Synchronizing circuit for use in a digital audio signal compressing/expanding system

Background of the Invention

Field of the invention

0001The present invention relates to a synchronizing circuit, and more specifically to a synchronizing circuit for a bit stream in a digital audio signal compressing/expanding system.

Description of related art

0002In the prior art, one example of this type synchronizing circuit includes a synchronizing circuit for a bit stream in a compressing/expanding system for a digital audio signal standardized accordance with ISO/IEC 11172-3 (called a "MPEG/Audio system" hereinafter). This MPEG/Audio system includes three systems, namely, Layer 1 system, Layer 2 system and layer 3 system. In the following, the prior art based on the Layer 2 system will be described.

0003However, before describing the prior art, a summary of the MPEG/Audio system will be described.

0004The number of bits included in one frame of the MPEG/Audio Layer 2 system is determined in accordance with the following equation from a bit rate and a sampling frequency:<maths id="math0001" num=""><math display="block"><mrow><mtext>{number of bits included in one frame} = {bit rate} × 1152 ÷ {sampling frequency}</mtext></mrow></math><img file="EP0735709A2_D0001.tif" /></maths>

0005In addition, it is defined that number of bits included in one frame is a multiple of "8". For example, if the bit rate is 192 kb/s and the sampling frequency is 48 KHz, the number of bits included in one frame is 4608 (bits).

0006One frame of the MPEG/Audio Layer 2 system under the above mentioned condition has the structure as shown in Figure 1, which includes a header, a CRC (Cyclic Redundancy Check) code, a bit allocation index, a scale factor selection information, a scale factor index, a sample and an ancillary data. The header is constituted of a synchronous pattern formed of continuous 12 bits of "1", and various information formed of 20 bits. These 20 bits include information used for determining the bit number of one frame, for example, a bit rate index, a layer, a sampling frequency, and padding. A first bit of various kinds of information included in these 20 bits is called an "ID bit", which is defined as "1" in the MPEG/Audio Layer 2 system.

0007In addition, the bit rate index is expressed by 4 bits. On the basis of the value of the bit rate index and the value of the layer, the bit rate is defined as shown in the table of Figure 2. However, use of "1111" is inhibited. The layer is expressed by 2 bits. "11" indicates the layer 1, and "10" indicates the layer 2. "01" designates the layer 3. However, "00" is undefined. The sampling frequency is expressed by 2 bits. "00" designates 44.1 KHz, and "01" indicates 48 KHz. "10" designates 32 KHz. However, "11" is undefined. If the padding is "1", the bit number of one frame is increased by one slot, and if the padding is "0", the bit number of one frame is unchanged. It is defined that the bit number of one slot is 32 bits in the case of the layer 1 and 8 bits in the cases of the layer 2 and the layer 3.

0008The above mentioned synchronous pattern is not only one in each frame, but there is possibility that the same pattern as the synchronous pattern is included in a portion other than the header. Because of this, it is not possible to conclude immediately from the detection of the synchronous pattern that the synchronism has been established. If an audio signal is decoded on the basis of a false or quasi synchronous pattern, a noise sound is generated. As a countermeasure for solving this problem, the synchronous circuit is configured to judge that the synchronism has been established, after a first synchronous pattern is detected, a normal synchronous pattern exists at a position after one frame. Furthermore, if it can be judged that a synchronous pattern exists in each of three or more continuous frames, it is possible to more precisely detect the synchronous pattern.

0009As seen from the above, it is not so easy to detect the synchronous pattern included in a bit stream. In the prior art, a specially designed synchronizing circuit as shown in Figure 3 has been used.

0010As shown in Figure 3, the prior art synchronizing circuit includes a serial parallel converter 2, a header register 3, a synchronous pattern detector 4, a controller 5, a frame counter 6, a frame length generator 7, a comparator 8, and an inconsistency detector 9, which are coupled as shown.

