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Why is the SDI signal not displayed when the SD card with BNC interface is used to capture the SDI signal?

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Update time : 2023-05-29 14:53:22
  When using a standard-definition USB capture card with a BNC interface to capture SDI video signals, there is no display or no signal on the display device. Why is this? .
  The BNC interface is a 10Base2 connector, that is, a coaxial thin cable connector. It can isolate the video input signal, reduce the interference between signals, and the signal bandwidth is larger than the ordinary 15-pin D-type interface, which can achieve better signal response effect, so it is often used on video equipment to transmit video signals. Ensure that the video signal is transmitted and used intact, for example, the transmitted video signal is broadcast live through the video capture card software, etc.
sdi capture card
  The BNC interface can transmit two kinds of video signals, namely SDI signal and CVBS signal. Although both can be transmitted by the BNC interface, they are two completely different video signals.
  SDI signal (HD signal)
  SDI interface is also called digital serial interface. A serial interface is an interface that sequentially transmits each bit of a data word and the corresponding data through a single channel. Due to the high data rate of the serial digital signal, it must be processed before transmission.
  CVBS signal (SD signal)
  The Chinese name is "Composite Synchronous Video Broadcasting Signal", which uses waveforms to transmit data. Composite video contains color difference (hue and saturation) and luminance (brightness) information and transmits them on the same signal, synchronized in pedestal pulses.
  Now back to the topic, why is there no signal displayed when the SD capture card with BNC interface captures SDI signals? It is because the video capture card at this time is a standard definition card, which can only be used to capture standard video signals. Therefore, the BNC interface on the card can only be used to receive and transmit SD signals, that is, CVBS signals.
  The capture card corresponds to the video interface on the card. If we use the BNC interface originally used to collect CVBS signals to collect SDI signals, it is wrong, because the two types of signals are different, and the BNC interface for collecting CVBS signals cannot be used. When collecting SDI signals, the BNC interface will not change as the video signal changes. Therefore, the standard-definition acquisition card with BNC interface cannot acquire SDI signals, and the corresponding signal display will not appear on the display device.