DMX (lighting technology)
DMX (old versions DMX512 , DMX512 / 1990 and current version DMX512-A ) is a digital control protocol that is used in stage and event technology to control lighting technology such as dimmers , "intelligent" spotlights, moving heads and effect devices. The abbreviation DMX stands for D igital M ultiple x .
DMX was first standardized by USITT ("USITT DMX512", "USITT DMX512 / 1990"), followed in 2000 by DIN 56930-2 (standard based on DMX512 / 1990) and in November 2004 by ANSI E1.11 (known as DMX512 -A; with innovations compared to the DMX512 / 1990).
Electrical specifications
DMX is based on RS-485 , a symmetrical transmission method is used , the differential levels are between ± 1.5 V and ± 5 V. Common-mode voltages between −7 V and +12 V must be tolerated. Thanks to the symmetrical transmission, DMX has a high level of interference immunity, since external interference has an even effect on both data lines and the level difference is evaluated at the receiver rather than the level.
Five-pin XLR plugs are required for the connection , but the three- pin version is often used because of the lower price. The free contact pair (pin 4 and 5) is intended for other tasks that are specified in more detail in the DMX512-A. Contrary to the usual practice in audio technology , DMX has a female contact on the transmitter and a male contact on the receiver. In the case of microphones, however, the mixer or the processing unit is the sender due to the phantom power supply. This also protects the output against short circuits when it comes into contact with metal parts. In addition, the signal lines (compared to audio signals) are swapped.
| pole | 5 pin | 3 pole international * | RJ45 T-568A | RJ45 T-568B (common in Germany) |
|---|---|---|---|---|
| 1 | Ground (shield) | Ground (shield) | Ground (shield) pin 7 (white / brown) and pin 8 (brown) | Ground (shield) pin 7 (white / brown) and pin 8 (brown) |
| 2 | Signal inverted (DMX, "Cold") | Signal inverted (DMX, "Cold") | Signal inverted (DMX−, "Cold") Pin 2 (green) | Signal inverted (DMX−, "Cold") Pin 2 (orange) |
| 3 | Signal (DMX +, "Hot") | Signal (DMX +, "Hot") | Signal (DMX +, "Hot") Pin 1 (white / green) | Signal (DMX +, "Hot") Pin 1 (white / orange) |
| 4th | optional Data 2 - | optional data 2 - pin 3 (white / orange) | optional data 2 - pin 3 (green) | |
| 5 | optional Data 2+ | optional data 2 + pin 6 (orange) | optional data 2 + pin 6 (white / green) |
(*) The 3-pole version does not conform to the DMX standard.
For older three-pole devices from the manufacturer Martin Professional, i. H. Built in 2000 and before, the DMX + and DMX− lines have been swapped. The standard assignment is used for newer Martin devices.
Temporal protocol
The data is transmitted via an asynchronous serial interface that is controlled by a Universal Asynchronous Receiver Transmitter (UART). With DMX, the data frame consists of the fixed parameters of 8 data bits, no parity bit and 2 stop bits ( 8N2 ) and a symbol rate of 250 kbaud . One bit is therefore 4 µs long, other data rates are not provided.
The transmission is byte-oriented due to the UART: The idle state of the bus (logical 1) is interrupted by the start bit (logical 0). This is followed by the eight data bits and the two stop bits (logical 1). The bus is then in the idle state again and the next byte can be transferred if necessary.
A DMX package begins with at least 88 µs (22 bit lengths) low level (logical 0) - this section is called "Break". This enables the beginning of the packet to be easily identified, since virtually every commercially available UART reports the break as an invalid data byte with missing stop bits. This is followed by “Mark after Break” with at least 8 µs (2 bit lengths) high level / idle state of the bus (logical 1). During this mark time, slower clocked controllers can adjust to a new DMX package. Then the start byte with the value 0 is transmitted. Then the channel bytes are sent, starting with the value of channel 1 (DMX channel counting starts with 1, not with 0). All 512 channel bytes can, but do not have to be transmitted. However, addressing the channel bytes is not possible - the first channel byte sent is for the first channel, the second channel byte for the second channel etc. Should the transmission be interrupted at any time, it can be resumed by sending a new DMX packet . The break sequence automatically resets all open transmissions.
With DMX-A, the start byte can also have values other than 0 (from 1 to 255). The receivers (dimmers or other control parameters) should ignore all packets that are initiated with a start byte other than 0. This means that the DMX bus z. B. the possibility to start RDM requests (Remote Device Management = feedback) or to load new software into the devices. However, since there are DMX devices on the market that ignore the start byte and pay attention to the channel bytes even if the start byte is not equal to 0, this functionality is not actually used in all applications. Serial data streams can also be transmitted on individual channels (e.g. for scrolling text displays). However, this usually requires that the input controller and the device are directly connected and that the signals are not interrupted by conversion or offsetting (e.g. HTP mixing).
Sources for exact specifications can be found in the web links.
Structure of the bus
The bus is structured in bus topology according to the EIA-485 standard (“ daisy chain ”). Up to 32 receivers can be connected to a transmitter, after which a repeater is required. DMX splitters must be used for branches . Each chain must be terminated at the end with a 110-ohm terminating resistor (also known as a termination resistor or terminator) to prevent signal reflections . The last DMX device in the chain can also take on this task (e.g. switchable terminating resistor or automatic detection as the last link in the chain).
