har-in-air/fpga_stereo_crossover

Stereo digital 2-way crossover filters processing I2S audio (16bit or 24bit) streams

FPGA_STEREO_CROSSOVER

• ESP32 reads .wav / .mp3 files on a micro-SD card and generates a I2S digital stereo audio stream (16-bit, 44.1kHz or 48kHz) as a master driving MCK, BCK and WS clocks.
• FPGA implements an I2S slave interface and stereo 2-way crossover filters. It generates two I2S data output streams that drive low-pass and high-pass channels on two TAS5753MD stereo I2S power amplifiers.
• Implemented in VHDL on Altera Cyclone IV EP4CE6E22 (WaveShare Cyclone CoreEP4CE6 board)
• Also implemented in Verilog and tested on Anlogic EG4S20BG256 (Sipeed Tang Primer board)
• Crossover filters are 4th order Linkwitz-Riley, implemented as cascaded identical 2nd order Butterworth filters. For Linkwitz-Riley filters, the sum of the low pass and high pass outputs is flat across the crossover frequency.
• The cascaded filters do not have to be identical. There is coefficient memory storage for four unique biquad filters, so we can implement equalization if necessary.
• Using Octave to compute 2nd order butterworth filter coefficients

• Implementation in direct form I to avoid overflow issues, at cost of more delay element memory usage.

• FPGA audio processing modules are clocked by the external MCK from the ESP32 board. This will vary depending on the audio source sample rate and data packaging.
• Top level, slave SPI interface and coefficient loader modules use the on-board system clock (50MHz for the Waveshare Cyclone IV board, 24MHz for the Sipeed Tang Primer).
• Audio samples are max 24bits. The I2S receiver can handle 16/16 and 24/32 bit packaging.
• Filter coefficients are 40bit (4.36) 2's complement. The 4 integer bits give us some headroom for equalization, and I want to be able to use the crossover filters at lower fc/fs values, e.g. for a sub-woofer crossover at 340Hz. In this case some of the filter coefficients are small enough to benefit from the 36bits of fractional resolution.

Filter coefficients @ Fc = 3400Hz

Filter coefficients @ Fc = 340Hz

• Altera Cyclone IV EP4CE6 resource usage

• ESP32 calculates the biquad filter coefficients based on the sample-rate of the audio file being played and updates the FPGA via an SPI interface.

Software development platform

• Intel Quartus Prime Lite 19.1 (Linux), for Altera FPGA
• Arduino 1.8.13 with arduino-ESP32 1.04 package
• Ubuntu 20.04 amdx64

Constraints

• The FPGA modules can handle I2S 16/16 or 24/32 data packaging, with sample rate 44.1kHz or 48kHz. The ESP32 code only reads wav/mp3 files encoded with 16bit data at 44.1kHz or 48kHz.

Credits

• FPGA Biquad IIR Filters
• ESP32 SD I2S Audio
• Cascaded Biquad filter coefficient calculation
• ST7032 LCD library

Useful links

• IIR filter design
• Biquad implementation
• Biquad filter formulae
• Cascaded biquad filters

Prototype

Top side of prototype board

• ESP32 breakout board
• Micro-SD breakout board
• KY-040 Rotary encoder with button for volume control and next track
• LCD 1602 (ST7032) with SPI interface
• Stacked TAS5753MD I2S power amplifier modules. Schematic and PCB layout
• The TAS5753MD power amplifier is rated for max 26V power supply. Tested with a 19.5V 4.7A laptop power brick.

Bottom side of prototype board

• Waveshare Core EP4CE6 FPGA development board.
• MP2307 dc-dc module (5V supply for FPGA board, max 24V input)
• AMS1117 LDO voltage regulator (3.3V supply for micro-SD and TAS5753MD)

Three-way speakers with 7" sub-woofer, 4" mid-woofer and ribbon tweeter.

When testing the crossover at fc = 3400Hz, the TAS5753MD amplifiers drive the mid-woofers and tweeters. The sub-woofers are disconnected.

When testing the crossover at fc = 340Hz, a passive 2-way 2nd order LC crossover circuit (fc = 3400Hz) drives the mid-woofer and tweeters. The TAS5753MD amplifiers drive the sub-woofers and LC crossover inputs.