BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to a digital broadcast receiver, and more particularly
to a digital broadcast receiver with a seek function of the type that when a seek
is instructed, a plurality of digital broadcast frequencies are sequentially tuned
in, and when a receivable digital broadcast station is found, the seek operation is
terminated.
2. Description of the Related Art
[0002] In Europe, so-called digital audio broadcasting (DAB) is prevailing in practice.
DAB uses orthogonal frequency division multiplex (OFDM) which is one kind of multi-carrier
modulation methods. Each transmission symbol is constituted of a guard interval and
an effective symbol to thereby allow reception highly resistant to ghosts. Each carrier
of DAB is DQPSK modulated.
[0003] DAB uses three bands: band II (87 to 108 MHz band), band III (175 to 250 MHz band),
and L band (1.452 to 1.492 GHz band). The band II and III utilize a transmission mode
1 having a transmission frame period of 96 ms and a carrier interval of 1 kHz. The
transmission mode 1 is highly resistant to multi-path and suitable for a single frequency
network (SFN), and is limited to only for use with the bands II and III. The L band
utilizes transmission modes 2, 3, and 4. The transmission mode 2 has a frame period
of 24 ms and a carrier interval of 4 kHz and is suitable for mobil reception. The
transmission mode 3 has a frame period of 24 ms and a carrier interval of 8 kHz and
is suitable for satellite broadcast or the like. The transmission mode 4 has a frame
period of 48 ms and a carrier interval of 2 kHz.
[0004] The format of a transmission frame signal in the transmission mode 1 of DAB is shown
in the upper portion of Fig. 3. There are a sync signal constituted of a NULL symbol
of 1.297 ms and a phase reference symbol (PRS: Phase Reference Symbol) in the initial
field, and seventy five OFDM symbols each of 1.246 ms in the following fields. Symbols
other than the NULL symbol are transmission symbols. A start period of 0.246 ms of
each transmission symbol constitutes a guard interval, and the remaining period of
1 ms constitutes an effective symbol.
[0005] The transmission symbol of S = 1 is PRS used for AFC (Automatic Frequency Control)
or the like, PRS being obtained through adjacent inter-carrier differential modulation
of a predetermined and specific code (called a CAZAC (Constant Amplitude Zero Auto
Correlation) code). The transmission symbols of S = 2 to 4 are FIC's (Fast Information
Channels) for transmission of information necessary for a receiver to tune in to a
desired program, auxiliary information for a program, and the like. The transmission
symbols of S = 7 to 76 are MSC's (Main Service Channels) for transmission of multiplexed
sub-channels of voices and data. Generally, one sub-channel corresponds to one program.
Information on how sub-channels are multiplexed in MSC is contained in FIC. Therefore,
by referring to FIC, a sub-channel of a program desired by a user can be located.
[0006] In the transmission mode 2, each symbol period shown in Fig. 3 is shortened by 1/4.
In the transmission mode 3, each symbol period shown in Fig. 3 is shortened by 1/8
and the number of OFDM symbols is increased. In the transmission mode 4, each symbol
period shown in Fig. 3 is shortened by 1/2.
[0007] Fig. 4 is a block diagram of a DAB receiver with a seek function.
[0008] A DAB broadcast signal (called ensemble) of, for example, in the band II, band III,
or L band caught with an antenna 1 is sent to a front end 2, and the reception signal
of the band II or III is input to an
a terminal of an RF switch 3. The reception signal of the L band is subject to a band
limitation by a BPF 4, and is passed through an AGC amplifier 5 to be input to a mixer
6 whereat it is mixed with a local oscillation signal L
0 input from a PLL circuit 7 to be frequency-converted into a band of the band III.
The signal L
2 output from the PLL circuit 7 has a frequency of f
1·(n
0/m
0), where f
1 is a frequency of a reference oscillation signal input from a reference oscillator
13 to be described later, and m
0 and n
0 take fixed values. An output of the mixer 6 is applied to a
b terminal of the switch 3.
