STANDARDS CONVERSION OF A VIDEOPHONE SIGNAL WITH 313 LINES INTO A TV SIGNAL WITH.625 LINES

Transcription

1 R871 Philips Res. Repts 29, , 1974 STANDARDS CONVERSION OF A VIDEOPHONE SIGNAL WITH 313 LINES INTO A TV SIGNAL WITH.625 LINES by M. C. W. van BUUL and L. J. van de POLDER Abstract A description is given of a standards conversion in which a videophone signal with 313 lines is converted into a TV signal with 625 lines. The conversion in its most simple form shows some irregularities. Some interpolation circuits are described that reduce these effects. The results show that with these interpolation circuits an acceptable picture quality can be obtained. Nearly the same picture quality is obtained when a 625-line picture is converted to 313 lines, transmitted via a I-MHz videophone network and reconverted to 625 lines after transmission. 1. Introduction For the videophone a standard has been proposed that is compatible with the normal TV broadcast standard I). Compatibility in this context means that a standards conversion from the videophone standard to the normal broadcast standard and vice versa can be effected fairly simply. Such a simple conversion is possible if the picture frequencies of the two standards are equal, and if the number of lines per picture of the videophone is equal to that odd number that is as near as possible equal to half the number of lines of the broadcast standard. In the case of the 625-line TV standard this results in 3i3 lines per picture for the videophone standard, and in the case ofthe 525-line TV standard it gives 263 lines per picture. The results of the standards conversion of a 625-line TV signal into a: videophone signal with 313 lines are described in ref. 1. These results show that a good picture quality can be obtained with this conversion. In this paper some results of the conversion from 313 into 625 lines will be given. 2. The conversion method. In this section only a brief description of the conversion method will be given. A more extensive explanation has been given in the paper referred to above I). The block diagram ofthe conversion circuit is shown in fig. 1. In the 625-line system the line period has about halfthe duration ofthat in the 3l3-line system. Hence, the video information on a line of the 313-line system must be compressed in time. This can be done with the shift registers Dl and D 2 (fig. I).

2 414 M. C. W. van BUUL and L. J. van de POLDER Input vkieo 313/i'nes IMHz Output video 625/ines 2MHz Fig. 1. The block diagram of the conversion circuit for the standards conversion of 313 lines into 625 lines; Dl and D2: shift registers; Sl' S2 and S3: electronic switches; k: clock pulses; 't': delay line of 64 (.l.s. During the write intervals the 313-line video information is shifted into the shift register by means of the writing-clock pulses; during the read intervals the information is shifted out of the register by means of the reading-clock pulses. The frequency of the reading-clock pulses is about double of that of the writingclock pulses; the exact ratio is 625 to 313. In this way the information during reading is compressed in time compared to the writing period. By means ofthe electronic switch SI the incoming 313-line video signal is fed alternately line by line to the two shift registers Dl and D 2 These registers are read out via the electronic switch S2' The writing and reading intervals are shown in fig. 2. In the picture period, as shown in this figure, the video lines numbered 1, 3, 5, and so on, of the 313- line input signal are converted by means of shift register D 1> and the lines numbered 2, 4, 6, and so on, are converted by shift register D 2 In ref. 1 it is Fig. 2. Principle of the conversion method. The odd-numbered video lines of the 313-line input signalof the picture period shown are converted by means of shift register Dl> the even-numbered video lines are converted by means of shift register D2

3 CONVERSION OF 313-LINE VIDEOPHONE SIGNAL INTO 62S-LINE TV SIGNAL 415 shown that this procedure can be carried out with only two shift registers such that no overlap of the writing and reading intervals occurs. The sum of the output signals of the two shift registers gives all the oddnumbered video lines of the 625-line system (fig. 2) and the even-numbered lines are missing. By means of a delay line of delay -c = 64 flsand the electronic switch S3 the output can be repeated, so that every even-numbered line gets the information of its preceding odd-numbered line. For one line, namely for line nr. 623, this procedure of repeating must be interrupted (fig. 2). This is done via the electronic switch S3 by omitting the delayed version of that line. By virtue of this omission the repetition of the 313 input lines now gives 625 output lines, as desired. 3. Interpolation In the preceding section the principle ofthe conversion method was reviewed; in this section we discuss some aspects of interpolation. In fig. 3a a part of a raster is shown. For the left side of the raster, namely for x = 0, the positions of the lines of the 313-line system are shown in the upper part of fig. 3b; the positions of the lines of the 625-line system are shown in the lower part of fig. 3b. The positions of the scanning lines during the first field are given by drawn lines, and those during the second field by dashed lines.!y a) -x Fig. 3. (a) Schematic representation of a scanned raster. (b) Representation of the positions of the centres of the scanning lines for the two systems, for the left side of the raster (x = 0); drawn lines show the positions during the first field and dashed lines during the second field. The arrows indicate the information shift.

