Patent Application
20070168408
1. Technical Field
The present disclosure relates to LMS based algorithms and, more specifically, to parallel architecture for acceleration of multiple channel LMS based algorithms.
2. Description Of the Related Art
An electronic filter is a device for eliminating unwanted frequencies from an electronic signal. Digital filters are electronic filters that are capable of filtering digital signals, for example, analog signals that have been converted into digital signals, for example, using an analog-to-digital converter.
One example of a digital filter is a line echo cancellation filter. The line echo cancellation filter attempts to reduce echo from digitally communicated audio signals. For example, line echo cancellation filters may be applied to multiple channels of telephone lines to reduce echo from the telephone lines.
Some digital filters, for example, some line echo cancellation filters, may be adaptive filters. Adaptive filters are digital filters that can analyze the filter output and use it to modify the filtering technique (e.g. the filter coefficients) to improve the digital filter quality in real-time. Adaptive filters may use feedback to refine the values of the filter coefficients and hence modify the adaptive filter's frequency responses.
Adaptive filters may refine the filter coefficients by analyzing multiple digital signals in an attempt to isolate the unwanted noise signals that may be present in the multiple digital signals.
Adaptive filters, as well as other digital filters, may utilize least mean square (LMS) algorithms in computing digital filter operations. LMS algorithms are optimization techniques that attempt to find a “best fit” to a set of data by attempting to minimize the sum of the squares of the difference (called residuals) between the fitted function and the data.
Line echo'cancellation filters are often implemented using digital signal processors (DSPs). DSPs are special-purpose microprocessors that have been optimized for the processing of digital signals. When using a DSP to implement digital filters, for example, line echo cancellation filters, it is ideal to maximize efficiency by reducing the number of cycles necessary to accomplish calculations and by minimizing the use of available memory. By increasing efficiency, a single DSP may be able to perform line echo cancellation on multiple line channels at the same time. Additionally, increasing efficiency may allow a single DSP to support multiple line channels using a limited amount of available memory with time division multiplexing (TDM). This is because TDM is commonly used to time-share available memory among multiple line channels. TDM therefore results in a lower memory cost per channel as system speed goes up or time used per channel goes down. Therefore increased DSP efficiency for handling digital filtering, for example, multi channel line echo cancellation, can lead to reduced implementation costs.
Embodiments of the present invention therefore seek to increase the efficiency for processing LMS algorithms, for example, to increase the efficiency by which DSPs can perform multiple channel line echo cancellation.
A parallel system for performing LMS coefficient adaptation includes a data memory, a tap memory, and two or more LMS hardware units. The LMS hardware units utilize data stored in the data memory and coefficients stored in the tap memory for performing multiple LMS coefficient adaptations in parallel.
A method for performing LMS coefficient adaptation using parallel architecture includes storing data in a data memory of the parallel architecture, storing coefficients in a tap memory of the parallel architecture, and performing multiple LMS coefficient adaptations. The multiple LMS coefficient adaptations are performed from the data stored in the data memory and the coefficients stored in the tap memory using two or more LMS hardware units, of the parallel architecture, in parallel.
A more complete appreciation of the present disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
In describing the preferred embodiments of the present disclosure illustrated in the drawings, specific terminology is employed for sake of clarity. However, the present disclosure is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents which operate in a similar manner.
Embodiments of the present invention seek to utilize multiple LMS computing units for calculating multiple LMS filter taps in parallel so that LMS filter calculation performance may be enhanced. By enhancing the efficiency of LMS computations, digital filtering, for example, multiple channel line echo cancellation, may be performed more quickly. Additionally, a single DSP supporting more channels may provide for more efficient usage of the available DSP memory.
Embodiments of the present invention also seek to use double-width single-port memory, so that Tap Memory (TM) can seemingly be both read and written to at the same time without requiring a two-port memory. This may provide an advantage as single-port memory may be more power efficient than two-port memory.
Moreover, by calculating multiple taps in parallel using a single filter with multiple LMS units, a single large Data Memory (DM) and a single large double-width Tap Memory may be used. this is advantageous over using multiple single-channel LMS processors as using a single large memory uses less space than multiple smaller memories.
ti y(n)=d(n)+e(n)
Wherein y(n) represents the output signal, d(n) represents the desired response signal and e(n) represents the error signal. Additional parameters may include the filter length (which means number of filter coefficient), a top and bottom position of a circulation data buffer in the data memory 12, a data starting point in the data memory 12, and the coefficient starting point in the tap memory 18.
The multiple LMS hardware units 13-16 may be connected in parallel to the LMS controller 17 such that any of the multiple LMS hardware units 13-16 may receive LMS processing tasks from and return results to the LMS controller 17. The hardware 10 may comprise any number of LMS hardware units 13-16. For example, the hardware 10 may comprise 4 LMS hardware units 13-16 as shown in
Each LMS hardware unit 13-16 may be comprised of a multiply-add component for coefficient adaptation and a multiplication and accumulation unit for convolution, as adaptation and convolution may be useful for LMS computation.
The LMS hardware units 13-16 may be connected, for example in parallel, to a Tap Memory (TM) 18. The tap memory 18 may be used by the LMS hardware units 13-16 for storing coefficients (taps) used in LMS computation. The tap memory 18 may support reading old tap and writing new tap in a single clock cycle. This may be accomplished using either dual-port SRAM (SRAM allowing for simultaneous read and write). Alternatively, single port SRAM may be used, for example using techniques described in U.S. Pat. No. 6,714,956 to Liu et al., which is incorporated by reference. Alternatively, a double-width single-port memory may be used so that the Tap Memory can seemingly be both read from and written to at the same time without requiring dual-port memory.
