Inovonics David IV, Installation & User Manual

Page: 34 / 67

— 30 —
The ITU Recommendation specifies the r.m.s. power of the
composite-multiplex signal (including the stereo pilot and any
audio or data subcarriers), as measured and integrated in a
‘floating’ 1-minute measurement window. It states that this
value should not exceed the r.m.s. power of a single-tone sine
wave modulated with a peak deviation of ±19kHz.
Now, ±19kHz is just about 12dB below the ±75kHz peak devia-
tion limit. This implies an average-to-peak ratio of the program
signal that, by contemporary broadcasting standards, gives a
rather pathetic showing… certainly not the in-your-face, highly-
compressed dynamics PDs have come to know and love(?) since
the mid-1960s!
The ITU
Recommendation has been mandatory in Germany,
Austria and Switzerland since 2004, but with some latitude.
Germany adheres to the letter of the Recommendation , “0dBr”
they call it. But Austria and Switzerland are able to exceed that
point by 3dB. In other words, their r.m.s. modulation is permit-
ted to go to “+3dBr.”
The DAVID IV provides compliance to the ITU standard by re-
defining the peak-limiting processing function when the
ITU
option is selected.
First, the PIPP* limiter is defeated in favor of more conservative
symmetrical peak control. Secondly, the Limiter Drive and Limiter
Density
sliders are re-scaled for less-aggressive overall pro-
cessing. The Limiter Density slider is, in fact, renamed in the
ITU
mode, becoming the
dBr BS.412-9
slider shown above.
This slider, calibrated to the multiplex r.m.s. “0dBr” value cited
in the ITU Recommendation , may be set at any dBr value be-
tween –1dBr and +5dBr, accommodating any ‘fudge factor’
permitted by the local regulating authority.
In operation, the DAVID IV continues to utilize the native algo-
rithm of the
Limiter Density
slider to anticipate a certain amount
of average-level modulation control based on audio program
dynamics. This approximation is supplemented with feedback
of the r.m.s. value of the multiplex signal to arrive at a final and
accurate correction factor.
Other ITU
Processing
Implications
The ITU mode will hold r.m.s carrier modulation to the pre-
scribed limit, but processing ahead of the peak limiter section
will have a second-order effect on how effective this utility will
be.
One caveat of the ITU Recommendation is that audio processing
used to satisfy the requirement must not create audible side-
effects (i.e.: ‘breathing,’ ‘pumping’ and obvious level shifts).