JPS60187144A - Optical transmitter and receiver - Google Patents

Abstract

PURPOSE:To eliminate the influence of the nonlinearity of a light emitting element upon the quality of a transmission signal by processing a video, a sound, and a digital signal by frequency-division multiplexing and previous pulsation FM modulation, and then converting signals into light. CONSTITUTION:A video signal V, FM-modulated sound signal, and FSK-modulated digital signal D are supplied to a multiplexer 9 for frequency division multiplexing. The frequency-division multiplexed signal is supplied to a pulsation modulator 10, which imposes pulsation FM modulation. The pulsation FM-modulated signal is amplified by a driving circuit 11 and a light emitting element 12 is driven to impose light intensity modulation. This light signal is transmitted through an optical fiber 3 and frequency-divided by reception-side filters 16, 18, and 21 to obtain the video signal V, sound signal A, and digital signal D. The demultiplexed signals are demodulated by respective demodulators and outputted.

Description

[Detailed Description of the Invention] V) Technical Field This invention relates to a video, audio, and data transmission/reception system,
Related to ATV systems, optical transmission systems, etc.

(B) Prior art and its problems (1) If video, audio, data signals, etc. are optically transmitted and received using the baseband method or the AM method, the drive current vs. the light emission is A wide linear range of output characteristics is required. This makes it expensive,
There is a drawback.

In the baseband and AM systems, as shown in FIG. 2, only the linear region can be used because nonlinear distortion occurs when the nonlinear region of the light emitting element is used. This is because if a non-linear region is used, the saturation, hue, or luminance level will change, deteriorating the transmission signal quality.

FIG. 2 is a graph illustrating the relationship between the drive current I and the light emission output P of the light emitting element. There is a linear region in the middle where I and P have a linear relationship.

(2) Although the operating range of the light emitting element changes due to changes in temperature, the light emitting element must always operate in a linear range.

Therefore, an adjustment circuit is required to always bring the operating range of the light emitting element into the linear range.

(3) When frequency multiplexing multiple signals and transmitting them, cross-modulation occurs between each signal.

In the case of baseband transmission, transmission line distortion, 76 elements r-1
This is because it is directly affected by i.

(4) When using the FM method, there is a drawback that the modulator has a high 411i and is not suitable for IC implementation.

The FM system requires a distortion-free sine wave oscillation circuit. This requires a crystal oscillator and a coil.

(1) Object of the Invention The present invention aims to overcome the above-mentioned drawbacks of transmitting video, audio, and digital fM9 using baseband, AM, or FM systems.

In the present invention, a video signal, an audio signal, and a digital signal are subjected to frequency division multiplexing (FDM), premodulated by pulsed FM, and the signals are charged, converted, and transmitted through an optical fiber. In this way, it is possible to simplify the system 1'ffj, lower the price, and improve the quality.

(:I-) Structure of the Invention The present invention provides an optical system for transmitting and receiving a video signal (color TV signal), an audio signal, and a digital signal (each may be of several types) using an optical signal. Provide transmitting and receiving equipment.

FIG. 1 shows a block diagram and frequency arrangement of an optical transmitter/receiver circuit according to the present invention.

The transmitting/receiving circuit of the present invention multiplexes the video signal, the audio electrical signal A, and the digital electrical signal by M frequencies (FD
M) an optical transmitter 1 that converts the optical signal into an optical signal and transmits it; an optical transmitter 1 that transmits the optical signal; and an optical transmitter 1 that transmits the optical signal; It consists of an optical receiver 2 that converts the signal into an electrical signal and outputs each signal.

As one transmission method, a pulsed frequency modulation method is adopted. Pulsed F'M (abbreviated as) is a type of FM that transmits a signal by changing the repetition frequency of pulses. Although the pulse repetition frequency changes in an analog manner, for each pulse, the "L i+ time" is equal to the "H" time of the pulse, and the duty is 50% of the pulse train.

The pulse ratio FM does not require a wide range of linearity in the drive current vs. light emission output characteristics of the electrical/optical conversion element. This is because the light emission output does not take values that change continuously, but only has two values, H and L.

FIG. 3 shows the relationship between the drive current I and the light emission output P of the light emitting element, and it is possible to set a wide allowable operating range including a non-linear portion. Since the difference between the two levels H and L can be made large, the optical carrier can be made large, and there is no need to readjust it due to changes in the degree of wetting.