[0009] An envelope of an output of the mixer 6 is detected by an envelope detector 9 and
output as an AGC voltage to the AGC amplifier 5. The AGC amplifier 5 lowers or increases
its gain in accordance with the AGC voltage so that an output of the mixer 6 has generally
a constant level irrespective of the antenna input level.
[0010] An output of the RF switch 3 is RF-amplifier by an RF amplifier 10 capable of changing
its gain with the AGC voltage and mixed at a mixer with a first local oscillation
frequency L
1 supplied from a PLL circuit 12 to be converted into a first intermediate frequency
signal having a center frequency of f
IF1. The output L
1 of the PLL circuit has a frequency of f
1·(n
1/m
1), where f
1 is the frequency of a reference oscillation signal supplied from a reference oscillator
13, m
1 takes a fixed value, and n
1 takes a value which is changed by a system controller made of a microcomputer to
be described later, n
1 being used for changing the tuned at a step of, for example, 16 kHz. The reference
oscillator 13 is a VCXO which changes its oscillation frequency in accordance with
an automatic frequency adjusting control voltage. The first intermediate frequency
signal is supplied to a SAW filter (elastic surface wave filter) 14 to limit a pass-band
to 1.536 MHz.
[0011] An output of the SAW filter 14 is supplied via an AGC amplifier 15 to a mixer 16
whereat it is mixed with a second local oscillation signal L
2 input from a PLL circuit 17 to be converted into a second intermediate frequency
signal having a center frequency of f
IF2 (< f
IF1). The signal L
2 output from the PLL circuit 17 has a frequency of f
1·(n
2/m
2), where f
1 is a frequency of a reference oscillation signal input from the reference oscillator
13, and both m
2 and n
2 take fixed values. The second intermediate frequency signal is supplied to an anti-aliasing
filter 18 to limit a pass-band to 1.536 MHz.
[0012] An envelope of the second intermediate frequency signal output from the anti-aliasing
filter 18 is detected by an envelope detector 19 and output as the AGC voltage to
the RF amplifier 10 and AGC circuit 15 (refer to
a in Fig. 3). The RF amplifier 10 and AGC circuit 15 lower or increase their gains
in accordance with the AGC voltage so that the second intermediate frequency signal
having generally a constant level independent from the antenna input level can be
obtained. An output of the envelope detector 19 is input to a NULL detector 20 to
detect a NULL symbol. The NULL detector 20 shapes the waveform of the NULL symbol
(refer to
b in Fig. 3), and measures a low level time Td which corresponds to the NULL symbol
period. If this low level time is coincident with a NULL symbol length of any transmission
mode defined by DAB, the NULL detector 20 outputs a NULL symbol detection signal ND
(refer to
c in Fig. 3) to a timing sync circuit 21, system controller, and the like, synchronously
with the rise timing of the envelope signal. According to the measured time period
Td, the NULL detector 20 also outputs a transmission mode detection signal TM which
represents the transmission mode (refer to
d in Fig. 3. It is assumed that Td = 1.297 ms so that the transmission mode detection
signal TM indicates the transmission mode 1).
[0013] The timing sync circuit 21 generates various timing signals during an ordinary operation,
by receiving carrier-components in the phase reference symbol PRS (effective symbol
period) input from an FFT circuit to be described later, calculating a carrier-power,
detecting a frame sync from a cepstrum obtained through IFFT of the carrier-power,
and outputting this sync detection signal to an unrepresented timing signal generator.
However, immediately after the start of ensemble reception, the timing sync circuit
21 detects the frame sync by using the NULL symbol detection signal ND input from
the NULL detector 20, and outputs a sync detection signal.
[0014] An output of the anti-aliasing filter 18 is A/D converted by an A/D converter 30.
