Abstract:
A noise shaper includes a first feedback loop for noise shaping a first feedback signal under normal operating conditions and having a first filter with a first signal transfer function and a second feedback loop that is stable under overload conditions and has a second filter having a second signal transfer function differing from the first signal transfer function. The noise shaper also includes an output circuit block including a quantizer and steering circuitry. The quantizer includes an input simultaneously responsive to outputs of the first and second filters. The steering circuitry steers a feedback from an output of the quantizer to input of the first and second feedback loops. The steering circuitry steers feedback from output of the quantizer to inputs of the first and second feedback loops, the steering circuitry including a first output for providing the first feedback signal to the first feedback loop and a second output for providing a second feedback signal to the second feedback loop.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates in general to delta-sigma modulators and, in particular, to noise shaping circuits and methods with feedback steering overload compensation and systems using the same. 
     2. Background of Invention 
     Delta-sigma modulators are particularly useful in digital to analog converters (DACs) and analog to digital converters (ADCs). Using oversampling, a delta-sigma modulator spreads the quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, the delta-sigma modulator performs noise shaping by acting as a highpass filter to the quantization noise; most of the quantization noise power is thereby shifted out of the signal band. 
     The typical delta-sigma modulator in an ADC includes an input summer which sums the analog input signal with negative feedback, an analog linear (loop) filter, a quantizer, and a feedback loop with a digital to analog converter (feedback DAC) coupling the quantizer output and the inverting input of the input summer. A delta-sigma DAC is similar, with a digital input summer, a digital linear filter, a digital feedback loop, a quantizer, and an output DAC at the modulator output: In a first order modulator, the linear filter comprises a single integrator stage; the filter in higher order modulators normally includes a cascade of a corresponding number of integrator stages. Higher-order modulators have improved quantization noise transfer characteristics over modulators of lower order, but stability becomes a more critical design factor as the order increases. For a given topology, the quantizer is either a one-bit or a multiple-bit quantizer. 
     One cause of instability in digital delta-sigma modulators is input overload. For example, input overload occurs when the gain of the input data is greater than one, when a digitized squarewave with significant Gibbs overshoot is received at the modulator input, or when a bad stream of data is fed from a preceding interpolator. Single-bit delta-sigma modulators are notoriously susceptible to input overload. Multiple-bit delta-sigma modulators are less susceptible to input overload, although overload will still often occur when the input stream approaches its maximum positive and negative levels. 
     Current techniques for handling overload in delta-sigma modulators are relatively complex and require detection of overload conditions and subsequent resetting or limiting of the modulator circuitry to avoid saturation and instability. However, modulator overload remains an important problem that must be addressed, especially in higher order modulators that provide higher quality noise shaping. Modulator overload is particularly troublesome in audio applications, in which an unstable modulator causes extremes in the output signal that damage the following processing stages and/or result in an unpleasant audible output to the listener. 
     SUMMARY OF INVENTION 
     According to the inventive concepts, methods and circuits are disclosed which provide noise shaper immunity to input overload. One representative embodiment of these concepts is a noise shaper including a first filter for noise shaping an input signal under normal operating conditions and a second filter that is stable under overload conditions. A quantizer responds to the sum of the outputs of the first and second filters. Signal steering circuitry steers feedback from the output of the quantizer to inputs of the first and second filters to maintain stability of the first filter under the overload conditions. 
     Circuits and methods embodying the inventive concepts directly address the problem of noise shaper input overload. When an overload condition occurs, the overload loop receives and bears the increased energy load while the energy being passed by the primary (high quality) noise shaping loop is sustained at a level to maintain primary loop stability. When the overload condition ceases, the primary loop resumes passing the majority of the energy and continues to provide the high quality noise shaping operation . The present invention does not require additional circuitry to either detect overload conditions or reset the noise shaper circuitry to avoid saturating the noise shaper output. Additionally, brief deviations of the input stream outside of the normal maximum limits of the noise shaper input do not substantially disrupt noise shaper operation. These circuits and methods are particularly useful in audio applications in which noise shaper overload causes damage in the following processing stages, such as the audio amplifiers and speakers, and even produce an audible output injurious to the hearing of the listener. 
