FI102799B - Improved Dolby Prologic decoder - Google Patents

Description

102799

Improved Dolby Prologic Decoder - Förbättrad Dolby Prologic -avkodare%

The Dolby surround sound system is widely used not only for movie but also for '5 video cassettes and other video recordings. The ice system is also spreading to stereo TV applications. There are four audio channels: left (L), right (R), center (C) and surround (S). They are encoded in the Dolby encoder such that L and R are transmitted normally, but the C and S channels are included in the phase encoded L and R channels. Both the center channel C and the surround channel S are divided equally between the L and R channels, but the surround channel S components to be added to the left and right channels have a 90 degree phase difference.

The decoder separates the four coded channels from each other. In a first generation passive decoder, the left channel L and the right channel R are output as such at the encoder output. These channels also contain information for channels C and S. The center channel is expressed as the sum of the L and R channels (the reverse phase surround information is canceled) and the S channel is expressed as the same phase center channel information is canceled.

20 If Dolby encoded audio is output from standard two-channel stereo equipment, the localized L and R audio will be normal. The center channel C is localized to the center of the stereo pair, and the surround channel S is not clearly localized anywhere, but comes from a vague direction.

The encoder is fairly simple to implement and consists mainly of a simple «difference amplifier», a delay element and filters. The disadvantage of the encoder is that although the difference between the main channels L and R, as well as the difference between the center channel C and the surround channel S, is more than 40 dB, the difference between adjacent channels (e.g. S and L) is only 3 dB. Therefore, the area where the stereo image is good and the center channel sound seems to come mid-30s is quite small. Also, front and rear isolation is not good because: surround speakers placed behind or to the side of the listening space also reproduce the separation information for the left L and right R channels. Therefore, passive decoders cannot accurately position the sounds, i.e., the sound does not come in the correct direction, but the directional effect is vague.

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The disadvantages of passive decoders have been reduced by the active decoder. An active decoder may be thought of as a passive decoder associated with a processing circuit. The active decoder is called Dolby Prologic. This process 2 102799 examines the balance on the L-R and C-S axes. Prologic processing aims to keep the total sound power constant. Dominant sound is emphasized by attenuating other channels. This apparent channel separation can be significantly increased.

Figure 1 is a block diagram of a known Dolby Prologic decoder. The encoder input is left channel Lt and right channel Rt encoded by center channel C and surround channel S. The level difference between the input signals is corrected in blocks 1 and 2. Adaptive matrix block 6 of the prolo-gic circuit produces all channels L, R, C and S. Surround the channel S is passed through an anti-crease filter 7, a body 9 adjustable delay element 8 and a low-pass filter 10 to a modified B type Dolby noise suppressor 11. The modified Dolby B noise suppressor is developed by Dolby and differs from conventional noise reduction only normal. The suppressed output surround signal S is then applied to the baseline level controller 12, to which also the left L, right R and center channel signals provided by the adaptive 1S matrix block 6 of the Prologic circuit are also applied. The output of the level controller provides all four channels as encoder output signals. The noise sequencer 3 can be used to adjust the levels of different channels.

The core of the decoder, and at the same time what distinguishes it from the passive decoder, is the adaptive matrix block 6. It comprises two main parts: an adaptation logic and a matrix of voltage-controlled amplifiers VCA which improve the channel separation. The adaptation block continuously monitors incoming Rt and Lt audio channels, from which it calculates, in a known manner, the sum and difference of the audio channels, utilizing the signal level ratios of the R and L channels and the C and S channels. In doing so, it examines the direction in which their instantaneous sound field dominance is. The determination is made by full-wave rectification, first by applying each of the main signals (L, R, C, S), performing a logarithmic transformation of the resulting DC voltages in pairs, and then taking their differences. This gives two independent control signals, one representing dominance on the left-right axis and the other on the center-surround axis. The control signals are further processed in the adaptation section of block 6 to obtain a set of (16) control voltages applied to the amplifier portion of block 6, which is thus formed by a matrix of a plurality of voltage controlled amplifiers (VCAs). The left-hand Lt and right-hand Rt are also fed to the VCA matrix as its input signals. The control voltages control the gain of the VCA amplifiers, and the matrix 35 outputs its signals R, L, C and S, which include directional information amplified in the direction of dominance calculated by the adaptation block and proportional thereto. The L, R, and C signals are then passed, as is the case with the passive Dolby decoder, to the level master 12, where S is also passed through the side branch, as shown in Figure 1.

However, this known Dolby Prologic decoder has some drawbacks, particularly in 5 television applications. Television receivers are increasingly equipped with Dolby Surround Sound decoders. As stated above, four audio channels L, R, C and S are available after the encoder for any additional audio signal processing performed by the television manufacturer. Thus, additional processing should be performed on each audio channel separately, which results in very expensive practical implementations. An object of the invention is to provide an arrangement to minimize the work required for further processing.

This is accomplished by applying the dual-channel audio signal Lt and Rt in the adaptive Prologic matrix to a linear filtering and / or equalization signal processing block that processes the signals of each channel separately. The two-channel output signal of said block is the input signal of the matrixing section. The Dolby B decoder part is arranged in parallel with said signal processing block, whereby the surround signal is excluded from the operations of the signal processing block.

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Thus, from the point of view of the input signals Rt and Lt, they are not led directly to a VCA matrix block, as in the prior art encoder, but first to a signal processing block that performs linear operations (DSP). The output of the DSP block produces the processed R and L signals, which are then applied to 25 VCA matrix blocks. Thus, only two signals must be processed instead of four, but the effect of processing is visible in each of the four output channels of the decoder. Thus, the invention is particularly suitable for use when all four channels would have to undergo a similar operation, such as filtering.