According to embodiments of the present disclosure, the DSP may have read and write access to the data memory and the tap memory. The DSP may not access the data memory or the tap memory at the same time as the LMS HW units. The width of the data memory may be 4×the word length of the data samples. The number of words in the data memory may be n/4 where n is the length of the filter.
According to embodiments of the present invention using 2-port memory, the width of the tap memory may be 4×the word length of the filter coefficients (taps). The number of words in the tap memory may be n/4.
According to embodiments of the present invention using single port memory, the tap memory size may be 8×the word length of the filter coefficients. The number of words in the tap memory may be n/8.
For example, in the case of 4 LMS HW units, to be able to read 4 data samples during the same clock cycle, the width of the data memory is required to be 4 times the width of the data samples. When using a 2-port memory for the tap memory, the tap memory size requirements are the same as the data memory. When using a single port memory for the tap memory, the tap memory width must be 2 times the data memory width (to allow for double width reads and writes).
Although the present invention apparently requires wider memories, an implementation may decide to divide these wide memories into several narrower memories. The reason to do this is so that the memory width could match the word length of the DSP, which will make it easier to map the data memory and tap memory into the DSP's memory address map.
The DSP may load parameters into the LMS HW controller. The DSP may load a frame of data into the data memory, and the initial filter coefficients values into the tap memory. The LMS HW units may perform the LMS calculations. During the LMS calculations, the filter coefficients may be adapted and new values written to the tap memory. The accumulated result of the LMS filter calculation may be written to data memory, where it may be read by the DSP. The DSP may read the tap memory and save the filter coefficients for the current channel.
These steps may be repeated for as many channels as possible that may be calculated within the available sample rate according to principals of TDM (time division multiplexing). In this way, as many channels as possible may be supported while minimizing hardware and memory requirements. The number of channels that may be filtered may depend on the filter length, the sampling rate, and the clock frequency of the hardware.
In performing digital signal filtering such as line echo cancellation filtering, convolution and coefficient adaptation must be performed a great number of times. By employing multiple LMS hardware units, multiple convolutions and coefficient adaptations may be performed in parallel thereby expediting the signal filtering. The LMS Hardware Controller 17 distributes convolution and coefficient adaptation assignments to the various LMS hardware units 13-16.
Each LMS hardware unit may then perform the assigned convolution and coefficient adaptation. The adapted coefficients may then be written back into the Tap Memory.
A single convolution and coefficient adaptation assignment may be broken down into several steps. First, a data sample may be read from the data memory. Then coefficients may be read from the tap memory. Next, coefficient adaptation may be performed. Finally, convolution may be performed and the adapted coefficients may be written back into tap memory.
The LMS hardware units may perform these several steps in a pipeline. In this pipeline, each LMS hardware unit may be responsible for completing a single step during each clock cycle. For example, during one clock cycle, a first LMS hardware unit may be responsible for reading a data sample from the data memory and for reading coefficients from the tap memory. A second hardware unit may then be responsible for performing coefficient adaptation. A third hardware unit may then be responsible for performing convolution. A fourth hardware unit may then be responsible for writing the adapted coefficients back into the tap memory.
The width of the memory may determine the number of LMS hardware units that may be supported. The number of pipeline stages may indirectly affect the number of LMS hardware units that may be supported.
The pipeline maximizes efficiency by not having to wait until one convolution and coefficient adaptation process is completed before beginning the next. For example, the first LMS hardware unit may read a data sample from the data memory and read coefficients from the tap memory for a first coefficient adaptation process, then read a data sample from the data memory and read tap coefficients from the tap memory for the next coefficient adaptation process as the second LMS hardware unit performs adaptation for the first coefficient adaptation process, and so on.
Each LMS hardware unit may perform assignments for multiple coefficient adaptation processes at the same time. For example, the first LMS hardware unit may read data samples from the data memory and read tap coefficients from the tap memory for a set of four coefficient adaptation processes in a single clock cycle.
As the pipeline gets started (the initial phase) all LMS hardware units will not be functioning until the first set of coefficient adaptation process makes its way down the pipeline. After that, all LMS hardware units may be functioning in parallel (normal iteration phase). During the normal iteration phase of the pipeline, as many coefficient adaptation processes may be performed in a single clock cycle as there are coefficient adaptation processes in a set. For example, where there are four coefficient adaptation processes in a set, four coefficient adaptation processes may be completed in a single clock cycle.
For example, coefficients (t) may be adapted according to the following expression:
tnew(n)=told(n)+ConvergenceFactor* x(n)
ACC=ACC+x(n)*tnew(n)
Where tnew(n) is the adapted coefficient, told(n) is the previously used coefficient, x(n) is data in the circulation buffer. The ACC is the accumulation register for convolution.
For example, accumulation registers may be summed up according to the following expressions:
ACC1=ACC1+ACC2
ACC3=ACC3+ACC4
Then, accumulation registers may be summed up according to the following expression:
ACC1=Saturation{Round[ACC1+ACC3]}
The values for ACC1 and ACC3 that were summed may be combined and rounded. Saturation may then be provided, where needed, to prevent the accumulation registers from taking a value greater than the maximum possible value. The accumulator register may be moved to data memory and made available to the DSP and/or the LMS accelerator controller.
Embodiments of the present invention may utilize dual port SRAM at tap memory. Alternatively single port SRAM may be used. When using single port SRAM, a method for simultaneously reading and writing to and from tap memory may be used. For example, the method for reading and writing to and from tap memory using even-odd memory described in Liu et al. may be used. Alternatively, a method for simultaneously reading and writing to and from tap memory based on double width read during odd clock cycles and double width write during even clock cycles may be used.
The above specific embodiments are illustrative, and many variations can be introduced on these embodiments without departing from the spirit of the disclosure or from the scope of the appended claims. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims.