Figure 4 shows the pulse fhi FM modulated wave (
b). The original signal fluctuates slowly and continuously.

The pulse repetition rate of the modulated wave increases in proportion to the voltage level of the original signal. Duty is 50%,
Since the pulse width (on width and off width are equal) varies, the average power of the pulse is large, but the DC component is always H,!
: The value is 1/2 of the L level and does not change at all.

In this way, the process of modulating the human input signal from the Shinso source before performing light intensity modulation using an electro-optic conversion element such as an LD or LED is called premodulation.

Premodulation formats include PPM (pulse position modulation), P
WM (Pulse Width Modulation), PFM (Pulse Frequency Modulation, J)
, PIM are also known.

However, all of these modulation schemes do not use pulses with coincident on and off times. For this reason, 10 streams will always fluctuate.・Another good thing about pulsed FM is that its oscillation waveform is a rectangular wave train with a duty cycle of 50%, so when it is Fourier transformed, there are no even-order distortion components.
That's what it means.

FIG. 5 is an explanatory diagram for Fourier transforming a pulse train with a duty of 50%. The X-axis is set between H and L, and calculations are made with H set to 1 and LQ set to -1. Figure 5 (
A) is taken with the y-axis aligned with the pulse.
The Fourier class i f(x) is expressed as an odd function, (1). FIG. 5(b) With the +d 3' axis set at the center of the pulse, the Zulier series f (x) is (2). In any case, the duty cycle is 50%
The Fourier transform components of the rectangular wave pulse train include only odd-order harmonics, including one even-order harmonic.

When a harmonic is received, it becomes a 51) component when demodulated. Therefore, harmonics can also be called distortion components.

Harmonics have a frequency that is an integer multiple of the signal component (j), but since the variable dt4 signal has a finite bandwidth, an integer multiple of the minimum frequency f min of the bandwidth B is the maximum frequency f +++ a
It may even be smaller than the xO value. In this case, the received modulated signal will contain harmonics near the upper limit of the bandwidth B.

Assuming that the lowest order harmonic other than the fundamental wave (first order) of the modulated signal is as follows, ・To exclude all harmonics from B, l fmin ≧ fmax (Must be 3J.

Let fc be the frequency of the carrier wave. Since this is the center of the modulation (No. 4 occupied band), fmin = fc - B/2 (4) f'max =
fc + B/2 (5). Substituting (4) and (5) into (3) yields. Bandwidth B is determined by the amount of signal to be transmitted. It is convenient that the carrier frequency fc is lower. This is because there are restrictions on the speed of electronic circuit elements, and the faster the electronic circuit elements operate, the more expensive they become.

The part related to e in equation (6) decreases as e becomes larger. If there is a second harmonic, fc is (3/2)
Must be greater than B. However, as in the present invention, the lowest harmonic is 3rd order ((1!=8), so to speak, f
It is good if c is larger than B. In other words, since there are no secondary harmonics, it is possible to selectively filter out harmonics with a lower frequency by B/2.

Figure 6 shows the power spectrum 2 with respect to the frequency f of the modulation signal.
11 is a diagram schematically illustrating. K) is the case where the fundamental wave component and the third harmonic component do not overlap (f'c--EI
S −P−−−′, + /I+1 + 胃u′ −+ k
bl-Q'/+6'mM oil, J=11'
When there is a claw in the shaded area of LAZ (fc(B) is shown).

Such a pulsed FM modulator is a voltage controlled oscillator based on emitter-coupled multi-by-break as shown in FIG.
This can be realized by using a scillator (VCO).

This circuit does not require components such as crystal oscillators or coils. It is best suited to be constructed using a monolithic IC. In fact, there are already some ICs on the market. For example, it is a VCO with Motorola MC4024:', r.

Control voltage vd is applied to the base of Tr5 at Ij. Tr
The emitter of 5 is connected to four transistors Tr6 to Tr9.
is connected to the base of The emitters of these four transistors are grounded by resistors Rθ of the same value.

Since they are emitter followers, the emitter voltages of these four transistors are all equal, and the control γ11゛shovd
From, base, emitter city 1f: descent (忽10.7■
) minus 2vbe. Then, the minimum value of the collector voltages of the four transistors is limited by this common emitter voltage ve Me = Va - 2Vba (7).

