JP3296721B2 - Set top converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a set top converter serving as a CATV terminal, and more particularly to a set top converter capable of receiving digital broadcasting and analog broadcasting.

[0002]

2. Description of the Related Art Recently, CATV has been receiving attention as a core of multimedia. In particular, in the United States and other countries around the world, experiments on integrated service of communication and broadcasting through CATV are being actively conducted. Under such circumstances, the level of the function of the set-top converter, which is a CATV terminal, is being improved, and in particular, it is going to evolve to a system capable of receiving digital broadcasting.

FIG. 3 is a block diagram showing the overall configuration of a set-top converter capable of receiving digital broadcasting in addition to analog broadcasting. A set-top converter for demodulating digital and analog broadcasts according to the present invention,
In FIG. 3, it corresponds to the part shown by the dotted line. Figure 3 below
An outline of the functions of the entire digital set-top converter will be described.

[0004] An input signal from a broadband network is first divided into two by a distributor 1, and the main tuners 2, 3 are connected to a QPS.
It is sent to the K data receiver 4. The QPSK receiver 4 is a message addressed to the subscriber controlled by the head-end controller,
It receives downloadable memory and other control and information data signals related to other administrative and subscriber related functions. The tuners 2 and 3 have a function of tuning all analog and digital signals, and have a function of receiving signals in a range that can be used differently for each country and for each customer.

[0005] The main digital video circuits are from the tuner 2 to the QAM receiver 5, where the digital carrier is detected and usually 27 Mb / s multiplexed with 5 or more MPEG2 compressed video signals and their associated audio data. Converted to a data stream. The MPEG bit stream is demultiplexed by the MPEG2 transport board 6.

An audio / video decompressor 7 constituting an audio / video decompression circuit performs MPEG2 decoding in real time to produce NTSC video and audio baseband signals. The video / audio multiplexer 8 handles a complete bitmap graphic and other video / audio special effects. This graphics is controlled by an internal bus directly connected to the CPU and the main memory 9. Finally, the selected signal or graphic passes through copy protection in the circuit 9 such as encoding, copy protection, and RF output modulator, and enters the output modulator. The subscriber's TV or VTR is connected to a baseband output, or RF output.

On the other hand, an analog program is sent from the tuner 3 to the analog demodulator / decoder circuit 10 and the NTSC
The baseband video / audio signal is sent to the video / audio multiplexer 8. The CPU and memory 9 all handle non-MPEG software functions and handle various applications and set-top physical functions such as tuner control, status monitor, and two-way communication. The bus interface circuit 11 controls input / output functions of a printer or other devices connected to the external communication jack.

FIG. 4 is a block diagram showing a conventional example of a set-top converter indicated by a dotted line in FIG. FIG.
In the above, the upper circuit is a demodulation decoder for digital channel reception, and the lower circuit is a demodulation decoder for analog channel reception. The digital channel is the current analog CA, as shown in FIG.
450-above 50-450MHz of TV channel
It is generally arranged in the 750 MHz band.

A CATV input signal is first divided by a distributor 1 into a digital demodulation system and an analog demodulation system. The digital channel is selected by a tuning circuit 23 including a local oscillator using a PLL circuit and a mixer via a band-pass filter (hereinafter referred to as BPF) 20, an AGC circuit 21 and an RF amplifier 22, and a first intermediate frequency. (955.75
MHz), the signal is converted to a signal of I
The signal is amplified by an F-amplifier 25, and further amplified by a frequency converter 26 comprising a local oscillator using a PLL circuit and a mixer.
It is converted to an intermediate frequency (45.75 MHz).

The received signal converted to the second intermediate frequency is
After being amplified by the IF amplifier 27, the sideband is removed by the BPF 28 composed of a SAW filter and converted into a signal in the baseband region of the third intermediate frequency by the fixed local oscillation signal f _OSC3 and the mixer 29. BPF
The signal is converted into a digital signal by an A / D converter 32 via the A / D converter 31 and then QAM demodulated by a QAM demodulator 33.

On the other hand, analog signals are BPF34, AG
After being tuned by the tuning circuit 37 composed of a local oscillator using a PLL circuit and a mixer via the C circuit 35 and the RF amplifier 36 and converted into a signal of the first intermediate frequency (955.75 MHz), the BPF 38 , Via the IF amplifier 39, by the fixed local oscillation signal f _OSC6 and the mixer 40 to be converted into a signal of the second intermediate frequency (45.75 MHz).

The received signal converted to the signal of the second intermediate frequency is demodulated into an NTSC signal by an NTSC demodulation circuit 43 after a sideband is removed by a SAW filter 42 comprising an IF amplifier 41 and a BPF 42.

