Abstract:
An analog-to-digital converter is provided for converting multiple analog inputs into corresponding digital values. An output interface circuit uses differential signaling to reduce noise and interference induced in the analog portions of the analog-to-digital converter.

Description:
FIELD OF INVENTION 
     The present invention relates in general to analog-to-digital converters, (ADCs) and, in particular, to methods and apparatuses for communicating high bit-rate data from an ADC to another device, such as a digital signal processor. 
     BACKGROUND OF INVENTION 
     Analog-to-digital converters convert analog signals into digital signals, enabling the signal to be manipulated and processed using numerical techniques. ADCs are utilized, for example, to convert an analog audio signal into a digital form so that digital processing techniques may be utilized to provide features, such as volume control, frequency equalization, encryption, filtering, surround sound decoding, and ambiance effects. Additionally, digital signals are suitable for recording or transmitting to another location with no loss in signal quality. 
     Typically, an ADC converts an analog signal into digital words representing the amplitude of the audio signal at fixed intervals determined by the sampling rate or frequency. The data words are then transmitted to a digital signal processor (DSP), which manipulates the data words to provide various desired features. The ADC may transmit the digital words in a parallel format, e.g. eight or sixteen bits at a time; however, commodity DSPs often provide circuitry for receiving data serially, e.g., one bit at a time. Accordingly, integrated circuit ADCs often provide the digital data words in a compatible serial format. 
     In mixed signal integrated circuits, switching transients in the digital circuitry are known to create considerable noise and interference in the analog circuits. In an ADC, the noise may taint the accuracy or reduce the audible signal to noise ratio of the device. Because of the rapid switching involved, the actual transmission of data on the serial data (SDATA) line is a major source of noise and interference for sensitive analog circuitry within an ADC. One method to minimize such noise and interference, is to have the switching activity occur at specific times relative to noise sensitive operations. Previously, this has been accomplished by re-timing the operation of various circuits within the ADC as is described in U.S. Pat. No. 4,746,899, which is incorporated herein in its entirety. Retiming the serial transmission requires the master clock (MCLK) frequency be at least twice serial clock (SCLK). 
     In conventional stereo audio systems, a pair of audio channels are converted into digital form. Typically, a stereo ADC, having a pair of ADCs in a single integrated circuit package, is utilized to convert stereo audio into a digital form. A serial clock (SCLK) signal synchronizes the transmission and reception of each bit, and a channel clock (LRCK) signal differentiates between the left and right channels of data which are alternately transmitted. A master clock (MCLK) controls the internal operation of the ADC and DSP. Stereo audio uses a pair of audio channels, which provide a left channel and a right channel of audio; however, newer audio formats use more than two audio channels to provide a richer audio experience. For example, 5.1 channel audio, which is found on some DVD movie soundtracks, is utilized to create up to six channels of audio: a center front channel, left and right front channels, left and right surround channels, and a subwoofer channel. Digital audio formats capable of providing seven, eight, or more audio channels have also been developed. 
     For such high channel-count audio systems, the frequencies required to retime serial channel operation become more difficult, particularly at high sample rates. For example, an 8-channel system with 24-bit samples and a 192 kHz sample rate requires an SCLK frequency of about 36.8 MHz and a corresponding MCLK greater than about 73.7 MHz. Circuits using such high clock frequencies are difficult to design and use and, therefore, are not practical nor desirable. High bit rates also exacerbate the noise and interference problems in the analog portion of an ADC. 
     It would, therefore, be desirable to provide methods and apparatuses for serially transmitting digital audio signals with a high channel count and a high bit-rate without re-timing. 
     It would also be desirable to provide methods and apparatuses for serially transmitting digital audio signals with a high channel count and a high bit-rate while minimizing noise and interference. 
     It would also be desirable to provide methods and apparatuses for serially transmitting digital audio signals with a high channel count and a high bit-rate without the need for an excessively high master clock frequency. 
     SUMMARY OF INVENTION 
     Methods and apparatuses are provided for serially transmitting audio signals with a high-channel count and a high bit-rate without re-timing and without the need for an excessively high master clock frequency, while minimizing noise and interference. 
