DE2541688C2 - Image transmission system - Google Patents

Description

The invention relates to a transmission system for in Pulse-code-modulated moving images ao present with a first converter for converting the pulse code modulated signals into differential pulse code modulated signals (DPCM coder). with a second Converter for converting the differential pulse code modulated signals of constant word length into differential pulse code modulated Signals of variable word length (VWL coder), with a transmit-side buffer memory, with a Transmission channel, with a receiving-side buffer memory, with a third converter for conversion the differential pulse code modulated signals of variable word length into differential pulse code modulated signals constant won length (VWL decoder) and with a reception filter, in which the DPCM coder also has a first subtraction circuit, whose plus input with the DPCM code input and whose output with the Connected to the input of a first delay circuit, a second subtracter circuit whose positive input with the output of the first delay circuit and its output with the input of a Quaniization attitude is connected, an estimator, its input with the output of the quantization circuit and the DPCM coder output and whose output is connected to the negative input of the second subtracting circuit, a first adding circuit, whose first input to the output of the quantizing circuit and whose second input to the Minus input of the / wide subtracting circuit is connected, and contains a second adding circuit, whose first input to the output of the first adding circuit and its output via a second Delay circuit on the one hand with the negative input of the first subtracting circuit and on the other hand connected via a third delay circuit to the second input of the second adder circuit is.

In VWL coding, the code words are of constant length according to the frequency of their occurrence evaluated. Frequently occurring code words become shorter code words and rarely appearing code words converted into longer code words in such a way that they receive (> o are recognizable on both sides without a separator. Because the DPCM coder often has long sequences of code words supplies that represent the value »0«, there are additional options for further reduction of the bit flow, such as cluster or block coding. <>. ■>

The transmit-citige buffer memory of this transmission system has only a limited capacity with reasonable effort and sisnallaufzeii. The statistical properties of the code words output by the DPCM coder give way for a longer period of time deviates in an unfavorable way from those on which the economics coding is based, then there is a buffer memory overflow possible.

This problem has hitherto been solved by undersampling when there is a risk of buffer memory overflow of the pixels to be transmitted only every second sent and the missing pixels on the Receiving side were obtained from a linear combination of the amplitudes of the neighboring points (The Bell System Technical Journal, 50 (1971) 6, pages 1889-1917).

An improvement of this method is the inclusion of subsampling of certain To make conditions dependent, which from the spatial environment of the image to be sent (The Bell System Technical journal, 53 (1974) 6, pages I) 37-1173).

The object on which the invention is based is, with little effort and little signal run-time through the transmission system a buffer storage overflow with subjectively no or only little Prevent noticeable loss of quality of the transmitted image / 11.

This object is achieved according to the invention in that at least one transversal filter with approximately rotationally symmetrical low-pass character is provided, the input of which with the output of the first Subtraction circuit is connected that a changeover switch is provided, the first input of which is connected to the output the first delay circuit is connected and its further inputs each to the output of a Transversal filters are connected to that one transmission line Buffer memory is provided with at least one output for a buffer memory filling level signal, and that a switch control is provided, the at least one input with the at least one Output for buffer memory filling level signals of the sending-side buffer memory and their output with is connected to the control input of the switch.

A rotationally symmetrical character of a filter is given if the two-dimensional impulse response in the image plane is only approximately a function of the absolute distance from the origin.

In the known system, the implementation of differential pulse clock modulated signals is of constant word length in such a variable word length adapted to picture / .en with weak or medium movement. the The invention makes it possible to overflow of the transmit-side buffer memory when there is a strong movement the mean word length of the code words to be transmitted and thus the current bit flow of the DPCM coder to reduce. Without any noticeable loss of quality of the received image, this is done quickly Overflow of the buffer memory on the sending side prevented.

For the practical implementation of the subject matter of the invention, it is advantageous that a transversal filter mi approximately rotationally symmetrical T'efpaßcharakte is provided, which is input from a delay containing circuits formed chain, dcien An catch, their taps and their end each via a coefficient circuit with one input each Summing circuit is connected, the output of which is used as a filter output.

It is advantageous if four delay shoes are provided, of which either the first and fourth have a delay time τ / - τ, and the second

and third a delay time τ λ or the first and fourth a delay time 2 Ta and the second and third a delay time tz-ta (τ ?. means the line duration and ta the scanning duration) and if five coefficient circuits are provided, of which the first , second, fourth and fifth has a coefficient 1/8 and the third has a coefficient 1/4.
In an advantageous embodiment of the invention

Transversal filter.

Fig. 1 shows with the exception of the elements 21 and 24 with a known transmission system for moving images equivalent system. The switch position I of the switch 13 is through in the known system replaced a cable run.

