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How Does LT101 behave, when you connect it to a computer graphics card"?

  1. Is the LT-101 the same as a monitor? We are not exactly like a general purpose monitors. Unlike most monitors we have a single DVI connector for both analog and digital signals. Monitors typically have a VGA (DB15) input for analog signals and a DVI-D/HDMI input for digital signals. Monitors do this to avoid confusing for the lay user.

    Connection scenarios.

    1. Graphics card connected to a monitor via DVI-I cable: The graphics card will output digital video as the analog pins on the monitor’s DVI connector are not connected internally. The graphics card had also attempted to detect an analog receiver but never found one.
    2. Graphics card connected to a monitor via DVI-D cable: Obviously the graphics card will output digital video as the analog signals are not present in the cable. Again the graphics card had also attempted to detect an analog receiver but never found one.
    3. Graphics card connected to a monitor via VGA cable: Of course the graphics card never detected a digital receiver as there is none connected to the DVI port. It will output analog.
    4. Graphics card connected to LT-101 via DVI-I cable. Here the graphics card detects both an analog receiver and a digital receiver. What will it choose? Our tests have shown that it can go either way. And there is nothing we can do about it. It’s entirely up to the graphics card.
    5. Graphics card connected to LT-101 via DVI-D cable. Here the graphics card detects on the digital receiver as the analog signal are not present in the cable . The graphics card has, of course, attempted to detect an analog receiver as well but never found one.
    6. Graphics card connected to LT-101 via VGA cable. Here the graphics card detects an analog receiver as there are no digital signals.
  2. How does the graphics card detect the presence of an analog or digital receiver? The usual technique is to simply check for a load. The graphics card will output video to the signals for which it has detected a load. If it detects an analog load, it outputs analog video. The same applies to a digital load. As we will see this load detection is not done continuously.
  3. So when does the graphics card check for a load present? In all probability this is the crux of the issue. It depends on the graphics card and its driver. Most graphics cards rescan the loads when they are reinitialized (resolution change, switch for dual screen to single screen, etc.) and of course when you power cycle or reboot the host system. Unfortunately, disconnecting or reconnecting the cable, changing monitors, or switching from a monitor to the LT-101 does not cause the graphics card to rescan the loads (at least with all the test we have performed). In fact, there is probably no way of externally forcing the graphics card to perform a rescan and thus change the output from analog to digital or vice versa. Some scenarios (they assume no re-initialization of the graphics card and consequently no rescan of the loads):
    1. The graphics card output is (or was) connected to an analog monitor (analog load) and then the output is connected to the LT-101. There will be no signal at all if you have a DVI-D cable. You will need to have a DVI-I or VGA cable and have the LT-101 set to input analog to get a signal. If you do use a DVI-I cable beware that the next time the graphics card is re-initialized it may decide to switch the output over to digital and as the LT-101 input would be set to analog – no signal.
    2. The graphics card output is (or was) connected to a digital monitor (digital load) and then the output is connected to the LT-101. There will be a signal if you have a DVI-D or DVI-I cable and the LT-101 has been set to digital. If you do use a DVI-I cable beware that the next time the graphics card is re-initialized it may decide to switch the output over to analog and as the LT-101 input would be set to digital – no signal.
  4. What should I be doing?
    1. Do to use DVI-I cables with the LT-101 as the graphics card could go either way.
    2. Use DVI-D cabled for digital video.
    3. Use VGA cable for analog video.
    4. If you switch the output from a monitor to the LT-101, always use the same type of signal unless you reinitialize the graphics card.
    5. A Graphics card’s output will not switch from analog to digital (or vice versa) unless it is reinitialized.
    6. The EDID has no effect on the output type.
    7. The LT-101 (or a monitor) has no control over the output type once the graphics card has been initialized.