Analog sync and digital/analog timing relationships

In analog interlaced video, 0V denotes the start of either field. In digital video, 0V for the second field is unimportant; some people use 0V to denote the start of a frame.

To determine whether a line that is a candidate for a digitized ancillary signal actually contains such a signal, examine every chroma/luma pair for {200h, 40h}: If any pair is unequal to these values, a digitized ancillary signal is present.

In analog video, sync is conveyed by video levels “blacker than black.” Line sync is achieved by associating, with every scan line, a line sync (horizontal) datum denoted 0H (pronounced zero-H) defined at the midpoint of the leading (falling) edge of sync. Field and frame sync is achieved by associating, with every field, a vertical sync datum denoted 0V (pronounced zero-V).

Sync separation recovers the significant timing instants associated with an analog video signal. Genlock reconstructs a sampling clock.

The relationship between the digital and analog domains is established by the position of 0H with respect to some TRS element. Table 38.6 overleaf summarizes several standards for digital representation of component 4:2:2 video; the rightmost column gives the number of luma sample intervals between the first word of EAV and the 0H sample (if it were digitized).

Ancillary data

In 4:2:2 SD, and in HD, ancillary data is permitted immediately after any EAV (HANC), or immediately after SAV (VANC) on a line containing neither active picture nor digitized ancillary data. (In 576i, ancillary data is limited to lines 20 through 22 and 333 through 335.) An ancillary packet is introduced by an ancillary data flag (ADF) comprising the three-word sequence {0, 3FFh, 3FFh}.

SMPTE ST 291, Ancillary Data

Packet and Space Formatting.

An ANC packet must not interfere with active video, or with any TRS, SAV, or EAV. Multiple ANC packets are allowed, provided that they are contiguous. Certain ANC regions are reserved for certain purposes; consult SMPTE ST 291.

An ancillary packet comprises the three-word (4:2:2) or one-word (4fSC) ADF, followed by these elements:

•A one-word data ID (DID)

•A one-word data block number (DBN) or secondary DID (SDID)

•A one-word data count (DC), from 0 to 255

•Zero to 255 user data words (UDW)

•A one-word checksum (CS)

Each header word – DID, DBN/SDID, and DC –

carries an 8-bit value. Bit 8 of each header word is parity, asserted if an odd number of bits 7 through 0 is set, and deasserted if an even number of bits is set. Bit 9 is coded as the complement of bit 8. (Codewords having 8 MSBs all-zero or all-one are thereby avoided; this prevents collision with the 0h and 3FFh codes used to introduce TRS and ANC sequences.)

Two types of ANC packet are differentiated by bit 7 of the DID word. If DID7 is asserted, the packet is Type 1; DID is followed by data block number (DBN). There are 128 DID codes available for Type 1 packets.

Details of the application of SDI in the studio are found in Chapter 7 of Robin, Michael, and Michel Poulin (2000), Digital Television Fundamentals: Design and Installation of Video and Audio Systems,

Second Edition (New York: McGraw-Hill).

Figure 38.4

BNC connector

SMPTE ST 292, Bit-Serial Digital Interface for High-Definition Television Systems.

a low clock rate so as to minimize cost. The choice of a clock rate is a compromise between these demands.

SDI uses scrambled coding, where the data stream is serialized, then passed through a shift register arrangement with exclusive-or taps implementing a characteristic function x9 +x4 +1.

Scrambling techniques using a single scrambler are well known. But the SDI and HD-SDI scrambler has

a second-stage scrambler, whose characteristic function is x+1. The two cascaded stages offer improved performance over a conventional single-stage scrambler. The scrambling technique is self-synchronizing; there is no need for initialization.

The data rate at the interface is the word rate times the number of bits per word. It is standard to serialize 10-bit data; when coding 8-bit video, the two LSBs are forced to zero.

No provision is made to avoid data sequences that would result, after serialization and scrambling, in serial bit sequences with long runs of zeros or long runs of ones. But a long run of zeros or ones provides no signal transitions to enable a receiver to recover the clock! In practice, such pathological sequences are rare.

SDI is standardized for electrical transmission at ECL levels through coaxial cable, using a BNC connector as depicted in Figure 38.4. Distances between 200 m and 400 m are practical. SDI is also standardized for transmission through optical fiber. Fiber-optic interfaces for digital SD are straightforward adaptations of SDI.

HD-SDI coding

The SDI interface at 270 Mb/s was adapted to HD by scaling the bit rate by a factor of 5.5, yielding a bit rate of 1.485 Gb/s (or in 24.976, 29.97, or 59.94 Hz systems, 1.485⁄1.001 Gb/s) – call these both “1.5 Gb/s.” HD-SDI is standardized in SMPTE ST 292. The interface is modeled after SD SDI, but there are two significant changes:

•Chroma and luma are encoded in separate streams, each with its own TRS sequence. The streams are multiplexed, then scrambled.

•Coded line number and a CRC are appended to the EAV portion of the TRS sequence (giving what is called EAV+LN+CRC).