The known transmission system contains an input 1, a DPCM coder 2 (without the elements 21 and 24), a VWL coder 3, a sendcscitigen

are a first and a second transversal filter with the io buffer memory 4, a transmission channel 5, a

Buffer memory 6 on the reception side, a VWL decoder 7, a reception filter 8 and an output 9.

The known DPCM coder 2 contains a subtracting circuit 10, a delay circuit U, a

18 and delay circuits 19 and 20. In this exemplary embodiment, the estimator 16 contains an adder circuit 23 and a delay

The chain of delay circuits described above is provided. With the first transversal filter the first, second, fourth and fifth coefficient circuits have a coefficient 1/16 and the third

Coefficient circuit on a coefficient 3/4. 15 subtraction circuit 14, a quantization circuit 15,

In the second transversal filter, these coefficients are an estimator 16, adding circuits 17 and replaced by the values 1/8 and 1/4.

An advantageous development is that a
a switchable transversal filter with an approximately rotationally symmetrical low-pass filter is provided, 20 approximation circuit 22.
With the means by the changeover switch control By the following extensions, the controlled changeover switches are not only the two transversal known to the transmission system according to the invention, filter by coefficient switching but also the delay circuit 11 between the output of the first delay circuit by separating the subtraction circuit 10 and the input of the first , second, fourth and fifth as well as by 25 subtraction circuit 14 is replaced by a circuit short-circuit of the third real-order 24 circuit. This contains a transversal filter. 12 with an approximately rotationally symmetrical Ticfpasscha-It is also advantageous if a switch control, possibly a further transversal feeder 12 ', is provided that switches the switch from the first delay circuit 11 with a delay time input to the / wide input when a 30 τ of the transit time of the transversaifiller 12 or the buffer storage level signal from the output of the 12 'and a switch 13 with inputs I, II and III. The send-side buffer memory arrives and the surrounding The sendescilige buffer memory 4 receives an output switch switches back when this buffer memory- «1, the filling level signal at a certain buffer memory filling level is absent. emits a signal. Another output b indicates that an embodiment is advantageous in which the transmission-side buffer memory 4 is characterized in that the transmission-side buffer memory can have additional outputs which, when further buffer memories are used, threshold input circuits of a number of transversal filters 12 'are further buffer memory filling procedures that can just indicate the number of transversal filters. Finally, a + 1 corresponds to the fact that these Schwcllcnwertkreiscn switching control 21 are provided, which emit the switching output signals, which on the one hand indicate 40 ter 13 depending on the buffer memory filling

if the Buffercrspcichcr-Creasing degree has reached this threshold and on the other hand indicate whether when full or emptying, and that a switchover control is also provided which, with increasing buffer memory

degree of the scndcseiiigcn buffer memory 4 controls.

To explain how it works, the Delay elements 11 and 20 are replaced by short circuits, since these are only used to adapt to the running / eil

filling when a threshold of an ordinal number 45 of the transversal filter 12, 12 'is used, which / wiper

the output of the subtraction circuit 10 and the input of the subtraction circuit 14 are to be inserted depending on certain .Slcucrkritcricr still to be explained. The prerequisite for these simplifications is that a delay time of one image duration for the inserted transversal filters 12, 12 'is assumed to be a delay time of NuI for the delay element 19, although it is physically not possible. These transversal filters 12, 12

with the exception of the lowest atomic number threshold - counting starting with the threshold lowest buffer storage capacity - the switch from the input of the next lower ordinal number switches to the input with the same ordinal number and that when the buffer storage tank filling is falling when a threshold of an ordinal number is reached, the switch from the input of the next higher ordinal number on the input of the same ordinal number

toggles or in the absence of an input of higher 55 should be as rotationally symmetrical as possible! Ordinal number does not switch.,,, "" Have impulse response, so also future picture

use points to form the impulse response.

The samples of the image amplitudes present at the input 1 in the form of PCM code words are protected in advance by those that are exactly one image duration back. The resulting contactor error of the first type is formed by the subtracter circuit 10, namely as the difference between the coded amplitudes of the current image

F1 g. 2 shows a transversal filter used according to the invention for the transversal filter at the output of the delay element 19; from the previous image. This Schötzfeh

Fig. 3 shows a pixel grid and ler. also called Blld-Blld-Dlfferenz, forms there

Fig. 4 shows an input signal of an intraframe encoder (Innerblldco

As a result, the buffer storage tank is filled with oscillation around a threshold avoids constant switching, which under certain circumstances can cause picture disturbances can.

The invention is explained in more detail below on the basis of exemplary embodiments.

