BACKGROUND
The present invention relates to a multi-channel decoding scheme, and more particularly to a multi-channel decoding system capable of reducing noise when switching between different channel modes, and method thereof.
Generally speaking, when a multi-channel decoding system switches from a multi-channel mode to a single-channel mode or from the single-channel mode to the multi-channel mode, users often hear a “pop” noise from their earphones or speakers. This is primarily because the output DC voltage levels of a channel output signal in the multi-channel and single-channel modes, which is generated from the multi-channel decoding system, are different. Another reason may result from the Glitch phenomenon during switching between the multi-channel and single-channel modes. Taking a stereo decoding system as an example, the pop noise is usually heard by human when the stereo decoding system switches between stereo and mono modes.
Please refer to FIG. 1 in conjunction with FIG. 2. FIG. 1 is a diagram of a related art stereo decoding system 100, and FIG. 2 is a timing diagram illustrating a clock signal SCLK having a frequency 38 KHz, a mode switching signal SMODE, and a clock signal SCLK′. As shown in FIG. 1, the stereo decoding system 100 comprises a decoding circuit 105 and a clock generating circuit 110, where the decoding circuit 105 further includes a mixer 115, voltage-to-current (V/I) converters 120a and 120b, a separation module 125, operational amplifiers OP1 and OP2, resistor units 130a and 130b, and low pass filters (LPFs) 135a and 135b. The decoding circuit 105 is utilized for receiving an input audio signal Sin to generate a left channel output signal LOUT and a right channel output signal ROUT according to the clock signal SCLK′. When the mode switching signal SMODE is at a high logic level, it indicates that the stereo decoding system 100 is in the stereo mode; when the mode switching signal SMODE is at a low logic level, this indicates that the stereo decoding system 100 is in the mono mode.
Ideally, the DC voltage level of the channel output signal LOUT or ROUT should be identical whether the stereo decoding system 100 is in the stereo mode or the mono mode. Practically, however, an equivalent offset voltage source Vos exists within the decoding circuit 105 and the DC voltage level of the channel output signal LOUT/ROUT in the stereo and mono modes are different due to the offset voltage source Vos. For example, in stereo mode, a voltage of the offset voltage source Vos is equal to V1; in mono mode, the voltage of the offset voltage source Vos will become V2. Consequently, the DC voltage level of the channel output signal LOUT/ROUT is changed while the stereo decoding system 100 switches from mono mode to stereo mode and the above-mentioned pop noise is thus introduced.
SUMMARY
It is therefore one of the objectives of the present invention to provide a multi-channel decoding system and related method capable of reducing the noise, to solve the above-mentioned problems.
According to an embodiment of the present invention, a multi-channel decoding system is disclosed. The multi-channel decoding system comprises a decoding circuit and a clock generating circuit. The decoding circuit is utilized for receiving an input signal to generate a first channel output signal and a second channel output signal. The decoding circuit has a mixer used for mixing the input signal with a specific clock signal. The clock generating circuit is utilized for generating the specific clock signal and arranged to gradually change an amplitude of the specific clock signal from a first value to a second value when receiving a mode switching signal instructing a switching from a first mode corresponding to a first number of channels to a second mode corresponding to a second number of channels.
According to the embodiment of the present invention, a multi-channel decoding method is disclosed. The multi-channel decoding method comprises the following steps: receiving an input signal to generate a first channel output signal and a second channel output signal, wherein the input signal is mixed with a specific clock signal; and gradually changing an amplitude of the specific clock signal from a first value to a second value when switching from a first mode corresponding to a first number of channels to a second mode corresponding to a second number of channels.
According to another embodiment of the present invention, a multi-channel decoding system is disclosed. The multi-channel decoding system comprises a decoding circuit, a test signal generating circuit, and a calibration circuit. The decoding circuit is utilized for receiving an input signal to generate a channel output signal, and the test signal generating circuit is utilized for providing a first test signal serving as the input signal in a first calibration mode. The calibration circuit is utilized for adjusting a DC voltage level of the channel output signal outputted by the decoding circuit with a first calibration signal by reducing a difference between a first predetermined reference signal level and a DC voltage level of the channel output signal generated from the first test signal.
According to the embodiment of the present invention, a multi-channel decoding method is disclosed. The multi-channel decoding method comprises the following steps: receiving an input signal to generate a channel output signal; providing a first test signal serving as the input signal in a first calibration mode; and adjusting a DC voltage level of the channel output signal outputted by the decoding circuit with a first calibration signal by reducing a difference between a first predetermined reference signal level and a DC voltage level of the channel output signal generated from the first test signal.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a related art stereo decoding system.
FIG. 2 is a timing diagram illustrating a clock signal SCLK, a mode switching signal SMODE, and a clock signal SCLK′ shown in FIG. 1.
