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Standards - ISP-BD25

DB-25 Connector

The DB-25 connector connects laser graphics signal sources to laser graphics projectors. The DB-25 pinout is designed so that the signals needed by most users are grouped together. It provides a logical ordering of signals to facilitate easy recollection in the field without a diagram.

DB-25 Connector Pinouts 

• Signal sources such as computer or tape playback devices shall use a DB-25 female.
• Projectors shall use a DB-25 male.

Signal Specification

The DB-25 graphics connector standard sets fourth certain signals. These are:

• Signals required to be present for proper projector operation;

• Signals which are optional; and

• User-defined signals, which are not required but are provided for flexibility.

Required Signals

The required signals are those which would typically be present on tape playback systems and laser graphics computer systems. These required signals are X, Y, R, G, B, Shutter, Ground and Interlock loop.

Optional Signals

These signals are not required by the ILDA Standard Projector but are provided for flexibility and use of the DB-25 connector by other projectors.

User-Defined Signals

These signals are not required by the ILDA Standard Projector but are provided for flexibility when using the DB-25 connector. These are paired as differential analogue signals. Although they can be used for any user application which meets the maximum voltage specifications, it is likely that they will be used for semi-standard purposes. The following recommendations are provided for these applications to ensure maximum compatibility between projector manufacturers.

Colour Channel Usage

The ILDA Standard Projector assumes that the colour device will be a polychromatic acousto-optic deflector (PCAOM) or a device with similar colour-control and fast speed characteristics (example: direct modulation of laser diodes). Today's PCAOMs can control four or more laser wavelengths (colours) simultaneously. This can be used to give a wider colour gamut (range) than if only three wavelengths -- red, green and blue -- are controlled.
Here's how this is done: Most full-colour laser displays at present are ion lasers strong in red and green, but deficient in deep blue. The strongest blue line is really blue-green (cyan) at 488 nm. When there are only three colour signals, it is impossible to get dark blues (476 nm) or vibrant violets (457 nm) alone. This is because the stronger cyan line predominates. Dark blues and violets look washed out. In some situations, this is unavoidable. One example is with the ILDA ISP-ADAT standard, where there are only three colour channels available on the ADAT tape. Depending on the laser wavelengths used, shows played from ADAT may be weak in dark blue and violet (without special hardware "tricks" on the projector).
Fortunately, many computer systems or custom playback systems (e.g. CD or DVD discs) can provide more than three colour channels. For example, a digitizer can draw a shape using a "deep blue" colour; when the show plays back, colour channel 4 is used to display the shape. The Colour Channel Usage chart below insures consistency when systems and projectors use more than three colours. In the example, it would not be good if each projector used colour channel 4 to control a different laser wavelength.

The Colour Channel Usage chart below shows the wavelengths to be used for 3-, 4-, 5- and 6-channel colour systems. Here is how to use it.

• System designers: If you are designing a computer or custom playback system, ensure that your colour output signals control the proper pins. Remember that the projector may have any number of channels (even just one if it does not have a colour laser), so there should be an output option for each choice (e.g., 1-, 3-, 4-, 5- or 6-channel colour).

• Projector manufacturers: Use a label next to the ILDA connector, or other means to clearly mark how many channels the projector can control. Think about "plug and play": a laserist should be able to plug their system into your projector, and instantly know how to set up the system to run your ILDA-compatible projector.
  Some advanced projectors may use circuits that map three colour inputs (red, green and blue) into more than three colour channels. (For example, 50% red and 50% blue might be displayed by a single violet line.) If so, add a switch or other means so this could be turned off or so the user can directly control more than three colour channels.

• Laserists: Ensure that you have selected the proper number of channels for the particular projector/laser combination you are using. If your laser projection has missing colours, or the colours are mapped incorrectly, recheck your settings.

Additional Notes About Signals

The absolute maximum voltage on any pin with respect to ground shall be positive or negative 25 volts. Signal inputs shall be designed to accept this voltage without any damaging results.

Signal outputs shall be current limited so that if they are shorted to ground, damage will not result. It is also recommended that the impedance of these outputs be between 100 and 150 ohms so that the cable impedance can be accommodated without ringing or other transmission line effects.

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