JPS5857606A - Pcm decoder - Google Patents

Abstract

PURPOSE:To prevent the disturbance of synchronism which is due to the dropout of a magnetic tape and to avoid assuredly a malfunction of the PCM demodulation, by delivering a horizontal synchronizing signal by means of a dropout detecting output. CONSTITUTION:The output of a carrier amplifier 1 is fed to a dropout detecting circuit 3 after passing through a limiter 2. The FM carrier component of the output is eliminated through a low pass filter 4, and the envelope is detected for the output. A drop-out driver 5 is driven with the envelope signal, and a mixer 6 is driven with the output of the driver 5. The output terminal of the amplifier 1 is connected to the input terminal at one side of the mixer 6. A signal obtained by giving a delay of a horizontal period to the output of the mixer 6 through a delay line 7 is fed to the input terminal at the other input terminal of the mixer 6. In other words, the mixer 6 transmits the output of the amplifier as it is for a dropout compensating circuit if no dropout occurs. If the dropout is detected, the 1-horizontal period preceding signal is made up for the period of the drop- out via the line 7. Thus the dropout of a video signal is compensated.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a PCM decoder that uses a video tape recorder (hereinafter referred to as VTR) or a part thereof to reproduce PCM signals compliant with standard television signals, and relates to a PCM decoder that uses a video tape recorder (hereinafter referred to as VTR) or a part thereof to reproduce PCM signals compliant with standard television signals. The purpose of this is to prevent synchronization errors that occur when a signal drops out, and to prevent malfunctions in PCM demodulation.

Figure 1a is the Japan Electronics Industry Association standard (STC-Oo7)
This shows the signal arrangement within one horizontal opening period section of the PCM signal determined by , and the period of each part takes a value that is an integral multiple of the master clock period.

To read the PCM data in this PCM signal, it is determined that the horizontal synchronization signal exists when a LOW period of several clocks continues from the fall of the horizontal synchronization signal, and a data of "1010" is detected at the 26th bit from the fall of the horizontal synchronization signal. 3/...- If a synchronization signal is present, the subsequent data is normally read using logic such as PCM data.

However, with this reading method, if a dropout occurs on the magnetic tape, for example, and the data part is disturbed as shown in Figure 1, and there is a LOW period of several bits or more,
It is assumed that a pseudo-horizontal synchronization signal exists, and it happens that the 26th bit from the pseudo-horizontal synchronization signal is 11o1o.
If there is no PC, M data, it is determined that it is the beginning of the PCM data, even though the data is actually in the middle of horizontal synchronization of the PCM data, so Il[l of the data array
If the k order is incorrect, large noise will occur in the PCM reproduction signal.

The present invention allows VTRs to be used even if such dropouts occur.
Using the output of the dropout detection circuit of the carrier amplification system, the PCM signal is detected during the dropout period.
The present invention provides a PCM decoder which prevents the generation of pseudo-horizontal synchronization signals by using a PCM decoder.

An embodiment of the present invention will be described below with reference to FIGS. 2 to 6.

FIG. 2 is a block diagram of a well-known dropout compensation circuit used in a carrier amplification system of a VTR.

In Fig. 2, the output beam of the carrier amplifier 1 passes through the ε vine 2 and then is input to the dropout detection circuit 3, and the output is filtered by the low-pass filter 4 to remove the FM carrier component and detect the envelope. Autodon' lime 5 is driven, and its output drives mixer 6. The output of the phantom carrier amplifier 1 is connected to one input terminal of the mixer 6, and a signal obtained by delaying the output of the mixer 6 by one horizontal period by a delay line 7 is input to the other input terminal. In other words, the dropout compensation circuit shown in Fig. 2 allows the output of the mixer 6" carrier amplifier 1 to pass through as it is when there is no dropout, and when there is a dropout, the delay line 7 is passed through for only that period.
The missing video signal is compensated for by filling in the signal from one horizontal period ago.

This is a circuit that eliminates pseudo-horizontal synchronization signals as shown in FIG.

In FIG. 3, 8 is a terminal to which the output of the dropout driver 5 of FIG. 2 is input, and when a dropout occurs, a pulse as shown in FIG. 4a is generated only during that period. Reference numeral 9 denotes an inverting amplifier that inverts the above pulse, and its output is as shown in Figure 4.The output is integrated by an integrating circuit consisting of a resistor 10 and a capacitor 11, and is input to an inverter 12 [C]. This results in a delayed pulse like this.

