Table of Contents
Advertisement
In case of stereophonic broadcast in accordance to the IEC 244-13 rule the reference power is
moved highwards of 1.9 dB in relation to the corresponding REC. ITU-R BS.412-7.
7.5
There are many people who would like to resolve over-crowding of the frequencies by synchronizing
two or more FM transmitters on the same frequency to cover ever-expanding zones in case of diffusion on
the same program. The idea in itself isn' t bad, but there are many technical-theoretical difficulties which in
our opinion advise against the creation of such a system except in extreme and exceptional situations such as
those described below.
Most importantly, FM modulation, unlike AM, cannot be synchronized on an analogue-type transmitter. As
such, the creation of an FM-synchronous system on all traditional transmitters is invalid even if input for
external synchronization via GPS or reference to rubidium or caesium is taken into account. This category
of transmitters also includes the 500W TX, upon which, for the reason previously described, input for external
synchronism has not been implemented. (The internal reference has a level of stability which more than
satisfies all regulations, and the frequency can also be fine-tuned by remote). If two analogue FM
transmitters are synchronized with a common reference, the two transmitters will have the same carrier
frequency. The modulations won' t ever be in phase for theoretical problems, however, and as such won' t
ever be able to verify the synchronous vector sum of the two signals overlapping at the listening point. This
could cause unsatisfactory results on the listening end in the area of reciprocal interference.
Recently, digital transmitters have been introduced to the market which many say should resolve the
problem. These devices have the generation of a modulated carrier with a digital synthesis which resolves
all previous theoretical problems but which, apart from the difficulty of respecting the most rigorous
regulations regarding spurious emissions from the digital process, must also be confronted with other
irresolvable difficulties. The velocity of the light propagation (approx. 300 meters per microsecond) creates
insurmountable limits on the creation of a synchronous system. Having multiple synchronized transmitters
over a distance of a few kilometers which serve intersecting areas (which occurs in most documented cases),
it is theoretically impossible for signals to overlap at the same point and not have reciprocal delays. To reach
the user, starting from the transmitting antenna, the radio signal takes 100 microseconds if the distance is
between two and 30km. A few microseconds of delay reduces listening quality. Resolving this problem is
often attempted by artificially delaying the radio signal by adding adjustable digital delay lines in the
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
Adjust the output of the LF generator at <1KHz to a level
corresponding to a frequency deviation of 40 KHz including
pilot tone.
Measure the peak value in channel B after the demodulator
and stereo encoder by means of the noise meter (without the
weighting network ).
For the remaining procedure , see the method used for
monophonic operation
ABOUT FM SYNCRONOUS
Pag. 57
A07B500S manual
Advertisement
Table of Contents