FIG. 3 is a diagram of a multi-channel decoding system according to a first embodiment of the present invention.
FIG. 4 is a timing diagram illustrating an example of a predetermined reference clock signal SCLK, a mode switching signal SMODE, a control signal Sc, and a specific clock signal SCLK′ shown in FIG. 3.
FIG. 5 is a diagram of a particular example of a clock signal generator shown in FIG. 3.
FIG. 6 is a schematic diagram of a multi-channel decoding system according to a second embodiment of the present invention.
FIG. 7 is a timing diagram illustrating an example of a calibration process for the DC voltage level of the channel output signal LOUT in the mono mode according to the second embodiment of the present invention.
FIG. 8 is a timing diagram showing calibration processes for the DC voltage level of the channel output signals LOUT and ROUT in the mono and stereo modes, respectively.
DETAILED DESCRIPTION
Certain terms are used throughout the description and following claims to refer to particular components. As one skilled in the art will appreciate, electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.
FIG. 3 is a diagram of a multi-channel decoding system 300 according to a first embodiment of the present invention. The multi-channel decoding system 300 includes a decoding circuit 305 and a clock generating circuit 310. The decoding circuit 305 is utilized for receiving an input signal Sin to generate at least a first channel output signal (e.g. a left channel output signal LOUT) and a second channel output signal (e.g. a right channel output signal ROUT); other operations and functions of decoding circuit 305 are similar to those of the decoding circuit 105 and not detailed for brevity. The clock generating circuit 310 is utilized for generating a specific clock signal SCLK′ and for gradually changing the amplitude of the specific clock signal SCLK′ from a first value to a second value when switching from a first mode to a second mode, such as receiving a mode switching signal SMODE instructing a switching from the first mode corresponding to a first number of channels to the second mode corresponding to a second number of channels. For instance, the clock generating circuit 310 gradually changes the amplitude of the specific clock signal SCLK′ from a specific value to substantially zero or from substantially zero to the specific value when switching happens. In this embodiment, the multi-channel decoding system 300 is a stereo decoding system; the first mode and the second mode are respectively meant to be a stereo mode and a mono mode. More specifically, the clock generating circuit 310 further includes a signal processing unit 3101 and a clock signal generator 3103. The signal processing unit 3101 generates a control signal Sc according to the mode switching signal SMODE. Particularly, when the mode switching signal SMODE instructs the switching between the stereo and mono modes, the signal processing unit 3101 generates the control signal Sc having a gradual transition from a first level to a second level, where one of the first and second levels is a low level (e.g. zero) and the other is a high level. The clock signal generator 3103 is utilized for generating the specific clock signal SCLK′ according to a predetermined reference clock signal SCLK and the control signal Sc. In practice, the clock signal generator 3103 can be implemented by a multiplier used for combining the predetermined reference clock signal SCLK and the control signal Sc to generate the specific clock signal SCLK′.
As mentioned above, the noise resulting from the equivalent offset voltage source Vos is easily heard if a conventional multi-channel decoding system, such as the system 100 of FIG. 1, abruptly switches from stereo mode to mono mode or vice versa. In the first embodiment, the mode switching signal SMODE originally having sharp transitions is processed by the signal processing unit 3101, such that the control signal Sc having gradual transitions is generated. The clock signal generator 3103 then generates the specific clock signal SCLK′ which also has gradual transitions according to the predetermined reference clock signal SCLK (usually has a frequency 38 KHz) and the control signal Sc. Taking an example of the multi-channel decoding system 300 switching from the stereo mode to the mono mode, a timing diagram regarding the control signal Sc and specific clock signal SCLK′ is illustrated in FIG. 4. As shown in FIG. 4, when the mode switching signal SMODE instructs a switching from the mono mode (MN) to the stereo mode (ST) at timing T1, the control signal Sc gradually transits from a low level to a high level before timing T2; in this example, the waveform of the control signal Sc during timing T1 to T2 looks substantially linear. However, gradually linear transition is not intended to be a limitation of the present invention. The clock signal generator 3103 then multiplies the predetermined reference clock signal SCLK by the control signal Sc to generate the specific clock signal SCLK′. Obviously, it is shown that the amplitude of the specific clock signal SCLK′ is increased (from zero to A1) gradually during timing T1 to T2. Accordingly, although the DC voltage level of the channel output signal LOUT or ROUT is different in the stereo mode and mono mode, the noise is not easily perceived by human since the multi-channel decoding system 300 does not switch rapidly from the mono mode to stereo mode. Similarly, when the mode switching signal SMODE instructs a switching from the stereo mode (ST) to the mono mode (MN), the control signal Sc also gradually transits from the high level to the low level and therefore the amplitude of the specific clock signal SCLK′ is not rapidly decreased (from A1 to substantially zero). A detailed description is not explained for brevity.
