642 PDM Pitch Filter
642 : PDM Pitch Filter
- Author: Armaan Gomes
- Description: It uses a moving average filter and decimator to filter out a specific frequency
- GitHub repository
- Open in 3D viewer
- Clock: 0 Hz
How it works
The Customizable PDM Pitch Filter can filter a specific pitch and its harmonics from a PDM(Pulse Density Modulation) microphone input stream. First, the microphone signal is passed through a variable decimator. This alters the sample rate and allowsthe filtering of lower frquency waves. Second the signal is passed through a variable length moving average filter. Because the bitstream is in PDM format, (1 or -1 at 3.072 Mhz) it is possible to nullify a signal of specific freuqncy by passing the signal through a moving average filter of a specific length. Here is the calculations to find the filter length: samplerate/target_frequency. From there the output is passed through an I2S bus to an audio reciever. Additionally, the decimation ratio and filter length can be customized through an SPI port.
How to test
1. Initial Setup
- Connect Power Supply
- Ensure the module is correctly powered
- Connect Clocks
- Connect a
3.072Mhzclock toclk(ui_in[0])
- Connect a
- Connect Microphone
- Connect the microphone's
VCCandGNDpins to its respective sources - Connect its
clkpin to a3.072Mhzclock - Connect its
datapin tomic_data(ui_in[3])
- Connect the microphone's
- Connect I2S reciever
- Connect an I2S reciever of your choice like a configured Raspberry Pi or supported speaker to the
outpin (uo_out[0])
- Connect an I2S reciever of your choice like a configured Raspberry Pi or supported speaker to the
- Connect SPI port
- Connect the
MOSIwire tospi_mosi(ui_in[1]) - Connect the
CS_N(Chip Select) wire tospi_cs_n(ui_in[2]) - Conect the other side of the SPI port to a microcontroller, Raspberry Pi or something similar
- Connect the
Configure Settings
- Procedure
- Set
CS_Nto low - Set decimation ratio
- Pass in
8h00then pass in the decimation ratio as 8 bits. Ex:8h08This should be a 16 bit input in total - Set
CS_Nto high - Wait at leaset 1 clock cycle and set
CS_Nto low - Set filter length
- Pass in
8h01then pass in the filter length as 8 bits. Ex:8h80This should be a 16 bit input in total
- Pass in
- Set
Monitor Output
- Procedure
- Enable I2S reciever
- Enable Clock Signals
- Record data from I2S reciever (Varies based on chosen reciever)
External hardware
- Digital PDM Microphone
- SPI Ouput (Can be a microcontroller or Raspberry PI)
- I2S Input (Can be a microcontroller, Raspberry Pi, or speaker)
IO
| # | 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 |