    
    
     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: 
     FIG. 1 is a diagram of a representative audio system application of a digital to analog converter (DAC) according to the principles of the present invention; 
     FIG. 2 is a high-level block diagram of an exemplary delta-sigma digital to analog converter (DAC) generally embodying the principles of the present invention and suitable for use in such applications as the DAC shown in the system of FIG. 1; 
     FIG. 3 is an operational block diagram depicting one particular exemplary delta-sigma DAC with feedback steering overload control embodying the principles illustrated by the general example of FIG.  2 . 
    
    
     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. 1-3 of the drawings, in which like numbers designate like parts. 
     FIG. 1 is a diagram of a typical audio system application of a digital to analog converter (DAC) subsystem  100  according to the principles of the present invention. In this example, DAC subsystem  100  forms part of an audio component  101 , such as a compact disk (CD) player, digital audio tape (DAT) player or digital video disk (DVD) unit. A digital media drive  102  recovers the digital data, such as 1-bit audio data in the Sony/Philips 1-bit format or multiple-bit PCM in multiple-bit audio applications, from the given digital data storage media, and passes the data along with clocks and control signals to DAC subsystem  100 . The resulting analog (audio) data is further processed in analog/audio processing block  103  prior to amplification in amplifier block  104 . Audio amplifier block  104  then drives a set of conventional speakers  105   a  and  105   b , a headset, or similar device. 
     Digital audio data is received as serial words through the SDATA path timed by the sampling clock (SCLK). The left and right channel data are alternately processed in response to the left-right clock signal (LRCK). The LRCK signal is normally at the same rate as the data input rate (i.e., the sampling rate). The master clock signal (MCLK) synchronizes the overall timing of audio component  101  and has an oversampling frequency of a given multiple of the audio sampling rate. Control signals DF 1  and DF 0  allow for the selection of the input format, such as a right or left justified format, a 20-bit or 24-bit word width format, etc. . When 1-bit data is being input, the SDATA port receives left channel data and the DF 1  port right channel data. 
     As discussed above, higher order delta-sigma modulators (e.g. third order or higher) typically provide better noise shaping over lower order delta-sigma modulators (e.g. first or second order). However, as the order of the modulator increases, modulator stability becomes a more critical design factor. One particular cause of instability is input overload, in which deviation of the input signal beyond the maximum positive or negative modulator input limits causes one or more of the modulator filter stages to saturate and the entire loop to oscillate. 
     In typical digital delta-sigma modulators, when the input stream exceeds a given maximum positive or negative value, the quantizer output is driven to its corresponding maximum or minimum value, at which point the throughput data steam is clipped (limited). In turn, the clipped output of the quantizer limits the amount of negative feedback available to the modulator input summer and loop filter. With insufficient feedback, the integrators of the loop filter saturate to their maximum or minimum values, and the modulator becomes unstable. In turn, when the integrator stages saturate, the following circuits, such as the DAC in a delta-sigma digital to analog converter, are overdriven. The result of overdriving is extreme transitions in the analog output signal, which in audio systems may damage the audio speakers and/or cause discomfort or injury to the listener. 
     One common technique for addressing overload in digital delta-sigma modulators is to reset the integrator stages of the loop filter to zero when overload is detected. Integrator overload detection and reset, however, is relatively difficult to implement. For example, the modulator has to be designed to be immune from disruptions due to occasional brief deviations of the input signal beyond its maximum values and at the same time still detect true overload conditions at the modulator input and reset accordingly. 
     FIG. 2 is a high-level block diagram of an exemplary delta-sigma digital to analog converter (DAC)  200  with feedback steering overload control embodying the principles of the present invention. DAC  200  is suitable for use in such applications as DAC subsystem  100  of FIG.  1 . DAC  200  includes two delta-sigma loops  201  and  202  and a shared quantizer  203 . Generally, primary delta-sigma loop  201  is a higher order filter that provides the desired noise shaping operation during normal (low level) operation. Delta-sigma loop  202  generally is a lower order “overload” data path that is unconditionally stable under overload conditions. Steering circuitry  204 , which is discussed further below, controls the negative feedback from quantizer  203  to the inputs of delta-sigma loops  201  and  202 . By steering the feedback to the inputs of loops  201  and  202 , the amount of energy passed through the corresponding loop  201 / 202  is controlled. 
