IMPROVED DEVICE FOR DCT ENCODING OF DIGITAL VIDEO SIGNALS

Abstract: The digital video signal is decomposed in intrafield blocks each comprising a set number of adjacent image elements, and each intrafield block, or an interfield block or an interframe block derived therein, is subjected to a Discrete Cosine Transform and the transformed signal is applied to quantizer to generate a quantized signal which is encoded at variable length and loaded to a transmission buffer, the quantization resolution being inversely proportional to the buffer fill rate. According to the invention, a digital low-­ pass filter is connected to filter the block to be subjected to Discrete Cosine Transform, and is subjected to the buffer so that its upper-frequency cutoff lowers as the buffer fill rate increases.

Description: The present invention relates to an improved device for the DCT encoding of digital video signals. As it is known, the conventional color digital video signal comprises a luminance component Y and two chrominance components Cr and Cb. The luminance component is sampled at 13.5 MHz, while each of the chrominance components is sampled at 6.75 MHz (CCIR, Recommendation 601, Dubrovnik 1986). Each sample of the signal components is made of 8 bits, thus the required bit per second flow rate, for transmitting such a signal is 216 Mbit/sec (i.e. 8x(13.5+6.75+6.75)) and, even eliminating the line and frame synchronisms transmission, the bit timing reduction cannot be over 25%. In the case of high definition video signals, the bit timing will be even 4 or 5 times the described one. It is thus apparent how the transmission of those signals would require high capacity transmission channels. In order to reduce the bit timing of those digital video signals without significantly reduce the image quality, it is known to use the Discrete Cosine Transform (DCT) for encoding the video signal reducing the redundancy. In such encoding (as it will be more apparent from the following description ) at least partial data are generated that are cleared of the information connected to the spatial correlation of the image or also cleared of the information connected to the time correlation of the image with respect to former images (frame or field) of the video signal. Consequently the generated data flow is not uniform, it being very low in case of static images or very high in case of rapidly variable images. In these encoding systems, it is therefore provided an output buffer feeded with a variable data flow which flows into the transmission channel with constant speed which depends on the transmission channel capacity. See for example the system disclosed by US-A-4672441. In these encoding systems , the buffer is inevitably going to saturate occasionally, with a greater frequency the lower is the transmission channel capacity. This must be avoided because it may cause data to be lost with consequent decoding errors; for this purpose it is known to have a rough quantization of the DCT coefficients as the buffer is filled up. This naturally causes an increase of quantization noise and a deterioration of the reconstructed image. This effect is naturally more evident when channels of reduced capacity are used, corresponding to an encoding of 1 to 1.5 bit/pel, for example channels of 10-15 Mbit/sec, for normal definition video signals (CCIR Recommendation N.601), or of 50-70 Mbit/sec for high definition video signals. Main object of the invention is to provide an improved encoder that, when using low capacity channels, lowers the mean number of bits at the buffer inlet without excessively increase the quantization noise of the DCT coefficients and, at the same time, can operate with no lesser performances than the known encoders when dealing with high capacity channels. The invention achieves the above mentioned object as well as other aims and advantages as will be apparent by the following description, with a device for the DCT encoding of video signals, in which a digital video signal is split in intrafield blocks each comprising a set number of adjacent image elements, each of said intrafield blocks, or an interfield or interframe block derived threin, is subjected to a Discrete Cosine Transform and the transformed signal is applied to a quantizer for generating a quantized signal which is encoded at variable length and loaded to a transmission buffer, the quantization resolution being inversely proportional to the buffer fill rate, characterized in that the device further comprises a digital low-pass filter connected to filter the block to be subjected to Discrete Cosine Transform, the filter being subjected to the buffer so that its upper-frequency cutoff lowers as the buffer fill rate increases. The invention will be now described in more detail with reference to some of its preferred embodiments, illustrated in the enclosed drawings, given by way of illustrative and not restrictive example, and in which: Fig. 1 is a block diagram of a DCT encoder according to the prior art; Fig. 2 is a block diagram of a DCT encoder according to a first preferred embodiment of the invention; Fig. 3 is a partial block diagram, more in detail, of a portion of the encoder of Fig. 2; Fig. 4 is a partial block diagram of a DCT encoder according to a further preferred embodiment of the invention. With reference