DEVICE FOR TIME RESETTING IN DIGITAL SIGNAL TRANSMISSION FOR NETWORKS

DISPOSITIF DE REMISE A L'HEURE DANS LA TRANSMISSION DE SIGNAUX NUMERIQUES POUR RESEAUX

English Abstract

In a time-resetting device in a terrestrial digital transmission
network of the type used by digital radio or television broadcasting, the
time information of the digital signal is modified in the transmission site
before the modulator in order to replace it by a time information that is
computed on the basis of the UTC time given by a GPS receiver and that
includes the transmission delay of the concerned site. The invention can
be applied to terrestrial digital transmission networks for digital audio
broadcasting (DAB) or digital video broadcasting (DVB-T).

Claims

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CLAIMS
1. A device for resetting the time in a digital signal transmission
network, said network comprising studio digital encoding sites, collection
networks, a multiplexing site that sets up the "digital bouquet", a network
for the primary distribution of the digital bouquet and transmission sites,
each transmission site comprising a modulator, a radio frequency amplifier
and a GPS receiver,
wherein the time-resetting device is positioned in the modulator and
comprises:
means to compute the time to be transmitted to the users,
means to extract the time in the digital bouquet, and
means to substitute the computed time for the extracted time.
2. A time-resetting device according to claim 1 wherein the
computation means comprise:
a module to memorize the UTC time given by the GPS receiver,
a module to memorize the delay introduced by the concerned
transmission site,
a module to add vup the UTC time memorized by the module and the
delay memorized by the module, and
a module to memorize the computed time.

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3. A time-resetting device according to claim 1 or 2 wherein the
means for the extraction of the time in the digital bouquet comprise:
a localization module to identify the envelope of the Service
Information elements, and
a demarcation module for the identifying, in the envelope of the
Service Information elements, of that information element which conveys
the UTC time.
4. A time-resetting device according to claim 1, 2 or 3 wherein
the substitution means comprise a multiplexes.

Description

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DEVICE FOR 'TIME RESETTING IN DIGITAL SIGNAL
TRANSMISSION NETWORKS
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to digital type transmission networks and,
s more particularly, in such networks, to a device to reset the time at which
the information is transmitted to the receivers.
In analog type transmission networks, the period of transmission of
the analog signals are brief and are practically not detectable by a radio
station listener or a television station viewer.
io In the case of dilrital type transmission networks, whether these are
radio or television networks, the analog signals are encoded in binary
numbers on which a certain number of digital processing operations are
carried out. These processing operations give rise to delays which
accumulate from the beginning to the end of the transmission chain or
is sequence.

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DESORIPTION OF THE PRIOR ART
Figure 1 is a drawing of a digital type transmission sequence that
begins with radio or television studios 10, 20 and 30 whose audio and/or
video analog signals are encoded in binary numbers by encoders 12, 22
s and 32 respectively. '.Chese binary codes come from the studios 10, 20 and
30 and are transmitted: to a multiplexing site 40 by means of collection
networks 14, 24 and 34.
In the multiplexing site 40, the binary codes coming from the
different studios are "mixed" to form what is called a digital "bouquet".
io This bouquet is distributed to transmission sites 16, 26 and 36 by means of
a distribution network 42 known as a "primary" network. In these
transmission sites 16, 26 and 36, each of which includes a
modulator/transmitter assembly 18, 28 and 38, the bouquet is sent to the
receivers of the users o~f the radio or television broadcasting network by a
is distribution network knows as a «secondary» network.
In one structure accordilig to the drawing of Figure 1, each
encoding, multiplexing and trmsmission site carries out a digital
processing operation whose duration is not negligible while each primary
distribution, secondary distribution and collection network introduces a
2o variable delay that too is not negligible.

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The adding up of these digital processing periods and these
transmission periods leads to a time lag between the instant at which the
sound or the image is picked up and the instant at which this sound or this
image is received by the user's receiver.
Furthermore, the new radio or television RF digital broadcasting
standards stipulate the integration, .into the bouquets, of service
information in addition to the audio and/or video programs. Now this
service information, known as SI, includes the date and time which is
usually introduced into the studio 10, 20 and 30 or by the operator of the
io multiplexing site 40 using information given by a receiver knows as the
GPS (Global Positioning System) receiver.
This information given by the GPS receiver includes a one-pulse-
per-second time reference, a frequency reference given by a 10 MHz
frequency signal and the UTC date and time information, UTC being the
is acronym for Universal Time Coordinated or Greenwich Mean Time using
the Julian calendar. Furthermore, to take account of the fact that the UTC
information corresponds to the Greenwich Mean Time, the «time lag»
or «time offset» between the Greenwich meridian and the zone served
by the bouquet is taken into account by using the local time information
2o given by the "LTO" (Local Time Offset) signal.