     In the illustrated embodiment of DAC  200 , primary loop  201  is a sixth (6 th ) order loop and includes an input summer  205 , which sums the digital input signal with negative feedback from steering circuitry  204 , and also a sixth (6 th ) order primary loop filter  206 . Primary loop filter  206  preferably has a conventional topology, such as a feedforward or feedback topology. A general discussion of the design and construction of various delta-sigma loop filter topologies are found in various publications such as Norsworthy et al.,  Delta - Sigma Data Converters, Theory, Design and Simulation , IEEE Press, 1996. 
     Exemplary overload delta-sigma modulator loop  202  is a second (2 nd ) order loop and includes an input summer  207  summing a fixed input value (in this case zero) with feedback from steering circuitry  204 , and also a second (2 nd ) order loop filter  208 . Second (2 nd ) order delta-sigma loops are relatively immune to overload and generally straightforward to implement. In other words, second order loop filter designs are able to operate at or up to one hundred percent (%100) of their input range and still remain stable. Additionally, the stability of second order filters in general is provable. Hence, in the illustrated embodiment of DAC  200 , a second (2 nd ) order loop  202  is selected for overload loop  202 . In general, the state variables of the second order stage are clipped or limited to insure that finite word length registers are able to be used. 
     The outputs of primary loop  201  and overload loop  202  are summed into shared quantizer  203  by summer  209 . In the illustrated embodiment, quantizer  203  is a nine (9)-level quantizer with limiting or truncating capabilities. In illustrated quantizer  203 , the maximum positive truncated (quantized) digital output value is plus four (+4) and the maximum negative output value is minus four (−4). Steering circuitry  204  controls two feedback streams: one stream from the output of shared quantizer  203  to input summer  205  of primary loop  201  and another stream to input summer  207  of overload loop  202 . The output stream from quantizer  203 , which is equal to the sum of the energy of the two feedback streams, drives a conventional switched-capacitor or current steering DAC  211  through dynamic element matching (DEM) circuitry  210 . DAC  211  typically has eight (8) elements, which are nominally equivalent to each other, and DEM  210  guarantees equal usage of the elements to remove noise due to mismatch. 
     In normal operation, quantizer  203  provides an output without clipping and therefore steering circuitry  204  directs the majority of the feedback from quantizer  203  to primary loop  201 . Consequently, input summer  205  at the input to 6 th  order loop filter  206  receives sufficient negative feedback to maintain primary loop  201  in the stableoperating regime. In this case, depicted nine-level limiting quantizer  203  outputs digital values in the range of negative four (−4) to positive four (+4). If the modulator input into primary loop  201  remains sufficiently small, feedback values in the range of minus four (−4) to plus four (+4) will provide sufficient feedback to maintain stability of the primary loop  201 . 
     As the input to modulator loop  201  increases and overload approaches, steering circuitry  204  steers sufficient negative feedback to the input of primary loop  201  to maintain stability. At the same time, a compensating level of feedback is sent to the negative input of summer  207  of low-order, unconditionally stable overload loop  202 . For example, if the limiting quantizer  203  clips its output at a value of +4, but the input requires feedback with a value of +5 to maintain stability, steering circuitry  204  feeds back a stream with a value of +5 to the input of primary loop  201  and a compensating stream with a value of −1 to the input of overload loop  202 . The total value out of feedback steering circuitry  204  thus remains equal to the value out of quantizer  203 . In order to minimize signal degradation under overload conditions, the operation of steering circuitry  204  guarantees that the two outputs from steering circuitry  204  sum to the output of quantizer  203 . Also, under low signal conditions, a minimal or no amount of the signal is returned to the input of low order modulator loop  202 . 
