Széchenyi István University
Department of Telecommunications
STUDENT’S GUIDE FOR LABORATORY EXERCISES
Analog Terrestrial Television Broadcast
using
the PAL and the TXT generator
Objectives
The goal is to analyze the analog broadcast method (in PAL).
Equipment
For the measurement you can use the measurement equipment and the CRT
television.
Introduction
Goal of the exercise is to analyze and measure the analog television transmission
and signals using a television, a signal generator and the oscilloscope. The
analog television signal contains a synchron at the beginning of every line and
the signal for the television picture (active time). In case of PAL television we
have:
Number of lines
n = 625
Number of active lines
n=575
Frames
50 Hz
Horizontal frequency
fs = 15625 Hz
Ratio
4:3
Time intervall for one line
T = 64 microseconds.
The timing for one line:
The timing of the synchron signal:
Spectrum of an analog TV signal:
Check the spectrum. Analog television uses AM-VSB modulation for the blackand-white information (luminance). This means, after the carrier, the upper side
band of the modulated signal goes up to 3-4 MHz, but below the carrier, the
lower side band only to -1,5 MHz. Using PAL, the color information uses
analog QAM modulation at another carrier frequency of 4,43 MHz (relative to
the AM-VSB carrier). They can overlap and cause different errors. The last big
peak is the audio signal using FM (as a radio station). If you have two separate
peaks, it means stereo sound. The whole spectrum must fit in an 8 MHz channel.
Procedure
Turn on the television and the generators as well as the oscilloscope.
1. Find the synchron signal on the oscilloscope and read the timing values
1.offset time
tl = 100 ns
tf =80 ns
Ut =108 mV
2. onset time
3. overshoot
2. Calculate the actual
upper bandlimit of the
synchron:
fmax = 0,4 / tf = 0,4 / 80ns = 5000000 Hz = 5 MHz
3. Check other lines (zoom out) on the oscilloscope. Find the 25 lines where the
interlaced picture is changing from field 1 to field 2 or back. In case of an
interlaced picture, one frame is made of two fields: the odd and the even lines
added together.
1600ns
4. Analyse the composite TV signal using the signal generator. Check the
picture and the oscilloscope together and compare the different measurement
signals.
If you have an antenna cable with signal, you can connect it to the measurement
chain instead of the signal generators. Watch the real TV signal on the screen
and on the oscilloscope.
5. Turn on the Teletext signal generator and repeat the measurement. Search for
the TXT lines (around the 7-23 lines). Calculate the bitrate and the frequency
band.
TXT before the modulator and after.
Check the signal form of the bits before and after the modulator. There is
distortion and latency of about 1 microsecond.
Every TXT data in every lien begins with a synchron of 8-8 bits of „1” and „0”.
Measure the time for that and divide it by 16, so you can get the time needed for
one bit (on average). The bitrate is 16/tbit.
t = 2.3 μs
tbit = 1,4375 * 10-7
Bitrate = 1 / tbit = 6,956 MHz
As mentioned above, you can calculate the bandwidth by using the equation:
fmax = 0,4 / tup
where tup is the onset time of the bit „1”
Transmitter side:
tup= 72 ns
fmax= 0,4 / tup = 0,4 / 72 * 10-9 = 5,555 MHz
Reciever side:
tup= 98 ns
fmax= 0,4 / tup = 0,4 / 98*10-9 = 4,0816 MHz