An I/Q demodulator 31 demodulates I/Q components to recover the transmission frame
signal shown in Fig. 3. The demodulated I/Q components are subject to a FFT (Fast
Fourier Transform) process by an FFT circuit 32 constituted of a dedicated processor
to thereby derive carrier-dependent components (complex number data representative
of an amplitude and phase of each carrier) of each of
n carriers constituting an OFDM modulated wave, in the unit of symbol, where n = 1536
for the transmission mode 1, n = 384 for the transmission mode 2, n = 192 for the
transmission mode 3, and n = 768 for the transmission mode 4. The FFT circuit 32 outputs
the carrier-dependent components during the effective symbol period of PRS to a frequency
error detector 33 in response to predetermined timing signals. The frequency error
detector 33 comprises a digital signal processor having a decoding software and decodes
the carrier-dependent components of PRS through inter-carrier differential demodulation
(for PRS, a predetermined fixed code was subject to the inter-carrier differential
modulation on the transmission side), and thereafter calculates a correlation function
between the decoded carrier-components and a predetermined reference code (e.g., conjugate
of CAZAC code). The correlation function is shown in the graph of Fig. 7). A frequency
error of the tuned frequency from the DAB broadcast signal is calculated from this
correlation function. While AFC is enabled by the system controller, the frequency
error detector 33 outputs frequency error data to an integrator 34 (while AFC is disabled,
data indicating that the frequency error is zero is output). Data integrated by the
integrator 34 is D/A converted by a D/A converter and output to the reference oscillator
13 as the automatic frequency adjusting control voltage. In accordance with this control
voltage, the reference oscillator 13 changes its oscillation frequency to thereby
change the reference oscillation signal frequency f
1 and cancel the frequency error.
[0015] The FFT circuit 32 outputs FFT carrier-components (complex number data representative
of an amplitude and phase of each carrier) of each symbol (effective symbol period)
of S = 2 to 76 shown in Fig. 3 to a channel decoder 36. The channel decoder 36 performs
frequency deinterleaving, DQPSK symbol demapping, and FIC/MSC separation, and outputs
packet data called an FIG (Fast Information Group) to the system controller, the FIG
including twelve FIB's (Fast Information Blocks) obtained through error detection/correction
(Viterbi decoding) and descrambling of three effective FIC symbols each divided into
four.
[0016] MSC effective symbols are classified into eighteen symbols to reconfigure four CIF's
(Common Interleaved Frames). Each CIF contains a plurality of sub-channels each corresponding
to one program.
[0017] When a user selects a desired program by using a program select key of an operation
panel 40, the system controller 37 performs a predetermined program selection control,
and outputs information of designating a sub-channel corresponding to the desired
program, by referring to FIC information. The channel decoder 36 derives the sub-channel
designated by the system controller 37 from four CIF's, and thereafter performs time
deinterleaving, error detection/correction (Viterbi decoding), error count, and descrambling
to output the demodulated DAB audio frame data to a MPEG decoder 38.
[0018] The MPEG decoder 38 decodes the DAB audio frame data and outputs audio data of two
channels. This audio data is D/A converted by a D/A converter 39 and output as an
analog audio signal.
[0019] The operation panel 40 is also provided with a seek key. A memory 41 stores therein
broadcast frequency data of a plurality of ensembles. When a seek command is given
upon depression of the seek key of the operation panel 40, the system controller 39
performs a seek control. This seek control process will be described with reference
to the flow chart of Fig. 5.
[0020] Upon reception of a seek command, the system controller 37 supplies an AFC disable
command to the frequency error detector 33 to make the latter output data indicating
that the frequency error is zero and to fix the oscillation frequency of the reference
oscillator 13 (Step S1 in Fig. 5).