On the other hand, two sets of Darlington-connected transistors Trl and Tr4 and Tr2 and Tr3 constitute an oscillator.
is selectively turned on or off.

In this example, Tri, Tr4 are on, Tr3, Tr
2 is off. It is assumed that Vcc (power supply) is 5V. Di, D2 are Tr3, Tr
The 40 base pressure is increased so that it does not fall below 4.3v.

The base of Tr4 is 5V since Tr2 is off. The base of Tr3 is 4 when Trl is on.
．． It is 3v.

The base of Trl is 0.0% lower than the base of Tr4. Since the voltage is lowered by the base and emitter of the TV, it is 4.3V (slightly higher than this for serious people). The emitter of Trl is 3.6V because it is lower than this by the base emitter.

Assuming that the emitter current is 2 summers, half of I flows through Tr6. However, the remaining ■ flows into capacitor C and charges it.

Since the other end of capacitor C is connected to the emitter of Tr2 which is off, the voltage is 1. Determined by Tr9.

The two transistors Tr5 and Tr9 are given the same forward voltage and have the same emitter resistance, so their collector currents should be equal.Therefore, the capacitor C has the collector current of Trl)6ε,2 A current of -minutes I flows.Of course, the value of I changes with charging.

As charging progresses, the emitter voltage IE of Trl increases, so Tri is turned off. The base pressure of Tr3 is 1 per 5v.
; Therefore, Tr3 and Tr2 are turned on. Tr4 is turned off. The capacitor C connected to the emitter of Tr2 is lifted up, and Tri and Tr4 are turned off more reliably. Capacitor C (which undergoes positive feedback in a very short period of time) is charged in the opposite direction. In this way, the capacitor is repeatedly charged and discharged in both directions. The time required for charging and discharging is equal to the pulse width.

When capacitor C is charged, one voltage is 3.6V and the other voltage is Ve in equation (7). Should be charged and discharged
The voltage is (3,6-Ve). If there is less of this, charging and discharging can be done in a shorter time, so the pulse repetition frequency will be higher.

In this way, it is possible to obtain a pulse train with a frequency proportional to vd.

Although the capacitor C is external, the transistor, diode, and resistor are integrated into a monolithic IC.

Although Trl, Tr2, Tr(3, and Tr9 are symmetrical to each other, the charging and discharging of the capacitor C is also performed symmetrically. Therefore, the length of the H pulse and the pulse L time are equal. Therefore, the duty A 50% pulse train is obtained.

FIG. 8 is a pulse waveform diagram of the output vO. 4.3v and 5
It becomes a pulse that takes two values of v.

Next, the transmitter 1, the receiver 2, and the optical fiber 3 are connected to the transceiver (transmitter 1, receiver 2, and optical fiber 3) to transmit the video signal V to the required level. It is amplified by amplifier 4.

The audio signal A is amplified to an appropriate level by an amplifier 5,
FM modulation by FM modulator 7, 11. ＋I am ejaculated. This is AM
You may decide to modulate it.

The digital signal is FSK (Frequency,
y5hif'tKeying) is modulated. This uses a device that generates two types of pulses of the same wave number, such as an oscillator and a frequency divider, to make ``H'' correspond to a pulse of fh frequency and L' to a pulse of f1 frequency.

Digital / l / signal or P S K (Pha
seShift Keying) may be modulated.

A video signal V, an 8-multiple signal A subjected to FM modulation (or AM modulation), and a digital signal subjected to FSK modulation (or PSK modulation) are frequency-division multiplexed in a synthesizer 9.

The frequency spectrum in the synthesizer 9 is shown in the lower left of FIG. The horizontal axis represents the frequency, and the signals are synthesized so that the video signal v1, audio duplex signal, A1 digital signal, and the like are included in order from the lowest frequency to the lowest. -1 of the frequency; the limit is 7MH2.

In this way, the frequency division multiplexed (FDM) signal is
A pulsed FM modulator 10 performs pulsed FM modulation. The reason for pulsed FM modulation is as described above.

The pulsed FM modulated signal is amplified by the drive circuit 11 and drives the light emitting element 12 . The light emitting element 12 is
Light emitting diode (LED) or laser diode (LD)
). The light intensity of the light emitting element 12 is modulated. This optical transmission signal is transmitted through the optical fiber 3.

