JP3194317B2 - Variable rate coding device for image signal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a digital transmission apparatus of the image signal, capable image signal changes in the encoding rate
The present invention relates to a variable rate coding device for signal processing .

[0002]

2. Description of the Related Art In a system for digitally transmitting an image signal by connecting to a network capable of variable-rate transmission,
The encoding apparatus prepares a plurality of quantization tables having different quantization step sizes, and switches the quantization table according to the congestion state of the network to change the encoding rate.

FIG. 4 shows an example of the configuration of a conventional variable rate encoding apparatus for image signals. In the figure, quantization table 4
1 is a table including a range from a large quantization step size to a small quantization step size. The encoding control unit 42 selects one quantization table, and the encoding unit 43 encodes the input image signal a at an encoding rate corresponding to the quantization step size. Buffer memory 44
Transmits the encoded data b to the network after temporarily storing it.

In the conventional apparatus, a corresponding coding rate is set by selecting a quantization table 41. That is, if a quantization table having a large quantization step size is used for the same input signal, the number of output quantization levels is reduced, and the code length associated therewith can be shortened. As a result, the coding rate can be reduced. is there. Conversely, if a quantization table with a small quantization step size is selected, the coding rate increases.

The selection of the quantization table 41 is performed based on the control of the encoding control unit 42. The encoding control unit 42 monitors the data amount c in the buffer memory 44,
When the data amount is larger than a predetermined value, a quantization table having a large quantization step size is selected, and when the data amount is smaller than a predetermined value, a quantization table having a small quantization step size is selected. Control the rate.

[0006] The encoding control unit 42 receives the encoding rate instruction signal d from the network, and controls the encoding rate by using a quantization table having a corresponding quantization step size. For example, when the network is in a congested state, a coding rate instruction signal d for reducing the coding rate is received from the network side, and the coding rate is reduced by switching to a quantization table having a large quantization step size. It has become. In addition, the coding rate instruction signal d includes an instruction to increase the coding rate or to set a predetermined coding rate according to the congestion state of the network.

[0007]

As described above, the coding rate in the conventional variable rate coding apparatus for image signals is changed by switching to a quantization table having a different quantization step size. .

However, the quantization table cannot be changed for data being encoded. For this reason, a processing delay occurs from the instruction to reduce the coding rate until the coding rate is actually reduced, and during that time, data may be discarded due to network congestion. Also, the problem of processing delay is the same for coded data already stored in the buffer memory, and even if the quantization table is switched quickly, the coding rate is not actually changed in the buffer memory. Has been transmitted after all of the stored data. As described above, in the conventional apparatus, the response to the instruction to change the coding rate is slow, and the delay in the buffer memory is added. During this time, data is discarded on the network, thereby deteriorating the image quality.

Further, in the method of transmitting the quantization table itself from the encoding device to the decoding device, the information of the quantization table itself must be transmitted according to the change of the quantization table. The encoded data cannot be transmitted. In this case, the image quality is rather deteriorated, and the quantization table cannot be frequently changed to reduce the coding rate. That is, in the method of transmitting the quantization table itself, the encoding rate cannot be flexibly changed.

[0010] The present invention provides a variable rate encoding apparatus for image signals which can flexibly and quickly change the encoding rate in response to an encoding rate instruction signal from the network side .
It is intended to provide a device .

[0011]

The invention according to claim 1 has a plurality of quantization tables having different quantization step sizes, and receives a coding rate instruction signal corresponding to a congestion state from a network. Encoding means for selecting a quantization table to be encoded and encoding an image signal at an encoding rate corresponding to the quantization step size, and a buffer memory for temporarily storing encoded data of the image signal and sending it to a network. in the variable rate coding apparatus for image signals comprising, receiving a coded rate indication signal, when it is instructed to reduce the coding rate, frame in the buffer memory
Discarding the lower frame data priority from the beam data, and characterized in that only the high frame data priority with data discarding processing means for sending to the network.