[0021] Broadcast frequency data of the first ensemble is read from the memory 41, and if
the reception signal is the band II or III, the RF switch 3 is turned to the terminal
a, whereas if it is the L band, the RF switch 3 is turned to the
b terminal. The n
1 corresponding to the first ensemble frequency is set to the PLL circuit 12 to tune
in to the first ensemble (Step S2). Next, it is checked whether the NULL symbol detection
signal is supplied from the NULL detector 20 (Step S3). If the ensemble is captured
at the present reception frequency, an output of the envelope detector 19 lowers at
the NULL symbol. The NULL detector 20 shapes the waveform of the output of the envelope
detector 19, and outputs the NULL symbol detection signal at the rise timing of the
envelope signal. Upon reception of the NULL symbol detection signal, the system controller
32 judges as YES at Step S3. Since there is a DAB broadcast signal at the present
reception frequency, the system controller 37 supplies an AFC enable signal to the
frequency error detector 33 to thereafter terminate the seek control process (Step
S4).
[0022] An output of the front end 2 is I/Q demodulated by the I/Q demodulator 31, and is
subject to FFT at the FFT circuit 32. The carrier-components of PRS are decoded through
inter-carrier differential demodulation by the frequency error detector 33, and thereafter
a correlation function between the decoded carrier-components and a predetermined
reference code is calculated. An example of this correlation function is shown in
the graph of Fig. 7 whose abscissa represents a frequency and ordinate represents
a correlation value. In accordance with this correlation function, a frequency error
of the tuned frequency from the DAB broadcast signal frequency can be calculated.
[0023] If the center of spectrum distribution of a received ensemble relative to the first
intermediate frequency is shifted toward a frequency higher than the normal center
frequency f
IF1, as shown by a solid line A in Fig. 6 (one-dot chain line in Fig. 6 indicates the
attenuation characteristics of the SAW filter 7), the corresponding correlation function
becomes as shown in the graph of Fig. 7. While the AFC enable command is supplied
to the frequency error detector 33, it outputs frequency error data representative
of the frequency error calculated from the correlation function. This frequency error
data is integrated by the integrator 34, D/A converted by the D/A converter 35, and
supplied to the reference oscillator 13.
[0024] The reference oscillator 13 changes its oscillation frequency in accordance with
the supplied control voltage, and changes the first and second local oscillation signals
L
1 and L
2 so as to cancel the frequency error. Therefore, the spectrum distribution of the
received ensemble relative to the first intermediate frequency signal shifts to the
lower frequency (refer to an arrow C in Fig. 6), and ultimately enters the pass-band
of the SAW filter 7 as indicated at A' in Fig. 8. It is therefore possible for the
channel decoder 36 to correctly recover the information of FIC and MSC. As a user
selects a desired program by using the operation panel 40, the system controller 37
instructs the channel decoder 36 to supply the DAB audio frame data of the desired
program to the MPEG decoder 38. In this manner, the desired program can be listened.
[0025] If NO at Step S3, there is no ensemble capable of being received at the presently
tuned frequency, and the system controller 37 checks by referring to the memory 41
whether there is broadcast frequency data of the next ensemble (Step S5). If not,
the seek control process is terminated, whereas if present, a corresponding n
1 is set to the PLL circuit 12, and after the new ensemble is tuned in, the above processes
are repeated (Step S6).
[0026] With the DAB receiver with a conventional seek function described above, when the
band II or III is to be received, the RF switch 3 is turned to the contact
a. Isolation between the terminals
b and
c is about 50 dB. This isolation of the RF switch 3 is not sufficient because high
AGC is incorporated in order to receive an antenna input of a minimum of - 90 dBm
according to the DAB specification. In the case of an ensemble A shown in Fig. 9A,
although the reception signal frequency-converted by the mixer 6 is attenuated by
50 dB by the RF switch 3 (refer to B in Fig. 9B), it is amplified by the RF amplifier
10 and AGC amplifier 15 (refer to C in Fig. 9C).
[0027] While an ensemble of the band II or III is sought at some tuning frequency, an ensemble
in the L band cannot be tuned with this frequency. However, if a reception signal
of an ensemble of the L band frequency-converted by the mixer 6 enters a pass band
of the SAW filter 14, the receiver operates to erroneously pull in this ensemble of
the L band and the ensemble of the band II or III cannot be correctly sought.
