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TELE-audiovision International — The World‘s Leading Digital TV Industry Publication

— 03-04/2015

—

TELE-audiovision.com

DVB-T

Echo Measurement

25. With the Spaun Sparos 711

it is possible to measure the

DVB-T/T2 echo that is caused

by simultaneous reception of

the same frequency from two

transmitters at different dis-

tances. The signal that has to

travel further is characterised

by a higher delay time, which

is graphically shown on the

meter’s screen and the distance

is presented using seconds or

kilometres. If the echo exceeds

the limits of the so-called guard

interval the tuner cannot offset

the echo any longer and video

errors will occur.

26. We had to zoom out to see

the vertical line that represents

the guard interval, as it is clear

for all to see that we are not

affected by echoes at our test

center. It would take a transmit-

ter with a distance of more than

67 km to cause interference

with our DVB-T signal.

27. This view lists all echoes in

a table, allowing quick analysis.

DVB-T MER Carrier

28. Here we are measuring a

perfect DVB-T signal with a

frequency of 144.5 MHz that

was created by a lab modula-

tor. Please note the BER value

of 37.6 – it gives the average

modulation error rate of all car-

riers on the transponder.

29. Obviously, this perfect

video is free from artefacts.

30. We will soon create inter-

ference with our CB radio,

generating a narrow-bandwidth

signal at precisely 144.5 MHz,

which will cause interference

with our DVB-T transponder

right in the center.

31. We are currently not broad-

casting with our CB radio and

since we are still dished up a

perfect DVB-T signal the MER

vs. carrier measurement shows

a constant MER across all car-

riers. This results in an average

MER of 38.2 dB – the result of

an MER measurement that is

available with most meters.

32. Now we are transmitting

a radio signal with 144.5 Mhz.

The measurement results are

still all good. At closer ins-

pection we can notice that the

pre-BER (BERi) is now slightly

lower than before, even though

the MER result is still perfectly

okay.

33. Yet all of a sudden we see

artefacts in our video, espe-

cially with fast camera move-

ments and when there is a lot of

change in screen content – in

other words, whenever the bit

rate increases. What’s going

on? Our measurements do not

provide a clue. Or do they?

34. Look closely: Do you notice

the narrow-band interfering

signal right at the center of the

144.5 MHz frequency?

35. This is the source of all evil

that interferes with individual

carriers in the center of the

transponder. The affected

carriers show an MER of 0

dB, yet on average the MER

across the entire transponder

remains almost unchanged.

After all, only a few carriers

experience interference, while

all others work just fine. Those

problematic carriers cannot be

identified with a regular MER

measurement, even though

they become more frequent,

not least due to the large-scale

rollout of LTE coverage.

36. This is a screenshot show-

ing an actual DVB-T transpon-

der that is subjected to an MER

vs. carrier measurement. No

interference is visible, and even

though the MER values are

slightly below average this can

be explained with transmission

attenuation.

DVB-T LTE Filter

Measurement

37. This spectrum shows two

DVB-T transponders in imme-

diate vicinity to an LTE signal.

Could an LTE filter do any good

in this circumstance?

38. With the Spaun Sparos

711 you don’t have go to great

lengths installing an LTE filter

to find our whether or not it is

of any use. Instead, you can

simply use the LTE function

of the meter, which simulates

an LTE filter in the spectrum.

As expected, in the case at

hand a filter would result in a

general attenuation in the 3 to

4 dB range and would therefore

completely filter out two of the

three LTE transponders. The

LTE transponder at 785 MHz,

however, would only be filtered

partially. Information like this

is immensely valuable and can

potentially save a great deal of

time and effort.

all channels broadcast their

signals on the same frequen-

cy from all transmitters, so

that receivers can be moved

from one cell to another with-

out having to look for a new

frequency for the channel

they are receiving. What’s

more, receivers can be dis-

tributed with pre-set channel

allocations, no matter where

they are used in a given re-

gion or country.

As always, however, all that

glitters is not gold and that’s

why SFN technology comes

with a major drawback. In

case a receiver is located at

a position that receives sig-

nals simultaneously from

two transmitters at different

distances, the correspond-

ing transponder is received in

duplicate and the signal from

the transmitter that is further

away arrives with a slight de-

lay, causing what technicians

call echo. When DVB-T/T2

was originally developed this

issue was already taken into

consideration and a so-called

guard interval was introduced

to the signal that allows re-

ceivers to compensate for

minor runtime discrepancies.

Once the echo becomes big-

ger than the guard interval,

however, the tuner is no long-

er able to offset that discrep-

ancy and interference is una-

voidable.

In case you need to deter-

mine the distance to a trans-

mitter (given in seconds or