Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1998-08-03
2003-04-29
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S345000, C375S346000, C375S230000
Reexamination Certificate
active
06556636
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a digital communication receiving apparatus used suitably for a digital audio broadcast receiving apparatus.
2. Description of Related Art
DAB (digital audio broadcasting) has been known as a digital communication using phase modulation. The DAB is practically used according to EUREKA 147 standard in Europe, the signal processing on the transmission side is described herein under.
(1) A digital audio data having the maximum of 64 channels is compressed according to the MPEG audio layer II for every channel.
(2) Each channel data resulted from the above-mentioned (1) is subjected to error correction encode processing by convolution coding and interleaving of the time axis.
(3) The result of the above-mentioned (2) is multiplexed to one channel. When, auxiliary data such as PAD is added.
(4) The result of the above-mentioned (3) is subjected to interleave processing on the frequency axis and a symbol for synchronization is added simultaneously.
(5) The result of the above-mentioned (4) is subjected to OFDM (Orthogonal Frequency Division Multiplex) processing and subsequently subjected to D/A conversion.
(6) The main carrier signal is subjected to QPSK modulation (Quadrature Phase Shift Keying) depending on the result of the above-mentioned (5), and the QPSK signal is transmitted.
The DAB receiving apparatus may therefore have the structure as shown in
FIG. 3
, for example.
In detail, in
FIG. 3
, an antenna
11
receives a DAB broadcast wave signal, the received signal is supplied to a mixer circuit
15
through a signal line comprising, in the order of passing, a band pass filter
12
, a high frequency amplifier
13
, and bandpass filter
14
, and a local oscillation circuit
16
supplies a local oscillation signal having the predetermined frequency which is variable depending on the received frequency to the mixer circuit
15
, and the received signal is subjected to frequency conversion and converted to an intermediate frequency signal SIF having a predetermined frequency.
The intermediate frequency signal SIF is supplied to the mixer circuits
21
I and
21
Q through a band pass filter
17
for intermediate frequency filtration and amplifier
18
for intermediate frequency amplification. A local oscillation circuit
22
generates a local oscillation signal having a frequency equal to the intermediate frequency of the intermediate frequency signal SIF and having a phase which is different by 90 degrees from that of the intermediate frequency signal SIF, and the local oscillation signal is supplied to the mixer circuits
21
I and
21
Q. As described herein above, in the mixer circuit
21
I and
21
Q, the intermediate frequency signal SIF is subjected to frequency conversion and the intermediate frequency signal SIF is converted to an I signal SI and a Q signal SQ, and the signals SI and SQ are outputted.
The signals SI and SQ are supplied to gain control amplifiers
23
I and
23
Q, in which the signals SI and SQ are converted to signals Si and Sq having a predetermined level, and these signals Si and Sq are supplied to A/D converter circuits
24
I and
24
Q and converted to digital data DI and DQ. The data DI and DQ are supplied to an FFT (Fast Fourier Transform) circuit
31
through digital low-pass filters
25
I and
25
Q described herein after and subsequently through amplifiers
26
I and
26
Q and are subjected to OFDM demodulation, and the OFDM demodulated data is supplied to a Viterbi decoder circuit
32
, in which deinterleaving and error correction are performed and a program (channel) is selected, and thus the digital audio data of the desired program is selected.
Subsequently, the selected data is supplied to an expansion circuit
33
, in which MPEG data expansion is performed, the data expansion circuit
33
expands the digital audio data of the desired program to the data having the original data length and outputs it, the outputted digital audio data is supplied to a D/A converter circuit
34
, in which the digital audio data is subjected to D/A conversion and is converted to an analog audio signal, and the signal is outputted to a terminal
35
.
At this time, a programmable gain control amplifier which is capable of gain controlling with a digital control signal is used as the variable gain amplifiers
23
I and
23
Q. The signal DI and DQ from the amplifiers
26
I and
26
Q are supplied to level detection circuits
27
I and
27
Q, in which the signal level (the signal level obtained when the signal DI and DQ are D/A converted) of the signal DI and DQ is detected, the detected outputs are supplied to the gain control amplifiers
23
I and
23
Q as a gain control signal, and the signal SI and SQ supplied to the A/D converter circuits
24
I and
24
Q are controlled to a predetermined constant level.
Accordingly, the signal level of the signals SI and SQ to be supplied to the A/D converter circuits
24
I and
24
Q is maintained at a constant level which matches to the dynamic range of the A/D converter circuits
24
I and
24
Q even though the received signal level from the antenna
11
changes, and thus the signals SI and SQ are A/D converted correctly to the data DI and DQ.
The above-mentioned description is the outline of the DAB receiving apparatus.
In the conventional receiving apparatus described herein above, the digital low-pass filters
25
I and
25
Q are provided to compensate the band pass filter
17
for processing.
For example, as shown in
FIG. 4A
, if there is a disturbance signal SUD at a frequency (fD+&Dgr;f) near the broadcast wave signal SD (center frequency fD) desired to be received, for example as shown in
FIG. 4B
, the output signal from the mixer circuit
15
contains undesirably the signal component SIFUD which is resulted from the disturbance signal SUD through frequency conversion at the frequency (fIF+&Dgr;f) in the case of down heterodyne conversion in addition to the intermediate frequency signal SIF (center frequency of fIF) which is resulted from the desired wave signal SD through frequency conversion.
The inclusion of the disturbance component SIFUD in the output signal from the mixer circuit
15
as described herein above results in undesirably inclusion of the signal component SBBUD as shown in
FIG. 4C
generated from the disturbance component SIFUD through frequency conversion at the position of frequency &Dgr;f in addition to the I signal SI and Q signal SQ of the base band, and the disturbance component SBBUD affects adversely following data processing as a matter of course.
To remove the disturbance component SIFUD contained in the output signal from the mixer circuit
15
, the band pass filter
17
having a passing characteristic as shown with a dashed line in
FIG. 4B
is provided on the step next to the mixer circuit
15
as described herein above.
However, the band pass filter is an analog circuit, therefore the center frequency and passing characteristic disperse. It is difficult to prescribe the temperature characteristic to a desired characteristic. Depending on the model, because the intermediate frequency fIF of the intermediate frequency signal SIF usually is in a range as high as from several ten MHz to several hundred MHz, if the disturbance signal SUD has a frequency near that of the intermediate frequency signal SIF, the dispersion becomes the more significant. Even if the band pass filter
17
were prescribed to a desired characteristic, such transmission receiving apparatus is disadvantageous in that parts cost is high and the apparatus size is large.
Because the band pass filter
17
can not remove the disturbance component SIFUD sufficiently, the digital low-pass filters
25
I and
25
Q are provided to remove the disturbance component SBBUD. In this case, because the low-pass filters
25
I and
25
Q comprise a digital circuit, required characteristic is obtained easily and stably. Further, the frequency of the I signal SI and Q signal SQ is low and, the disturbance component SBBUD is easily removed even if the frequency o
Kumar Pankaj
Maioli Jay H.
Pham Chi
Sony Corporation
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