Pulse or digital communications – Synchronizers – Frequency or phase control using synchronizing signal
Reexamination Certificate
1998-07-29
2001-08-14
Pham, Chi (Department: 2631)
Pulse or digital communications
Synchronizers
Frequency or phase control using synchronizing signal
C375S344000
Reexamination Certificate
active
06275551
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an apparatus for receiving digital information signals, and more particularly, to a digital information signal receiving apparatus which is operative to receive a digital information signal, such as a digital audio broadcasting signal, and to obtain a reproduced information signal, such as a reproduced audio signal, based on the digital information signal received thereby.
2. Description of the Prior Art
Although an analog audio broadcasting system which includes an amplitude-modulated (AM) audio broadcasting system in which audio signals are transmitted through the form of an AM audio information signal and a frequency-modulated (FM) audio broadcasting system in which audio signals are transmitted through the form of a FM audio information signal, have been put to practical use for a long time in the field of audio broadcasting, there has been recently proposed to introduce a digital audio broadcasting system in which audio signals are transmitted through the form of a digital audio information signal for the purpose of improving quality of audio information transmitted or received in the system. Especially, in the region of Europe, the digital audio broadcasting system called “DAB” has been already put to practical use in some countries.
The digital audio information signal transmitted from a broadcasting station under the digital audio broadcasting system is called a digital audio broadcasting signal. The digital audio broadcasting signal carries not only audio information data forming a digital audio signal but also service information data representing service information, such as weather forecast, traffic information and so on, and further carries control information data which are necessitated for reproducing the digital audio signal based on the audio information data and the service information based on the service information data on the receiving side. Then, the digital audio broadcasting signal is a modulated wave signal obtained by modulating a carrier wave signal with digital data, such as audio information data, service information data, control information data and so on, in accordance with the Orthogonal Frequency Division Multiplexing (OFDM) system, and the digital data transmitted by the digital audio broadcasting signal is composed of a series of frame units, each of which is called a transmission frame.
The transmission frame has a time duration of, for example, 96 ms and contains 77 segmental periods each named Symbol and identified with Symbol
0
to Symbol
76
, respectively, as shown in FIG.
1
. The Symbol
0
is a null Symbol where the modulated wave signal does not exist in actual fact. The Symbol
1
is a reference Symbol for synchronization. The Symbol
0
which is the null Symbol corresponds to about 1.3 ms and each of the Symbol
0
to the Symbol
76
corresponds to about 1.25 ms.
A channel named Synchronous Channel is allocated to the Symbol
0
and the Symbol
1
, a channel named Fast Information Channel (FIC) is allocated to the Symbol
2
to the Symbol
4
following the Symbol
1
, and a channel named Main Service Channel (MSC) is allocated to the Symbol
5
to the Symbol
76
following the Symbol
4
, as shown in FIG.
1
. The MSC is composed of a series of frames each named Common Interleaved Frame (CIF) for transmitting the audio information data and the service information data. The FIC is composed of series of blocks each named First Information Block (FIB). The FIB contains a couple of portions of a FIB data field and an error checking word, and the FIB data field is composed of a series of groups each named Fast Information Groups (FIGs). The FIC thus formed transmits the control information data representing multiplex configuration information (MCI) and other information.
Each of the Symbol
1
to the Symbol
76
contains a couple of portions Gi and Se. The portion Gi is named Guard Interval and corresponds to about 0.246 ms. The portion Se is named Effective Symbol and corresponds to about 1 ms. An end portion Se′ of the Effective Symbol Se contains the same data as those contained in the Guard Interval Gi.
In a digital information signal receiver which is used for receiving the digital audio broadcasting signal which is the modulated wave signal obtained by modulating a carrier wave signal with digital data, such as audio information data, service information data, control information data and so on, in accordance with the OFDM system and obtaining a reproduced audio signal based on the digital audio broadcasting signal received thereby, the digital data, such as the audio information data, the service information data, the control information data and so on, obtained based on the received digital audio broadcasting signal, may be accompanied with errors resulting from undesirable frequency offset of the received digital audio broadcasting signal. Accordingly, it has been previously proposed to detect the frequency offset of the received digital audio broadcasting signal on the strength of the digital data obtained based on the received digital audio broadcasting signal and to provide a frequency synchronizing control to the received digital audio broadcasting signal in response to the detected frequency offset, so that the frequency offset of the digital audio broadcasting signal is reduced.
The detection of frequency offset of the received digital audio broadcasting signal which is required for the frequency synchronizing control to the received digital audio broadcasting signal has been previously carried out with use of the reference Symbol for synchronization contained in each of the transmission frames constituting the digital data. That is, the reference Symbol is obtained from the digital data and caused to be subjected to frequency analysis and the frequency offset of the received digital audio broadcasting signal is calculated based on the result of the frequency analysis to which the reference Symbol is subjected, so that the frequency offset of the received digital audio broadcasting signal is detected.
Such a detection of frequency offset of the received digital audio broadcasting signal as carried out with use of the reference Symbol for synchronization is only practiced at every transmitting frame. Therefore, in the frequency synchronizing control to the received digital audio broadcasting signal, it is feared that a relatively long time is required to put the received digital audio broadcasting signal into a frequency synchronizing condition and further feared that the received digital audio broadcasting signal put in the frequency synchronizing condition is easily influenced adversely by undesirable variations in the digital audio broadcasting signal resulting from fading thereon or the like.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an apparatus for receiving digital information signals, by which a digital information signal for transmitting digital data composed of a series of transmission frames each containing a plurality of segmental periods each named Symbol and including a reference Symbol for synchronization is received, frequency offset of the received digital information signal is detected on the strength of digital data obtained from the received digital information signal, and the received digital information signal is subjected, in response to the detected frequency offset, to a frequency synchronizing control so that the frequency offset of the received digital information signal is reduced, and which avoids the aforementioned disadvantages encountered with the prior art.
Another object of the present invention is to provide an apparatus for receiving digital information signals, a digital information signal for transmitting digital data composed of a series of transmission frames each containing a plurality of segmental periods each named Symbol and including a reference Symbol for synchronization is received, frequency offset of the recei
Fukami Tadashi
Nakamura Jin
Nomura Kiyoshi
Tsuruoka Tatsuya
Burd Kevin M
Maioli Jay H.
Pham Chi
Sony Corporation
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