4 416 M. C. W. van BUUL and L. J. van de POLDER The arrows between the two schemes indicate the information shift for the conversion system as discussed in the preceding section. During the first field the shift of the average position of the information due to the conversion is equal to d 625, where d 625 is the distance between two scanning lines of the 625-line system. However, during the second field the shift of the average position of the information is equal to 2d 625 (fig. 3b). This shows that in the converted 625-line picture the average position of the information of the second field is shifted in the vertical direction over a distance d 625 with respect to the average position of the information of the first field. Such a position error is also encountered for the inverse conversion from 625 into 313 lines with the conversion method in its most simple form 1). As will be shown in the next section the results obtained with the conversion method according to fig. 3b are rather poor: the position error and the repeating of the lines in this simple way gives a picture in which oblique, nearly horizontal, lines are somewhat deteriorated. Some more ingenious interpolation methods are therefore investigated in order to improve matters. It may be noted that the repeating of a line, as applied in fig. 1, is itself a kind of interpolation, but it is a very simple method of interpolation. The circuit part of fig. 1, containing the delay line 7: and the switch S3 might be called the first interpolation circuit. A first requirement to be fulfilled by the new interpolation methods is that the position error is eliminated. This problem can be solved by applying different interpolation coefficients during the two field periods. This method is also used for the conversion from 625 into 313 lines. A second requirement is posed by the number of delay lines to be used for the interpolation: this number should be as low as possible Interpolation with one delay line The first method to eliminate the position error is given in fig. 4. With this method the shift of the average position of the information in both fields is made equal to 2d 625 This can be carried out in the way shown in fig. 4a. During the first field the odd-numbered lines of the 625-line system are composed of two components: the first component is equal to half the value of the line directly delivered by the convertor, the second component is equal to half the value of the line that is delayed over two line periods of 64!lsoThe information for all the other lines of the 625-line system is not changed from that of fig. 3b. The block diagram of a circuit with which (in principle) the correction method of fig. 4a can be carried out is shown in fig. 4b. Thé circuit contains two delay lines 7: of 64!ls and the adder A. The electronic switch S4 is switched every field during field blanking and the electronic switch S3 is switched every line during line blanking (except once during every second field-blanking period, as in the circuit of fig. I). The switch S4 is drawn in the position it adopts during the

5 CONVERSION OF 313-LINE VIDEOPHONE SIGNAL INTO 62S-LINE TV SIGNAL 417 ~..m I I I I I I ~J.:.ljl)_e_sy.. ~!D. 6 Output video 625 lines Fig. 4. The first correction method. (0) As in fig. 3b; the values marked at the arrows indicate the fractions of the original 313- line signal used to compose the new 625-line information. (b) Block diagram showing the principle of the correction method; or: delay line of 64 (Ls; A: adder; S3 and S4: electronic switches. (c) Block diagram of the practical correction circuit; Ss: electronic switch. second field and then the circuit is identical to that of fig. 1. In the case of the first field (switch S4 in the opposite position), the output of the adder A is used for every second line. Every second-line period the convertor gives an output signal so that the delay lines 1" of fig. 4b are used for the delaying of a video signal only every second-line period, the delay lines being unused for the remainder of the time. This offers the possibility to eliminate one of the two delay lines of fig. 4b, applying the method of recycling. The application of this method is shown in fig. 4c: the part of the circuit within the dashed lines replaces the part of the circuit of fig. 4b also within dashed lines; the remaining parts of the two circuits are identical. By means of the electronic switch Ss the output signal of the delay line is once again fed to the input of this delay line, Ss is switched every line. In this way it is possible to carry out the correction with only one delay line 1".