     A high bit-rate ADC with a serial interface for transmitting digital data words to another device is provided. In one embodiment, the ADC is a multi-channel ADC, and the serial interface uses a plurality of pins to serially transmit the digital data words. Each pin is utilized to transmit a different set of data channels so that multiple channels are concurrently transmitted. In a second embodiment of the multi-channel ADC, the serial interface is configured to transmit the data from all of the analog channels over a single serial data path, preferably using differential signaling. Multi-channel audio ADCs incorporating the principles of the present invention are able to transfer digital data at high bit rates without requiring excessive clock frequencies, while minimizing noise and interference to the analog circuitry. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which like numbers designate like parts throughout, and in which: 
     FIG. 1 is a schematic block diagram of an illustrative multi-channel ADC in accordance with the principles of the present invention; 
     FIG. 2 is a block diagram of a single ADC channel of FIG. 1; 
     FIG. 3 is an illustrative timing diagram of signals used to communicate data in a first mode of operation of the present invention; 
     FIG. 4 is an illustrative timing diagram of signals used to communicate data in a second mode of operation; 
     FIG. 5 is a simplified block diagram of the illustrative circuitry implementing the serial output interface of FIG. 1; and 
     FIG. 6 is a simplified block diagram of an exemplary audio processor receiver using a multi-channel ADC constructed in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in FIGS. 1-6 of the drawings, in which like numbers designate like parts. 
     FIG. 1 is a high level operational block diagram of a single-chip, multi-channel audio analog-to-digital converter (ADC)  100  in accordance with the principles of the present invention. Multi-channel ADC  100  includes ADC channels  1  to  8 , which receive analog audio signals AIN 1 -AIN 8  and convert them into corresponding digital signals, which are sent to serial output interface  110 . Serial output interface  110  organizes the eight digital signals and transmits them to another device, such as a microprocessor or digital signal processor (DSP), using data lines SDOUT 1 -SDOUT 4 , serial clock SCLK, and left-right channel clock LRCK. Master clock MCLK is utilized to control and synchronize the operation of multi-channel ADC  100 . Additional inputs may be provided for power and ground connections, voltage references, discrete electronic components, mode configuration, and the like; however, for clarity, these connections are omitted from FIG.  1 . 
     Referring now to FIG. 2, an analog signal is connected to differential inputs AIN+ and AIN− to gain stage  200  of illustrative ADC channel  1 . The amplified signal is then passed to delta-sigma modulator  201 , which includes summer  202 , low pass filter (LPF)  203 , comparator (quantizer)  204 , and digital to analog converter (DAC)  205  in the feed back loop. In an alternative embodiment, gain stage  200  is omitted, and the input(s) is connected directly to delta-sigma modulator  201 . The outputs of the delta-sigma modulator are passed through decimation filter  206 , which reduces the oversampling rate, and then through high pass filter (HPF)  207 , which attenuates any out-of-band noise. The output of ADC channel  1  is a digital word or sample, typically 16 to 24-bits in length, representative of the amplitude of the analog signal at a discrete time. The digital samples are obtained at fixed intervals determined by the sample rate or sample frequency (F s ), which is typically in a range of about 40 kHz to about 200 kHz. 
     Referring back to FIG. 1, the outputs of ADC channels  1  to  8  are provided to serial output interface  110 . Serial output interface  110  accepts the digital sample data from the ADC channels, organizes the data according to the selected mode of operation of multi-channel ADC  100 , and serially transmits the data samples over serial data output pins SDOUT 1  to SDOUT 4 . As described above, the presence of multiple data channels requires the data to be transmitted at a high bit rate. In a first mode of operation, the data channels are organized as four stereo pairs of data, which are then transmitted over the four output pins, each pin alternating between two channels of data. For example, data words from ADC channel  1  are transmitted through pin SDOUT 1 , alternating with data words from ADC channel  2 , as shown in timing diagram  300  of FIG.  3 . Similarly, ADC channels  3  and  4  are transmitted through pin SDOUT 2 ; ADC channels  5  and  6  are transmitted through pin SDOUT 3 , and ADC channels  7  and  8  are transmitted through pin SDOUT 4 . The channels sharing an output data line are differentiated by the status of clock LRCK. For example, when the signal for clock LRCK is high, the odd channel in each pair is being transmitted whereas, the even channel in each pair is transmitted when the signal for clock LRCK is low. 
