Introduction to Digital Terrestrial Television

ITU Transmission System Model

The first step, as explained when referring to the MPEG, is common to all DTV systems. This input block compresses and multiplexes the signals that the driver must process to modulate the transmitter.

The second step, multiplex and transport, is a process that is done differently in ATSC and is governed by Digital Television Standard A/53E, which was amended to cover new services and so as not to harm the existing receivers on December 27, 2005 and published on March 21, 2006.

Modulation and transmission are done in 8-VSB in one of 36 formats of screens, depending on the choice made by the TV broadcaster.

The receiver must receive all formats, but for output on the screen it must do so in one of four formats, as indicated below.

Picture Scanning

It is the same as for analogue television.  There are two types of screen scanning:

INTERLACED (i): First, the odd lines and then the even lines are traced alternately, creating two fields.  

PROGRESSIVE (p):  If it involves 1080 p, the frame is shaped by a scanning of 1080 lines where the even lines are traced after the odd lines line by line from top to bottom on the screen.

PROGRESSIVE (p): It provides higher definition and less flickering of the picture.

Remember: In NTSC, each frame has four fields because in each frame the color burst changes 90o. PAL has eight fields.

In DTV 1080i, the odd field contains 540 odd lines, and the even field has 540 even lines.

In 60-frame systems, there is less flickering because the picture is projected twice per second.  If the system consists of 30 frames per second, the projection of a moving picture involves choppiness as in the movies of the early twentieth century.  In an interlaced system, compression is less efficient.

So many formats have proliferated in ATSC because, at first, the team did not have enough specialists to standardize criteria.  More formats mean more electronics, which raises both producer and user costs.

• First, the lines of the odd field (green lines) are scanned or traced.

• Then the even field (magenta lines) is scanned.

• After tracing each odd line and each even line, the electronic beam is switched off, travels back to the left edge of the screen and starts drawing the next line, just like when we write on a sheet of paper.  In the picture, the scan lines are represented by dotted white lines. 

• When the two fields of the picture have been scanned, the electronic beam travels back to the upper part of the screen and starts to draw or trace the following frame (yellow diagonal line). 

• The time it takes for the beam to travel back is called the horizontal blanking interval (HBI) (white line) if it occurs after each line or vertical blanking interval (VBI) (yellow line) if it occurs at the end of each field.

• In analogue television, synchronizing pulse signals are sent during these blanking intervals.

• In digital television, instead of synchronizing pulse signals, ancillary, complementary or exogenous data are transmitted.  In other words, in addition to television, other services from which the operator earns extra income are transmitted.

Asociación Colombiana de Ingenieros
Colombian Association of Engineers
(ACIEM)