6 418 M. C. W. van BUUL and L. J. van de POLDER Compared to the interpolation circuit of fig. 1 the interpolation circuit offig. 4c now contains only two extra switches and one extra adder, S4 and Ss and A respectively. Results obtained with the interpolation circuit of fig. 4c will be discussed in the next section. A modification of the circuit of fig. 4 is shown in fig. 5. The interpolation coefficients used in this circuit for the two fields are symmetrical. As shown in ref. 1 this has the result that the signal-to-noise ratios of the two fields are equal. With the circuit of fig. 5 the shift of the average information in both fields is equal to 2 5 d 62S ' The functions ofthe various circuit parts of fig. 5 are identical to those of fig. 4; the only difference between the two circuits is that the circuit of fig. 5 contains one extra adder. In b) In c) Fig. 5. The second correction method. (a) As in fig. 4a. (b) As in fig. 4b; Al and A 2 : adders. (c) As in fig. 4c. Output video 625/ines

7 CONVERSION OF 313-LINE VIDEOPHONE SIGNAL INTO 62S-LINE TV SIGNAL Interpolation with two delay lines A third correction method is shown in fig. 6. With this solution the applied interpolation function is broader than that of the methods of figs 4 and 5. In Output video 625/ines Fig. 6. The third correction method. (a) As in fig. 4a. (b) As in fig. 4b; Al' A2' and A 3 : adders. This results in a decrease of the vertical resolution, but also in a decrease of the disturbing effects, as will be shown in the next section. The shift of the average information in both fields is equal to 3d 625 (fig. 6a). The functions of the different circuit parts offig. 6b are analogous to those offig. 4b; the switch 8 4 is again switched every field. The two delay lines in the part of the circuit within the dashed lines can again be replaced by one single delay line plus an electronic switch. As in the interpolation circuit of fig. 4 the circuit of fig. 6 can be modified in order to make the interpolation coefficients symmetrical for the two fields. As, however, this modified circuit gives nearly the same performance as the circuit of fig. 6, this modification will not be discussed here. 4. Results In this section the results will be given as obtained with the four interpolation methods described in the preceding section. The standards conversion is carried out with digital signals. The I-MHz 313-

8 420 M. C. W. van BUUL and L. J. van de POLDER a) Fig. 7a. Original 313-line picture, bandwidth I MHz. b) Fig. 7b. Converted picture, with 625 lines; use is made of the circuit of fig. 1.

9 CONVERSION OF 313-LINE VIDEOPHONE SIGNAL INTO 625-LINE TB SIGNAL 421 Fig. T«. Enlargement of fig. 7a. line video signal is therefore first fed to an analogue-to-digital convertor. This convertor works with a sampling frequency of about 2 5 MHz, with 8 bits per sample. For the line stores (fig. 1) digital shift registers are used. The delay lines i also consist of digital shift registers. After the interpolation, the 2-MHz 625-line video signal is obtained by means of a digital-to-analogue convertor (sampling frequency about 5 MHz). The results of the various interpolation methods of the standards conversion are given by the photographs in figs 7 and 8. These photographs show an overall view of the pictures and of every picture an enlargement of an interesting area, in order to demonstrate more clearly the disturbing effects. The result obtained with the circuit of fig. I, using the special test slide, is shown in fig. 7b (the original I-MHz 3l3-line picture is shown in fig. 7a). This result shows that some disturbing effects are encountered in oblique, nearly horizontal, transients. Furthermore it may be noted that the actual picture on the monitor shows a rather annoying jitter in vertical direction, due to the position error as discussed in sec. 3; of course this effect is invisible on the photograph. The result obtained with the interpolation circuit of fig. 4 is shown in fig. 7c, and the result of the circuit of fig. 5 is shown in fig. 7d. These results show that an important improvement is obtained compared to the result of fig. 7b. Moreover, the vertical jitter due to the position error is eliminated, so that the picture on the monitor is much more improved than it appears from the photographs. The subjective differences between the results of the circuits of figs 4 and 5 are small; perhaps a slight preference exists for the circuit of fig. 5.

10 422 M. C. W. van BUUL and L. 1. van de POLDER cl Fig. 7c. Converted picture, with 6251ines; use is made ofthe interpolation circuit of fig. 4. Fig. 7d. Converted picture, with 625lines; use is rna-"c ofthe interpolation circuit offig. 5.