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When this date and time information reaches the user's receiver, the
earlier it has been introduced into the transmission sequence, the more
erroneous will it be.
The aim of the present invention therefore is to obtain a device that
can be used to reset the information on time that is transmitted to the
user's receiver so that it corresponds to the real time of the place of
reception.
This aim is achieved by implementing a time-resetting device
positioned in each transmission site 16, 26 and 36 and more particularly at
io the input of the modulator of each site, this time-resetting device using
the
UTC information given by the GPS receiver as well as the information on
the duration of processing in the transmission site and on the duration of
transmission to the user's receiver.
is SUMMARY OF THE INVENTION
The invention therefore relates to a device for resetting the time in a
digital signal transmission network, the network comprising studio digital
encoding sites, collection networks, a multiplexing site that sets up the
"digital bouquet", a network for the primary distribution of the digital
ao bouquet and transmission sites, each transmission site comprising a

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modulator, a radio frequency amplifier and a GPS receiver, wherein the
time-resetting device is position in the modulator and comprises:
means for computing the time to be transmitted to the users,
means for extracting the time in the digital bouquet, and
means for substituting the computed time for the extracted time.
BRIEF' DESCRIPTION OF THE DRAWINGS
Other features and advantages of the present invention shall appear
from the following description of a particular embodiment, said description
io being made with reference to the appended drawings, of which:
Figure 1 is a drawing of a structure of a digital type transmission
system to which the invention can be applied,
Figure 2 is a functional drawing of a modulator/transmitter
comprising a time-.resetting device according to the invention,
is Figure 3 is a. functional drawing of the time-resetting device
according to the invention,
Figure 4 is a. functional drawing showing the computation of the
time that will be transmitted to the user, and
Figure 5 is a functional drawing of the substitution module 72 of
Zo Figure 3.

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MORE DETAILED DESCRIPTION
Each transmission site 16, 26 or 36 respectively comprises a
modulator/transmitter 18, 28 or 39 which consists (Figure 2) of a
modulator 50, a transmitter 52, a transmission antenna 54 and a GPS
receiver 56. The transmitter comprises a power amplifier 66 and, as the
case may be, a filter 68. In the known way, the modulator 50 comprises
an input interface 58 for the uiput of the digital signal given by the primary
distribution network 42, and OFDM (Orthogonal Frequency Division
Multiplex) type modulator 60 proper that carries out baseband modulation
io and a frequency converter 62 that carries out a baseband frequency
change.
According to the invention, a time-resetting device 63 is positioned
between the input interface 58 and the modulator 60 and carries out a
resetting of the time contained in the digital signal of the distribution
is network by introducing the current time or real time after having corrected
it by the duration of processing in the transmission site.
The GPS receiver 56 gives the modulator 50 the following
information in order that it may be used in a manner that is known: the
one-pulse-per-second tame reference information and the 10 MHz
Zo frequency reference information. According to the invention, the GPS

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receiver furthermore gives the UTC time information which is used only
by the time-resetting device 64.
The time-resettilig device 64 comprises (Figure 3) a module 70 to
compute the time information to be transmitted on the basis of the UTC
time information, the one-pulse-per-second time information elements
given by the GPS receiver 56 and the information on duration of
processing Td in the transmission site. This correct time information
replaces the information contained in the digital signal of the bouquet in a
module 72.
io The time information of the bouquet is localized in the input digital
signal by the modules 74 and 76, the module 74 being used to localize the
Service Information elements SI while the module 76 is used for the
demarcation, in these Service Information elements SI, of those
information elements that correspond to the information of the received
is bouquet.
As can be seen in Figure 4, the module 70 comprises a first register
78 for the memorizing, at each second, of the real UTC time information, a
second register 80 for the memorizing of the processing duration Td of the
transmission site considered, an adder 82 to add up the contents of the first
2o and second registers 78,, 80 and a third register 84 to store the result of
the
addition which constitutes the substitution time information.

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The modules 74 and 76 are adapted to the digital signals of the
bouquet to be broadcast. Tlus bouquet is given to the modulator in a form
known as the "transport multiplex" which is defined by the ETI standard
(ETS 300 799) for digital audio broadcasting (DAB) and by the MPEG-TS
s (ISO/IEC 13818-1) standard to digital video broadcasting television
(DVB-T).
The Service Information elements SI and their transport envelope
are constituted by information elements that are defined by the ETS
standard 300 401 for digital audio broadcasting (DAB) and by the ETS
io standard 300 468 for digital video broadcasting (DVB-T).
The localizing module 74 is designed to identify the envelope of the
Service Information elements SI in the transport multiplex. For digital
audio broadcasting, the envelope is conveyed by the frame ETI to a fixed
location. For digital video broadcasting, the transport envelope is defined
is by a TDT (Time and Date Table) that is conveyed in packets provided
with an identifier having the value 14.
The UTC time demarcation module is designed for the identification
in the envelope SI, localized beforehand by the module 74, of the
information element conveying the UTC time. For digital audio
2o broadcasting, this is a group having the identifier 0/9 (FIG 0/9, FIG being

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the acronym for "Fast Information Group"). For digital video
broadcasting, it is a TDT table having the identifier OX70.
The substitution module 72 essentially comprises a multiplexer 86
that is controlled by a substitution signal given by the localizing module
74. Tlus substitution module 72 replaces the UTC bytes of the distribution
network by the bytes of the UTC value computed by the module 70.
The description that has just been given of the invention shows that
the time-resetting device comprises:
means 70 to compute the time to be transmitted to the users,
io means 74 and 7~5 to extract the time in the digital bouquet, and
means 72 to substitute the computed time for the extracted time.

Representative Drawing