xilinx-kintex-audio-processing

Implementing a phase vocoder for time-stretching in Verilog on an FPGA with Xilinx Vivado requires careful design of both the hardware components and the algorithm, as it is computationally complex. Here's a high-level breakdown of how we can start with Verilog and Xilinx Vivado:

1. High-Level Design:

We'll break down the process into smaller components, each of which will be implemented separately in Verilog:

• FFT Module: Perform the Fast Fourier Transform (FFT) on each frame of the input audio.
• Magnitude and Phase Extraction: Extract magnitude and phase from the FFT output.
• Noise Reduction: Subtract the noise spectrum from the magnitude.
• Phase Vocoder Time-Stretching: Interpolate the magnitude and modify the phase for time-stretching.
• Inverse FFT (IFFT): Reconstruct the time-domain audio signal from the modified magnitude and phase.
• Overlap-Add: Combine the modified audio frames with overlap.

2. Modules Breakdown:

2.1 FFT Module (Verilog)

We will need a pipelined FFT core, which is typically provided by Vivado IP. To avoid manually writing the FFT logic, Vivado’s FFT IP core is recommended, as it is optimized for FPGA. You can configure this IP in Vivado.

// Example Verilog instantiation of FFT IP core
fft_ip fft_inst (
    .clk(clk),
    .s_axis_config_tdata(config_tdata),
    .s_axis_config_tvalid(config_tvalid),
    .s_axis_data_tdata(input_audio_frame),
    .s_axis_data_tvalid(input_valid),
    .m_axis_data_tdata(fft_output),
    .m_axis_data_tvalid(fft_valid)
);

2.2 Magnitude and Phase Extraction

The FFT produces complex output, and we need to extract the magnitude and phase:

// Magnitude calculation
always @(posedge clk) begin
    mag <= sqrt(real_part * real_part + imag_part * imag_part);
    phase <= atan2(imag_part, real_part);  // Phase calculation
end

2.3 Noise Reduction Module

You can store the noise spectrum during the initialization phase and subtract it from each incoming frame.

always @(posedge clk) begin
    if (valid_frame) begin
        mag_denoised <= (mag > noise_spectrum) ? (mag - noise_spectrum) : 0;
    end
end

2.4 Phase Vocoder Time-Stretching

This module adjusts the phase and magnitude. You can interpolate between frames to stretch the time:

always @(posedge clk) begin
    if (frame_valid) begin
        delta_phase <= next_phase - prev_phase;
        adjusted_phase <= prev_phase + (stretch_factor * delta_phase);
    end
end

2.5 Inverse FFT (IFFT)

Similar to the FFT, you can use Vivado's IFFT IP core to convert back to the time domain.

// Example Verilog instantiation of IFFT IP core
ifft_ip ifft_inst (
    .clk(clk),
    .s_axis_data_tdata(fft_output),  // Input is the modified spectrum
    .s_axis_data_tvalid(fft_valid),
    .m_axis_data_tdata(time_domain_output),
    .m_axis_data_tvalid(output_valid)
);

2.6 Overlap-Add for Reconstruction

After converting each frame back to the time domain, the frames need to be overlapped and added:

always @(posedge clk) begin
    if (output_valid) begin
        overlap_add_result <= overlap_add_result + time_domain_output * window;
    end
end

3. System Integration and Vivado Setup

Here’s the workflow in Vivado:

1. Create a New Project in Vivado targeting your FPGA board (e.g., Spartan S7-25).
2. Add the FFT and IFFT IP cores:
   • Go to Tools > IP Catalog and search for "FFT".
   • Configure the FFT core with appropriate parameters like the FFT size (1024 points).
3. Design the Verilog Modules for:
   • Magnitude and phase extraction.
   • Noise reduction.
   • Phase interpolation for time-stretching.
   • Overlap-add.
4. Testbench:
   • Write a Verilog testbench to simulate the system and verify each module.
5. Synthesize and Implement the design.
6. Generate Bitstream and upload to your FPGA.

4. Example Testbench (for FFT module)

module testbench;

reg clk;
reg [15:0] audio_frame;
wire [15:0] fft_output;

// Instantiate the FFT module
fft_ip fft_inst (
    .clk(clk),
    .s_axis_data_tdata(audio_frame),
    .s_axis_data_tvalid(1'b1),
    .m_axis_data_tdata(fft_output)
);

// Clock generation
always begin
    #5 clk = ~clk;  // 100MHz clock
end

initial begin
    clk = 0;
    audio_frame = 16'd0;
    #10;
    audio_frame = 16'd200;  // Simulate an audio input frame
    #20;
    audio_frame = 16'd400;
    #100;
    $stop;
end
endmodule

5. Testing and Debugging

Once the system is synthesized and the bitstream is generated:

• Test it with pre-recorded audio or live input via ADC.
• Debug the timing and ensure that the time-stretching effect is applied correctly.

6. Next Steps:

• Optimize the Design: Ensure that the design meets timing constraints and has minimal latency.
• Add External Interfaces: Interface with ADC/DAC for audio input/output.
• Improve Noise Reduction: Implement more sophisticated noise filtering techniques if needed.

This Verilog-based FPGA implementation will likely involve iterative design and testing to meet real-time processing requirements. Let me know if you'd like assistance with specific modules or Vivado IP configurations!