11 CONVERSION OF 3l3-LINE VIDEOPHONE SIGNAL INTO 625-LINE TV SIGNAL 423 Fig. 7r. Enlargement of fig. 7c. Fig. 7ö. Enlargement of fig. 7d. The result obtained with the interpolation circuit of fig. 6 is shown in fig. Te. This result shows that the disturbing effects have diminished but also the sharpness in vertical direction is decreased. In general the opinion of observers is that the decrease of the disturbing effects is more important than the decrease of the sharpness, so from a subjective point of view a preference exists for the circuit of fig. 6. Results obtained with a normal slide are shown in fig. 8. The originai313-line picture is given in fig. 8a; the result obtained with the circuit of fig. I is given in fig. 8b and the result of the circuit of fig. 5 is given in fig. 8e. It should be noted that the photographs give a rather good impression of the quality of a non-moving picture. However, with pictures moving in vertical direction some of the disturbances become more visible, but the quality rating for the various interpolation circuits found with the non-moving pictures is not affected. 5. A 625-line signal via a 313-line network In those cases where a reduction of the picture quality is allowed, one might use a I-MHz 3I3-line videophone network for the transmission of a picture from a 625-line camera to a 625-line monitor or receiver. A possible method then is that at the transmitting end the 625-line video signal is converted to 313 lines, in the manner described in ref. I, and at the receiving end the 313-line signal is converted back to 625 lines, in the way as described in the preceding sections. Results obtained with this method will be discussed below.

12 424 M. C. W. van BUUL and L. J. van de POLDER e) Fig. 7e. Converted picture, with 625 lines; use is made ofthe interpolation circuit offig. 6. Fig. 7s. Enlargement of fig. 7e. Fig. 8a. Enlargement of fig. 8a.

13 CONVERSION OF 313-LINE VIDEOPHONE SIGNAL INTO 625-LINE TV SIGNAL 425 Fig. 8a. Original 3l3-line picture, bandwidth 1 MHz. a) The experiments show that the overall picture quality, obtained with the system 625 via 313 to 625, is practically identical to the picture quality obtained with the conversion from 313 into 625lines. This means that the extra reduction in picture quality due to the conversion from 625 to 313 lines is small indeed: the conversion from 313 to 625 lines appears to be the determining step for the picture quality. If the most simple method for the conversion from 625 into 313 lines is used, then the difference in information shift between the two fields is equal to d 625 1); with the conversion from 313 into 625 lines, applying the circuit of fig. 1, the information shift is also d 625. If now these two simple conversion methods are used in cascade, then two possibilities exist: either the reading intervals and the switching of switch S2 are such that the information shifts counteract each other, or the errors add together. With the first possibility the information error is eliminated; with the second possibility the total error is equal to 2d 62S ' the result being unacceptable. In principle this ambiguity can be avoided by transmitting some additional information. If this information is available at the receiving end then the reading intervals and the switch S2 can be put in the right phase. Even if this is so the picture quality obtained with the two simple conversion methods in cascade is hardly acceptable. It is therefore always preferable that the circuit of fig. 5, or that of fig. 6, is used at the receivingend (assum-

14 M. C. W. van BUUL and L. J. van de POLDER Fig. 8b. Converted picture, with 625lines; use is made ofthe interpolation circuit of fig. 1. b) Fig. 8e. Converted picture, with 625 lines; use is made ofthe interpolation circuit of'fig. 5. c)

15 CONVERSION OF 313-LINE VIDEOPHONE SIGNAL INTO 62S-LINE TV SIGNAL 427 Fig. 8f3. Enlargement of fig. 8b. Fig. 8y. Enlargement of fig. 8c. ming that the same interpolation circuit is always used at the receiving end, irrespective of whether the system 313 to 625 or 625 via 313 to 625 is used). If one of these circuits is used, then no information shift between the two fields is introduced at the receiving end. In that case it is also preferable to use one of the compensation methods at the transmitting end 1), at the cost of an extra delay line of 64 fls. It is found that the phase of the reading intervals and of the switch at the receiving end is not critical at all if the two conversion systems, both using a compensation circuit, are cascaded. In both phases one obtains practically identical pictures. No information about the phase of the switch at the transmitting side needs therefore to be transmitted. The pictures obtained with the system 625 via 313 to 625 are practically' identical to those obtained with the conversion from 313 to 625 lines. Figures 7 and 8 therefore give a good representation of the picture quality that can be expected for a 625-line system making use of the I-MHz 3l3-line videophone network. 6. Conclusion The results obtained with the standards conversion from 313 into 625 lines demonstrate that an acceptable picture quality can be obtained. By means of interpolation circuits some disturbing effects can be reduced, at the cost of a slight reduction of the sharpness in the vertical direction. For the case of a videophone application this reduction in sharpness is certainly acceptable.

16 428 M. C. W. van BUUL and L. J. van de POLDER Nearly the same picture quality is obtained when a 625-line picture is converted to 313 lines, transmitted via a I-MHz videophone network and reconverted to 625 lines after transmission. Eindhoven, September 1974 REFERENCES 1) M. C. W. van Buul and L. J. van de Polder, Philips Res. Repts 28, , 1973.