The above-described delayed pulse is input to the NAND gate 13, which is the first logic gate, together with the output of the dropout driver 5 (FIG. 4a) inputted from the terminal 8, and its output becomes as shown in FIG. 4d. . The muting transistor 1 is connected to the muting transistor 1 via the resistor 14 by the output of the NAND gate 13.
If you drive 5, the FM wave will be detected by the FM detector 16,
The PCM signal from which the carrier component has been removed by the low-pass filter 17 is muted at least during the period in which drop-out occurs, as shown in FIG. 5a, and is treated as a signal whose level is fixed to the DC level of the PCM signal. It is output to terminal 18. If this is done, a pseudo horizontal synchronization signal as shown in FIG. 1 will not be generated, so that there will be no error in reading PCM data.

Note that the delay 11 in FIG.
0, which is caused by the capacitor 11.

By the way, if the dropout timing occurs at the timing of the horizontal synchronization signal as shown in Figure 5, the falling edge of the horizontal synchronization signal will be delayed isometrically, and the data synchronization will be delayed as explained in Figure 1a. The signal is 26 from the horizontal sync signal
Since it arrives before the th bit, an error occurs in reading the PCM data.

A portion 19 surrounded by a dotted line in FIG. 3 is a circuit for gating such a pseudo-horizontal synchronizing signal to ignore it. In other words, the output of the dropout driver 5 input from the terminal 8 and the NAND gate 13
If the output is inputted to the second logic gate (AND gate) 20, its output becomes a pulse as shown in FIG. The Q output has a waveform shown by the dotted line in FIG. 4f. However, the time constant of the monostable multivibrator should be selected to be sufficiently large. On the other hand, the horizontal synchronizing signal shown in FIG. If a short length is chosen, the waveform will be as shown in the fourth diagram. Since this output is connected to the reset terminal of the first monostable multi-bip plate 21, the Q output of the first mono-stable multi-bip plate 21 becomes seven solutes as shown by the solid line in Fig. 4f. be done. Therefore, if the Q output of the first monostable multi-by break 21 and the horizontal synchronizing signal input to the terminal 22 are input to the OR gate 24, which is the third logic gate, the output is In this case, the next horizontal synchronization signal after the dropout occurs will disappear.In this case, the PCM signal will not be read for one horizontal synchronization period, but redundant data for error correction will be transmitted to each horizontal synchronization signal. Since it is located within the section, it is possible to demodulate PCM without any problem even in practical use.

As described above, the present invention utilizes the dropout detection output to ensure that the PC
Since it applies meeting to the M signal and outputs a horizontal synchronization signal for removing the PCM0M delta by using the memo dropout detection output and the horizontal synchronization signal in the PCM signal, it is possible to use a magnetic tape. It is possible to prevent synchronization disturbance due to dropout of PCM and reliably prevent malfunction of PCM demodulation.

[Brief explanation of the drawing]

Figure 1 a and b are PCMs defined by the Japan Electronics Industry Association.
A diagram showing the signal arrangement of signals and how dropouts occur, 2nd Nd, conventional dropout 9, -, block diagram of out compensation circuit, 3rd is a block diagram of an embodiment of the present invention, 4th Figures a-i are time charts for explaining the operation, and Figures 5a and 5b are waveform diagrams for explaining the operation of the above embodiment. 1o...Carrier amplifier, 2...Limiter, 6...Mixer, 7...Delay line, 8...Dropout Drino (output terminals to which are input, 9...inverting amplifier, 10.
11...Integrator circuit, 12...Inverter, 13...First logic gate, 16...
...Meeting transistor, 16...
FM detector, 17...Low pass filter, 18
....PCM signal output terminal, 20...
Second logic gate, 21...First monostable multi-byte gate, 22...Terminal into which a horizontal synchronization signal is input, 23...Second monostable multi-bicycle gate. Break, 24...Third logic gate. Figure 1 Figure 2 Figure 4 Figure 5 Figure 1 Flash

Claims (1)

[Claims] Dropout detection means for detecting a dropout of a PCM signal compliant with a standard television signal recorded on a magnetic tape for a video tape recorder, and logic for the output of the dropout detection means and its delayed output. and a first logic gate that takes FM detection at the output of the first logic gate, at least during a period in which dropout occurs.
a muting means for muting a CM signal;
a second logic gate that takes a logic between the output of the dropout detection means and the output of the first logic gate; and a first monostable multivibrator driven by the output of the second logic gate. , a second monostable multivib break driven by a horizontal synchronization signal in the PCM signal and whose output resets the first monostable multivibrator; and an output of the first monostable multivibrator and the horizontal synchronization. - a third logic gate that takes the logic of the signal;
A PCM decoder characterized in that a horizontal synchronizing signal for reading PCM data in the PCM signal is output from the logic gate.