     In other words, the increased feedback into summer  205  of primary loop  201  sums with the increased (overload) digital input signal of DAC  200  and maintains the stages of primary 6 th  order loop filter  206  out of saturation. The compensating feedback into overload loop  202  increases the energy through loop  202 . 
     Consequently, primary loop  201  is prevented from overloading and remains stable. Overload loop  202  passes the majority of the overload energy but remains stable due to its lower order. When the overload condition ceases, the majority of the feedback energy is redirected to primary loop  202  which returns to generating a high quality output signal. Second (2 nd ) order loop  202  is simple to construct and implement, since it does not have any input signals and only has quantized feedback signals. Therefore, the wordlength of the registers may be made to be very short. 
     A number of ways exist for implementing feedback steering overload compensation, such as shown in DAC  200  of FIG.  2 . FIG. 3 is an operational block diagram depicting one particular exemplary delta-sigma DAC  300  with feedback steering overload control. Delta-sigma DAC  300  includes a high-order (6 th  order) primary loop filter  301  and a low-order (unconditionally stable) (2 nd  order) overload loop filter  302 . For illustrative purposes, primary loop filter  301  is a sixth (6 th ) order filter, and low order filter  302  is a second (2 nd ) order filter. Again, a second (2 nd ) order topology is selected for low order filter  302  since second (2 nd ) order loop filters are provably stable under overload conditions. In this example, low order filter  302  is the overload filter. 
     Primary 6 th  order loop filter  301  provides the high quality filtering of the input signal under normal (low level) operating conditions. The signal output of primary loop filter  301  is quantized by a non-limiting quantizer  303 , which in turn feeds one input to summer  304 . Summer  304  is placed after quantizer  303 , as the output of a simple second order loop filter is also an integer since the input is always driven with an integer and hence does not participate in the truncation. The output of non-limiting quantizer  303  also provides negative feedback to input summer  305  to close the primary deltasigma modulator loop, which also includes a delay (Z −1 ) block  306  for signal timing. 
     A second input to summer  304  is fed by overload filter  302 . The input to overload filter  302  is a fixed value, such as a logical zero (0) in this example. The negative feedback to summer  307  from the output of overload filter  302 , which is delayed by delay (Z − 1) element  308 , is discussed further below. 
     The sum of the outputs from respective primary and overload filters  301  and  302  generated by summer  304  is passed through a limiter  309  which performs a clipping (truncation) operation. The resulting output signal from limiter  309  drives DEM circuitry  310  and DAC  311  at the output of DAC  300 . 
     The feedback to input summer  307  is generated by summer  312 . The inverting (negative (−))_ input FB 1  to summer  312  is driven by the output of non-limiting quantizer  303 . The non-inverting (positive (+)) input of summer  312  is driven by the output of limiter  309 . 
     As long as the output from non-limiting quantizer  303  remains below the maximum (positive to negative) output from limiter  309 , the overload feedback FB 2  from summer  312  remains at zero (0). The majority of the energy is therefore passed through high-quality, 6 th  order loop filter  301 . On the other hand, as the output from quantizer  303  exceeds the positive or negative maximum output values from limiter  309 , the overload feedback FB 2  from summer  312  increases accordingly. The full feedback FB 1  from non-limiting quantizer  303  to the input of sixth (6 th ) order loop filter  301  maintains 6 th order loop filter  301  stable by insuring that the stages of loop filter  301  do not saturate. The overload feedback FB 2  to the input of second (2 nd ) order filter  302  ensures that more energy passes through loop filter  302 , which remains stable under overload conditions. The total feedback into summers  305  and  307  equals the output from limiter  309 . 
     Other steering mechanisms may also be used in alternate embodiments of the present invention, such as a system that uses the overload filter path only when overload is severely affecting the operation of the main loop filter, but allows short, transient overloads to be clipped in the quantizer. Additionally, the feedback steering may be based upon the level of the input signal. 
     The principles of the present invention were described above with respect to exemplary digital delta-sigma modulators in exemplary DACs  200  and  300 . Feedback steering overload control according to these principles, however, are also applicable to analog delta-sigma modulators and related applications such as analog to digital converters. 
     Although the invention has been described with reference to a 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.