Digital audio broadcasting receiver

Details Digital audio broadcasting receiver Digital audio broadcasting receiver

1999-09-29

2003-03-25

Corrielus, Jean (Department: 2631)

Pulse or digital communications

Receivers

C375S259000

Reexamination Certificate

active

06539065

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a digital audio broadcasting receiver. In particular, the present invention relates to a digital audio broadcast receiver for receiving digital audio broadcast signals, e.g., Eureka 147 digital audio broadcasting (DAB).
2. Description of the Related Art
Conventional digital audio broadcasting receivers have been proposed that are compatible with Eureka 147 DAB system based on an OFDM (orthogonal frequency division multiplex) method, as disclosed in General-purpose and application-specific design of a DAB channel decoder, EBU Technical Review Winter 1993, pp.25-35 and Japanese Laid-Open Publication No. 10-126353.
FIG. 9
illustrates a conventional digital audio broadcasting receiver
2000
based on the OFDM method, which includes an RF circuit
100
for converting a radiofrequency signal received from a digital audio broadcast transmitter into an analog base band signal; an analog-digital (AD) converter
101
for converting the analog base band signal into a digital base band signal via sampling; a null symbol detector
102
for detecting a null symbol from the power envelope of the analog base band signal for determining a position at which to start frame processing for the first transfer frame at the time of receiving; an OFDM demodulator
103
for subjecting each symbol to OFDM demodulation by sequentially extracting, with a predetermined symbol cycle, a predetermined number of samples of null symbols, reference symbols, and data symbols from the digital base band signal which is output from the AD converter
101
and sequentially applying FFT (fast Fourier transform) thereto; a digital demodulator
104
for subjecting the output from the OFDM demodulator
103
to a &pgr;/4 shift DQPSK (differential quadriphase phase shift keying) demodulation; an error correction circuit
105
for performing error correction for the output of the digital demodulator
104
; and an audio decoder
106
for extracting from the output of the error correction circuit
105
the audio data which has been compressed on the transmitter side and expanding the audio data into a PCM signal so as to generate audio data which contains plurality of audio samples. The audio data which is output from the audio decoder
106
is reproduced by an audio reproducer (not shown) into sounds.
The conventional digital audio broadcasting receiver
2000
further includes a CIR calculator
107
for calculating the power characteristics of a channel impulse response (hereinafter “CIR”) of a transfer path based on the result of the FFT performed for the reference symbols; a VCXO controller
108
for detecting a difference in frequency between the clock signal on the transmitter side and the clock signal on the receiver side based on the calculation results by the CIR calculator
107
for controlling the voltage for a voltage controlled crystal oscillator (hereinafter “VCXO”) on the receiver side so as to equalize the clock signal on the receiver side to the clock signal on the transmitter side; a digital-analog (DA) converter
109
for converting the control data from the VCXO controller
108
into an analog signal; a VCXO
110
which is capable of oscillating at various frequencies in accordance with a control voltage that is based on the output from the DA converter
109
; and an AD clock signal generator
111
for dividing the clock signal for the VCXO
110
so as to generate a sampling clock signal that defines the sampling cycle of the AD converter
101
.
As shown in
FIG. 10
, one transfer frame TF includes a null symbol TFN which has a very low signal level for indicating the start position of a transfer frame; a reference symbol TFR containing known information; and a plurality of data symbols TFD which represent data for transfer. The digital audio broadcasting receiver
2000
is operable, when starting reception, so as to start a FFT process responsive to the OFDM demodulator
103
receiving the null symbol detection signal from the null symbol detector
102
via a switch
120
which may be controlled by a CPU (not shown). The null symbol TFN, reference symbol TFR, and data symbol TFD which have been output from the AD converter
101
are sequentially subjected to FFT processes by the OFDM demodulator
103
, preferably from a central portion of a guard interval of the null symbol TFN, at intervals corresponding to symbols (TFN, TFR, TFD). The reference symbol which has been subjected to the FFT process by the OFDM demodulator
103
and converted into a frequency signal is sent to the CIR calculator
107
. In the CIR calculator
107
, the reference symbol is multiplied by a conjugate complex number of a known reference symbol, and its result is subjected to an IFFT (inversion fast Fourier transform), whereby the channel impulse response (CIR), which represents the transfer path characteristics along the time axis, is calculated. By calculating the CIR power characteristics, the temporal relationships between a plurality of received waves, e.g., a direct wave and reflected waves, can be known.
As shown in
FIG. 11
, a direct wave
1101
and reflected waves
1102
are detected from the CIR power characteristics. As shown in
FIG. 10
, each symbol (TFN, TFR, TFD) has a guard interval (GI) at the beginning for attaining tolerance for the reflected waves
1102
. A guard interval GI is a copy of the last ¼ of each symbol (TFN, TFR, TFD) excluding the guard interval GI. Accordingly, the number of samples of each symbol (TFN, TFR, TFD) is {fraction (5/4)} times as many as the number of samples to be subjected to FFT.
If any reflected waves
1102
are present, the reflected waves
1102
will interfere with a subsequent symbol because the reflected waves
1102
will be delayed behind the direct wave
1101
. Accordingly, the OFDM demodulator
103
applies FFT to subsequent symbols so as to ensure that they do not contain any delayed components of preceding symbols, thereby reducing inter-symbol interference and enabling substantially error-free reception. By utilizing the fact that each symbol has a length which is {fraction (5/4)} times the number of samples which need to be subjected to FFT, the FFT may be performed for a portion extracted from the center of the guard interval GI such that no reflected waves
1102
due to any preceding symbol are contained in that portion. As a result, at least those delayed waves which are within ½ of the guard interval length are prevented from interfering with subsequent symbols.
In order to ensure that the center of gravity of the CIR power characteristics shown in
FIG. 10
is located at the center of the guard interval GI, the VCXO controller
108
controls the clock for the VCXO
110
in the following manner: If the center of the guard interval GI is located temporally before a point which corresponds to ½ of the guard interval GI, then the FFT will be performed for a portion which is extracted too late; therefore, the clock for the VCXO
110
is made faster so that an adequately “earlier” portion is extracted. Conversely, if the center of the guard interval GI is located temporally after a point which corresponds to ½ of the guard interval GI, then the FFT will be performed for a portion which is extracted too early; therefore, the clock for the VCXO
110
is made slower so that an adequately “later” portion is extracted. If the first impulse coincides with the center of the guard interval GI, then at least those delayed waves which are within ½ of the guard interval length are prevented from causing intersymbol interference.
Thus, by controlling so that the impulse position in the CIR power characteristics coincides with the center of the guard interval GI, intersymbol interference due to reflected waves
1102
can be suppressed. Also, the fixed impulse position means the same DAB transfer frame length for both transmission and reception, which in turn means stable reproduction of audio signals due to synchronization of the receiver-side audio reproduction cl

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