Fixed-length delay implementation

First, implement the 131,072-sample delay shown in Figure 1. Store the samples in a buffer in SDRAM with alignment 0x40000h. Do this by adding a line in your userlinker.cmd file to create a new section in memory: Allocate a buffer of the size you need in the assembly file. Be sure to place it in the section you just created. Since the memory address is now greater than 16-bits, you will need to do a MOV dbl() in order to get the full address into an auxiliary register.

.bigbuffer:  align=0x40000 {} > SDRAM

Figure 1: Fixed-Length Delay

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?) If you want, you may assume the delay will be a power-of-2 for easier implementation.

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