By PURPLELEC | 09 June 2023 | 0 Comments
What is SDI signal
SDI interface is the abbreviation of Serial Digital Interface, that is, serial digital interface. The 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. Use scrambled non-return-to-zero inversion (NRZI) to replace the early block coding, the standard is SMPTE-259M and EBU-Tech-3267, the standard includes digital composite and digital component signals including digital audio.
Before transmission, the original data stream is scrambled and transformed into NRZI codes to ensure reliable restoration of the original data at the receiving end. In this way, the digital serial interface can be understood as a baseband signal modulation in concept. The SDI interface can pass serial digital component signals of 270Mb/s, and for 16:9 format images, it should be able to transmit 360Mb/s signals.
The NRZI code is a polarity-sensitive code, which uses "1" and "0" to indicate the high and low levels. If there is a long continuous "1" or continuous "0", it will affect the receiving end to extract the clock from the digital signal. Because the serial digital signal interface does not transmit the clock signal separately, the receiving end needs to extract the clock signal from the digital signal stream, so the NRZI code with "1" and "0" to indicate whether there is level conversion is used.
When receiving the NRZI code stream, as long as the level change is detected, the data can be recovered. Even if it is all "1" signals, the resulting signal frequency is only half of the original clock frequency. After scrambling, the chance of continuous "1" is reduced , which further reduces the high-frequency components. At the receiving end of the data stream, the original data stream is restored from the NRZI code stream by the SDI decoder.
The SDI interface cannot directly transmit compressed digital signals. After the compressed signals recorded by digital video recorders, hard disks and other equipment are replayed, they must be decompressed and output through the SDI interface before they can enter the SDI system. Repeated decompression and compression will inevitably cause image quality degradation and delay increase. For this reason, various digital video recorders and non-linear editing systems in different formats have stipulated their own interfaces for direct transmission of compressed digital signals.
{C}(a){C}Sony's serial digital data interface SDDI (SerialDigital Data Interface), used for Betacam-SX non-linear editing or digital news transmission system, through this interface, can be uploaded from tape to disk.
{C}(b){C}Sony's 4x-speed serial digital interface QSDI (QuarterSerial Digital Interface), in the DVCAM video recorder editing system, uploads from tape to disk at 4x speed, downloads from disk to tape or Data is copied from disk to disk.
{C}(c){C}Panasonic's compressed serial digital interface CSDI (CompressionSerial Digital Interface), used in DVCPRO and Digital-S digital video recorders, nonlinear editing systems, from tape base to disk base or between disk bases Data can be transferred at 4x speed.
The above three interfaces are incompatible with each other, but they are all compatible with SDI interface. In the 270Mb/s SDI system, high-speed transmission is possible. These three interfaces are designed for the establishment of digital audio and video networks. This type of network does not use handshake protocols like computer networks, but uses synchronous network technology, and there will be no delay due to different paths.
People often embed digital audio signals in SDI signals, that is, insert digital audio signals into the line and field sync pulses (line, field blanking) of video signals and transmit them simultaneously with digital component video signals.
The 3G in 3G-SDI refers to the data transmission rate of the SDI signal at 3Gbit per second. Since HDTV can support 30 frames per second progressive scanning 1920×1080 resolution format, and 3G can support a frequency twice as high as the highest frame scanning frequency of HD video signals, that is, 3G can support HD signals at 60 frames per second, This makes a huge difference when watching dynamic video.
SDI signals are currently mainly used in the broadcasting industry. It is quite different from other video signals. SMPTE424M defines the physical layer of 3G-SDI and the characteristic performance of such electrical signals. The transmitted signal should have a voltage swing of 800mV, and the rise and fall times must be less than 135ps. Some overshoot is allowed, but it cannot exceed 10%, that is, 80mV.
The definition of clock jitter and adjustment jitter is specified in SMPTERP184. The jitter parameters they require are less than 2UI and 0.2UI respectively. The adjustment jitter parameters actually require 0.3UI, but SMPTE strongly recommends the parameter requirement of 0.2UI, so this article will use 0.2 UI parameter requirements. In the receiver, in the frequency band from 10Hz to 297MHz, the input jitter tolerance is from 2UI to 0.2UI. Both the output of the transmitter and the input of the receiver should be optimized for return loss.
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