FFT Spectrum Display Demo#

Introduction#

The FFT example in the current media chapter has been adjusted to a real-time audio spectrum display demo. This example collects mono audio from AI, calls the FFT HAL to perform hardware FFT computation, and then uses the OSD layer to draw the spectrum bar chart on the display screen.

Unlike the older sample_fft that only does FFT/IFFT regression tests, the current example focuses more on complete media pipeline integration, covering the following modules:

Audio capture (AI / I2S)
FFT HAL call
VB/MMZ buffer usage
OSD layer drawing and screen display
Automatic landscape layout and frequency axis display

If you only want to verify the basic correctness of the FFT HAL, please refer to the FFT HAL Example in the peripheral chapter.

Function Description#

Real-time audio data capture and hardware FFT execution
Automatically use the landscape layout of the display panel to obtain larger spectrum horizontal resolution
Draw frequency scale and Hz axis label at the bottom of the spectrum
Support custom sampling rate, FFT points, rotation angle, display size, and display gain

Code Location#

Demo source code location: src/rtsmart/examples/mpp/sample_fft_display/main.c

Build and Run#

Build#

After building the firmware in the SDK root directory, the example ELF will be generated in the RT-Smart examples directory.

Run#

After starting the development board, enter the /sdcard/app/examples/mpp directory and run:

./sample_fft_display.elf -c <connector_type> [options]

For example, a 01Studio 480x800 screen can use:

./sample_fft_display.elf -c 20 -s 44100 -p 512

Parameter Description#

You can view the help by running ./sample_fft_display.elf -h. The current version supports the following parameters:

Parameter	Description	Default
`-c <type>`	Connector type, required; use `list_connector` to check	None
`-w <width>`	OSD width	Automatically derived from the rotated panel size
`-h <height>`	OSD height	Automatically derived from the rotated panel size
`-r <deg>`	Rotation angle, supports `0/90/180/270`	Auto landscape
`-s <rate>`	Audio sample rate	`44100`
`-p <points>`	FFT points, supports powers of 2 from `64~4096`	`512`
`-g <gain>`	Display gain, in dB	`0`

Running Effect#

After startup, the program will output logs similar to the following:

./sample_fft_display.elf -c 20 -s 44100 -p 512
FFT Spectrum: connector=20, panel=480x800, OSD=800x480, rotate=90, rate=44100, points=512, gain=0.0 dB
main start connector=20 panel=480x800 osd=800x480 rotate=90 rate=44100 points=512
before drv_fft_open
after drv_fft_open inst=0x7f9f2030
Running... press Ctrl+C to stop

At this point, the screen will continuously display:

Spectrum bar chart
Frequency axis at the bottom
Title and real-time refresh rate at the top

Press Ctrl+C to exit the example.

Tip

The current example uses the FFT HAL interface drv_fft_* and no longer uses the legacy kd_mpi_fft* API. For interface details, please refer to the FFT API Documentation.