JPH0444874B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an interframe coding method for television (TV) signals, and particularly to a method suitable for synchronization between an encoder and a decoder.

[Background of the Invention] In order to improve transmission efficiency when encoding and transmitting a TV signal, an interframe encoding method is known. That is, interframe coding is a method of encoding and transmitting a region in which the signal of the current frame changes compared to the signal of the previous frame.
In this method, encoded signals are not generated uniformly in time and location. Therefore, in order to make the transmission rate uniform, buffer memories are placed at the exit of the transmitter and the entrance of the receiver. Since the time the signal remains in the buffer memory varies depending on the signal generation situation, the time delay between the transmitter and receiver is not uniform. When a receiver needs to periodically repeat the received signal for display, such as with a TV signal, it is difficult to determine how to reproduce the frame timing on the receiving side.

One solution to this problem is a method in which only the timing information of the information to be transmitted is bypassed and retrieved without passing through the buffer memory.
``Synchronous signal transmission method for interframe coding of image signals'').

[Object of the Invention] An object of the present invention is to realize a new synchronization signal reproduction method in interframe coding of TV signals.

[Summary of the Invention] In the present invention, the time-consuming synchronization method described above is not used, and the TV camera on the transmitting side and the TV receiver (decoder) on the receiving side are based on signals from independent frequency sources. and make it work. however,
The relative error ε between them is, for example, at most 0.5×
10 ^-4 or 10 ^-5 (in the case of a normal crystal oscillator, the accuracy of the frequency source is
10 ^-6 to ^{10 -3} is guaranteed, and this condition is not particularly severe).

Now, as will be described later, when the frequency on the receiving side is lower, the number of frames on the camera side is greater, so data is left over and the buffer memory on the receiving side overflows. Therefore, for safety, it is sufficient to encode at a lower frequency than both the transmitting and receiving frequencies.
Specifically, for example, 1 every 10 ⁴ or 10 ⁵ frames.
Pause encoding at a frame rate. This number is the reciprocal 11/(kε) of the value kε, which is the product of the possible relative error ε and the safety factor k (k: for example, 5).
This will be explained using FIGS. 1, 2, and 3.

Figure 1 shows the case where the frame frequency _S on the transmitting side is lower than the frame frequency on the receiving side. The difference is only about 10 ^-3 to 10 ^-5 . Also, transmission delay in a broad sense is not shown).
In this case, encoded information generated in frame A is decoded in frame A' on the receiving side via a buffer. The same applies to B.

On the other hand, frame C cannot be decoded due to the frame phase relationship (this is because the delay time in the buffer memory cannot be negative). Therefore, this problem can be solved by decoding at the time of frame C''.In addition, due to the characteristics of interframe coding, in frame C', the information from B' is repeated without decoding. Goodbye.D
The following frames are also similar to A and B.

In this way, there is no major problem when _S < _R.

On the other hand, when _S < _R , something like Figure 2 occurs.

That is, since decoding according to the frame is slower on the receiving side than on the transmitting side, the buffer memory becomes full at frame D, and the information transmitted during frame E, for example, is lost (that is, it is not decoded). No matter how large the buffer memory capacity is, this phenomenon is bound to occur someday since it is limited.

In this way, when _S < _R , data is missing and there is a problem. If this is normal PCM information, there is no problem even if it is missing, as described in Tokusho No. 40-27142. However, when transmitting information that has changed from the previous frame, as in interframe coding, if one frame is lost, it will affect subsequent frames (until they are reset), resulting in a significant deterioration of image quality. .

Figure 3 describes this solution. From the above, there is no major problem when _S < _R. The present invention is to force this to happen.

Therefore, on the transmitting side, TV frames A, B, C,
Encoding is performed up to frame D, but encoding is stopped at frame E. This pause period depends on the frequency accuracy between transmitting and receiving, but once every 10 ⁴ frames is sufficient, for example. During a pause, the transmitting side buffer memory may become empty, but you can fill it with appropriate fill information. This is something that is done in normal interframe coding and is not related to the present invention.

