README

	For USB audio operation see below.

This sample program is a I2S to PWM sound converter. It receives I2S sound data
(2 channels with 32 bit width and 24 bit signed audio samples) via the I2S
interface exposed on the 40 pin GPIO header. The sound data will be converted to
PWM signals and you can listen to it on the 3.5" headphone jack.

For a simple test two Raspberry Pi computers can be used. Install
sample/34-sounddevices on the transmitting computer and create a file
cmdline.txt on the SD card with this contents:

	soundev=sndi2s

Install this sample program on the receiving computer and connect a headphone to
the 3.5" headphone jack.

Both computers have to be connected via the 40 pin GPIO header this way (GPIO
numbers are chip numbers, not header positions):

	Transmitter		Receiver
	GPIO			GPIO

	18 (CLK)	-->	18 (CLK)
	19 (FS)		-->	19 (FS)
	21 (DOUT)	-->	20 (DIN)
	GND		<-->	GND

The configured SAMPLE_RATE (default 48000) in the file config.h of both sample
programs must match, but can be modified.

After powering on both computers and completed boot a modulated 440 Hz tone can
be heard on both channels of the headphone.


USB AUDIO

The input sound data for this program can also come from an USB audio streaming
device. This is currently only supported on the Raspberry Pi 4. You have to
define USE_USB in the file config.h to enable this.

USB audio devices often support setting the input jack (LineIn or Microphone)
and the input volume in dB. You can enable this in config.h too. Without these
settings, the default input jack and volume is used.


SOUND RECORDER

If you enable the setting "ENABLE_RECORDER = 1" in the Makefile, this sample acts
as a digital sound recorder, which saves the received I2S or USB sound data to
the SD card in a .wav file. It can be imported into many applications for further
processing.

The sound recorder is started and stopped using a simple button, which connects
the pin GPIO17 to GND. If you want to use an other pin, you can change this in
the file config.h.

Before build you should add the following system options to the file Config.mk:

	DEFINE += -DNO_BUSY_WAIT -DNO_SDHOST

If you want to use this program on the Raspberry Pi 4, without a connected
display, you also have to add this option to the file Config.mk:

	DEFINE += -DSCREEN_HEADLESS

Building the sound recorder program requires the libraries in addon/SDCard/ and
addon/fatfs/ to be built manually before.

Be sure that you have enough free space on your SD card (about 15 MB/min with
24-bit Stereo mode).