0011Now, operation of the prior art synchronizing circuit will be described. First, a reset signal 102 is applied to the controller 5, so that the controller 5 is initialized in response to the reset signal 102, and outputs a counter reset signal 111 to the frame counter 6 and starts to output a read clock 103 to the serial parallel converter 2. On the other hand, a stream of bits 101 are supplied to the serial parallel converter 2, and a bit in the bit stream is received by the serial parallel converter 2 in response to a rising of the read clock 103, so that the bit stream 101 is converted in accordance with the read clock 103 into a parallel value of 32 bits which are the same in number as the bit number of the header. The parallel value of 32 bits is outputted as data 107 to the synchronous pattern detector 4 and the header register 3. The data 107 has such a format that the most significant bit is the oldest in time.

0012The synchronous pattern detector 4 generates a synchronous pattern detection signal 108 when the received data 107 is consistent with the synchronous pattern, and the synchronous pattern detection signal 108 is supplied to the controller 5 and the consistency detector 9.

0013Here, the synchronous pattern detector 4 is constituted of a combinational circuit configured to generate the synchronous pattern detection signal 108 only when all of the following conditions are satisfied. In the following synchronous pattern detection conditions, the most significant bit of the 32-bit parallel value of the data 107 is indicated by "31" and the least significant bit is expressed by "0". For example, the value expressed by 12 bits counted from the most significant bit is expressed as "31:20".

Synchronous Pattern Detection Conditions

0014<dl id="dl0001" compact="compact"><dt>Synchronous pattern "31:20"</dt><dd>= all "1"</dd><dt>ID "19"</dt><dd>= "1"</dd><dt>Layer "18:17"</dt><dd>= other than "00"</dd><dt>Bit rate index "15:12"</dt><dd>= other than "1111"</dd><dt>Sampling frequency "11:10"</dt><dd>= other than "11"</dd></dl>

0015After the controller 5 is initialized, if a first synchronous pattern detection signal 108 is generated, the controller 5 outputs a latch signal 109 to the header register 3 so as to cause it to latch the information determining the frame length, extracted from the data 107 which is outputted from the serial parallel converter 2. The latched information is outputted from the header register 3 as header information 110 to the frame length generator 7.

0016This frame length generator 7 seeks the bit number of one frame on the basis of the header information 110, and outputs it as a frame length data 112 to the comparator 8. On the other hand, the frame counter 6 is reset by the counter reset signal 111 outputted from the controller 5, and counts the read clock 103 supplied from the controller 5 so that a count value 113 of the frame counter 5 is supplied to the comparator 8. Here, the counter reset signal 111 is being generated without discontinuance until the synchronous pattern detection signal 108 is supplied to the controller 5, and therefore, the frame counter 6 does not carry out the counting operation at all during a period in which the counter reset signal 111 continues to be supplied.

0017The comparator 8 generates a count end signal 114 when the frame length data 112 and the count value 113 are consistent with each other, and the count end signal 114 is supplied to the consistency detector 9. If the synchronous pattern detection signal 108 is supplied from the synchronous pattern detector 4 to the consistency detector 9 at the same time as the count end signal 114 is supplied to the consistency detector 9, the consistency detector 9 judges that the synchronism has been established, and outputs a synchronous signal 116, which is supplied not only to an external but also to the controller 5. If the synchronous signal 116 indicating that the synchronism has been established, is supplied from the consistency detector 9 to the controller 5, the controller 5 stops outputting the read clock 103.

0018If the synchronous pattern detection signal 108 is not generated at the same time as the count end signal 114 is generated, it is judged that the synchronism has not yet been established, and the consistency detector 9 outputs an inconsistency signal 115, which is supplied to the controller 5. A first synchronous pattern in this case will be called a "quasi synchronous pattern" in the following. In this case, namely, when the inconsistency signal 115 is applied to the controller 5, the circuit is caused to return to the initial condition, so that in response to the inconsistency signal 115, the bit stream 101 of the amount supplied until that time and corresponding to one frame is disposed. Namely, the bits of the number corresponding to the frame length data 112 outputted from the frame length generator 7 are disposed. Here, it is to be noted that the number of bits disposed at this time is the bit number value obtained by construing as the header the 32 bits starting from the quasi synchronous pattern, namely, is different from an actual frame length. Therefore, the timing for establishing the frame synchronism has to be delayed.

