Chapter 6 - TV Measuring Range
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BER measurement (Bit Error Rate)
6.2.2.5.3
The measurement of the bit error rate aids in the determination of the quality of a DTMB signal.
To determine the bit error rate, the error correction mechanisms in the digital receiver are used.
The data stream is compared before and after correction and the number of corrected bits is
determined from that. This number is placed in a ration to the total throughput of bits and the BER
is calculated based on that. With DTMB there are two independent error protections that work
together.
The inner error protection comes from LDPC (Low Density Parity Check), the external error
protection uses BCH (Bose Chaudhuri Hocguenghem).
The instrument measures the Bit Error Rate before LDPC (CBER) and after LDPC (LBER). Both
results will be shown on the display in exponential form. The measurement depth is for CBER
1•10
6
Bits and for LBER 1•10
8
Bits.
MER measurement (Modulation Error Rate)
6.2.2.5.4
In addition to measurement of the bit error rate, it is established practice with digital transmission to
also measure MER. It is defined in ETR290. MER is calculated from the constellation points. It is
the counterpart to S/N measurement with analogue transmission methods. The measuring range
goes up to 32 dB with a resolution of 0.1 dB.
Noise Margin (NM)
6.2.2.5.5
In case of white noise a limit value of MER for the minimum signal quality (QEF) can be determined
dependent on the modulation type, the FEC and the other parameters.
The difference of MER to this limit value corresponds to the system reserve NM (noise margin).
Just as MER it is displayed in dB with a resolution of 0.1 dB. For an easier assessment of the
signal quality the NM is shown in the colors red for bad, yellow for limited and green for good signal
quality. The limit values for the BER (>2e-4 for bad and <1e-6 for good) will also be included in this
assessment. This assessment also considers the limit values of LBER (>1e-7 for bad).
Important! The noise margin cannot be shown in DTMB during active PE-measurement.
Impulse response
6.2.2.5.6
For the measurement of the impulse response, the measuring instrument must receive a DTMB
signal. For this, you must adjust the instrument to the appropriate channel.
By selecting the menu point
IMPULSERES
the instrument will show the impulse response on the
display. Simultaneously you will see a further menu for adjustments.
With
FREEZE
you can "freeze" the picture. With
ZOOM
the impulse response can be spread
horizontally. You can define the unit of x-axis with
µs
or
km
. Time and length values are related
via the speed of light c:=3•10
8
m/s. Via the menu point
BACK
you can end the display of the
impulse response.