Flg. Fig. 1 shows the overlay device according to the invention;

dierers), consisting of the subtraction 14, the Quantizing circuit 15 and the estimator 16 with the input 25 and the output 26. Possible Estimators for an intraframe coding are in the journal "The Bell System Technical Journal", 45 (1966) 5, pages 689-721 and 50 (1971) 3, pages 1049-1061. The over the switch 13.1 The guided first estimation error is now in turn predicted, with the amount at the output 26 of the Schälzwertbildncrs 16 pending estimated values. The resulting estimation error of the second kind - as Output of subtracter circuit 14 - becomes a generally non-uniform quantization Subjected in the Quanlizerschallung 15. This quantized estimation error 25 is on the one hand to Receiver transmitted and on the other hand used as an input variable for the estimator 16. Furthermore, from this and the output signal 26 of the estimator 16 with the aid of the adding circuit 17 the estimation error of the first kind recovered, but disturbed by a quantization process Quaniizing noise produced. (This signal 27 may also be available directly in the latter, depending on the estimator, so that the adding circuit 17 is omitted.) The values present at the output of the delay sound 19 and for the purpose of estimation image amplitudes serving the current image amplitudes of the previous image are now reconstructed with the exception of the quantization errors The first type corrects in such a way that the current image amplitudes also appear in the output of the adding circuit 18 except for the quantization errors and for estimating those of the following Image can be used. By increasing the signal transit time through the DPCM coder by the Running the transversal filter can do this for the purpose Better description, simplified circuit diagram with the help of elementary transformations into the one in l'i g. I shown be reshaped.

As mentioned, the c | uantisicrtc treasure / error / wciier kind the output signal of the I) PCM coder 2 Depending on the respective C'odewoi ι, it is possible, compared to a transmission mn of constant code word length, to reduce the / u transmitted BiIfIuU by coding with a variable word limit. The VWI.-Codcr X tier takes care of this, however, a non-continuous flow of images so that the buffer memory 4 / ur (ΊΙίιΗιιημ. Which hcillt to convert the. Non-continuous in a continuous bit flow suitable for transmission via a Kttnal 5 must be used.

The receiving-side buffer storage and VWl. Decoders deliver on the receiving side when there is no interference Transmission is identical to signal 25, only through the transmission line delayed signal for the Receiving filicrB.

The circuit diagram of the receive filter can be taken from the reciprocal transfer function of the transmission filter, that is, the lincarisierlcn equivalent circuit of the DPCM coder 2 can be determined, z. B. mü the help of algebra the block diagrams (L Merz. Basic course in control engineering, R. OI d c η b ο u r g - Munich and Vienna, 1970. p. 126).

The size of the bit stream to be transmitted depends on the extent to which quantization noise can be permitted for moving and still images and the extent to which the image scene changes. In addition, measures not shown here have to be provided which reduce the bit flow of the DPCM coder in the event of an impending buffer memory overflow, which is associated with additional corruptions of the image signal to be transmitted. This case cannot be avoided in principle, since any coding with variable Codcwortlängc can only be adapted to an average signal statistic and there, as soon as the

ίο actually present compared to this in unfavorable Way, for a longer duration, each cache deviates because of its limited capacity overflows.

The measures according to the invention are now aimed

is aimed at that case of incipient overflow to reduce the image flow of the DPCM-Codcrs 2 in a way that is not subjective to the human eye or is only slightly perceived as a disturbance on the receiving ropes. Because adulterations in the dormant Image background can be noticed much more easily than in moving parts of the image, these measures are allowed in the essentially only cause a reduction in the current bit flow in the latter. This reduction is achieved by positively influencing the current signal statistics in the direction of the mean Signal statistics on which the economics coding was based, specifically by means of a suitable one based on the (Spatial) filtering of the image-image difference relating to brightness distributions in the image plane. the Spatial filtering has a low-pass characteristic which causes the spatial correlation in the moving image parts is increased, the quantizer is thus controlled weaker and the large estimation errors, which are also transmitted with longer code words due to the mean signal statistics, less has to be sent frequently.

The spatial filtering is carried out by the transversal filter 12 with approximately symmetrical impulses causes. This is switched on when the scndcscitigc buffer memory 4 over its output </ emits a buffer storage degree signal and the Umschallersieuerung 21 switch 13 to the Input II controls.

In the case of other variants, there are 4

two outputs u and Miir Pulferspeicher filling basic signals are specified, which not only signal when a threshold has been reached, but also indicate from which side. The transversal filter 12 is switched on when the threshold is higher than the envelope

s <) des is reached and is then switched off when the The threshold of the lower degree of increase has been reached.