[0012]

[0013]

In the encoding apparatus according to the first aspect of the present invention, the low priority data among the data stored in the buffer memory is discarded, and only the high priority data is transmitted to the network. The conversion rate can be changed directly. As a result, a delay in switching the quantization table and a change in the coding rate caused by the buffer memory is eliminated, and it is possible to quickly respond to a coding rate reduction instruction from the network side. The switching of the quantization table is performed simultaneously with the reduction of the encoding rate accompanying the data discarding or after a predetermined time has elapsed.

[0014]

[0015]

1 shows the configuration of an embodiment of a variable rate encoding apparatus for an image signal according to the present invention.

In the figure, the same components as those of the prior art shown in FIG. That is, the quantization table 41,
The encoding control unit 42, the encoding unit 43, and the buffer memory 44 are the same as the conventional one.

In this embodiment, a data discard processing unit 11 and a data discard control unit 12 are provided. Data discard processing unit 1
1 fetches the coded data b output from the buffer memory 44, discards low-priority data from the coded data b in response to the discard data instruction signal e, and outputs the coded data f discarded to the network. Is output. The data discarding control unit 12 receives the coding rate instruction signal d from the network, determines the priority of data to be discarded in accordance with the specified coding rate, and gives the data discarding unit 11 the priority. The corresponding discard data instruction signal e is transmitted.

Here, the priority order of the encoded data b stored in the buffer memory 44 will be described with reference to the MPE shown in FIG.
Description will be made by taking encoded data according to the G (Moving Picture Expert Group) method as an example. The MPEG system has become one of the international standards for moving picture coding in which a plurality of moving picture frames are subjected to coding processing as a set.

In FIG. 3, (1) shows bidirectional predictive encoding of two middle frames. Note that I is intra-frame coded data, and an image signal can be restored using only I-frame data. P is forward prediction encoded data, and since prediction encoding is performed using I frame data or P frame data, reproduction of these data is necessary for reproduction. B is bidirectionally predictive encoded data, and since predictive encoding is performed using I frame data and P frame data, or P frame data before and after, the reproduction of these data is necessary.
(2) shows an example of the transmission order of each data by this predictive coding, and I, P, B, B, P,..., I, B, B in accordance with the order of decoding processing on the receiving side. Are transmitted in this order. Each data contains, at a fixed position, side information used for prediction of a synchronization signal, a data type, a motion vector, and the like. The data is detected by detecting a break from the middle of the data, and decoding processing of encoded data is performed. Has become possible.

For these three types of data, I, P, B
Prioritize in frame order. That is, since the I frame serves as a reference in the decoding processing of a set of encoded data, the highest priority is set.

When receiving a coding rate reduction instruction from the network, the data discard control unit 12 sends a discard data instruction signal e to the data discard processing unit 11 so as to discard the B frame data having the lowest priority. In accordance with the discard data instruction signal e, the data discard processing unit 11
First, the transmission of the B frame data is stopped and discarded. Therefore, only the data of the I and P frames is sent to the network, and the coding rate can be reduced quickly. That is, the response to the coding rate reduction instruction from the network is quick, and it is possible to prevent data discarding in the network.
Note that it is possible to discard the P frame following the discard of the B frame, and it is possible to further reduce the coding rate.

On the other hand, the discarded frames are thinned out on the receiving side. That is, since the number of reproduced frames is smaller than the number of normal decoded frames, interpolation processing is required. Therefore, a decoding device corresponding to the image signal variable rate encoding device of the present invention is required.

FIG. 2 shows a variable rate decoding apparatus for an image signal .
An example of the configuration will be described. In the figure, a received coded data f is input to a decoding unit 22 via a buffer memory 21, and a quantization table 2 corresponding to the processing on the coding device side.
3 is decoded. The decoding control unit 24 identifies the quantization table used for encoding from the received encoded data f, and selects the corresponding quantization table 23. Further, the decoding control unit 24 monitors, for example, the sequence number of the coded data f, detects data loss caused by data discard on the coding device side, and notifies the data interpolation control unit 25 of it. On the other hand, the signal decoded by the decoding unit 22 is recorded in the frame memory 26, the missing part is interpolated and reproduced under the control of the data interpolation control unit 25, and the decoded image signal a is generated.

