Custom Piano Tuner

Environment

apt packages

• libasound2-dev (for compilation only)
• libasound2 - underlying ALSA support
• linux-libc-dev - Linux headers for SPI

Remove pulseaudio.

Todo: move to headless mode and blow away a pile of unneeded packages.

/boot/config.txt

# See https://rpf.io/configtxt
dtparam=spi=on

dtoverlay=spi0-1cs

arm_64bit=1

# todo: turn off video

Audio USB device details

dmesg:

usb 1-1.2: new full-speed USB device number 3 using xhci_hcd
usb 1-1.2: New USB device found, idVendor=0d8c, idProduct=013c, bcdDevice= 1.00
usb 1-1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=0
usb 1-1.2: Product: USB PnP Sound Device
usb 1-1.2: Manufacturer: C-Media Electronics Inc.      
input: C-Media Electronics Inc.       USB PnP Sound Device as /devices/platform/scb/fd500000.pcie/pci0000:00/0000:00:00.0/0000:01:00.0/usb1/1-1/1-1.2/1-1.2:1.2/0003:0D8C:013C.0001/input/input0
hid-generic 0003:0D8C:013C.0001: input,hidraw0: USB HID v1.00 Device [C-Media Electronics Inc.       USB PnP Sound Device] on usb-0000:01:00.0-1.2/input2

lsusb:

Bus 001 Device 003: ID 0d8c:013c C-Media Electronics, Inc. CM108 Audio Controller

arecord -l:

**** List of CAPTURE Hardware Devices ****
card 1: Device [USB PnP Sound Device], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

arecord -L:

default:CARD=Device
    USB PnP Sound Device, USB Audio
    Default Audio Device
sysdefault:CARD=Device
    USB PnP Sound Device, USB Audio
    Default Audio Device
hw:CARD=Device,DEV=0
    USB PnP Sound Device, USB Audio
    Direct hardware device without any conversions
plughw:CARD=Device,DEV=0
    USB PnP Sound Device, USB Audio
    Hardware device with all software conversions

Examine these lines in /usr/share/alsa/alsa.conf; the defaults should correspond to the correct card and device listed above:

defaults.ctl.card 1
defaults.pcm.card 1
defaults.pcm.device 0
defaults.pcm.subdevice 0

Audio input test

$ arecord -D hw:CARD=Device,DEV=0 -c 1 -f S16_LE -r 48000 -d 1 -v output-test
Recording WAVE 'output-test' : Signed 16 bit Little Endian, Rate 48000 Hz, Mono
Hardware PCM card 2 'USB PnP Sound Device' device 0 subdevice 0
Its setup is:
  stream       : CAPTURE
  access       : RW_INTERLEAVED
  format       : S16_LE
  subformat    : STD
  channels     : 1
  rate         : 48000
  exact rate   : 48000 (48000/1)
  msbits       : 16
  buffer_size  : 24000
  period_size  : 6000
  period_time  : 125000
  tstamp_mode  : NONE
  tstamp_type  : MONOTONIC
  period_step  : 1
  avail_min    : 6000
  period_event : 0
  start_threshold  : 1
  stop_threshold   : 24000
  silence_threshold: 0
  silence_size : 0
  boundary     : 1572864000
  appl_ptr     : 0
  hw_ptr       : 0

Verifying SPI

$ raspi-gpio get | grep SPI0
GPIO 9: level=0 fsel=4 alt=0 func=SPI0_MISO pull=DOWN
GPIO 10: level=0 fsel=4 alt=0 func=SPI0_MOSI pull=DOWN
GPIO 11: level=0 fsel=4 alt=0 func=SPI0_SCLK pull=DOWN

$ lsmod | grep spi
spidev                 24576  0
spi_bcm2835            24576  0

$ ls -l /dev/spi*
crw-rw---- 1 root spi 153, 0 Dec  5 17:06 /dev/spidev0.0

# First, guarantee sync; this requires at most two bytes
echo -ne "\x00\x00" > /dev/spidev0.0

# Channel 1: 1V, 205: 0b001_000_00 0b1100_1101
echo -ne "\x20\xCD" > /dev/spidev0.0

# Channel 2: 2V, 410: 0b010_000_01 0b1001_1010
echo -ne "\x41\x9A" > /dev/spidev0.0

# Channel 5: 3V, 614: 0b101_000_10 0b0110_0110
echo -ne "\xA2\x66" > /dev/spidev0.0

# Channel 4: 4V, 26, 0b100_11001
echo -ne "\x99" > /dev/spidev0.0