SUMMARY OF THE INVENTION
[0028] A digital broadcast receiver according to the present invention comprises reception
means for tuning a selected broadcast frequency to receive a digital broadcast signal
of an OFDM modulated wave in the tuned broadcast frequency; deriving means for deriving
carrier-components from an output of the reception means; program information demodulating
means for demodulating information part (FIC, MSC) of the derived carrier-components
to recover a program desired by a user; frequency error detecting means for detecting
a tuning frequency error by referring to a correlation function calculated from control
part (PRS) of the derived carrier-component and a reference code; frequency adjusting
means for adjusting the tuning frequency in the reception means to eliminate the detected
tuning frequency error; NULL detecting means for detecting a NULL symbol in the output
of the reception means; and control means for in response to a seek instruction controlling
the reception means to sequentially tune each of broadcast frequencies of the digital
broadcast signal and stop the seek operation when the NULL detecting means detects
the NULL symbol at one of the sequentially tuned broadcast frequencies and then controlling
the frequency adjusting means to conduct the tuning frequency adjustment at said one
of broadcast frequency, wherein
said control means is response to the NULL symbol detection further examines the
transmission mode and controls the reception means to stop the seek operation when
the examined transmission mode is a transmission mode aimed by the seeking.
[0029] In the above a digital broadcast receiver according, said NULL symbol detecting means
generates a transmission mode signal which represents the NULL symbol period and send
the transmission mode signal to said control means.
[0030] In the above digital broadcast receiver, said control means further judges whether
or not the tuning frequency error adjusted by the frequency adjusting means when the
seek operation is stopped is less than a predetermined value after a preselected time
period has elapsed, and resumes the seek operation if the tuning frequency error is
not less so that the reception means tunes the next broadcast frequency.
[0031] In the above digital broadcast receiver, said control means turns off the tuning
frequency adjustment operation by the frequency adjusting means during the seek operation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032]
Fig. 1 is a block diagram of a DAB receiver with a seek function according to an embodiment
of the invention.
Fig. 2 is a flow chart illustrating a seek control process to be executed by a system
controller shown in Fig. 1.
Fig. 3 is a diagram illustrating the format of a DAB transmission frame signal and
an operation of detecting a NULL symbol.
Fig. 4 is a block diagram of a conventional DAB receiver with a seek function.
Fig. 5 is a flow chart illustrating a seek control process to be executed by a system
controller shown in Fig. 4.
Fig. 6 is a graph showing a frequency spectrum of an ensemble relative to a first
intermediate frequency signal.
Fig. 7 is a graph illustrating an operation of an frequency error detector.
Fig. 8 is a graph showing a frequency spectrum of an ensemble relative to the first
intermediate frequency signal.
Figs. 9A to 9C are graphs showing a frequency spectrum of an ensemble of the L band.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0033] An embodiment of the invention will be described with reference to Fig. 1.
[0034] Fig. 1 is a block diagram of a DAB receiver with a seek function according to the
embodiment of the invention. In Fig. 1, like elements to those shown in Fig. 4 are
represented by using identical reference numerals.
[0035] A system controller 37A constituted of a microcomputer performs a predetermined seek
control process upon reception of a seek instruction entered by depressing the seek
key of an operation panel 40, and performs a predetermined program selection control
upon reception of a program selection instruction entered by the program select key.
The conditions of terminating the seek control process are that a NULL symbol is detected,
and that the transmission mode of an ensemble to be sought is coincident with the
transmission mode detected by a NULL detector 20.
[0036] The other structures are quite the same as those shown in Fig. 4.
[0037] The seek operation of the above embodiment will be described with reference to Fig.
2 which is a flow chart illustrating the seek control process to be executed by the
system controller 37A.
[0038] It is assumed that a memory 41 stores in advance broadcast frequency data of ten
ensembles of the bands II and III and the L band, in memory channels CH1 to CH10.
[0039] Upon reception of a seek command entered from a user by depressing the seek key of
the operation panel 40, the system controller 37A supplies an AFC disable command
to a frequency error detector 33A to make the latter output data indicating that the
frequency error is zero and to fix the oscillation frequency of a reference oscillator
13 (Step S11 in Fig. 2).
