640 PDM Correlator :: Quicker, easier and cheaper to make your own chip!

How it works

The Customizable PDM Cross/Auto Correlator performs a correlation operation of two PDM (Pulse Density Modulation) audio signals. If microphone signals from two diffeerent microphones are pass into the chip, it can find the time difference of arrival between the two and calculate the direction of arrival betweent he two microphones (Cross COrrelation. If the signal from the same microphone is passed into both channels, the pitch can be detected. It peroforms correlation by storing both signals in shift registers and summing the pairwaise XOR between data vlues. The lower this value, the higher the correlation. A decimator has beeen included to help with low-frequency signals. Additionally setting like length and decimation can be configured through an SPI port. It performs an XOR on two input bitstreams and sums the result. The lower this value is the higher correlation. Explain how your project works

How to test

1. Initial Setup

Connect Power Supply
- Ensure the module is correctly powered
Connect Clocks
- Connect a 3.072Mhz clock to clk(ui_in[0])
Connect Microphone
- Connect the microphone's VCC and GND pins to its respective sources
- Connect its clk pin to a 3.072Mhz clock
- Connect its data pin to mic_data_1(ui_in[3])
- If you have a second microphone connect its data pin to mic_data_2(ui_in[4]) other wise connect mic_data_1 to mic_data_2
Connect I2S reciever
- Connect an I2S reciever of your choice like a configured Raspberry Pi or supported speaker to the out pin (uo_out[0])
Connect SPI port
- Connect the MOSI wire to spi_mosi(ui_in[1])
- Connect the CS_N (Chip Select) wire to spi_cs_n(ui_in[2])
- Conect the other side of the SPI port to a microcontroller, Raspberry Pi or something similar

Configure Settings

Procedure
- Set CS_N to low
- Set decimation ratio
- Pass in 8h00 then pass in the decimation ratio as 8 bits. Ex: 8h08 This should be a 16 bit input in total
- Set CS_N to high
- Wait at leaset 1 clock cycle and set CS_N to low
- Set filter length
  - Pass in 8h01 then pass in the filter length as 8 bits. Ex: 8h80 This should be a 16 bit input in total

Monitor Output

Procedure
- Pins uo_out carries the correlation value (lower is bette, from 0 to filter_length)
- uio_out[1] is high if a positive delay improves correlation
- uio_out[2] is high if a negative delay improves correlation

External hardware

Microphones, clock generator, spi port

#	Input	Output	Bidirectional
0	Bit Clock (3.072 Mhz)	PCM Out Mic 0	Delay Select 0
1	LR Clock (48Khz)	PCM Out Mic 1	Delay Select 1
2	PDM Input Mics 0,1	PCM Out Mic 2	Delay Select 2
3	PDM Input Mics 2,3	PCM Out Mic 3	Delay Select 3
4	PDM Input Mics 4,5	PCM Out Mic 4	Delay Select 4
5	PDM Input Mics 6,7	PCM Out Mic 5	Beamformed PCM Output
6		PCM Out Mic 6	Mic Clock
7		PCM Out Mic 7

640 PDM Correlator