0019Referring to Figure 4, there is shown a timing chart illustrating the timing for establishing the frame synchronism, when a counting operation starts from a normal synchronous pattern. Figure 5 shows a timing chart illustrating the timing for establishing the frame synchronism, when a counting operation starts from a quasi synchronous pattern. "A" in Figures 4 and 5 indicates the bit stream 101, and "B" in Figures 4 and 5 shows the synchronous pattern detection signal 108. "C" in Figures 4 and 5 indicates the synchronous signal 116. The hatched portion in "A" of Figures 4 and 5 designates the synchronous pattern.

0020In "A" of Figure 4, if it is constructed that the frame included in the bit stream 101 is the layer 2, the sampling frequency is 48 KHz, and the bit rate is 192kb/s, the bit number of one frame is 4608 bits. In "A" of Figure 5, if it is constructed that the frame included in the bit stream 101 is the layer 2, the sampling frequency is 48 KHz, and the bit rate is 256 kb/s, when the inconsistency signal 115 is generated in the consistency detector 9, the number of bits disposed is 6144 bits.

0021To add, in the example shown in Figure 4, at the moment a second normal synchronous pattern is inputted, the synchronous signal 116 is generated and outputted. However, in the example shown in Figure 5, since the quasi synchronous pattern exists at a heading, two normal synchronous patterns just after the quasi synchronous pattern are missed in reading, and the synchronous signal 116 is firstly generated at the moment a fourth normal synchronous pattern is inputted. Namely, because the quasi synchronous pattern is detected, the two normal frames are disposed extra. In certain cases, the number of normal synchronous patterns disposed further increases, dependently upon a mutual relation between the number of the normal frames and the bit number of a frame based on the quasi synchronous pattern.

0022As mentioned above, since the frames in a heading portion are disposed, a heading portion is missing in a sound corresponding to a digital audio signal.

0023Furthermore, if the disposal of the bit stream based on the quasi synchronous pattern is repeatedly performed until the synchronous signal is generated, the period of the missing in the sound corresponding to the digital audio signal becomes long.

Summary of the Invention

0024Accordingly, it is an object of the present invention to provide a synchronizing circuit, which has overcome the above mentioned defect of the conventional one.

0025Another object of the present invention is to provide a synchronizing circuit, capable of minimizing the missing in the sound corresponding to the digital audio signal.

0026The above and other objects of the present invention are achieved in accordance with the present invention by a synchronizing circuit receiving a bit stream including a synchronous pattern and various kinds of information bits, for outputting a synchronous signal, the circuit comprising a synchronous pattern register which is previously set with known information bits of the various kinds of information bits, and means for generating the synchronous signal when there is inputted the bit stream which is consistent with not only the synchronous pattern and the known information bits set in the synchronous pattern register.

0027More specifically, there is provided a synchronizing circuit configured to receive a bit stream including a synchronous pattern and a frame length determining information in each one frame, and to detect a frame boundary so as to generate a frame synchronous signal corresponding to the synchronous pattern, the synchronizing circuit comprising: <ul id="ul0001" list-style="none" compact="compact"><li>a synchronous pattern holding means for holding, as a synchronous pattern value, an expected value corresponding to at least a portion of the frame length determining information;</li><li>a data converting means receiving the bit stream for serial-parallel converting the received bit stream into a parallel data;</li><li>a synchronous pattern detecting means receiving from the parallel data a partial bit train corresponding to the synchronous pattern and the synchronous pattern value, for generating a synchronous pattern detection signal when the partial bit train is consistent with the synchronous pattern and when a portion of the partial bit train corresponding to the synchronous pattern value is consistent with the synchronous pattern value; and</li><li>a synchronous pattern discriminating means receiving the synchronous pattern detection signal, for generating a predetermined synchronous signal.</li></ul>

