6 SPDIF to I2S decoder :: Quicker, easier and cheaper to make your own chip!

How it works

SPDIF audio is a well known and commonly used industry standard for audio distibution on a single optical or electical interface. It signal both audio data and the clock in the same signal. I2S is a well known and commenly used industry standard for audio distibution on a 3 wire interface. It has clock(BCK), left/right sync(WS) and data signal(D0).

Digial audio amplifiers and DAC's often use I2S as input interface.

I have coded up a spdif converter that oversample the spdif using a 27Mhz system clock. One process regenerates the i2c_bck clock signal by counting number of system clock between spdif input data edges. The spdif singal is a phase mark kind of encoding with a couple of sync words. This decoder only looks for Left and right sync word that indecate when to toggle the i2c_ws signal.

The audio PCM samples will get decode by looking for phase/mark changes and the pcm bits are shifted in to a left and right pcm_sample fifo. From here the i2s_d0 signal is generate by shifting out of the non active fifo in reversed order.

Code base

The system consist of 3 major block:

A register bank
I2C interface to read/write to the register bank
The audio interface

The register back is written in Golang and generates HDL systemVerilog to support packed structs and packages. All systemVerilog sources are converted to verilog using sv2v during the FPGA build script used for testing.

The register map has a seciton for the audio interface - and holds 8 registers for write operation through a dedicated I2C interface to the amplifier. Default it will set amplifer volume on address 0x40 and amplifer audioformat to std i2s. More commands can be added by software through the system I2C interface.

FPGA test implementation

How to test

If no smoke coming out after supply has been applied all good :-)

Apply optical audio from a spdif source - if sounds good - it works :-)

External hardware

Amplifier module MA12070p and ftdi usb to i2c module :-)

#	Input	Output	Bidirectional
0	rx_in	amp_i2s_bck	i2c_scl
1	debug_in	amp_i2s_ws	i2c_sda
2		amp_i2s_d0	amp_i2c_scl
3		amp_nenable	amp_i2c_sda
4		amp_nmute
5
6
7		pwm_out

6 SPDIF to I2S decoder