The frequency spectrum of the optical transmission signal is shown in the lower right of Figure 1. The horizontal axis is frequency (MHz).

- Give an example.

Maximum frequency of pulsed FM signal fmax 30 MHz
Minimum frequency of pulsed FM signal fmin IQ MH2
Carrier wave frequency fc 20 MH2 Variation θ1 Signal occupied bandwidth 8 20 MH2 Maximum frequency deviation Δf6 MH2 The highest frequency of the frequency division multiplexed signal fh 7 MH2 can be selected. Between these degrees, B = fm
ax −fmin (3) B = 8f + '2fh
(9) There is the following relationship.

The optical transmission signal is transmitted to a desired location via an optical transmission path such as an optical fiber 3 and guided to an optical receiver 2 .

The optical receiver 2 includes a photoelectric conversion element f13 such as a photo diode or a d-avalan photo diode APD. The photoelectric conversion element 13 converts the optical transmission signal into an electrical signal.

conversion? [The signal is amplified by preamplifier 14.]

The amplified signal is demodulated by a pulsed FM demodulator 15. This results in a composite signal having a frequency spectrum as shown in the lower left of FIG.

Since the video signal vs. the audio signal A1 and the digital signal have different frequencies, they are frequency-divided and separated by filters 1B, 1B, and 21. That is, the video signal Vsft is separated into audio signal A1 and digital signal components.

For example, if duplex '72' A is F'M modulated, then
The FM demodulator 19 demodulates the signal. If AM modulated, AM de-jd is performed.

The digital signal is demodulated into the original digital signal by the FSK demodulator 22.

Amplifiers 17, 20, 23 output video signals v1 audio duplex A1
Amplify digital signals and output them to external circuits.

In the example shown in FIG. 1, the frequency axes are assigned in the order of digital signal, single-tone multiple signal, A, video signal, and V. However, the present invention is not limited to this, and the digital signal and the audio signal A may be interchanged.

Further, the number of video, audio, and digital signals is not limited to one each, and there may be two or more.

For example, an optical transmitter/receiver can be constructed in the same manner even if the combination is video, audio, audio, secondary audio, and digital.

Effects (1) The light-emitting element used in the transmitter does not need to have a drive tube: current vs. light emission output characteristic that is linear over a wide range.

As a transmission method, a frequency modulation method is adopted as E modulation before optical intensity modulation. Therefore, the nonlinearity of the light emitting element does not affect the quality of the transmitted signal.

Requirements for light emitting elements are relaxed, and it is not necessarily necessary to use expensive light emitting elements.

(2) The operating point of the light emitting element, which is 2=1 due to changes in temperature, changes in required light exposure, etc., is not necessary. Therefore, the adjustment circuit CAGC) for this purpose is also unnecessary.

Furthermore, the carrier wave power can also be increased. Frequency modulation (
Since the pulsed FM) method is used, linearity is not required, and the allowable operating range is expanded as shown in FIG. Therefore, the difference between the on and off light amounts can be increased, and the carrier wave power can be increased.

(3) There is an effect of improving S/N due to wideband gain.

In addition, no Fl' f+lli S/N for the video signal
Compared to this, the J1ζ value S/N is approximately 10 dB larger.

This is because pulsed FM is also a type of frequency modulation, and is either a noise spectrum or a noise spectrum that enters the discriminator.

(4) As a spectral component of the transmitted waveform, there is always a second-order distortion component, so as shown in Figure 6, even if the carrier frequency fc is set low (f'c≧B), it will not be equal to the fundamental wave. The overlapping of harmonics (3'6<1-) can be prevented as explained with reference to FIG.

As shown in Figure 5, the pulsed FM onset waveform is:
Since it is a rectangular wave with a due-recycle of 50%, there is no chance that even-order components will be included in the two-reel components.

(5) Since there is no baseband component, there is no need to worry about cross-modulation between the signal and modulation signal components due to transmission path distortion.

The average value of a rectangular wave with a frequency of 50% is constant (average value of fCx - constant). Therefore, there is no DC level fluctuation. Therefore, there is no baseband component.

This is shown in Figure 91. The broken line represents the name of the Earl of Bassvant. Modulation signal components are shaded. If it is a baseband pompous component, there is a risk that harmonics will be generated due to (1<transmission il"t'I distortion, and this and the modulation signal component have a humidity i:a.