to Fig. 1, in a tipical DCT encoder device, illustrated by way of example, a conventional digital video signal SV is applied to a line-block converter RBl, which splits each field of SV into groups of 8 lines, usually called "stripes", and then each stripe into groups of 8x8 pels, called "blocks", giving an output of a course of block signals SB. Such splitting is separately performed for the chrominance and luminance components, using the same coding algorithm, but for the sake of simplicity the sole luminance component will be herewith considered. The signal block SB at the converter 1 output is applied to an adder 21 which computes the difference between each block of SB and a prediction block SP providing a difference block SE of 8x8 pels which contains the prediction error; as it will be explained later, the prediction block SP is obtained by the signal treated by the preceeding field (interfield mode) or by the signal of the preceeding frame (interframe mode). For a simpler illustration, only the interfield block is referred in the description and drawings, being intended that the circuits will be normally applied to both the properly called interfield block and the interframe block. Where the video signal has been generated according to a non interlaced standard, such a distinction is lost and the sole interframe block remains. The 8x8 pel choice is tipical but it is indicated purely by way of example, being it possible, within the invention to choose arbitrary numbers n and m for the two block dimensions. Both the signal block SB (intrafield) and difference block SE (interfield) are applied to the two inlets of a switch 31 and to two inlets of a per se known selector DEC 2 of the interfield/intrafield modes. The selector 2 compares the two signals for each block and emits a signal CS to control the switch 31 to choose the one of the two blocks that minimizes the generated bit number. The block SB contains both space and time correlations of the image and it is called "intrafield block", while the block SE has been cleared by the time correlations, thus mainly containing space correlations between successive images, and it is called "interfield block" or "interframe block" depending whether the two compared fields belong to the same frame or to two different frames. The intrafield or interfield (interframe) blocks SB or SE leaving the switch 31 as signal SS are applied to a circuit 3 which generates the bidimensional Discrete Cosine Transform DCT of the signal block 8x8 (per se known), giving out transformed blocks SD, i.e. 8x8 blocks of DCT coefficients. Signal SD is then applied to an adaptative quantizer 4 (also per se known) which gives out a corresponding block SQ of quantized DCT coefficients. Block SQ enters a variable length encoder 9, where it is converted into a variable word length code, for example as described in Rafael C. Gonzales and Paul Wintz, "Digital Image Processing", Second Edition, Addison Wesley, pages. 262-268. According to the criteria described therein, the code words constituted by few bits are assigned to the least probable amplitude levels in order to reduce the mean number of bits necessary for transmitting the coefficients. Furthermore, a diagonal scanning of the signal SQ is used to produce many consecutive null coefficients and thus to utilize a shortened encoding of zero sequences (Gonzales and Wintz, op. cit., pages 288-289). The generated code words SC are then loaded to a buffer 10 and, with a constant timing, therefrom transferred to a transmission encoder 11, which generates the data to be transmitted on the channel, tipically introducing codes for correcting errors and other selected control codes. To obtain the prediction signal SP (interfield or interframe block) a decoding chain is enclosed in the encoding system and comprises an inverted quantizer 5 and a Discrete Cosine Antitransform circuit 6, adapted to generate a signal SID which is the reconstruction of the inlet signal of circuit DCT 3, and also an adder 22, a frame memory 7, a predictor circuit 8, all per se known, with a switch 32 between the outlet of the predictor 8 and one of the inlets of adder 22. If the signal SID is an intrafield block, switch 32 is open by command of signal CS of predictor 2 and is directly memorized as signal SU on the frame memory 7. If instead signal SID is an interfield block (difference), switch 32 is closed by the predictor 2 in order to add SID to SP to obtain signal SU which enters the field memory 7. As said in the introduction, since the buffer 10 can saturate in cases of image high information content, it is known to regulate the quantization resolution in quantizer 4 (and parallely in the inverted quantizer 5) as schematically illustrated by the dashed line connection 62, in order to make a rougher quantization when the buffer is close to saturation. Practically, a group CB, of bits more significant of the current course of buffer 10, is used as address in a not illustrated memory containing a table of the possible amplitudes of the quantization step, in a per se known manner and thus not described for the sake of brevity in the present description. By using a rougher quantization, the quantized coefficient levels are reduced and the consecutive zero sequences are lengthened. Naturally the treated and reproduced image