0028In one embodiment, the synchronous pattern holding means includes a synchronous pattern register for holding the synchronous pattern value. The synchronous pattern detecting means includes: <ul id="ul0002" list-style="none" compact="compact"><li>a header register responding to the synchronous pattern detection signal to extract and hold the frame length determining information from the parallel data;</li><li>a frame counter responding to the synchronous pattern detection signal to count the number of bits included in the bit stream supplied after the synchronous pattern detection signal;</li><li>a frame length generator for outputting the bit number of one frame from a value held in the header register;</li><li>a comparator comparing a count value of the frame counter with the bit number of one frame outputted from the frame length generator, and for generating a count end signal when both are consistent with each other; and</li><li>a consistency detector for generating the predetermined synchronous signal when the synchronous pattern detection signal and the count end signal are concurrently generated.</li></ul>

0029The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.

Brief Description of the Drawings

0030<ul id="ul0003" list-style="none" compact="compact"><li>Figure 1 illustrates the structure of one frame of the MPEG/Audio Layer 2 system;</li><li>Figure 2 is a table illustrating the bit rates determined by the bit rate index and the layer value;</li><li>Figure 3 is a block diagram of the specially designed synchronizing circuit in the prior art;</li><li>Figure 4 is a timing chart illustrating the timing for establishing the frame synchronism, when a counting operation starts from a normal synchronous pattern;</li><li>Figure 5 is a timing chart illustrating the timing for establishing the frame synchronism, when a counting operation starts from a quasi synchronous pattern;</li><li>Figure 6 is a block diagram of an embodiment of the synchronizing circuit in accordance with the present invention; and</li><li>Figure 7 is a timing chart illustrating the timing for establishing the frame synchronism in the embodiment of the present invention, when a counting operation starts from a quasi synchronous pattern.</li></ul>

Description of the Preferred embodiments

0031Referring to Figure 6, there is shown block diagram of an embodiment of the synchronizing circuit in accordance with the present invention. In Figure 6, elements similar to those shown in Figure 3 are given the same Reference Numeral. This embodiment is constructed as a specially designed synchronizing circuit.

0032As shown in Figure 6, the embodiment of the synchronizing circuit includes a synchronous pattern register 1, a serial parallel converter 2, a header register 3, a synchronous pattern detector 4, a controller 5, a frame counter 6, a frame length generator 7, a comparator 8, and an inconsistency detector 9, which are coupled as shown.

0033As seen from comparison between Figures 3 and 6, the shown embodiment is different from the prior art example in that, the shown embodiment newly adds the synchronous pattern register 1 receiving a write signal 104 and a synchronous pattern setting value 105 and for outputting a synchronous pattern value 106 to the synchronous pattern detector 4.

0034Now, operation of the embodiment of the synchronizing circuit will be described. First, a reset signal 102 is applied to the controller 5, so that the controller 5 is initialized in response to the reset signal 102, and outputs a counter reset signal 111 to the frame counter 6 and starts to output a read clock 103 to the serial parallel converter 2. On the other hand, a stream of bits 101 are supplied to the serial parallel converter 2, and a bit in the bit stream is received by the serial parallel converter 2 in synchronism to a rising of the read clock 103, so that the bit stream 101 is converted in accordance with the read clock 103 into a parallel value of 32 bits which are the same in number as the bit number of the header. The parallel value of 32 bits is outputted as data 107 to the synchronous pattern detector 4 and the header register 3. The data 107 has such a format that the most significant bit is the oldest in time.

