Reduction of Control Bits for Adaptive Sub-Nyquist Encoder

The introduction of new television services such as DBS (direct broadcasting satellite) offers the opportunity of introducing new television standards. It is desirable that any new standard provide a means for backward-compatible extended definition, i.e. viewing a high definition picture on a standard monitor. Most of the new systems being contemplated provide an auxiliary data channel as part of the video format. Therefore this disclosure provides a means for utilizing the auxiliary data channel for picture quality enhancement.

In U.S. Pat. No. 4,323,916 "Data Rate Reduction for Digital Video Signals by Subsampling and Adaptive Reconstruction" (R. A. Dischert and G. A. Reitmeier) a means was disclosed to enhance the resolution of a transmitted digital video signal by including "steering bits" in the data stream that indicate to a decoder which output of the outputs of several postfilters should be applied to reconstruct a deleted sample in the optimal way. The same concept can be applied to extending the resolution of an analog transmitted DBS signal by sampling it and computing intermediate samples in accordance with instructions in steering bits transmitted with the signal. In general, the data rate of the auxiliary channel is not high enough to support the number of bits required to control the filtering on a pixel-by-pixel basis.

In the previously mentioned patent, the encoder produces reconstruction filter control or steering bits according to which filter produces the "best" estimate, but the control bits may change to indicate a change in the reconstruction filter when the difference between the current "best" estimate and the previous "best" estimate may be as little as one quantization level and as a result, the control bits occur in a very random fashion, since they respond to noise and very small changes in picture content. This invention provides for modifications to the encoder of the aforementioned patent, that modifyies the generation of bits in order to greatly reduce the randomness of the filter control bits, which allows for a great amount of data reduction by further encoding the control bits.

A system that transmits a control byte to indicate which of combinations of surrounding bytes is a best match to an omitted byte. The control byte is repeated for subsequent omitted bytes unless it no longer indicates a match within a selected tolerance.

FIG. 1 is a block diagram of a first embodiment of a transmitter of the invention;

FIG. 2 is a block diagram of a second embodiment; and

FIG. 3 is a receiver for use with FIGS. 1 and 2.

FIG. 1 shows a modified encoder wherein an 8-bit digital video signal supplied from a digital video signal source 102 is received at 8-bit terminal 32. As in the encoder of said patent, filters 34, 36, 38 and 40 (see FIGS. 6-9 of said patent) provide horizontal, vertical, left diagonal, and right diagonal averages or estimates of the sample output from equalizing delay 41. Absolute value difference circuits 42, 44, 46 and 48 provide a measure of the reconstruction error of each filter estimate, i.e. the difference between the actual value from delay 41 and the estimates from filters 34, 36, 38, and 40. Minimum error logic 50 (see FIG. 5 of said patent) examines said reconstruction errors and encodes which filter produces the best estimate, i.e. provides a code signal to indicate which estimate is the best match to the omitted digital word. Modifications to the basic encoder consist of MUX (multiplexer) 139 with inputs from the four absolute difference circuits 42-48, and an output controlled by the signal from 2-bit latch 153, which contains the control bits used for the previous deleted sample. The control bits from the output of latch 153 thus select in MUX 139 the difference between the last omitted digital word and its best estimate. The output signal from MUX 139 is fed to a magnitude comparator 141 along with a threshold T (generally a small number). The threshold number T may be provided by a suitable threshold signal source 103 coupled to input 106 of comparator 141. The output of magnitude comparator 141 is a 1 whenever the reconstruction error for the new omitted digital word exceeds the threshold T. When this occurs the reconstruction error of the filter indicated by the previous control bits is too large, and latch 153 is enabled so as to clock in new control bits generated by minimum error logic 50. If the reconstruction error indicated by the previous control bits is smaller than T, then latch 153 is disabled, and holds the previous control bits, despite the generation of a marginally "better" choice by minimum error logic 50. In other words, if the error is within a threshold, control bits from a previous omitted word are repeated instead of transmitting new control words. The output of latch 153 is now a set of control bits with great redundancy, which are fed to data encoder 159 to reduce this redundancy, and hence reduce the required data rate. Data encoder 159 may use any technique already in the art, although run length coding may be particularly appropriate in this case. It eliminates every other video data word from delay 41 and can also comprise a D/A (digital-to-analog) converter for the video bits.

Delay 155 is a field delay, for delaying by one field signal the samples provided by delay 41 for the purpose of allowing the encoded control bits to be inserted in the vertical interval preceding the field in which the samples selected by encoder 159 will be used. D/A converter 157 (optional) converts the samples into an analog signal if analog operation is desired. Data encoder 159 has sufficient buffering capacity to insert encoded filter control bits into the vertical interval of the transmitted signal. It should be understood that said encoded filter control bits need not be transmitted during the vertical interval if another means (e.g. a frequency multiplexed data carrier) is provided for their transmission. It should also be understood that threshold T may be modified by data encoder 159 in order to prevent the repetition of an overload condition in said encoder. Thus, the threshold may change as a function of the steering bit data rate.

A modification to the encoder of said patent is shown in FIG. 2. The outputs of absolute difference circuits 42, 44, 46 and 48 are fed to multiplexers 140 and 142. MUX 140 is controlled by the output of minimum error logic 50, so that its output is the reconstruction error of the best estimate. MUX 142 is controlled by the output of latch 153, so that its output is the reconstruction error of the previously indicated filter. Both errors are applied to absolute difference circuit 144, whose output signal is the difference in reconstruction errors between the best estimate and the previously indicated estimate. This difference is fed to magnitude comparator 146 along with threshold T, and the output of said comparator enables the clocking of latch 153 as previously described. The remaining portions of FIG. 2 also function as described for FIG. 1.

FIG. 3 shows the decoder, which is essentially the same as the standard decoder of said patent. Data decoder 200 extracts the encoded control bits and decodes them, generating the same sequence as occurs at latch 153 in the 2+2 encoder. A/D 202 (optional) digitizes the transmitted video signal (consisting of the retained samples). The rest of the decoder is standard, and using the control bits, will reconstruct a signal with substantially greater resolution than the transmitted signal by use of the transmitted steering bits.