     Typically, the signals on SDOUT 1 - 4  are single-ended signals in which the logical value of a signal is determined by the voltage of the signal with respect to a circuit ground. In another mode of operation all of the digital channels are serially transmitted over a single communication path using differential signals in which the logical value of a signal is determined by the relative voltage of a pair of signals. For example, eight ADC channels are transmitted over SDOUT 1  in the following order: channel  1 ,  3 ,  5 , and  7  followed by channels  2 ,  4 ,  6 , and  8 . However, other channel orderings are also possible. As in the first mode of operation described above, clock LRCK is utilized to identify the channel being transmitted. FIG. 4 shows this use of clock LRCK in illustrative timing diagram  400 . Clock LRCK is high for the odd numbered ADC channels and low for the even numbered channels. Thus, the data for ADC channel  1  follows a low-to-high transition clock on LRCK, and the data for ADC-channel  2  follows a high-to-low transition on clock LRCK. Because each channel has a know word size, the position of the remaining channels of data are determined by counting bit positions as shown in the inset in FIG.  4 . Alternatively, LRCK is high for ADC channel  1  and low for all the other channels. 
     When this mode of operation is used, pins SDOUT 1  and SDOUT 2  are used together to create differential signals. In other words, the signal transmitted from pin SDOUT 2  is the logical complement of the signal from pin SDOUT 1 , e.g., pin SDOUT 2  is low when pin SDOUT 1  is high, as shown in the inset of FIG.  4 . Using this technique, the value of a data bit is determined by the level at pin SDOUT 1  relative to the level at pin SDOUT 2 . When the level at pin SDOUT  1  is higher than the level at pin SDOUT 2 , the data bit is a ‘ 1 ’; conversely, when the level at pin SDOUT 1  is lower than the level at pin SDOUT 2 , the data bit is a logical ‘ 0 ’. Differential transmission enables higher transmission speed and also reduces noise and interference coupled from the output signal lines to sensitive analog circuitry within the ADC. This is because noise from the positive and negative signals couple equally to the analog circuits and tend to cancel each other. 
     Illustrative circuitry implementing serial output interface  110  is shown in FIG.  5 . Serial output circuitry  110  includes latches  500   a - 500   h  and corresponding shift registers  501   a-h . ADC channels  1  to  8  convert their respective analog input signals into digital data words, and are synchronized by clock MCLK. Upon completion of an analog-to-digital conversion, the data words are transferred to corresponding latches  500   a - 500   h , which serve to double buffer the data until shift registers  501   a - 501   h  are empty. When shift registers  501   a - 501   h  become available, the shift registers are loaded with the data words from corresponding latches  500   a - 500   h.    
     The data words are then serially shifted from the shift registers and sent to outputs pins SDOUT 1  to SDOUT 4  as determined by control circuit  502  according to the selected mode of operation. In a first mode of operation, data words from a pair of ADC channels are transmitted alternately on an output as described above with reference to FIG.  3 . In this mode of operation, control circuit  502  first enables shift registers corresponding to odd numbered ADC channels, e.g., shift registers  501   a ,  501   c ,  501   e  and  501   g . Control circuit  502  also selects the input corresponding to ADC channel  1  on 8:1 multiplexer (MUX)  503 , selects the input corresponding to OR gate  505  on 2:1 mux  506 , and drives clock LRCK high to indicate the odd numbered channels are being transmitted. On each cycle of serial clock SCLK, a bit is shifted out of the enabled shift registers and routed to the appropriate output. For example, a bit shifted out of shift register  501   a  is sent to output pin SDOUT 1  via 8:1 mux  503  and output driver  504 . At the same time, a data bit corresponding to ADC channel  3  from register  501   c  is sent to OR gate  505 . Because the output of shift register  501   d  is not enabled by control circuit  502 , its output does not affect OR gate  505 , and only data from shift register  501   c  is sent to 2:1 mux  506  and then to output driver  507 . Data from ADC channels  5  to  8  are similarly sent to outputs pins SDOUT 3  (not shown in FIG. 5) and SDOUT  4 . 
