Systems : NTSC, PAL, SECAM, MPEG, and DOCSIS

There are several systems that are used for video distribution. The system (standards) used in CATV systems include: NTSC, PAL, SECAM, MPEG, and DOCSIS.

National Television Standards Committee (NTSC)
The NTSC system is an analog video system that was developed in the United States and is used in many parts of the world. The NTSC system uses analog modulation where a sync burst precedes the video information. The NTSC system uses 525 lines of resolution (42 are blanking lines) and has a pixel resolution of approximately 148k to 150k pixels.

The NTSC system uses 6 MHz wide radio channels that range from 54 MHz to 88 MHz (for VHF channels 1-6), 174 MHz to 216 MHz (for VHF channels 7-13) and 470 MHz to 806 MHz (for UHF channels 14-69). Initially, the frequency range of 806 MHz to 890 MHz was available for UHF channels 70 to 83. The FCC reallocated these channels for cellular and specialized mobile radio (SMR) use in 1983.

When used in the United States, NTSC systems have a maximum transmitter power level that varies from 100 kWatts for low VHF channels (1-6), 316 kWatts for high VHF channels (7-13) to 5 million Watts for UHF channels (14-69). Television transmission limits are also established based on the class of service (local or wide area) for the authorized television broadcast company.

Phase Alternating Line (PAL)
The PAL system was developed in the 1980’s to provide a common television standard in Europe. PAL is now used in the Middle East and parts of Asia and Africa. The PAL system uses a phase alternation process to enhance the video signal’s resistance to chromatic distortions as compared with the NTSC video signal. Although PAL and NTSC systems are similar in function, they are not compatible. A converter box is required between the two systems.

The system provides 625 lines per frame, 50 frames per second. A modified version of PAL (PAL-M) is used for the Brazilian television system. PAL-M provides 525 lines per frame and 60 frames per second. The PAL system uses 7 or 8 MHz wide radio channels.

Sequential Couleur Avec Memoire (SECAM)
Sequential Couleur Avec Memoire (SECAM) is a video transmission system that was developed by France and the former Union of Soviet Socialist Republics to improve on the NTSC video transmission system. This translates to “sequential color with memory.” SECAM is a color video transmission system that provides 625 lines per frame and 50 frames per second. This system transfers color difference information sequentially on alternate lines as a FM signal. The SECAM system requires 8 MHz of bandwidth.

Motion Picture Experts Group (MPEG) Compression
There are several digital video compression systems. The most common form of digital video compression of video signals conforms to the motion picture experts group (MPEG). There are various levels of MPEG compression; MPEG-1 and MPEG-2. MPEG-1 compresses by approximately 52 to 1. MPEG-2 compresses up to 200 to 1. MPEG-2 ordinarily provides digital video quality that is similar to VHS tapes with a data rate of approximately 3.2 Mbps. MPEG-2 compression can be used for HDTV channels, however this requires higher data rates.

Data Over Cable Service Interface Specifications (DOCSIS)
The data over cable service interface specifications (DOCSIS) is a standard used by cable systems for providing Internet data services to users. The DOCSIS standard was primarily developed by equipment manufacturers and CATV operators. It details most aspects of data over cable networks including physical layer (modulation types and data rates), medium access control (MAC), services, and security. The DOCSIS cable modem specifications are available from CableLabs® at http://www.cablemodem.com/specifications.html.

The downstream information flows to all users that are tuned to a specific RF channel on the cable system. There may be several RF channels used to serve many cable modem users in a system. Each individual cable modem decodes their portion of the data on a specific RF channel. For transmitting on the upstream side, each user is assigned time of a few milliseconds each where the user can transmit short bursts of data. Dividing the channel into small slices of data is well suited for short delays to keyboard commands.

To convert the Internet data into a format suitable for delivery on a cable channel, a CATV upconverter is used at the head-end of the cable system. The CATV upconverter handles both digital and analog television signals. Usually 10-20 upconverters are installed into a single equipment chassis. To allow cable modems to connect to data networks (such as the Internet), a cable modem termination system (CMTS) is used. The CMTS an interface device (gateway) that is located at the head-end of a cable television system to send and adapt data between cable modems and other networks.

A single 6 MHz wide television channel is capable of 30-40 Mbps data transmission capacity. This is because coaxial cable offers a communication medium that is relatively noise free (compared to radio or unshielded twist pair cable) that allows the use of complex modulation technologies (combination of amplitude and phase modulation). These modulation technologies can transfer several bits of data for each Hertz of bandwidth (bits per Hertz). In 2001, cable modems could transmit data using 64 QAM modulation technology. To increase the data rate, even more complex modulation technologies such as 256 QAM or even to 1024 QAM have been demonstrated [13].

The DOCSIS system is focused around packet service such as Internet Protocol (IP) and asynchronous transfer mode (ATM) to provide a variety of services (e.g., variable bit-rate, constant bit-rate) with the ability to offer varied levels of quality of service (QoS). This allows the DOCSIS system to offer multiple channels to a home or business that can provide for various services such as voice (constant bit-rate), data (high reliability), and video (high-speed data).