0035On the other hand, as mentioned above, the write signal 104 and the synchronous pattern setting value 105 are supplied to the synchronous pattern register 1, so that the synchronous pattern setting value 105 is held in the synchronous pattern register 1 in response to the write signal 104, and then, outputted as the synchronous pattern value 106 to the synchronous pattern detector 4.

0036The synchronous pattern detector 4 generates a synchronous pattern detection signal 108 when the received data 107 is consistent with the synchronous pattern and also consistent with items designated by the synchronous pattern value 106. The synchronous pattern detection signal 108 is supplied to the controller 5 and the consistency detector 9.

0037After the controller 5 is initialized, if a first synchronous pattern detection signal 108 is generated, the controller 5 outputs a latch signal 109 to the header register 3 so as to cause it to latch the frame length determining information, extracted from the data 107 which is outputted from the serial parallel converter 2. The latched information is outputted from the header register 3 as header information 110 to the frame length generator 7.

0038This frame length generator 7 seeks the bit number of one frame on the basis of the header information 110, and outputs it as a frame length data 112 to the comparator 8. On the other hand, the frame counter 6 is reset by the counter reset signal 111 outputted from the controller 5, and counts the read clock 103 supplied from the controller 5 so that a count value 113 of the frame counter 5 is supplied to the comparator 8. Here, the counter reset signal 111 is being generated without discontinuance until the synchronous pattern detection signal 108 is supplied to the controller 5, and therefore, the frame counter 6 does not carry out the counting operation at all during a period in which the counter reset signal 111 continues to be supplied.

0039The comparator 8 receives the frame length data 112 and the count value 113, and generates a count end signal 114 when the frame length data 112 and the count value 113 are consistent with each other, and the count end signal 114 is supplied to the consistency detector 9. If the synchronous pattern detection signal 108 is supplied from the synchronous pattern detector 4 to the consistency detector 9 at the same time as the count end signal 114 is supplied to the consistency detector 9, the consistency detector 9 judges that the synchronism has been established, and outputs a synchronous signal 116, which is supplied not only to an external but also to the controller 5. If the synchronous signal 116 indicating that the synchronism has been established, is supplied from the consistency detector 9 to the controller 5, the controller 5 stops outputting the read clock 103.

0040If the synchronous pattern detection signal 108 is generated at the same time as the count end signal 114 is generated, it is not judged that the synchronism has not yet been established, and the consistency detector 9 outputs an inconsistency signal 115 to the controller 5.

0041Here, it can be so constructed that, if the synchronous pattern detection signal 108 is generated at the same time as the count end signal 114 is generated, continuously over three or more frames, the synchronous signal 116 is generated.

0042When the inconsistency signal 115 is applied to the controller 5, the controller 5 is caused to return to the initial condition, so that in response to the inconsistency signal 115, the bit stream 101 of the amount supplied until that time and corresponding to one frame is disposed. Namely, the bits of the number corresponding to the frame length data 112 outputted from the frame length generator 7 are disposed.

0043Here, one example of the value set in the synchronous pattern register 1 will be explained with reference to the following system: In this system, it is assumed that it is judged as the synchronous pattern only when the set value of the bit rate index is consistent with the bit rate index value included in the header, the set value of the sampling frequency is consistent with the sampling frequency included in the header, and the set value of the layer frequency is consistent with the layer included in the header. In this system, the synchronous pattern detector 4 is constituted of a logic circuit configured to generate the synchronous pattern detection signal 108 only when all of the following conditions (1) and (2) are satisfied. In the following synchronous pattern detection conditions, the most significant bit of the 32-bit parallel value of the data 107 is indicated by "31" and the least significant bit is expressed by "0". For example, the value expressed by 12 bits counted from the most significant bit is expressed as "31:20".