After dematrization, the Surround channel is subjected to Dolby B decoding, such as: depicted in Figure 1. This is a non-linear operation whose effect changes if the value of the signal changes. Because the signal processing block of the invention changes the value of the surround signal, it cannot be derived to a Dolby B decoder. Therefore, in the embodiment of the proposed solution, the level of the surround signal is detected 35 before the linear DSP block, and this information is used to control the Dolby B process, whereby the linear operations in the DSP block no longer adversely affect the operation of the Dolby B block.

4, 102799

The invention will be described with reference to the accompanying schematic drawings, in which Figure 1 illustrates a known block diagram of a Prologic decoder, and Figure 2 illustrates the placement of a linear DSP block in the Figure 15 decoder according to the invention.

The structure of Figure 1 has already been described above. Fig. 2 shows an adaptation and VCA amplifier matrix block 6 of Fig. 1, to which a linear signal processing block 23 of the invention has been added 23. The input signals of the prior art block 6 (Fig. 1) are 10 audio channel signals Rt and Lts encoded in center C and surround S.

channels (Figure 2). Using the prior art, the encoded two input signals would be fed to both the Dolby Prologic adaptation block, which defines the direction with the instantaneous dominant signal, and to the amplifier block, which consists of a matrix formed by the voltage-controlled amplifiers VCA. The adaptation block generates control signals that control the gain of each VCA amplifier. Four audio channels R, L, C and S are output from the matrix output.

According to the invention, after the point at which the left Lt and right Rt channels are branched into the adaptation block 21, but before the channel entry into the VCA-20 matrix block 26, a block 23 performing linear signal processing operations is included. The block contains at least one digital signal processor DSP.

In the DSP block 23, the desired operations such as filtering, equalization, etc. for the left and right channel audio signals Rt and Lt can be performed, and the desired operations are application specific and freely determined by the television manufacturer. Since there are only two channels to process, only two filters are needed, and not four, as would be the case if the audio signals were processed after the Dolby decoder.

From the DSP block, the processed signals of the left and right channels are applied to the block 22 of the VCA-30 amplifiers which form four audio channels in a known manner.

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The surround signal from the amplifier block 22 cannot be derived to a non-linear Dolby B decoder because the value of the surround signal has been changed by the DSP block 23. Therefore, the input signals are also applied to the surround signal level detector 24. 35 The detected surround signal is now the "correct" surround encoded in the input signals, so this signal is fed to the Dolby B decoder. This decoder is now working properly.

5, 102799

The invention has a number of advantages over post-decoder processing of audio signals. First of all, only two channels (Lt, Rt) need to be processed and yet all four decoder output channels are affected. Second, the hardware requirement is reduced if the processing is performed by the hardware, or the signal processor-5 code is less if the implementation is programmatic. Third, two-channel processing does not require as much processing power as four-channel processing. Performing linear processing according to the invention does not interfere with the operation of the Dolby decoder itself.

Claims (3)

6 102799 A decoder which decodes the resulting dual-channel audio signal (Lt, Rt) into a four-channel (left, right, center, surround) audio signal, and S having a Dolby Prologic adaptive matrix block (6) comprising: - a voltage controlled amplifier (VCA) matrix section (22) which amplifiers are individually controlled by amplifier gain control signals and whose output signals are a four-channel audio signal (L, R, C, S), an adaptation section (21) having input signals as a dual-channel audio signal (Lt, 10Rt) and output amplifier control signals, in addition to the decoder including a Dolby B decoder portion for processing a surround signal expressed from a two-channel audio signal, characterized in that the two-channel audio signal (Lt, Rt) is applied to a signal processing block (23) performing linear filtering and / or equalizing, and that the two-channel output signal (R ', L') of said block is an input signal of the matrixing section (22) and that said Dolby B decoder section is arranged in parallel with said signal processing block (23), wherein the surround signal is out of operations of said signal processing block (23) . 20 2. Avkodare enligt patent krav 1, audio coding channel audio channel (Lt, Rt), audio leds (en), surround signal (S), color detectors utsignal är en insignal i Dolby B-avkodardelen. 15 A decoder according to claim 1, characterized in that the dual channel audio signal (Lt, Rt) is also applied to a surround signal detector (24) which separates the surround signal (S) encoded into the input signals (Lt, Rt), wherein the detector output signal is Dolby B. input signal of the decoder section. 25 Decoder according to Claim 1, characterized in that the signal processing block (23) is a digital signal processor. 30 *: 1. Avkodare som avkodar en tillförd audiosignal med tvä kanaler (Lt, Rt) account en audiosignal med fyra kanaler (crank, höger, mitt, surround) and vars Dolby Prologic-adaptive matrisblock (6) innefattar - en matriseringsdel (22 ) bestaende av spänningsstyrda förstärkare (VCA), stem 35 förstärkare styrs individellt med styrsignalema för förstärkning och vars utsignaler utgörs av en audiosignal med fyra channeler (L, R, C, S), - en anpassningsdel (L) en audiosignal med tvä channeler (Lt, Rt) och utsignaler av these styrsignaler for förstärkning av förstärkama, 7 102799 colors avkodaren dessutom innefattar en Dolby B-avodardel för bearbetning av en surround-signal detonter med i audiosignalen med. tvä channeler (Lt, Rt) leds till ett signal-behandlingsblock (23) som utfor linear filtering och / eller equalization och behand-5 lar signalema frän de tvä channelema skilt , och att utsignalen (R't, L't) med tvaner frän this block is insignal i matriseringsdelen (22) och att utnda dolby B-avodardel anordnats parallellt med these signalbehandlingsblock (23), color surround-signal utesluts frännn signalbehandlingsblocks (23) operationer. 10 A digital signal processor according to claim 1, wherein the call signal is an attalbehandlingsblocket (23).