Analog Television - Digital Video and Audio Broadcasting Technology - page 21

for 100% white. Its nominal amplitude is 700 mV. The "roof" of the white
pulse is 10 Μs long and should be flat and without overshoots. This is fol-
lowed by the 2T pulse which is a so-called cos 2 pulse with a half-
amplitude period of 2T 2 • 100 ns = 200 ns. The main components of its
spectrum extend to the end of the luminance channel of 5 MHz. It reacts
very sensitively to amplitude response and group delay distortion from 0 ...
5 MHz and can thus be used for assessing linear distortion both visually
and by measurement. The next pulse is a 20T pulse, a cos 2 pulse with su-
perimposed color subcarrier and with a half-amplitude period of 20T = 20
• 100 ns = 2 Μs. It clearly shows linear distortion of the color channel with
respect to the luminance channel.
Linear distortion of the color channel with respect to the luminance
channel is

• Differential gain of the color channel with respect to the lumi-

nance channel

• Luminance-chrominance delay caused by group delay

Non-linear distortion can be easily identified by means of the 5-step
gray scale. All five steps must have identical height. If they do not have
equal height due to nonlinearities, this is called static nonlinearity (lumi-
nance nonlinearity). In test line 330, the gray scale is replaced by a stairca-
se on which the color subcarrier is superimposed. This can be used for
identifying non-linear effects on the color cubcarrier such as differential
amplitude and phase. The color bursts superimposed on the staircase
should all be ideally of the same amplitude and must not have a phase dis-
continuity at the transition points of the steps.
Teletext is well known by now (Fig. 2.13. and 2.14.). It is a data service
offered in analog television. The data rate is about 6.9 Mbit/s, but only in
the area of the lines really used in the vertical blanking interval. In actual
fact, the data rate is much lower. In each teletext line, 40 useful characters
are transmitted. A teletext page consists of 40 characters times 24 lines. If
the entire vertical blanking interval were to be used, just short of one tele-
text page could be transmitted per field. Teletext is transmitted in NRZ
(non-return-to-zero) code. A teletext line begins with the 16-bit-long run-
in, a sequence of 10101010… for synchronizing the phase of the teletext
decoder in the receiver. This is followed by the framing code. This hexa-
decimal number 0xE4 marks the beginning of the active teletext. After the
magazine and line number, the 40 characters of a line of the teletext are
transmitted. One teletext page consists of 24 text lines.