Pulse F''lV modulation (P F M ) on No. 41 (C)
The waveform of is shown. Pulse IIIM is constant and very narrow. In this case, the iU flow component shows changes similar to the original borrowing. Since the DC component fluctuates, a baseband component is generated.

(6) Pulse (TISFM modulator is suitable for IC implementation.

A modulator for pulsed FM can be realized by a VCO using an emitter-coupled multi-high break (illustrated in FIG. 7). Since this does not require a crystal oscillator or a coil, it can be easily integrated into an IC.

[Brief explanation of drawings]

FIG. 1 is a block diagram of the 14-piece optical transceiver of the unexploded optical transceiver. FIG. 2 is a characteristic diagram showing the relationship between the drive current (2) of the light emitting element and the light emission output P. The d'1 capacity operating region in the case of baseband and AM systems is shown as a linear region. Figure 3 is the same characteristic diagram as Figure 2. The consummation operation area of the present invention is shown. Figure 4 is a diagram for explaining the pulsed FM modulation method.
(a) shows a waveform diagram of the original signal, (b) shows a waveform diagram of pulsed F'M modulation, and (C) shows a waveform diagram of pulsed FM modulation (PFM). Figure 5 shows a pulse with a duty cycle of 50% (R1
? An explanatory diagram for calculating 61 Fourier transforms (shaped waves). (
(a) shows the case where X=Q is taken at the beginning of the pulse, and (b) shows the case where X-0 is taken at the center of the pulse. Figure 6 shows the spectrum explanation when the fundamental wave component and the harmonics (triple components) are in one phase (a) and l[1y!
1゜Figure 7 shows a voltage controlled oscillator (Voltage Controlled 0) used for pulsed FM modulation.
scillator? VCO) circuit example diagram. #0 KAIA-EkQl: VIIT1+1. ;blindness 1
-L cape-C・111VLjJ4-+ahl is a rectangular wave with 5enV and L of 4.3■. FIG. 9 is a spectrum diagram for explaining that there is no risk of cross-modulation because the signal of the present invention does not include the baseband component. 1 ...... Light transmitter 2 ...
・・・・・・ Light receiver 3 ・・・・・・・
... Optical fibers 4 to 6...
Amplifier 7...FM modulator 8...FSK modulator 9...
...... Synthesizer 10 ...... Pulsed FM modulator 11
...... LD or LED drive 1Hil path 12 ...... Light emitting element r・13 ・
・・・・・・・・・ Photoelectric conversion Yonago 14 ・・・ ・・
・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・
... Pulsed FM demodulator 19 ......
FM repeater 23 22 ...... FSK restorer, 1!1 device fc ...... Pulsed FM change i1! Carrier wave frequency fh of i ...... Maximum frequency f max of IM slope division multiplexing signal... ... Higher frequency f'min... of pulsed FM signal...・・・
・・Minimum frequency B of pulsed FM signal ・・・・・
−・Modulation f4 main occupation・jl) Bandwidth Δf...
... Maximum frequency deviation of pulsed FM I゛-K
Matsu 111η Ito Yokono King! l'! r, i'r Ilt l+, a'iノ(fl
, Tomichi Kigyo H5 Ceremony Meeting Figure 2 PFM Figure 6

Claims (2)

[Claims] (1) Frequency shift keying of the carrier wave using one or more input video signals V, a signal obtained by frequency modulating or amplitude modulating the carrier wave with one or more input audio signals A, and one or more input digital signals An optical transmitter 1 that pulses and frequency modulates a rectangular carrier wave with a duty cycle of 50% using a frequency division multiplexed signal with a modulated or phase shift keying modulated signal, and converts this signal into an optical signal by an electrical/optical conversion element. After converting the optical signal into an electrical signal by an optical/electrical conversion element, the pulsed frequency is returned to #! an optical receiver 2 which converts the signal into a frequency division multiplexed signal, separates the modulated audio signal and modulated digital signal with a filter, and demodulates them to obtain the original video signal v1 audio signal A and digital signal; - An optical transmitting/receiving device characterized by having an optical transmission line 3 connecting an optical transmitter 1 and an optical receiver 2. (2) The optical transmitting/receiving device according to claim (1), wherein the value of the rectangular carrier frequency fc is greater than or equal to the value of the pulsed FM signal occupied bandwidth B.