a shaky picture (analog) or system crashes (digital). In addition, there are problems with digital film projection that have to do with digital rights management and can lead to the system prematurely terminating a screening.
The following properties arise with regard to image reproduction in the cinema, which partly depend on the recording on film material, partly on the characteristics of the film copy in the cinema and the cinema projector:
- a resolution of " 2K ", ie around 2 million pixels;
- an “isotropic” visual appearance, ie without a preferred direction, due to the disordered structure of the film grains;
- a frequency of 24 frames per second due to the shooting of the film ;
- a playback frequency in the cinema projector of 48 images (sometimes 72) per second, since each image is displayed twice with a short interruption;
- a film-specific color space , which also depends on the recording material used and the coloring in post-production;
- a specific appearance of surfaces due to the grain of the individual images, which are shown one after the other in chronological order;
- a specific behavior with overexposure (highlights) and underexposure;
- an in-picture contrast of approx. 1000: 1 (i.e. around ten f-stops), d. H. the brightest displayable areas of a certain image currently being displayed can let about 1000 times as much light fall on the screen as the darkest displayable areas;
- temporal and spatial convergence of the image information, d. H. On a single image of the film, all color information for this image can be found in places that were also present in the recorded motif and that are also projected onto the screen at the same time.
For a cinema-like effect at home, it is obvious to strive for a reproduction that comes as close as possible to the above specifications. However, many home cinema users also see improved possibilities in the use of video projectors and in some cases prefer the image effect that can be achieved with video projectors.
None of the video projector technologies currently in widespread use permit a completely film-like reproduction. In detail:
- A resolution in the range of 2K is practically achievable with many video projectors available today, since 1080p is only slightly below 2K.
- Isotropic playback is no longer possible, as all technologies (except for CRT monitors) work with fixed, square pixels . There are visible gaps between these during projection, which lead to a fly screen effect . This is more or less pronounced depending on the technology used. With LCD, there are stronger structures, with digital light processing these are less pronounced and weakest with LCoS . In order to reduce the grid structure, so-called "smooth screens" (a type of optical filter in the projector beam path) are used.
- The recording frequency of 24 frames per second in combination with the usual exposure time of cinema cameras of approx. 1/48 of a second leads to a certain motion pattern typical of cinema films. Since the Blu-ray Disc standard allows digitization at this frequency ("1080P / 24), and a corresponding transmission from the player to the video projector via HDMI cable is also possible, the requirement of a 24p feed should be regarded as feasible. even if many Blu-ray discs are not encoded accordingly.
- It looks different on the playback side. Many video projectors work with a playback frequency of 60 or 120 Hz. In relation to 24p film sequences, this means that each film frame would have to be repeated up to five times (instead of two to three times as in the cinema) to compensate for the differences. Various methods are used to bring about an alignment, in addition to the most cinema-like, repetition, so-called pulldown and frame insertion methods. These are briefly described in the article on 24p . The situation becomes even more complicated if the film material is not sent to the projector as a 24p image sequence, but as a so-called field sequence (see interlacing ). Here the projector tries to recognize whether the film material was originally interlaced (i.e. was recorded by a classic video camera) or was originally full screen (progressive) and was split up into fields by the player before it was transferred to the video projector. In both cases, the video projector reassembles fields into full images before displaying them, but in the case of a full image broken down into fields, the lines of the (hopefully correctly assigned) fields are simply " deinterlaced "; in the case of original fields, however, intermediate images may be used to compensate arithmetically the time interval in which two fields were recorded. How well this works depends, as in the case of frame insertion, on the manufacturer-specific procedures used.