To deal with the activation of the transversal filter To make the associated falsifications as little perceptible as possible, it is advantageous to use the "must" of the

ss Ticfpaßfiltciungcn depending on the buffer storage level to control in such a way that with increasing buffer level the "measure" of the ticfoot filing increases. This can be done by appropriately switching the switch 13 to further transversal filters 12 '

At or more simply by varying the coefficients of a in Fig. 4 switchable transversal filter yet to be shown can be achieved.

F i g. 2 shows a transversal filter 12 according to the invention with an approximately rotationally symmetrical low-pass filter

This transversal filter is inserted into the transmission system according to FI g. I between the subtraction sound 10 and the aisle II of the switch 13 used. The filter contains delay shells

709 Θ2Β / 322

29 to 32, coefficient circuits 33 to 37, and a summing circuit 38.

F i g. 3 shows the image raster of a field with a current image point A, previous image points E and D and subsequent image points C and B. The scanning direction of the image points points to lines from left to right and that of the lines within an image from top to bottom. The dashed lines indicate the lines of the other field.

To take these pixels into account, the transversal filter according to FIG. 2 the delay circuit 29 has a delay time τ? - τ λ. τ / means the line duration and τ, \ the scanning time between two pixels. The delay time of the delay circuit 29 was chosen with a view to the time interval between the pixels and D. The delay circuit 30 obtains with respect to time interval of the Bildpunktc D and A is a delay time τ λ- The delay circuit 31 obtains with regard to the interval of the pixels A and C is a delay time τ λ and the delay circuit 32 is finally obtained with respect to the time interval of the pixels C and its delay circuit with a delay time τ ζ-τ α

The coefficient circuits 33, 34, 36 and 37 obtain coefficients 1/8 and the coefficient circuit iS has a coefficient of 1/4.

Knowledge of the impulse response is sufficient for the description of a linear system according to FIG. Because of the sequential image scanning, spatial intervals in the image curve correspond to temporal intervals, and only the propagation of a temporal pulse that repeats itself at the frame rate needs to be observed. This pulse passes through the delay circuit 29-32 one after the other and is each time weighted by the coefficients of the coefficient circuit 33-37 and fed to the output via the summing circuit 38. As a result, weighted unit impulses appear one after the other - practically simultaneously for the viewer - at the image raster points A - D , which in their entirety represent the two-dimensional impulse response in the image curve. With the choice of the coefficients, spatial frequency responses, here with a low-pass character, can be set, which generally increases the correlation between the image points.

4 shows a switchable transversal filter 24, that in the transmission system according to FIG. I between the

ίο subtraction circuit 10 and the subtraction circuit 14 can be used. Like the transversal filter according to FIG. 2 delay circuits 29 to 32, coefficient circuits 33 to 37 and a summing circuit 38. Further there are coefficient circuits 33 'to 37' and changeover switches 13 | provided to 13 ₅ . These switches are able to set defined coefficient values between zero and infinity.
This switchable transversal filter according to Fig.4

Depending on the position of the switch 13 to 13 ₅ acts like the delay circuit 11 or the transversal filter 12 or 12 ′ according to FIG. 1.

The set of coefficients 4 Vie + ³ Ai intended for a lower bit flow reduction has a coefficient sum of 1, with which the spatial low-pass filtering of the transversal filter is normalized to the surfaces of constant brightness, i.e. such surfaces are transmitted unchanged and only brightness jumps at contours are flattened. Because this filtering itself

.? o only affects image-image differences, occurs in dormant Büdteile no change on. Blurring only occurs on moving contours, but this is due to the Movement is barely perceived by the eye. The coefficient intended for a greater bit flow reduction

.15 cient theorem 4 ι / «+ V ₄ has a coefficient sum of Vt <!, So that even moving surfaces with constant brightness are subjected to a filtering, which is noticeable at the output of the transmission system as a reduction in the temporal resolution.

For this purpose 3 sheets of drawings

Claims (9)

