Long past are the days of simplistic, coax and BNC connections between video devices. In today's digital environment, each device must be properly interfaced, or the signal won't just be degraded; it's likely to be nonexistent. In order to lay some groundwork for our further discussions, let's look more closely at several key digital video interfaces.
CCIR 601 sets the digital stage
In 1982, the CCIR published Recommendation CCIR-601-1, now called ITU-R BT.601, a standard for encoding both NTSC and PAL television into an interchangeable digital form. Recommended Practice 601, usually just called Rec. 601, defines the digital encoding of the analog component video signal, which includes horizontal and vertical sync and blanking. Using a 13.5MHz sampling rate and the well-known 4:2:2 luminance/chrominance sampling structure, multiplexing the component video in the sequence CB-Y-CR-Y results in a data rate of 27 megasamples per second. While 8 bits of quantization were originally specified by Rec. 601, this was later extended to 10-bit precision to provide higher video performance.
Keep in mind that Rec. 601 is not a video interface standard, but a sampling standard. The electrical interfaces were defined by SMPTE as Standard 125M (for 525/59.94) and by EBU as EBU Tech 3267 (for 625/50). Both of these interfaces were subsequently codified into what is now known as ITU-R BT.656.
Because of the bandwidth limitations of digital processing at the time, these digital interfaces were parallel. The physical connection requires 11 twisted pairs (10 data signals plus synchronous clock) and 25-pin D-subminiature connectors. The twisted-pair balanced lines limit cable lengths to 160ft without equalization and up to 650ft with equalization. Video data is transmitted in NRZ (non-return-to-zero, where a “high” or “low” bit is simply encoded as a “1” or “0”) form in real-time blocks, each comprising one active television line.
A higher 18MHz clock can carry 960 × 480 16:9 pictures and is specified in SMPTE 267M. A digital version of the composite NTSC signal — with sampling rate 4ƒsc — was later specified in SMPTE 244M and provides a somewhat less-expensive interface, using the same connectors and electrical specifications.
While Rec. 656 does not specify the precise technology for the line drivers and receivers, any such components must be ECL-compatible. This means the balanced line drivers must have an output impedance of 110Ω, and the receivers must terminate the balanced lines in a 110Ω impedance.
While these interfaces may be treated with a certain amount of neglect, the ninth and 18th harmonics of the 13.5MHz sampling frequency fall exactly on the 121.5MHz and 243MHz aeronautical emergency channels. Therefore, the integrity of these connections is critical, especially from an RFI-emission standpoint.
SDI offers economy
It should come as no surprise that the use of parallel interfaces can be constraining in larger installations, where extensive cabling could have as much as a 10:1 impact on cost. For this and other reasons, serial digital video interfaces were developed. This allowed the use of less expensive 75Ω coax and standard BNC connectors.
The first such interface, SDI, is still widely used, and was initially specified in SMPTE 259M as a means to digitally encode 10-bit 4:2:2 component and 4ƒsc composite NTSC and PAL digital video signals. The bit rate for this data stream is 143Mb/s for NTSC, 177Mb/s for PAL, 270Mb/s for 13.5MHz sampled 4:3 component video, and 360Mb/s for 18MHz sampled 16:9 component video.
In order to eliminate the need for a separate clock, channel coding is employed in the form of scrambled NRZI (non-return-to-zero-inverted, where a “high” bit is encoded as a transition from “1” to “0” or vice versa, depending on the previous encoded bit). With such a scheme, a polynomial scrambling function ensures that the data stream looks random, with enough transitions to allow for clock regeneration from the stream itself. SMPTE 259M also supports four channels of AES/EBU digital audio when present in the NTSC data space. Depending on the type of cable used, a transmission distance of up to 980ft is possible, especially when equalizers are used. (See Figure 1.)
The related standard, SMPTE 294M, specifies serial digital encoding for 480p sources. A 4:2:2 video signal requires two SMPTE 259M links. A 4:2:0 signal can be carried on a single SMPTE 259M link at 360Mb/s. An HD-SDI version was standardized in SMPTE 292M and ITU-R BT.656. It supports up to 1.485Gb/s of uncompressed HDTV video.
SDTI compresses and quickens transfers
To handle compressed video, the serial data transport interface (SDTI) was developed and standardized in SMPTE 305. In order to provide backward compatibility with an existing SDI infrastructure, SDTI places the compressed bit stream within the space of normal active video on an SDI link, between the start of active video (SAV) and the end of active video (EAV). This provides 1440 10-bit words of data per video “frame” at 270Mb/s and 1920 8-bit words at 360Mb/s. The actual data payloads are 200Mb/s and 270Mb/s, respectively. Because these rates comfortably exceed those needed for even the most-demanding video quality, faster-than real-time video transfers are possible.