- For physical reasons, the color space of film material deviates from the color space that electronic devices can generate. In addition, in the home cinema sector, for reasons of historical compatibility, color spaces are used that originate from the field of classic television technology and that are adapted to the possibilities of tube televisions for color reproduction. The ITU-R BT 601 color space for SDTV material and the BT.709 color space for HDTV material should be mentioned here in particular . Today's home cinema projectors could in many cases display a larger color space; however, it fails to meet appropriate standards. Only the xvYCC color space is supported by some projectors (and many flat screen televisions); however, television programs are not broadcast with this color space, nor is it provided for in the Blu-Ray Disc standard. Only a few amateur video cameras with AVCHD offer the option of recording with this color space. The digital cinema initiative “ Digital Cinema Initiatives ” has therefore defined the entire CIE color space as the color space , even if today's cinema projectors cannot display it. The color space also includes the subtlety of the gradation in the representation of colors and gray levels. With the usual 8 bit this is a maximum of 256 color levels (in fact only the values from 16 to 235 are used for BT 601 and 709) for each basic color and for black and white films basically only 256 gray levels, which can lead to visible gradations in the image. Some projectors graduate the image more finely (also referred to as "deep color"), which is achieved by interpolating the image information. This can be imagined that with z. B. 10 bits the gradation values are no longer 16, 17, 18 ... 235, but 16, 16.25, 16.5, 16.75, 17, 17.25 ... 234.75, 235.
- The impression of the image due to the rapid projection of grainy images behind one another cannot be simulated in the home theater area either. Video projectors have their own noise behavior, which, however, differs from that of film grain. For example, DLP projectors show color noise in darker, but sometimes also in brighter image areas due to problems with the control of individual mirrors. LCoS projectors can blur the image in places due to the sluggishness of the LCD switchover, but this leads to smoothing rather than noise, etc.
- Even the in-image contrast of film material cannot yet be achieved by today's video projectors. A measurement method that measures this in-image contrast in projectors has been defined by ANSI . Here a still image with a checkerboard pattern of 16 maximum white and maximum black fields is displayed (projected) and the brightness of all fields is determined. The so-called ANSI contrast then results from the ratio of the average brightness of the white fields to the average brightness of the black fields. For DLP projectors, a manufacturer gives values of approx. 350: 1 to 800: 1, for LCoS projectors the values are around 300: 1 <, depending on the measurement. Projector manufacturers try to alleviate these limitations in contrast by providing projectors with a so-called dynamic iris, which can change the overall image brightness by automatically fading in and out. However, this does not improve the contrast within an image and, in the event of selective changes in the scene brightness (e.g. when panning or switching a lamp on or off), the image also becomes lighter or darker in the parts due to readjustment that have actually not changed at all, which can lead to an unnatural impression. (Note: the values given by the manufacturers do not generally refer to the ANSI contrast, but to the on / off contrast (on / off, native, static, etc.), in which a white image and a black image one after the other The image can be projected and measured, or the dynamic contrast in which the white image is also projected when the iris is fully open and the black when the iris is dimmed to the maximum).
- The temporal and spatial convergence of the image information in a single image cannot be simulated 1: 1 with current video devices either. Apart from the basic problems already mentioned above of the transfer of temporally recorded half images with many sources, the video projectors can only approximate this convergence for technical reasons. With LCoS and 3LCD projectors, three separate images for the three primary colors are generated by means of three micro-LCD panels, each of which is irradiated with light of different wavelengths (red, green and blue), which are then converted into one with the help of mirrors and prisms common beam path are merged. There may be deviations here due to manufacturing tolerances, so that the three images on the canvas do not exactly coincide (lack of vergence ). There is therefore a lack of spatial convergence (some projectors allow digital compensation in which the image of each color can be shifted pixel by pixel. However, vergence errors of less than half a pixel cannot be corrected due to the principle, and tilts with respect to the optical axis are not at all) . In DLP projectors with a single micromirror field (so-called one-chip DLPs), the three images for the primary colors are generated one after the other. Due to the use of a single mirror, these are always exactly on top of each other, but the color impression is only created by the inertia of the observer's eye, which perceives the three monochrome color images projected one behind the other as one. So there is a lack of temporal convergence. With rapid eye movements, the individual color images, which then hit the retina at different points, are perceived by many observers as flashes of color, which is known as the rainbow effect and can falsify the impression, especially with black and white films .
-  (PDF; 123 kB) Requirements for a digital film camera