Measuring Digital TV Signals in the Broadband Cable - Digital Video and Audio Broadcasting Technology - page 305

18.2.6 Error Vector Magnitude (EVM)

The error vector magnitude (EVM) is closely related to the modulation er-
ror ratio (MER), the only difference being the different reference used.
Whereas in the MER, the reference is the RMS value of the QAM signal, it
is the peak value of the QAM signal which is used as reference for the
EVM.
EVM and MER can be converted from one to the other with the aid of
table 18.1.

18.3 Measuring the Bit Error Ratio (BER)

In DVB-C and in J83A, C, the transmission is protected by Reed Solomon
forward error correction RS(204,188). Using 16 error protection bytes per
transport stream packet, this protection allows 8 single errors per TS
packet to be corrected at the receiving end. Counting the correction events
performed by the Reed Solomon decoder at the receiving end and assum-
ing that these are attributable to single errors, and relating them to the in-
coming bitstream in the comparable period (a transport stream packet has
188•8 useful bits and a total of 204•8 bits), provides the bit error rate, a
value between 1•10 -4 and 1•10 -11 .
However, not all the errors can be corrected by the Reed Solomon de-
coder. Errors in TS packets which are no longer correctable lead to er-
rored packets which are then marked by the transport error indicator in the
MPEG-2 transport stream header. Counting the non-correctable errors and
relating them to the corresponding data volume allows the post-Reed
Solomon bit error ratio to be calculated.
Thus, there are two bit error ratios in DVB-C and J83A, C:

•

Bit error ratio before Reed Solomon - the channel bit error ratio

• Bit error ratio after Reed Solomon

The bit error ratio (BER) is defined as:

BER = bit errors / transmitted bits;

The bit error ratio has a fixed relationship to the signal/noise ratio if
only noise is involved. This relationship is shown in the figure below. In
addition, the figure includes the equivalent noise degradation (END) and
the noise margin for an example.