[0013]

The above-mentioned conventional set-top converter is obtained by adding a demodulation circuit for receiving a digital channel to the circuit of an analog set-top box, and requires two systems of demodulation circuits for analog signals and digital signals. As a result, the circuit scale becomes large and complicated, and the cost increases. In addition, the CATV signal needs to be divided into two parts in the input system into an analog signal system and a digital signal system, and there is a problem that the loss (-4 dB) due to the distribution in the input system increases. The present invention has been made in view of the above problems, and has as its object to allow a single signal processing circuit to receive an analog signal and a digital signal.

[0014]

To achieve the above object, according to an aspect of, the present onset bright, than the first local oscillator and a first mixer equipped with a PLL circuit for converting an input signal into a first intermediate frequency signal A first frequency conversion circuit, a narrow band-pass filter for digital channel demodulation provided in the first intermediate frequency amplification circuit for amplifying the first intermediate frequency signal,
A second frequency conversion circuit comprising a second local oscillator having a PLL circuit for converting the output of the narrow band-pass filter into a second intermediate frequency signal, and a second mixer;
A switching circuit for switching the local oscillation frequency of the local oscillator by the PLL circuit according to the analog channel and the digital channel, and a distributor for dividing the output of the second frequency conversion circuit into an analog channel signal and a digital channel signal And one of the output paths of the distributor.
Band pass filter for the analog channel signal
Filter and a digital signal provided on the other output path of the distributor.
A low-pass filter dedicated to the channel signal and the above band
An analog signal demodulating circuit for demodulating an analog channel signals output from the pass filter, the low-pass fill
After the digital channel signals output from the data converting A / D, and a QAM demodulation circuit for demodulating an analog
With both channel and digital channel input signals
Is also characterized in that demodulation is performed by a double conversion method.

Therefore, CATV supplied from the input terminal
The analog channel signal and the digital channel signal of the AGC 5
2, an automatic gain control is performed, and the analog channel signal is converted to a first intermediate frequency signal of 955.7 MHz and the digital channel signal is converted to a first intermediate frequency signal of 954 MHz together with channel selection in the first frequency conversion circuit. Is done.

The analog channel signal and the digital channel signal converted into the first intermediate frequency signal have a BPF
, The analog channel signal is converted to a second intermediate frequency signal of 45.75 MHz, and the digital channel signal is converted to a second intermediate frequency signal of 5 MHz in the second frequency conversion circuit.

The second intermediate frequency signals of the analog and digital channels are divided by a distributor,
From the second intermediate frequency signal of the analog channel, a signal of a required band is taken out via a BPF, and the demodulator outputs NT signal.
Demodulated into SC video and audio signals. Also, as for the second intermediate frequency signal of the digital channel, a signal of a necessary band is taken out by an LPF, A / D-converted, and then QA-converted by a demodulator.
M demodulated.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. The present invention relates to a set-top converter for selecting both received signals of a digital channel and an analog channel by one system of receiving circuit, and FIG. 3 shows the configuration thereof. The received signal supplied from the broadband network is divided by a splitter 5 before entering the tuner.
The signal is guided to 0, and the distributor 50 divides the data I / O (QPSK RX.TX) signal and the video data.

The band of each signal to be received is going to be allocated as shown in FIG. 2, and the up line is 5-30M as a CATV telephone and a communication line with the center.
Hz, existing analog channels of CATV are arranged in a band of 50 to 450 MHz, and digital channels are arranged in a band of 450 MHz or more.

The other signal divided by the distributor 50 is 5
Through a BPF 51 having a pass band of 0 to 750 MHz, a 50 to 750 MHz wide band automatic gain control circuit (AG
C) Via a broadband high-frequency amplifier circuit (RF-AM) 52
P) 53.

The received signal amplified by the broadband high-frequency amplifier 53 is used to select a desired channel by a wideband mixer (MX) 54, a first local oscillator (f _OSC ) 55 and a PLL circuit 56, and the first intermediate frequency Is converted to In this case, the analog channel (NTSC system) is converted to 955.75 MHz as a specific example, and the digital channel (QAM system) is converted to 954 MHz. BPF5
7. The BPF 58 passes the digital channel through the pass band (954
MHz ± 3 MHz).
In particular, the BPF 58 is constituted by a SAW filter or a dielectric filter.