Moreover, another example of the clock signal generator 3103 is shown in FIG. 5. A signal SCLK1 is meant to be an inverse signal of the predetermined reference clock signal SCLK and a voltage VCM is a common mode voltage. The specific clock signal SCLK′ shown in FIG. 3 comprises output signals Sout and Sout1. For those skilled in this art, it should be well-known to appreciate the operation of the circuit (i.e. the clock signal generator 3103) shown in FIG. 5 and it is not illustrated for simplicity.
Please refer to FIG. 6. FIG. 6 is a schematic diagram of a multi-channel decoding system 600 according to a second embodiment of the present invention. As shown in FIG. 6, the multi-channel decoding system 600 comprises a decoding circuit 605, a clock generating circuit 610, a test signal generating circuit 640, and a calibration circuit 645, where the decoding circuit 605 further comprises a signal processing module 606 and a demodulator 608. The test signal generating circuit 640 is utilized for providing a first test signal in a first calibration mode and providing a second test signal in a second calibration mode. In this embodiment, the first and second test signals are identical square wave signals and are both called Stest. The test signal Stest is served as an input signal for the decoding circuit 605 in the first and second calibration modes. However, providing the same test signal Stest for two different calibration modes is not intended to be a limitation of the present invention. Moreover, the first and second calibration modes are respectively meant to be a stereo mode and a mono mode when calibrating DC voltage levels of channel output signals LOUT and ROUT. The demodulator 608 demodulates the test signal Stest into a DC signal Sin′, and then the signal processing module 606 generates the channel output signals LOUT and ROUT. Since operation and function of the signal processing module 606 are respectively similar to those of the decoding circuit 105, these are not detailed for brevity. Moreover, the clock generating circuit 610 can be the conventional clock generating circuit 110 in FIG. 1 whose specific clock signal SCLK′ has sharp transitions, or the clock generating circuit 310 in FIG. 3 whose specific clock signal SCLK′ has gradual transitions. Of course, it is assumed that an equivalent offset voltage source Vos also exists within the decoding circuit 605.
After the decoding circuit 605 outputs the channel output signals LOUT and ROUT generated from the test signal Stest, the calibration circuit 645 outputs a first calibration signal Scal and a second calibration signal Scal′into the decoding circuit 605 according to the channel output signals LOUT and ROUT, so as to respectively adjust DC voltage levels of the channel output signals LOUT and ROUT in the stereo and mono modes, for example, by separately reducing the difference between the DC voltage level of the channel output signal LOUT generated from the first test signal Stest and a first predetermined reference signal level Sref (e.g. ground level) with the first calibration signal Scal and reducing a difference between the DC voltage level of the channel output signal ROUT generated from the second test signal Stest and a second predetermined reference signal level Sref′ (e.g. ground level) with the second calibration signal Scal′. In this embodiment, the first and second predetermined reference signal level Sref and Sref′ inputted into a comparison module 6451 are designed to become identical. That is, the DC voltage levels of the channel output signals LOUT and ROUT are adjusted to be close to the same predetermined reference signal level whether in stereo or mono mode. In the following description, for simplicity, the first predetermined reference signal level Sref is used for illustrative purposes. Even though the multi-channel decoding system 600 suddenly switches between the stereo mode and mono mode, the noise due to the equivalent offset voltage source Vos can be therefore reduced and not easily perceived by human.
The calibration circuit 645 comprises a comparison module 6451 and a decision module 6453. The comparison module 6451 is used for comparing the DC voltage level of the channel output signal LOUT/ROUT with the predetermined reference signal level Sref, to output a first comparison result in the stereo mode and a second comparison result in the mono mode, respectively. In this embodiment, the comparison module 6451 is implemented by a comparator COMP, where an inverting input terminal of the comparator COMP is coupled to the predetermined reference signal level Sref while a non-inverting input terminal of the comparator COMP is coupled to the channel output signal LOUT/ROUT switched by switches SW2 and SW3, as shown in FIG. 6. The decision module 6453 is utilized for separately adjusting the first calibration signal Scal and the second calibration signal Scal′according to the first comparison result and the second comparison result. In this embodiment, the decision module 6453 comprises a decision unit 64531 and a digital-to-analog converter (DAC) 64535. The decision unit 64531 is utilized for determining a first adjusting signal Sadj and a second adjusting signal Sadj′ according to the first and second comparison results respectively, and the DAC 64535 is used for converting the first and second adjusting signals Sadj and Sadj′ into a first voltage VL and a second voltage VR serving as the first and second calibration signals Scal and Scal′respectively. More specifically, the first and second adjusting signals Sadj and Sadj′ can be digital codes, and the DAC 64535 converts the digital codes, e.g. first and second adjusting signals Sadj and Sadj′, into the first voltage VL and the second voltage VR.