As the signal contains high frequencies (125 kHz square wave, cut-off frequency ~ 2.5 MHz), an appropriately shielded cable with 110 Ω impedance must be used. Preferably comes twisted pair cable into consideration, but it is also special microphone cables on the market whose electrical properties are also suitable for DMX transmission. Nevertheless, it is common to use conventional microphone cables without a specified impedance, often in conjunction with a stage box .
The bus address must be set on each device; for devices that receive multiple channels, this is the start address (a 12-channel dimmer with start address 25 receives channels 25 to 36). However, this is a major source of error, especially when using intelligent light. Incorrect addressing of the devices usually leads to undesirable behavior because they react to control data that are not intended for them or channels are shifted. On the other hand, it is quite possible to deliberately address two devices the same, for example if there are too few channels available, or to simplify the programming of the light show. Identically addressed devices with the same channel assignment behave completely identically.
use
The original use for which DMX was designed was the control of lighting circuits via dimmers. The number of 512 channels and the resolution of 8 bits per channel (corresponds to 2 8 = 256 steps per channel) appeared to be sufficient for this. In the meantime, however, practically all stage and effect lighting devices are controlled via DMX. Examples are dimmers , color changers , stroboscopes , scanners and moving heads . The latter two in particular require multiple channels to control their diverse functions. In addition, the resolution of a channel is too low to allow smooth travel of a mirror or moving head. For this reason, two channels (e.g. Pan Coarse and Pan fine) are usually used for the two movement axes Pan and Tilt. This means that the pan attribute is resolved with 16 bits (corresponds to 2 16 = 65536 levels). This results in the large number of channels for many devices (example for a scanner: 2 channels pan, 2 channels tilt, lamp, brightness (dimmer), shutter, 1st gobo wheel, 2nd gobo wheel, gobo rotation, 1st color wheel, 2nd color wheel, effect wheel , Focus, zoom, device control = 16 channels for one device).
The DMX signals are usually generated by a controller, but there is also lighting control software that can output DMX via special computer interfaces (DMX built-in card or USB DMX devices).
USB DMX
USB-DMX is a control for DMX devices via computer ( USB connection)
The commands of a control software are converted into the DMX standard via a USB DMX interface. This means that in some applications there is no need for a light mixer. In addition, with some interfaces, DMX data can also be read into the PC and processed there, for example to control computer programs via DMX.
USB DMX controls can be divided into two categories:
- Classic lighting consoles that are connected via the USB port to a PC or notebook on which software that extends the function of the console is running. The advantage of these systems: buttons and faders on the desk allow quick and precise access to the functions.
- PC software that forwards the commands directly to a USB interface via the USB port and thus works without a classic desk. The advantage of these systems lies in the almost unlimited range of functions of PC software and the low price, since no extra desk is required. However, additional input devices in the form of a classic lighting desk are often used, as the control of the lighting software via sliders and buttons allows faster and more direct control. In addition, some software manufacturers offer additional functionalities that go beyond pure lighting control. One then speaks of ILPS systems (Integrated Light Planning and Steering Systems) if the light planning is an integral part of the overall system.
W-DMX
W-DMX (Wireless DMX) enables a wireless connection to a DMX device. A transmitting and a receiving station are required for this.
Alternatives
Ethernet is now widely used in the transmission sector. Here are must generally distinguish between the use of physical Ethernet cabling with conventional electric DMX signals and the use of protocols for transmitting DMX information on Ethernet from the data link layer .
When using Ethernet cables with electrical DMX signals, simple connection converters are used to bridge larger distances with cables. The level and protocol of the transmitted signals are identical to normal DMX and are neither electrically nor protocol-technically compatible with Ethernet, only the Ethernet cabling is used.
Another possibility for the transmission of DMX signals is the use of Ethernet from the data link layer , with which the signals can at least be switched and possibly also routed . The advantages lie in the widespread use of Ethernet technology, as well as in the very high transmission speed and flexibility and the possibility of generating this directly on a normal PC with a commercially available network card. This simplifies the use of PC-based control software and reduces the entry costs, but a converter is then required for the transition to native DMX infrastructure. Solutions such as B. Art-Net use this to transmit multiple DMX universes over a single Ethernet line, while z. B. ACN or PSI represent new developments with extended functionality. Since these solutions are based on the Internet Protocol (IP) (mostly using UDP / IP ), their use is not limited to Ethernet, but at least theoretically possible on all media that support IP.
Individual evidence
- ↑ Archive link ( Memento from August 22, 2011 in the Internet Archive )
literature
- Michael Ebner: lighting technology for stage and disco; A handbook for practitioners. 1st edition, Elektor-Verlag, Aachen, 2001, ISBN 3-89576-108-7
- Frank Burghardt: Lighting technology for beginners - professionally control your own light show with DMX. 1st edition, Elektor-Verlag, Aachen, 2009, ISBN 978-3-89576-188-1
Web links
General
- Basic information about DMX
- Basic control signals in lighting technology for event technology
- DMX signal recorded using measurement technology
- DMX splitter and merger (later in the article)
- Further information on the DMX standard