[0040] Broadcast frequency data of the first ensemble is read from the memory 41 in the
memory channel CH1, and if the reception signal corresponds to the band II or III,
an RF switch 3 is turned to
a terminal, whereas if it corresponds to the L band, the switch 3 is turned to a
b terminal. A value n
1 corresponding to the broadcast frequency data of the first ensemble is set to a PLL
circuit 12 to tune in to the first ensemble (Step S12). Next, it is checked whether
the NULL symbol detection signal ND is supplied from a NULL detector 20 (Step S13).
If NO, there is no possibility that the ensemble is received at the present reception
frequency. Therefore, broadcast frequency data of the next ensemble stored in the
memory 41 in the memory channel CH2 is read, and if the reception signal corresponds
to the band II or III, the RF switch 3 is turned to
a terminal, whereas if it corresponds to the L band, the switch 3 is turned to a
b terminal. The value n
1 corresponding to the broadcast frequency data of the second ensemble is set to the
PLL circuit 12 to tune in to the second ensemble (Steps S14 and S15).
[0041] When the ensemble or DAB broadcast signal is captured at the present reception frequency,
the front end 2 outputs the second intermediate frequency signal, and an output of
the NULL symbol from an envelope detector 19 lowers. The NULL detector 20 shapes the
waveform of the output of the envelope detector 19, and measures a low level time
Td. If this low level time is coincident with a NULL symbol length of any transmission
mode defined by DAB, the NULL detector 13 outputs a NULL symbol detection signal ND
synchronously with the rise timing of the envelope signal (refer to Fig. 3). By using
the NULL symbol detection signal ND, a timing sync circuit 21 detects a frame sync,
and outputs a sync detection signal to an unrepresented timing signal generator which
generates various timing signals.
[0042] Upon reception of the NULL symbol detection signal ND, the system controller 37A
judges as YES at Step S13. However, it is uncertain that the received ensemble is
the band II or III, or the L band as viewed from the output of the front end 2.
[0043] After Step S13, the system controller 37A fetches the transmission mode detection
signal TM from the NULL detector 20. If the ensemble to be sought is the band II or
III, the transmission mode 1 is used (if a transmission distance of a radio wave is
long and SFN is used, the transmission mode 1 is used in order to have a sufficient
length of the guard interval). It is therefore checked whether the transmission mode
designated by the transmission detection signal TM is the transmission mode 1 (Step
S16). If not, it is judged that the NULL symbol was accidentally detected because
an ensemble of the L band was frequency-converted to the band III, and the flow advances
to Step S14.
[0044] If the ensemble to be sought is the band II or III and the transmission mode designated
by the transmission detection signal TM is the transmission mode 1, there is a high
possibility that the presently received ensemble is an ensemble to be sought. The
AFC enable command is therefore supplied to the frequency error detector 33A, and
a timer for counting up a predetermined time is made to start (Steps S17 and S18).
[0045] If the ensemble to be sought is the L band, the permitted transmission modes are
modes 2, 3, and 4. It is therefore judged at Step S16 whether the transmission mode
designated by the transmission detection signal TM is coincident with one of the transmission
modes 2, 3, and 4. If not coincident, it is judged that the NULL symbol was accidently
detected because some ensemble of the band II or III leaked to the output side of
the RF switch 3, and the flow advances to Step S14.
[0046] If the ensemble to be sought is the L band and the transmission mode designated by
the transmission detection signal TM is one of the transmission modes 2, 3, and 4,
there is a high possibility that the presently received ensemble is an ensemble to
be sought. The AFC enable command is therefore supplied to the frequency error detector
33A, and the timer for counting up a predetermined time is made to start (Steps S17
and S18).