Patent claims: 1. Transmission system for moving images in a pulse code modulated form with a first converter for converting the pulse code modulated signals into differential pulse code modulated signals (DPCM coder), with a second converter for converting the differential pulse code modulated signals of constant word length into differential pulse code modulated signals of variable word length (VWLCoder) Transmit-side buffer memory, with a transmission channel, with a receiving-side buffer memory, with a third converter for converting the differential pulse code modulated signals of variable word length into differential pulse code modulated signals of constant word length (VWL decoder) and with a reception filter, in which the DPCM coder also has a first subtraction circuit , whose plus input is connected to the DPCM coder input and whose output is connected to the input of a first delay circuit, a second subtracting circuit, whose plus input is connected to the output of the first delay circuit and whose A output is connected to the input of a quantizing circuit, an estimator whose input is connected to the output of the quantizing circuit and the DPCM coder output and whose output is connected to the minus input of the second subtracting circuit, a first adding circuit whose first input is connected to the output of the quantizing circuit and the latter The second input is connected to the minus input of the second subtracting circuit, and contains a second adding circuit, the first input of which is connected to the output of the first adding circuit and whose output is connected via a second delay circuit on the one hand to the minus input of the first subtracting circuit and on the other hand via a third delay circuit to the second input the second adding circuit, characterized in that at least one transversal filter (12, 12 ') is provided with an approximately rotationally symmetrical low-pass filter, the input of which is connected to the output of the first subtracting circuit (10) and it is that a changeover switch (13) is provided, the first input (I) of which is connected to the output of the first delay circuit (11), and the further inputs (II, III) of which are each connected to the output of a transversal filter (12, 12 ' ) are connected that a transmit-side buffer memory (4) with at least one output (a. b) is provided for a buffer memory filling level signal, and that a changeover switch control (21) is provided, the at least one input of which with the at least one output (a, b) for buffer memory filling level signals of the transmitter-side buffer memory (4) and its output with the control input of the switch (13) is connected (Fig. 1). 2. Transmission system according to claim 1, characterized in that a transversal filter (12) with approximately rotationally symmetrical low-pass character is provided, the one from the input side Delay circuits (29-32) formed chain contains, their beginning, their taps and their Each end via a coefficient circuit (33-37) with one input each of a summing circuit (38) whose output serves as a filter output (Fig. 2). 3. Transfer system according to claim 2, characterized in that four delay circuits (29-32) are provided, of which the first (29) and the fourth (32) delay circuit have a delay time xz ~ Xa and the second (30) and the third ( 31) delay circuit have a delay time Va , where Xz is a line duration and Xa is a sampling duration. 4. Transmission system according to claim 2, characterized in that four delay circuits (29-3:>) are provided, of which the first (29) and fourth (32) delay circuits are one - Delay time 2 τ α and of which the second (30) and the third (31) delay circuit have a delay time χζ-χλ , where x _{z is} a line duration and χ α is a sampling period. 5. Transmission system according to claim 2, characterized in that five coefficient circuits (33-37) are provided, of which the first (33), the second (34), the fourth (36) and the fifth (37) Koeffuirntntn circuit a coefficient 1/8 and the third coefficient circuit (35) a coefficient 1/4 has. 6. Transmission system according to claim 1, characterized in that a first (12) and a second (12 ') transversal filter according to claim 3 or 4 are provided that the first transversal filter (12) a first (33 '), a second (34'), a fourth (36 ') and a fifth (37') coefficient circuit with a Contains coefficients 1/16 and a third coefficient circuit (35 ') with a coefficient 3/4 and that the second transversal filter (12 ') contains coefficient circuits according to claim 5. 7. Transmission system according to claim 6, characterized in that a switchable transversal filter (24) is provided in which not only the two transversal filters (12, 12 ') by _{means of the switching control (21) controlled changeover switches (13, -13 5)} Coefficient switching but also the first delay circuit (11) can be implemented by separating the first (33), second (34), fourth (36) and fifth (37) and by short-circuiting the third (35) coefficient circuit (Fig. 4 ). 8. Transmission end according to claim 1 with a transversal filter with an approximately rotationally symmetrical low-pass character, characterized in that a switch control (21) is provided which switches the switch (13) from the first input (I) to the second input (II) when a buffer memory -Filling level signal arrives at the output (a) of the buffer memory (4) on the transmitter side and switches back the switch (13) if this buffer memory level signal is missing. 9. Transmission system according to claim 1 mil at least one transversal filter with approximately rotationally symmetrical low-pass filter character, characterized in that in the transmit-side buffer memory (4) Threshold circuits are provided with a number corresponding to the number of transversal fil ter (112,12 ') +1 corresponds to that this threshold value circuits emit output signals that indicate when the buffer memory is full grad has reached this threshold and changes to indicate whether filling or emptying, and then a changeover switch control (21) is also provided is that with increasing buffer storage filling when reaching a threshold of an ordinal number with Except for the threshold of the lowest ordinal number _ with counting starting with the threshold * lowest buffer storage filling level - the switch (13) from the input next lower Ordinal number switches to the input of the same high ordinal number and the one with decreasing Buffer filling when a threshold of an ordinal number is reached, the switch (13) from the The input of the next higher ordinal number switches to the input of the same high ordinal number or in the absence of a higher ordinal number input, nichi. switches. > 5