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Audio Effects: Using External Memory

Module by: Douglas L. Jones, Swaroop Appadwedula, Matthew Berry, Mark Haun, Dima Moussa, Daniel Sachs

Summary: You will implement three audio effects: a fixed-length delay, a variable-length delay, and a feedback-echo. All require storing many samples in external memory.

Introduction

Many audio effects require storing thousands of samples in memory on the DSP. Because there is not enough memory on the DSP microprocessor itself to store so many samples, external memory must be used.

In this exercise, you will use external memory to implement a long audio delay and an audio echo. Refer to Core File: Accessing External Memory on TI TMS320C54x for a description and examples of accessing external memory.

Delay and Echo Implementation

You will implement three audio effects: a long, fixed-length delay, a variable-length delay, and a feedback-echo.

Fixed-length delay implementation

First, implement the 131,072-sample delay shown in Figure 1 using the READPROG and WRITPROG macros. Use memory locations 010000h-02ffffh in external Program RAM to do this; you may also want to use the dld and dst opcodes to store and retrieve the 32-bit addresses for the accumulators. Note that these two operations store the words in memory in big-endian order, with the high-order word first.

Figure 1: Fixed-Length Delay
Figure 1 (delay.png)

Remember that arithmetic operations that act on the accumulators, such as the add instruction, operate on the complete 32- or 40-bit value. Also keep in mind that since 131,072 is a power of two, you can use masking (via the and instruction) to implement the circular buffer easily. This delay will be easy to verify on the oscilloscope. (How long, in seconds, do you expect this delay to be?)

Variable-delay implementation

Once you have your fixed-length delay working, make a copy and modify it so that the delay can be changed to any length between zero (or one) and 131,072 samples by changing the value stored in one double-word pair in memory. You should keep the buffer length equal to 131,072 and change only your addressing of the sample being read back; it is more difficult to change the buffer size to a length that is not a power of two.

Verify that your code works as expected by timing the delay from input to output and ensuring that it is approximately the correct length.

Feedback-echo implementation

Last, copy and modify your code so that the value taken from the end of the variable delay from Variable-delay implementation is multiplied by a gain factor and then added back into the input, and the result is both saved into the delay line and sent out to the digital-to-analog converters. Figure 2 shows the block diagram. (It may be necessary to multiply the input by a gain as well to prevent overflow.) This will make a one-tap feedback echo, an simple audio effect that sounds remarkably good. To test the effect, connect the DSP EVM input to a CD player or microphone and connect the output to a loudspeaker. Verify that the echo can be heard multiple times, and that the spacing between echoes matches the delay length you have chosen.

Figure 2: Feedback Echo
Figure 2 (echo.png)

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