Analysis of the differences between SDI and HD-SDI

HD-SDI, or HD SDI, is an extension of the digital studio serial digital interface (SDI) in the field of broadcast television. SDI's proposal originated from the digital transmission needs of studio video.

The essence of SDI was back in 1982. In order to unify the digital television system around the world, the former International Radio Consultative Committee (CCIR) proposed the all-digital studio standard - CCIR 601 (because CCIR was later merged into the ITU International Telecommunications Union The Radiocommunication Committee, abbreviated ITU-R, and therefore this Recommendation is accordingly changed to ITU-R BT. 601), which specifies the encoding parameters for the studio component video signal, ie 13.5 MHz samples for the luminance signal, and for the two The color difference signals are respectively sampled at 6.75 MHz, thereby forming a 4:2:2 sample structure, and defining the synchronous positioning relationship between the sampling points and the analog video signal. However, in order to ensure the effective docking of studio video equipment, ITU-R is based on the Society of Motion Picture and Television Engineers (SMPTE) and the European Broadcasting Union (EBU) based on ITU-R BT. The relevant transmission standards developed by 601 (SMPTE 125M and EBU Tech 3267) further developed the international standard ITU-R BT for standardizing studio digital video transmission. 656.

ITU-R BT. The 656 standard requires the video data stream structure to satisfy the world's two largest scanning systems of “525 lines/59.94 Hz” and “625 lines/50 Hz”, and clearly specifies the digitization of the video signals of these two large scanning systems. The time reference of line and field blanking provides that the video data stream can be in both parallel and serial form, and then the specific format of the 8-bit video data in parallel transmission and serial transmission, respectively, is given. The bit order (pass low first, pass high); also gives the synchronization pulse signal frequency and square wave width, pulse edge jitter and other specific parameters. ITU-R BT. The latest interface standard of the 656 supports 10-bit-precision digital video signals. When used to transmit 8-bit data, it is necessary to add two zeros to the lowest bit.

Rec. The 656 standard specifies that the physical interface of studio digital video equipment uses 11 pairs of 25-pin "D" type jacks for twisted pairs (10 pairs of which are used to transmit 10 bits of parallel data, and 11 pairs of wires to transmit a 27 MHz clock) Another pair of wires is used for system ground, and one wire is used for cable shielding). In the transmission, the bits are transmitted in the order of Cb, Y, Cr, Y, Cb, and Y in units of digital video signals. The bit rate is 27 Mbps. At the beginning and at the end of each television active line, there are respectively a 4-bit active video start flag (SAV) and an end of active video (EAV). The entire digital active line includes 720 luminance words and 720 chroma words (360 each for Cb, Cr). Since the EAV and SAV of the digital component signal already contain synchronization information, it is no longer necessary to transmit the synchronization signal, and the digital audio signal and other auxiliary data can be transmitted during line and field blanking.

The Active Video section consists of 720 Y, 360 Cb, and 360 Cr (1440 samples in total), and the entire video frame structure includes NTCS 258 characters in addition to the 1,440 active video samples. Section or PAL 280-byte horizontal blanking (space between EAV and SAV codes) and vertical blanking (V=1 space).

SDI is actually putting ITU-R BT. The bits of the 10-bit data word specified by 656 and the corresponding data are sequentially transmitted (serial transmission) through a single channel, so the data rate of the serial digital signal formed after parallel-to-serial conversion is high, reaching 270 Mbps (vs. 4:2). : 2 samples of D1 format component video).

Before transmission, the original video data stream needs to be scrambled, that is, the input digital signal is divided by a polynomial X9+X4+1, and then inverted, and the digital stream after scrambling is removed by a polynomial X+1 to form an NRZI code to ensure reliable reception at the receiving end. Restore the original data. This process is qualitatively understood as a baseband signal modulation whose standards are SMPTE 259M and EBU Tech 3267, except for the above mentioned based on ITU-R BT. The 601 standard 4:2:2 structured digital component video also includes digital composite video including digital audio. The highest transmission rate of PAL digital composite video is 177.3 Mbps (D2 format composite video). The highest transmission rate of digital composite video to NTSC is 143.2 Mbps (D2 format composite video). Figure 2 shows SDI's use of a scrambled non-return-to-zero inversion code for transmission. The SDI signal is transmitted on a single coaxial cable with a maximum transmission distance of approximately 300 meters.

HD-SDI HD-SDI has the same basic electrical specifications as SDI, but its transmission bit rate is much higher than SDI. Due to ITU-R BT. The 1120-2 proposal specifies that the luminance sampling frequency of the HD video signal is 74.25 MHz, and the sampling frequency of the two color difference signals is 37.125 MHz, respectively, so the basic code rate of HD-SDI reaches 1485 Mbps. Taking into account the influence of transmission cable distribution parameters in the case of high-frequency transmission, the cable length will be greatly shortened when high-definition video signals are transmitted.

When the HD-SDI signal is transmitted, the effective transmission distance of the signal is still less than one-third of that when the ordinary SDI signal is transmitted. Therefore, some manufacturers promote the so-called upgrading of the traditional security video surveillance system to a high-definition monitoring system without resetting. The cable is not true, unless the original system has a small coverage and the cable length is within the allowable range. This means that when designing a high-definition monitoring system in accordance with the HDcctv specification, the transmission distance of the HD-SDI signal must be taken into consideration, and generally fiber transmission should be considered.

The HD-SDI data transmission format is basically the same as the SDI transmission format. The luminance signal Y and the time-multiplexed color difference signals Cb, Cr are treated as 20-bit words. Each 20-bit word corresponds to a color difference sample and a luminance sample. The multiplexing combination method is: (Cb1Y1), (Cr1Y2), (Cb3Y3), (Cr3, Y4).

In fact, the high bit rate of 1485 Mbps is only for interlaced 1080i video formats (25 frames/30 frames per second, or 50 fields/60 fields per second), and for higher quality progressive scan 1080p video formats (50 frames per second / 60 frames), SMPTE has further developed dual link HD-SDI (Dual Link HD-SDI) standard and 3G-SDI standard. This Dual Link HD-SDI or 3G-SDI standard is not only suitable for digital video in 1080p format, but also suitable for transmitting 3D stereoscopic video.