Measuring DVB-T Signals - Digital Video and Audio Broadcasting Technology - page 405

scattered pilots. But these are only available in intervals of 3 carriers and
between them it is necessary to interpolate.
In 2K mode, the center carrier is No. 852 which is a payload carrier or
sometimes a scattered pilot. Verifying I/Q errors will not present any prob-
lems, therefore. The situation is different in 8K mode where the center car-
rier is No. 3408 and is always a continual pilot. I/Q errors can only be ex-
trapolated in this case by observing the adjacent upper and lower carriers.
Each of the effects described has its own measurement parameters. In
the DVB-C cable standard, these parameters have been combined to form
an additional aggregate parameter called the modulation error ratio.

Resultant vector

Q

Error vector

Centrum of
decision field

Ideal vector

I

Fig. 21.25. Error vector for determining the modulation error ratio (MER)

The modulation error ratio (MER) is a measure of the sum of all inter-
ference effects occurring on the transmission link. Like the signal/noise ra-
tio, it is usually specified in dB. If only one noise effect is present, MER
and S/N are equal.
The result of all the interference effects on a digital TV signal in broad-
band cable networks, explained above, is that the constellation points ex-
hibit deviations with respect to their nominal position in the center of the
decision errors. If the deviations are too great, the decision boundaries are
crossed and bit errors occur. However, the deviations from the center of
the decision field can also be considered to be measurement parameters for
the magnitude of an arbitrary interferer. Which is precisely the aim of an
artificial measurement parameter like the MER. When measuring the
MER, it is assumed that the actual hits in the constellation fields have been