SDTI was designed to carry any valid data payload type registered with SMPTE. This includes DVCAM, DVCPRO, Digital-S, Betacam SX, and MPEG-2 program and transport streams. A 1.5Gb/s version, using HD-SDI, has been standardized in SMTPE 348M. Because of the widespread interest in interfacing MPEG-2 and ATSC transport streams, a separate interface was developed that removed the need for encoding and decoding functions to extract the transport stream. This serial interface is outlined in SMPTE 310M and provides a direct, point-to-point connection between DTV devices.
A word is in order about the terms synchronous and asynchronous. There has been considerable confusion in their application to the various interfaces mentioned above. Both terms describe the relationship between two entities, i.e. source and destination. SDI and SDTI are both synchronous interfaces in that there is a common clock signal (or timing reference) between the source and destination devices to coordinate their transmissions.
Furthermore, the video contained within the SDI interface is synchronous with the SDI carrier. This means there is a fixed relationship between the video timing (based on 27MHz) and the SDI clock (270MHz).
However, video contained within a compressed bit stream within an SDTI transmission can actually be asynchronous with the SDTI carrier itself. This is because the video is compressed and the timing is defined by a program clock reference (PCR) within the bit stream. The actual video timing may be locked or unlocked with respect to the 270MHz SDTI clock.
There is always competition, and interfaces are no exception. The DVB-ASI protocol is equally as popular as the 310M interface. DVB-ASI was originally developed to allow cable headend equipment to transfer DVB/MPEG-2 signals.
MPEG-2 transport packets are carried at a rate of 270Mb/s in 188- or 204-byte packets, simplifying the requirements on interface equipment. The data bytes are 8B/10B coded, which produces one 10-bit word for each 8-bit byte presented. The serial data is recovered by means of a unique 10-bit synchronization word that is prevented from occurring by the 8B/10B encoder. This code also provides error checking.
DVB-ASI was originally described in Annex B of ETSI EN 50083-9 and later updated by IEC 60728-9. Other variants include synchronous parallel and serial interfaces (DVB-SPI and DVB-SSI), as well as the use of optical fiber.
IEEE 1394 provides computer interfaces
Finally, we have a plethora of computer interfaces, one of which is finding wide use in professional applications — IEEE 1394, also known as FireWire (by Apple) and i.Link (by Sony). The bidirectional connection is made over two twisted pairs and an optional power-carrying pair. The cable is terminated in a small four- or six-pin connector.
In its simplest application, 1394 allows peer-to-peer device communication at 100Mb/s, 200Mb/s or 400Mb/s. Up to 63 IEEE 1394 peripherals can be connected in a hubbed network structure. A new IEEE 1394b specification supports 800Mb/s traffic on a nine-pin connector, as well as optical connections supporting up to 100m (330ft) in length and data rates up to 3.2Gb/s.
While SMPTE 396M has defined a standard method to carry DV-based video over 1394, various proprietary protocols have emerged to carry compressed video, audio and control. So, don't assume that because a device says it supports 1394, or another of its branded labels, that full compatibility is ensured.
When it comes to standards, the good news is that there are plenty of them. The bad news is that there are plenty of them. Be sure you understand how these professional interfaces operate and what your equipment needs to be properly connected to the rest of the world.
Aldo Cugnini is a consultant in the digital television industry.
Editor's note
Broadcast Engineering's new “Transition to Digital” columnist, Aldo Cugnini, is a DTV consultant. He recently served as the project manager for Maximum Service Television (MSTV) during the MSTV/NAB Terrestrial Digital Converter Project. He also had a leadership role in the development of the “Grand Alliance” digital HDTV system, which led to the ATSC DTV system. Previously, he held technical and management positions at Philips Electronics' Research and Consumer Electronics Divisions and at interactive-television developer ACTV. He also worked on audio and RF systems at Broadcast Technology Partners and CBS Laboratories. In addition, he was an RF specialist at RCA Broadcast Systems.
In addition, Cugnini served on the board of directors of the Advanced Television Technology Center and has been awarded six patents in DTV and broadcasting, as well as issued an FCC First Class Commercial Radiotelephone Operator's license.
He received his BS and MS degrees from Columbia University and is the author of numerous industry reports, technical papers and publications, including a new section in the upcoming 10th edition of the NAB Engineering Handbook, “Worldwide Standards for Digital Television.”
He is a joint recipient of a 1997 Engineering Emmy and R&D Magazine's 1998 R&D 100 Award, and was a finalist in the 2005 IEEE-USA Congressional Fellowship program.
Further, Cugnini is a member of the Academy of Digital Television Pioneers, the IEEE and Eta Kappa Nu. He is a past member of the American Association for the Advancement of Science and the Audio Engineering Society. Finally, he is a musician by avocation. Write him at aldo.cugnini@penton.com.
Send questions and comments to:aldo.cugnini@penton.com
