A07A2200S
is switched off. Next, a noise signal C+D replaces the
sinusoidal tone, and attenuator E is adjusted to obtain the
same peak-reading as before at the noise voltmeter. The quasi-
peak deviation is thus equal to 32 KHz. Since the pre-
emphasis has not been included in the level measurement,
the actual peak deviation is higher .The described adjustment
corresponds to the present-day broadcasting practice.
Note. – A normal sound-broadcasting programme without
compression is simulated by modulating the unwanted
transmitter with the standardized coloured noise signal using
a frequency deviation of 32 KHz. Therefore, the results
obtained with this method and this deviation are only valid
for sound broadcasting programmes without compression.
The not considering the pre-emphasis leads to a difference of 1dB about, whereas the audio
compressors installed now in every broadcast networks increase the modulation power of 2dB
further on.
If a stereophonic signal is being examined the Rec.ITU-R BS.412-7 is very clear and it makes no
distinction between the modulation power within monophonic and stereophonic signal:
Rec. ITU-R BS.412-7 2.3 The radio-frequency protection ratios assume that the
maximum peak deviation of 75 KHz is not exceeded. Moreover, it is
assumed that the power of the complete multiplex signal
including pilot-tone and additional signals , integrated over any
interval of 60 s is not higher than the power of a MPX signal
containing a single sinusoidal tone which causes a peak dev.
of 19 KHz (see Note 4 )
.............................................................................................
Note 4 – The power of a sinusoidal tone causing a peak dev. of
19 KHz is equal to the power of the coloured noise modulation
signal according to ITU-R BS.641 i.e. a coloured noise signal
causing a quasi-peak deviation of 32 KHz .
Whereas the IEC 244-13 makes a difference between monophonic signal (reference of 32KHz) and
stereophonic one (40KHz):
IEC 244-13 9.4 For monophonic operation
Check that the pre and de-emphasis filters are in circuit
Adjust output of the LF generator at <1KHz to a level witch
corresponds to a frequency dev. 7.4 dB below maximum rated
deviation ( 32 KHz for 75 KHz dev.)
Measure the peak value by means of the noise meter at the out
of the demodulator ( without weighting network).
Switch the LF generator out of circuit and the noise generator
in circuit and adjust the output of the noise generator , so that
the noise meter gives the same reading. The peak-dev. is now
correct.
...............................................................................................
...............................................................................................
For stereophonic operation
Check that the appropriate pre and de-emphasis are in circuit
380 221 R02 Pag. 42
To Next Page
Loading...