The received signal that has passed through the BPF 58 is amplified by an intermediate frequency amplifier 59 and then converted to a second intermediate frequency by a second mixer 60, a second local oscillator (f _OSC ) 61 and a PLL circuit 62. As a specific example, the analog channel sets the oscillation frequency (f _OSC ) of the second local oscillator 61 to 910M.
Hz, the second intermediate frequency becomes 45.7.
Converted to 5 MHz. The digital channel is the second
The oscillation frequency (f _OSC ) of the local oscillator 61 is 949 MHz
Is converted to the second intermediate frequency of 5 MHz.

The received signal converted to the second intermediate frequency is amplified by an intermediate frequency amplifier 63 and then divided by a divider 64. One analog signal is passed through an NTSC 45 MHz SAW filter composed of a BPF 65. Via NT
The signal is derived by the SC demodulation circuit 66 as a baseband signal. The other digital signal is passed through an LPF (low-pass filter) 67 having a pass band of 10 MHz or less, and then converted into a digital signal by an AD converter 68.
A QAM demodulator derives a demodulated output of the digital signal. This demodulated output is led to the next stage MPEG circuit.

As described above, according to the circuit of the present invention, CA
Both analog and digital reception signals received as TV signals can be processed by one system of reception circuit until they are converted into a signal of the second intermediate frequency.

That is, in the prior art, the analog circuit performs two-stage frequency conversion by the double conversion method, while the digital circuit performs three-stage frequency conversion by the triple conversion method. Although a receiving circuit of a system is required, in the present invention, since both the analog circuit and the digital circuit are of the double conversion type, both circuits can be formed into a single circuit. This is because the oscillation frequency of each of the first local oscillation circuit and the second local oscillation circuit is
This is because it can be set by a circuit.

In general, CATV companies switch from an analog set-top box (STB) to a digital set-top box (STB) to switch between three and four.
Admits twice the cost. This is because the digital demodulation circuit is directly added to the analog demodulation circuit as in the conventional example. However, in the present invention, it is easy to achieve both digital and analog compatibility by changing or modifying the conventional analog circuit. , And a receiving device for both analog and digital signals can be provided at low cost.

In the conventional example, since an input signal is supplied to each of the analog and digital demodulation circuits, a two-distribution circuit is required at the input end of the signal, and the performance of the video S / N deteriorates in the analog channel. However, in the present invention, since signal processing is performed by one circuit, a decrease in gain due to signal distribution at the input terminal can be suppressed.

[0028]

According to the present invention, the receiving circuit up to the demodulation circuit is a single system common to analog signals and digital signals. Therefore, the size and power consumption of both analog and digital set-top converters are reduced. Can be economical. Also, as in the conventional case, up / down
No need for two tuners for analog and digital,
The circuit configuration can be simplified by using one tuner. Furthermore, at the input end of the CATV, NTS
Since a distributor for dividing the analog TV signal and the digital TV signal of the C system becomes unnecessary, a decrease in gain due to the distributor of the high-frequency input unit can be suppressed.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention.

FIG. 2 is a diagram showing a spectrum of CATV broadcasting.

FIG. 3 is a block diagram illustrating an outline of an overall configuration of a conventional CATV receiving circuit.

FIG. 4 is a block diagram of a conventional set-top converter.

[Explanation of symbols]

 54 mixer 55 oscillator 56 PLL circuit 57 BPF 58 BPF 60 mixer 61 oscillator 62 PLL circuit 64 distributor 65 BPF 66 NTSC demodulator 67 LPS 68 AD converter 69 64 QAM demodulator

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 7/00 H04N 7/10 H04N ^7/ 14-7/173 H04N ^7/ 20-7/22

Claims (1)

(57) [Claims] 1. A first frequency conversion circuit comprising a first local oscillator and a first mixer having a PLL circuit for converting an input signal into a first intermediate frequency signal, and the first intermediate frequency signal A narrow band-pass filter for digital channel demodulation provided in a first intermediate frequency amplifier circuit for amplifying the signal; and a PLL circuit for converting an output of the narrow band band-pass filter into a second intermediate frequency signal. A second frequency conversion circuit comprising a local oscillator and a second mixer; a switching circuit for switching the local oscillation frequency of the second local oscillator by the PLL circuit according to an analog channel and a digital channel; a divider for dividing the output of the second frequency converting circuit into a signal of the signal and digital channels of analog channels, analog provided on one of the output paths of the distributor Yan'ne
And a digital channel provided on the other output path of the distributor.
A low-pass filter dedicated to a digital signal, an analog signal demodulation circuit for demodulating an analog channel signal output from the band-pass filter , and a QAM demodulation for A / D converting and demodulating a digital channel signal output from the low-pass filter. circuit
When the equipped, analog channels and digital channels
A set-top converter characterized in that both input signals are demodulated by a double conversion method.