Synchronous Pattern Detection Condition (1)

0044<dl id="dl0002" compact="compact"><dt>Synchronous pattern "31:20"</dt><dd>= all "1"</dd><dt>ID "19"</dt><dd>= "1"</dd><dt>Layer "18:17"</dt><dd>= other than "00"</dd><dt>Bit rate index "15:12"</dt><dd>= other than "1111"</dd><dt>Sampling frequency "11:10"</dt><dd>= other than "11"</dd></dl>

Synchronous Pattern Detection Condition (2)

0045<dl id="dl0003" compact="compact"><dt>Layer "18:17"</dt><dd>= Layer set value of Synchronous pattern setting value 104</dd><dt>Bit rate index "15:12"</dt><dd>= Bit rate index set value of Synchronous pattern register setting value 104</dd><dt>Sampling frequency "11:10"</dt><dd>= Sampling frequency set value of Synchronous pattern setting value 104</dd></dl>

0046Referring to Figure 7, there is shown a timing chart illustrating the operation when the bit stream 101 is applied, similarly to the case shown in Figures 5A, 5B and 5C. "A" in Figure 7 indicates the bit stream 101, and "B" in Figure 7 shows the synchronous pattern detection signal 108. "C" in Figure 7 indicates the synchronous signal 116. The hatched portion in "A" in Figure 7 designates the synchronous pattern.

0047In this example, such a condition is set in the synchronous pattern register 1 as to treat the received bit train as the synchronous pattern, only when the frame included in the bit stream 101 is the layer 2, the sampling frequency is 48 KHz, and the bit rate is 192 kb/s. In this condition, the set values are so that the layer="10", the bit rate index = "1010", and the sampling frequency="01".

0048A first inputted quasi synchronous pattern is "1100" indicating that the bit rate index is 256 kb/s. Since the first inputted quasi synchronous pattern of "1100" is different from the above set value "1010", the synchronous pattern detector 4 does not output the synchronous pattern detection signal 108. Next, when a first correct synchronous pattern is inputted, the synchronous pattern is completely consistent with all the items set in the synchronous pattern register 1, the synchronous pattern detector 4 outputs the synchronous pattern detection signal 108, so that the frame counter 6 starts its counting operation, with the result that when a second normal synchronous pattern is inputted, the synchronous signal 116 is outputted from the consistency detector 9.

0049In the above mentioned example, the bit rate index includes 15 kinds, the layer includes 3 kinds, and the sampling frequency includes 3 kinds. Therefore, by setting the synchronous pattern corresponding to the respective items (the actual bit rate index, the actual layer and the actual sampling frequency) to the synchronous pattern register 1, probability that the frame counter 6 erroneously starts its counting operation because of a quasi synchronous pattern under the same synchronous pattern detecting condition, is reduced as follows in comparison with the prior art example.<maths id="math0002" num=""><math display="block"><mrow><mtext>(1/15) × (1/3) × (1/3) = 1/135</mtext></mrow></math><img file="EP0735709A2_D0002.tif" /></maths>

0050In addition, in the case that the bit stream 101 includes a quasi synchronous pattern having the same bit rate index, the same layer and the same sampling frequency as those of the correct synchronous pattern, the bits included in the bit stream 101 are disposed, but the number of bits disposed is ceaselessly the same as the frame length set in the synchronous pattern register 1. Bits of the number corresponding to two or more frames are never disposed at one time, differently from the prior art.

0051As seen from the above, the present invention is applied to a synchronizing circuit for a bit stream in a compressing/expanding system for a digital audio signal, and is characterized by comprising the means for previously setting the values corresponding to the respective items of the layer, the bit rate and the sampling frequency, and a synchronous pattern detecting means configured not to recognize as a synchronous pattern, a synchronous pattern having the values other than the previously set values. With this arrangement, it is possible to remarkably reduce probability that the quasi synchronous pattern is detected as the synchronous pattern, and therefore, it is possible to minimize the missing in a sound corresponding to the digital audio signal.

0052The invention has thus been shown and described with reference to the specific embodiments. However, it should be noted that the present invention is in no way limited to the details of the illustrated structures but changes and modifications may be made within the scope of the appended claims.