Here, the interpolation method will be described. The first is a method of repeatedly displaying the previous frame. The second method is to use a value obtained by weighting and averaging the pixel values of the preceding and succeeding frames by distance. This is, for example, n, n +
Of the 1, n + 2, n + 3 frames, n + 1 and n +
When two frames are discarded, the pixel values F _{n + 1} (i, j) and F _{n + 2} (i, j) of both frames are represented by F _n , corresponding to the time distance between the previous and next frames. ₊₁ (i, j) = {2 · F _n (i, j) + F _{n + 3} (i, j)} / 3 F _{n + 2} (i, j) = {F _n (i, j) + 2 · Interpolation can be performed based on F _{n + 3} (i, j)｝ / 3. Here, i and j represent pixel positions.

The above equation is an interpolation method when two out of four frames are discarded (when the B frame is discarded in FIG. 3), but can be extended to an arbitrary pattern by changing the constant. is there. When a P frame is discarded, the P frame is discarded in a set of frame data.
The subsequent frames can be reproduced by repeatedly displaying the data of the previous frame or by interpolation using the next set of I-frame data. However, discarding the P frame may cause significant degradation in image quality.
There is also a conventional method of prioritizing the switching of the quantization table after discarding the B frame.

As described above, the conventional function of selecting a quantization table in accordance with the data amount c in the buffer memory 44 and in response to the coding rate instruction signal d from the network and controlling the coding rate is the present invention. It is possible to use the functions of the invention coding apparatus simultaneously or with a time lag.

[0027]

As described above, the present invention eliminates the delay of the coding rate change caused by the switching of the quantization table and the buffer memory, and flexibly and in response to the coding rate reduction instruction from the network side. It is possible to respond quickly and reduce the coding rate. Therefore, data discard due to congestion on the network can be prevented, and image quality in the variable rate coding system can be stabilized.

Further, in the encoding apparatus according to the present invention, since the discarding is performed in units of frames in accordance with the priority order, the interpolation processing on the receiving side becomes easy, and a corresponding decoding apparatus can be realized with a simple configuration. it can. In addition, this makes it possible to avoid discarding the I frame data that makes the entire set of frame data unreproducible, thereby preventing a significant deterioration in image quality.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of a variable rate encoding apparatus for an image signal according to the present invention.

FIG. 2 is a block diagram showing a configuration example of an image signal variable rate decoding device .

FIG. 3 is a diagram showing an example of an encoding process according to the MPEG system.

FIG. 4 is a block diagram showing a configuration example of a conventional image signal variable rate encoding device.

[Explanation of symbols]

 Reference Signs List 11 data discard processing unit 12 data discard control unit 21 buffer memory 22 decoding unit 23 quantization table 24 decoding control unit 25 data interpolation control unit 26 frame memory 41 quantization table 42 encoding control unit 43 encoding unit 44 buffer memory

Claims (1)

(57) [Claims] 1. A quantization table having a plurality of quantization tables having different quantization step sizes, receiving a coding rate instruction signal according to a congestion state from a network, selecting a corresponding quantization table, and selecting the quantization step size. Encoding means for encoding the image signal at an encoding rate according to, and a buffer memory for temporarily storing the encoded data of the image signal and sending it to the network, a variable rate encoding apparatus for an image signal, When the coding rate instruction signal is received and it indicates a reduction in the coding rate, the frame rate in the buffer memory is reduced.
Over the low frame data priority from the beam data is discarded and the higher priority frame data only image signals for variable rate coding apparatus characterized by comprising data discarding processing means for sending to the network.