Furthermore, the decision unit 64531 may utilize a digital successive-approximation algorithm to determine the first and second adjusting signals Sadj and Sadj′; of course, other approximation algorithms can also be applied to the embodiments of the present invention. In this embodiment, the values of the resistors RS, RS′, RF, and RF′ are designed carefully so that the voltage levels of the channel output signals LOUT and ROUT are compensated. For example, in order to compensate the voltage level of the channel output signal LOUT, the values of the resistors RS and RF are designed such that the value of the first voltage VL equals a specific value associated with the equivalent offset voltage of the channel output signal LOUT, which is illustrated in Equation (1):
where the value Vosl is indicative of the voltage level of the channel output signal LOUT before calibrated, i.e. the equivalent offset voltage. Through the calibration, the voltage level (DC voltage) of the channel output signal LOUT is therefore adjusted to become almost zero.
Similarly, for compensating the voltage level of the channel output signal ROUT, the values of the resistors RS' and RF′ are designed such that the value of the first voltage VR equals a specific value associated with the equivalent offset voltage of the channel output signal ROUT, which is illustrated in Equation (2):
where the value Vosr is indicative of the voltage level of the channel output signal ROUT before calibrated, i.e. the equivalent offset voltage. Through the calibration, the voltage level (DC voltage) of the channel output signal ROUT is also adjusted to become almost zero.
Please refer to FIG. 7. FIG. 7 is a timing diagram illustrating an example of a calibration process for the DC voltage level of the channel output signal LOUT in mono mode according to the second embodiment of the present invention. A control signal Sc1 is utilized for controlling a status of a switch SW1 shown in FIG. 6; when the control signal Sc1 transits from a low level to a high level at timing T0, the switch SW1 is turned on. Next, at timing T1, a control signal Sc2 for switch SW2 of FIG. 6 also transits from a low level to a high level and the calibration process for the channel output signal LOUT in the mono mode is enabled. Initially, the first adjusting signal Sadj equaling a code ‘10000’ corresponds to the first voltage V1 equaling zero. When the DC voltage level of the channel output signal LOUT is higher than the predetermined reference signal level Sref, such as in a time period from T2 to T3, the first adjusting signal Sadj is to be another code ‘11000’ to raise the first voltage V1 for decreasing the DC voltage level of the channel output signal LOUT (e.g. during timing T3 to T4). Otherwise, if the DC voltage level of the channel output signal LOUT is lower than the predetermined reference signal level Sref, the first adjusting signal Sadj will be adjusted to lower the first voltage V1 for increasing the DC voltage level of the channel output signal LOUT. In this example, the DC voltage level of the channel output signal LOUT is gradually decreased and finally is close to the predetermined reference signal level Sref. The decision unit 64531 then records the first adjusting signal Sadj finally determined (e.g. the code ‘11111’ in this example). The first adjusting signal Sadj is then applied in the signal processing module 606 when the multi-channel decoding system 600 receives an input audio signal instead of the test signal Stest to generate the channel output signal in the mono mode. A detailed procedure of how the first and second calibration signals Scal and Scal′change the DC voltage levels of the channel output signals LOUT and ROUT is well-known to those skilled in this art, and further description is omitted for simplicity.
A calibration process for the DC voltage level of the channel output signal LOUT in stereo mode or for the DC voltage level of the channel output signal ROUT in mono/stereo mode is similar to the above-mentioned calibration process shown in FIG. 7. Please refer to FIG. 8. FIG. 8 is a timing diagram showing the calibration processes for the DC voltage level of the channel output signals LOUT and ROUT in mono and stereo modes, respectively. Codes C1 and C2 are in the first adjusting signal Sadj finally determined by the decision unit 64531 for the mono mode and stereo mode, respectively. Codes C3 and C4 are in the second adjusting signal Sadj′ finally determined by the decision unit 64531 for the mono mode and stereo mode, respectively. Further description is not detailed here for brevity. Obviously, through the calibration processes, the DC voltage level of the channel output signal LOUT whether in mono or stereo mode is very close to the predetermined reference signal level Sref, and the DC voltage level of the channel output signal ROUT is also close to the predetermined reference signal level Sref whether in mono or stereo mode. Consequently, the problem caused by the pop noise due to the equivalent offset voltage source Vos can be solved.
Additionally, in another embodiment, if calibrating only the DC voltage level of the channel output signal LOUT or ROUT is considered, then the calibration circuit 645 can only output the first calibration signal Scal or the second calibration signal Scal′into the decoding circuit 605 according to the channel output signal LOUT or ROUT to adjust the DC voltage level of the channel output signal LOUT or ROUT in the mono and stereo modes. This also helps to reduce the pop noise caused by the equivalent offset voltage source Vos.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.
Patent Application
20090252337
