spectrumpaint: a picture on the waterfall

Paints a picture onto the waterfall display of an SDR receiver. The program reads a raw file one 320-byte row at a time and turns each byte into the strength of one point across the band. Code: spectrumpaint.go.

Fields

JSON	Go field	Type	Required	Rules
pictureFile	PictureFile	string	yes	Path to an existing raw file, 320 bytes per row
frequency	Frequency	float64	yes	Hz, 5000 to 1500000000
excursion	Excursion	*float64	no	Width of the picture in Hz, greater than 0; the program defaults to 100000

Making the picture file

The width of 320 is fixed in the rpitx program. ImageMagick can write the luminance plane on its own:

convert input.jpg -resize 320x100! -flip \
  -quantize YUV -dither FloydSteinberg -colors 4 \
  -depth 8 -interlace partition picture.yuv

-interlace partition writes three files, picture.Y, picture.U and picture.V. Use picture.Y. -depth 8 matters: a stock ImageMagick build writes 16 bits per sample without it, which makes each row 640 bytes and garbles the picture. Check the result with ls -l; a 320x100 picture is 32000 bytes. -flip turns the image upside down so it reads the right way on a waterfall that scrolls down. Leave out the 100! height to keep the aspect ratio.

gorpitx checks only that the file exists. It does not check the width or the size.

Example

{
  "pictureFile": "/home/pi/picture.Y",
  "frequency": 434000000,
  "excursion": 100000
}

Runs:

stdbuf -oL "$GORPITX_PATH/spectrumpaint" /home/pi/picture.Y 434000000 100000