As a result, the frame frequency of encoded information sent from the transmitter always satisfies _S < _R. Conversely, it is only necessary to decide the period of the frame at which encoding is paused so that this occurs.

[Embodiment of the Invention] An embodiment of the present invention will be described below with reference to FIG. In FIG. 4, the portion other than switch 1 and frequency divider circuit 2 surrounded by dotted lines is a conventional interframe encoder. To simplify the explanation, the simplest case of inter-frame DPCM is shown in a simplified manner.

The difference between the TV signal input from the input terminal 3 and the output of the prediction circuit 4 is determined by a difference circuit 5, and the difference is quantized by a quantization circuit 6. This output is given a code word by a variable length code word generator 7, and a buffer memory 8
added to. When the buffer memory is about to overflow, mode control such as threshold control or sub-sampling is normally performed, but since this is not directly related to the present invention, the explanation will be omitted. There is also a circuit for inserting a fill word when the buffer memory 8 becomes empty, but since this is not particularly added by the present invention, a description thereof will be omitted.

Horizontal and vertical synchronization signals are extracted from the TV signal to operate the timing circuit. This is also a conventional method. The timing circuit is a normal counter and does not require any special explanation. The timing circuit output includes a frame timing output that is output once in frame 1.

The above is the same configuration as the conventional interframe coding.

In the present invention, an electronic switch (AND gate) 1 and a frequency dividing circuit 2 are added to this. The frequency dividing circuit is
For example, it counts 10,000 frames and outputs only one frame out of every 10,000 frames. As a result, the interframe encoded output is cut off by switch 1.

As a result, encoding for one frame is stopped as shown in FIG. 3, and it becomes as if the TV frame frequency of the transmitting section has been lowered by 1/10000.

There may be other variations in the method of pausing encoding depending on the configuration of the interframe encoder.
For example, when a gate is provided at the input of the quantization circuit, when a gate is further provided at the input of the buffer memory 8, when the code word generation circuit 7 is stopped, when the clock is stopped, etc. Note that, as mentioned above, a problem arises only when _S < _R , so it is also possible to send a command to the receiving side and suspend encoding only when the buffer memory on the receiving side is likely to overflow.

[Effects of the Invention] According to the present invention, since it is guaranteed that the transmitting side and the receiving side each operate using independent frequency sources, there is an effect that the configuration of the encoder and decoder can be simplified.

Note that the following modifications are also possible.

(i) In the case of encoding a composite color TV signal, it is often easier to suspend color frames (two frames) as a set.

(ii) When paused, the amount of data stored in the sending buffer memory will rapidly decrease. It's a pain to send file language. Therefore, it is better to fill the buffer memory in the previous frame (frame 9999). for example,
It is also good to switch to direct mode (sending the PCM itself) or intraframe coding mode, as is often done in interframe coding.

(iii) If the maximum interval between paused frames is the value determined above, it does not necessarily have to be periodic. For example, there is a method of pausing when the transmission buffer memory is about to become full and counting the number of frames again from there.

[Brief explanation of drawings]

1 and 2 are time charts for explaining the meaning of the present invention, FIG. 3 is a time chart of the present invention, and FIG. 4 is a configuration diagram of an embodiment of the method according to the present invention. . 1...Electronic switch, 2...Frame frequency dividing circuit, 4
...Prediction circuit, 5...Difference circuit, 6...Quantization circuit, 7
...Variable length code word generation circuit, 8...Buffer memory.

Claims (1)

[Claims] 1. If kε is a value larger than the value ε that can be considered as the relative error of the frame frequency of the television signal (k is a value larger than 1 and sufficiently smaller than 1/ε), then once per 1/(kε) frame, An interframe encoding method for television signals characterized by pausing encoding at intervals.