NTi Audio XL2 Sound Level Meter - Manual (Page 120)

120
STIPA
13 STIPA Modulation Index MF1, MF2
For good speech intelligibility it is mandatory that the in-
tegrity of the transmitted voice signal modulations are pre-
served. Therefore STIPA is based on measuring the MTF
(Modulation Transfer Function). This function quantifies the
degree to which the voice modulations are preserved in
individual octave bands. STIPA determines the MTF by ana-
lyzing the seven frequency bands. Each band is modulated
with two frequencies, resulting in the modulation ratio
mr1 and mr2. All indexes together combined with psycho-
acoustic models provide the single-value STIPA result.
14 Error Detection
The built in error detection helps identifying faulty measure-
ments caused by the amount of impulsive ambient noise.
Such noise influences the accuracy of any STIPA measure-
ment. The error detection checks the following:
Invalid modulation indices in individual octave bands •
(mr1 or mr2 > 1.3)
Changes in ambient noise or any impulsive noise dur-
•
ing the measurement (by comparing the first half of the
measurement period with the second half)
Question Mark ? at individual Octave Bands
Individual octave bands are marked with ? in case of a
Missing test signal level •
Invalid modulation indices (mr1 or mr2 > 1.3) •
Changes in ambient noise or any impulsive noise during •
the measurement
Flashing Question Marks ?.?? at 2
The measurement result flashes in combination with the
question marks ??? in case of a
Question Mark • ? at one or multiple octave bands. See
the STIPA Table Result Page on the XL2 for details.
Changes in ambient noise or any impulsive noise during
•
the measurement
In case the question marks ?.?? are flashing,
then the measurement is faulty. Verify possible
causes and repeat the measurement.
For details see the chapter STIPA Measurement
Hints.