Extra Class question of the day: Television practices: fast scan television standards and techniques; slow scan television standards and techniques

August 19, 2012 Dan KB6NU

Although we are called “radio” amateurs, we can also send and receive television signals.  There are several ways that amateurs communicate by television. Perhaps the two most popular ways are standard fast-scan television and slow-scan television (SSTV).

The video standard used by North American Fast Scan ATV stations is called NTSC.(E2B16) The NTSC, or National Television Systems Committee, is the body that set standards for the analog television system that was used in the U.S. and many other parts of the world. After nearly 70 years of using the analog NTSC system, U.S. broadcasters switched over to a digital broadcasting system on June 12, 2009.

A fast-scan (NTSC) television frame has 525 horizontal lines (E2B02), and a new frame is transmitted 30 times per second in a fast-scan (NTSC) television system. (E2B01) NTSC systems use an interlaced scanning pattern. An interlaced scanning pattern is generated in a fast-scan (NTSC) television system by scanning odd numbered lines in one field and even numbered ones in the next. (E2B03)

In order for the scanning beam to only show the picture, a technique called blanking is used. Blanking in a video signal is turning off the scanning beam while it is traveling from right to left or from bottom to top. (E2B04)

NTSC signals are amplitude modulated (AM) signals, but use a technique called vestigial sideband modulation. Vestigial sideband modulation is amplitude modulation in which one complete sideband and a portion of the other are transmitted. (E2B06) The reason that NTSC TV uses vestigial modulation is to conserve bandwidth. Even using this technique, an NTSC signal is 6 MHz wide. One advantage of using vestigial sideband for standard fast- scan TV transmissions is that vestigial sideband reduces bandwidth while allowing for simple video detector circuitry. (E2B05)

Amateurs can transmit color TV as well as black-and-white TV. The name of the signal component that carries color information in NTSC video is chroma. (E2B07)

There are a number of different ways to transmit audio with an NTSC signal. The following are common methods of transmitting accompanying audio with amateur fast-scan television:

  • Frequency-modulated sub-carrier
  • A separate VHF or UHF audio link
  • Frequency modulation of the video carrier

All of these choices are correct. (E2B08)

Slow-scan TV
Because of the bandwidth requirements, amateurs can only transmit fast-scan TV above 440 MHz. FM ATV transmissions, for example, are likely to be found on 1255 MHz. (E2B18) In fact, one special operating frequency restriction imposed on slow scan TV transmissions is that they are restricted to phone band segments and their bandwidth can be no greater than that of a voice signal of the same modulation type. (E2B19) The approximate bandwidth of a slow-scan TV signal is 3 kHz. (E2B17)

SSTV images are typically transmitted on the HF bands by varying tone frequencies representing the video are transmitted using single sideband. (E2B12) The tone frequency of an amateur slow-scan television signal encodes the brightness of the picture. (E2B14)

128 or 256 lines are commonly used in each frame on an amateur slow-scan color television picture. (E2B13) Specific tone frequencies signal SSTV receiving equipment to begin a new picture line. (E2B15)

There are a number of different SSTV modes. The function of the Vertical Interval Signaling (VIS) code transmitted as part of an SSTV transmission is to identify the SSTV mode being used. (E2B11)

Digital Radio Mondiale is one way to send and receive SSTV signals. No other hardware is needed, other than a receiver with SSB capability and a suitable computer, is needed to decode SSTV using Digital Radio Mondiale (DRM). (E2B09) Just like any SSTV transmission, 3 KHz is an acceptable bandwidth for Digital Radio Mondiale (DRM) based voice or SSTV digital transmissions made on the HF amateur bands. (E2B10)

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