quality is thus made worse, because the quantization noise increases. In the case instead that the buffer tends to get empty, the quantization precision is enhanced and, if the case, filler words are generated for the sole purpose of feeding the channel. Fig. 2 shows the block diagram of a DCT encoding system similar to that of Fig. 1 and improved according to a first aspect of the invention. The system comprises the same modules and functional connections of Fig. 1, and designated by the same reference signs, but it is different in that a digital low-pass filter 12 is inserted downstream the swith 31, which is controlled by the selector 2; the outlet SS of the filter 12 is applied to the Discrete Cosine Transform circuit 3. This filter attenuates the high frequences in the signal SB or SE which has to be transformed, thus reducing the levels of the corresponding DCT coefficients, and consequently lengthening the zero sequences and reducing the timing of the bits applied to the buffer. The filter characteristic, i.e. its cut-off frequency, is controlled by the same quantization resolution control signal CB through connection 62 (shown by a dashed line), in order to lower the cut-off frquency according to the fill rate of the buffer. The filter is controlled in such a way that when the buffer fill rate exceeds a first threshold, so as to cause the onset of a stronger filtering, the filter gets back to the lower filtering only when the fill rate has gone below a second threshold which is lower than said first threshold. In other words, the transit thresholds of the filtering action are offset in rising or falling or, still in other words, the filter controlled with hysteresis. Naturally, when the quantizer is working at the highest resolutions, the filter is commanded for excluding any attenuation. Fig. 3 schematically and partially shows a preferred embodiment of the filter 12 used for attenuating the behaviour described above. In a known manner the filter comprises a plurality of multipliers 70, 72, 74, driven by the inlet signal SB or SE through a chain of delay elements 76, 78, with the multipliers outlets grouped in an adder 79, the output SS whereof is the selected filtered signal. The second inlets of the multipliers 70, 72, 74 are connected to respective outlets of ROM memories 80, 82, 84, which contain tables corresponding to the selected multiplier factors for the different operating modes, or different cutoff frequencies for the filter. For generating the address to be applied to the ROM memories 80, 82, 84, the filter, according to the invention, comprises a ROM memory 86 for selecting the filtering and having an outlet applied to a gate 88 which is in turn on a latch register 90. Gate 88 is adapted to transfer an incoming signal on the the latch 90 from a timing signal CK having the same timing of the "stripes" of the video signal. The outgoing signal CL of latch 90 controls the address inlets of all the ROMs 80, 82, 84, in parallel. The address which controls the filtering selector ROM memory 86 is formed by associating the bit lines constituting the outlet signal CL of latch 90 with the bits of signal CB coming from buffer 10 as indication of the fill level. By loading the ROM memory 86 with selected filtering values according to banks corresponding to the different values of CL, according to criteria known in the art, the desired behaviour "with hysteresis" is obtained, i.e. with different thresholds according to the raising or falling direction of the filter fill rate. With the use of filter 12, the reproduced image will not have an increase of noise caused by the quantization of the DCT coefficients, but just a loss of definition given by the attenuation of the high frequencies. Such a loss of definition is less annoying to the user than a high quantization noise which shows up in the reproduced image as spot alterations, or "grain", on the image. The filter operating mode on the signal is such that it is not necessary to give any information on the filter operation in the transmitted data, since in decoding at the other end of the channel no filter is necessary nor any operation connected to it: any modification in the filter choice and in the variation law of its characteristic concerns the sole decoder. According to a variated embodiment, the filter characteristic is subjected also to the inter/intra selector 2, in order to filter the intrafield blocks more strongly than the interfield blocks (and/or interframe). In fact, on average the intrafield blocks contain image parts having a strong time movement, while the interfield blocks contain image parts having a weak time movement. Since, as it is known, details are scarcely perceived by the eye in rapidly moving image parts, it is therefore acceptable a greater band limitation for these cases than for the static image cases, given the same image quality as subjectively perceived. Fig. 4 shows a second embodiment of the invention, more complicated but also more sophisticated. For semplicity, Fig. 4 shows exclusively the part of block diagram which differs from that of Fig. 2. Instead of inserting a low-pass filter 12 downstream the switch 31, as in the embodiment of Fig. 2, two filters 120 and 122 are herewith inserted, respectively for signal SB (intrafield) and for signal SE (interfield and