     After data from the odd numbered ADC channels has been sent, control circuit  502  disables the outputs of the previously enabled shift registers and enables the outputs of the shift registers corresponding to the even numbered ADC channels, i.e., shift registers  501   b ,  501   d ,  501   f , and  501   h . Control circuit  502  also causes 8:1 mux  503  to select the input corresponding to shift register  501   b  and drives the signal for LRCK clock low. On each cycle of serial clock SCLK, a bit is shifted out of the enabled shift registers to corresponding outputs pins SDOUT 1 -SDOUT 4  in a manner analogous to that described above. 
     In a second mode of operation, data from all of the ADC channels is serially transmitted over a single communication link using differential signals as described in connection with FIG.  4 . In this mode, control circuit  502  first enables shift register  501   a  corresponding to ADC channel  1 ; and disables all other shift registers. Clock LRCK is driven high, and 8:1 mux is configured to select the input connected to shift register  501   a . Control circuit  502  also disables any unused output drivers, e.g., output driver  509 . On each cycle of serial clock SLCK, a bit is shifted from shift register  501   a  and routed through 8:1 mux  503  and output driver  504  to output SDOUT 1 . 
     After an entire data word has been shifted from shift register  501   a , shift register  501   a  is disabled, and shift register  501   c  is enabled. 8:1 multiplexer  503  is also reconfigured to select its input from shift register  501   c . This re-configuration provides a path for data bits to be shifted from shift register  501   c , which corresponds to ADC channel  3 . Control circuit  502  continues to selectively enable shift registers  501   a - 501   h  one at a time, in the desired order, and configures 8:1 mux  503  appropriately so that the data from ADC channels  1 - 8  are serially transmitted to output pin SDOUT 1  in the order shown in FIG.  4 . Alternative arrangements of the data words are possible, and in an illustrative embodiment of the present invention, control circuit  502  is configurable to send the data in any desired sequence. After all eight channels have been transmitted, new data is loaded into shift registers  501   a - 501   h  from corresponding latches  500   a - 500   h , and the process is re-started. 
     In the second mode of operation, the data is transmitted using differential signaling by also routing the output of 8:1 mux  503  to output driver  507  through inverter  508  and 2:1 mux  506 , which is configured to select its input from inverter  508 . The output of output driver  507  is then the inverse of the output of output driver  504 . When the signal at pin SDOUT 1  is high, the signal at pin SDOUT 2  is low, and when the signal at pin SDOUT 1  is low, the signal at pin SDOUT 2  is high. Differential signaling advantageously minimizes noise coupling to sensitive analog circuitry within ADC  100 . 
     Referring now to FIG. 6, an exemplary audio processor is described that incorporates a multi-channel ADC according to the principles of the present invention disclosed herein. Audio processor  600  includes eight channel ADC  601 , which receives four analog stereo inputs on input jacks  602 . Illustratively, the analog stereo signals correspond to eight channels in a 7.1 channel format. ADC  601  converts the analog signals to corresponding digital signals. Typically, audio processor  600  also includes digital input circuitry  603  that receives digital signals from input connector  604 , e.g., a Sony/Philips Digital Interface (S/PDIF) optical or coaxial connector. 
     The digital signals, from ADC  601  or digital input  603 , are sent to DSP  605  for processing. Memory  606  stores programs and data used by DSP  605  to implement the features and operations to be provided by audio processor  600 . For example, programs and data to implement time delay and reverberation effects are utilized to selectively recreate the acoustic ambiance of a concert hall or night club. The processed data are then sent to DACs  607  for conversion to analog stereo signals to be reproduced by amplifiers  608  and speakers  609 . Controller  610  orchestrates the operation of audio processor  600  and provides additional operations such as an user interface. 
     Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. It is therefore contemplated that the claims will cover any such modifications or embodiments that fall within the true scope of the invention.