[0047] An output of the front end 2 is I/Q demodulated by an I/Q demodulator 31, and is
subject to a FFT process by a FFT circuit 32. Each time the carrier-dependent components
of PRS are received from the FFT circuit, the frequency error detector 33A received
the AFC enable command decodes the carrier-dependent components through inter-carrier
differential demodulation and calculates a correlation function between the carrier-dependent
components and a predetermined reference code. In accordance with the calculated correlation
function, a frequency error is calculated, and the calculated frequency error data
is supplied to an integrator 34. The frequency error data is integrated by the integrator
34, D/A converted by a D/A converter 35, and output as an automatic frequency adjusting
control voltage to the reference oscillator 13. The reference oscillator 13 changes
its oscillation frequency f
1 with this control voltage to change the frequencies of the first and second local
oscillation signals L
1 and L
2 to cancel the frequency error.
[0048] If the center frequency of the received ensemble is originally away from the center
frequency f
IF1 of the SAW filter 7 relative to the first intermediate frequency and the frequency
pull-in by AFC is impossible, then the frequency error does not become small even
if a time lapses after the AFC enable command and the ensemble cannot be received
correctly. If the detection of the NULL symbol is originated not from an ensemble
but from a dip formed during a mobile reception on the time axis of a TV broadcast
signal or the like other than DAB broadcast signals, because of fading phenomenon
or the like and if the maximum correlation value accidentally becomes equal to or
higher than the reference value S
c, the frequency error does not become small even if a time lapses after the AFC enable
command.
[0049] When the timer counts up the predetermined time, the system controller 37A checks
whether the current frequency error data fetched from the frequency error detector
33A has converged into a predetermined value or lower (Steps S19 and S20). If NO,
it is judged that the NULL symbol of the transmission mode 1 was detected because,
for example, a dip formed during a mobile reception on the time axis of a TV broadcast
signal because of fading phenomenon or the like was erroneously detected as the NULL
symbol. Then, the system controller 37A supplies the AFC disable command to the frequency
error detector 33A (step S21), and the flow advances to Step S14 whereat the next
ensemble corresponding to the memory channel CH2 is tuned in to repeat the above processed.
[0050] In this manner, a seek can be speeded up and performed correctly, without a wasteful
frequency pull-in operation.
[0051] On the contrary to the above, if YES at Step S20, the seek operation is terminated
because a program of the sought ensemble can be listen.
[0052] A channel decoder 36 recovers information of FIC and MSC from the carrier-independent
components of each symbol input from the FFT circuit 32. When a user selects a desired
program by using the operation panel 40, the system controller 37A instructs the channel
decoder 36 to output the DAB audio frame data of the desired program to a MPEG decoder
38. In this manner, the desired program can be listened.
[0053] In this embodiment, when the NULL symbol is detected at some reception frequency
of an ensemble during the seek operation and if the transmission mode detected by
the NULL detector 20 is coincident with the transmission mode 1 because if the ensemble
to be sought is the band II or III, the transmission mode is the mode 1, then the
AFC is enabled and the seek operation is terminated if the frequency error converges
to the predetermined value or lower in the predetermined time. In this manner, the
ensemble to be sought can be correctly received. If the ensemble to be sought is the
L band, the mode is only the transmission modes 2, 3, and 4. Therefore, only if the
transmission mode detected by the NULL detector 20 is coincident with the transmission
mode permitted for the L band, the AFC is enabled and the seek operation is terminated
if the frequency error converges to the predetermined value or lower in the predetermined
time. In this manner, the ensemble to be sought can be correctly received.
[0054] In the above-described embodiment and modifications, DAB broadcasting in Europe is
used. The invention is not limited only to the DAB broadcasting, but is also applicable
to other broadcasting and communications such as digital ground wave TV broadcasting
and digital satellite broadcasting.
[0055] According to the invention, when the NULL symbol is detected at some reception frequency
during the seek operation and if the transmission mode detected by the transmission
mode detecting means is coincide with the transmission mode permitted for the digital
broadcast signal to be sought, the seek operation is terminated to correctly receive
the ensemble to be sought.