interframe). Each of the two filters 120 and 122 is subjected to the quantization resolution rate in a way similar to that of filter 12 of Fig. 2. In this way the inter/intra selector 2 makes the selection on already filtered signals, and thus takes into account possible changes the filtering makes on signals SB and SE, such as to suggest a different selection. Generally, the efficiency and quality of the image can be improved at the cost of a greater circuit complication. It is apparent that where it is necessary to distinguish the interfield block from the interframe block, three filters will be used instead of two, with the same conditions described above. Both filter 12 and filters 120,122 can be linear or non-linear (with respect to the signal amplitude), according to criteria known in the art. Furthermore, the whole filter can be either monodimensional or bidimensional. The invention has been illustrated in some of its preferred embodiments, applied to a tipical known device, illustrated in Fig. 1, but the principles of the invention are applicable also to other similar devices, particularly to a device that does not provide for generating signal SE and comparing it to signal SB in the inter/intra selector 2. In that case there will not be the entire decoding chain 5, 6, 7, 8, 22, 32, selector 2, adder 21, and switch 31. It is apparent that, even in such a simplified device, the use of filter 12 yealds advantages similar to those described above. It is also apparent that filter 12 or filters 120,122 may be controlled, instead by the signal CB coming from buffer 10, by an adapted signal provided by the quantizer 4, indicating the current quantization resolution: also in this case the filter is in the end subjected to buffer 10 anyhow, given the tight connection between the buffer and the quantizer. At the end of the filtering operations, only the samples present in the 8x8 block to be filtered can be used, or, in order to eliminate the filter start transient state in the single block, samples taken from the adjacent blocks can be taken, as many as there are those of the impulsive timelapse of the filter. In this second case, part of the samples must be taken from the blocks preceeding the one to be filtered, and part from the following blocks. The memory will have to be increased in order to contain the added samples, and it will have to be introduced in the DCT treatment a delay equal to the number of samples taken from the blocks following the one to be filtered. Alternatively, to reduce the memory increase and to avoid delays in the treated signal, just the samples relative to the blocks preceeding that to be filtered can be added. It is thus eliminated the transient state from the start edges of the block but not from the end edges. In all these cases though, it is necessary to use filters having a very short pulse response time. In fact a long pulse response time of the filter requires a great deal of added memory and a great delay in case of filtering without transient state, while it can prevent the filter to get into working conditions in case of filtering with transient states on the edges. The above mentioned techniques for eliminating the transient states at the edges of the blocks are to be considered comprised in the definition of filtering of the blocks as described above and claimed in the claims.

Claims: Device for the DCT encoding of digital video signals, in which a digital video signal is decomposed in intrafield blocks each comprising a set number of adjacent image elements, and each intrafield block, or an interfield block or an interframe block derived therein, is subjected to a Discrete Cosine Transform and the transformed signal is applied to quantizer to generate a quantized signal which is encoded at variable length and loaded to a transmission buffer, the quantization resolution being inversely proportional to the buffer fill rate, characterized in that the device further comprises a digital low-pass filter connected to filter the block to be subjected to Discrete Cosine Transform, the filter being controlled by the buffer so that its upper-frequency cutoff lowers as the buffer fill rate increases. Device, according to claim 1, characterized in that the low-pass filter is also controlled by the quantizer to be substantially excluded when the quantization resolution is at a maximum. Device, according to claim 1 or 2, characterized in that the low-pass filter is also controlled by the quantizer in order that its upper-cutoff frequency lowers as the quantization resolution is reduced. Device, according to one of claims 1 - 3, wherein the quantized signal is reconverted, inverse transformed, reconstructed and memorized and eventually subtracted from the intrafield block for producing an interfield block, and an interfield/intrafield mode selector compares each intrafield block with the corresponding interfield block to choose among them the one with the least content of information, characterized in that the low-pass filter is furthermore controlled by said selector for more strongly attenuating the high frequencies in the intrafield than in the interfield blocks.

Inventors: RONCHETTI, LUIGI, STROPPIANA, MARIO
Applicants: RAI RADIOTELEVISIONE ITALIANA
Application year: 1989
Main classification: H04N7
IPC codes: H04N1, G06T9, H03M7, H04N7