High quality Raspberry Pi I2S HAT - part 5

High quality Raspberry Pi I2S HAT - part 5 - linux kernel patching

Kernel patching

At last, in order to make I2S Hat working properly you have to recompile the kernel.

Firstly download kernel source files, suitable to your distribution.
Secondly you can download the patches here. Copy and unpack in top directory kernel source.

What are the patches doing?

Here is the list:

p1.patch - support for 384k in file ./sound/core/pcm_native.c
p2.patch - support for 384 in file ./include/sound/pcm.h
p3.patch - support for 384k in file ./sound/soc/codecs/pcm1794a.c
p4.patch - gpio config in file ./arch/arm/boot/dts/overlays/rpi-dac-overlay.dts
p5.patch - main driver file ./sound/soc/bcm/rpi-simple-soundcard.c
p6.patch - support for 384k in file ./drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c

To apply the patch you can use patch command as below:

patch -p1 -i p1.patch

Repeat this for all six patches.

Then recompile and install new kernel and modules.

Don't forget to add the line to config.txt

Thats all :)

Comments

High quality Raspberry Pi I2S HAT - part 1 - overview

As it is known the Raspberry Pi can output I2S signals but there are some big disadvantages resulting in poor sound quality. First of all, Rpi cannot output exact audio frequencies (44.1/48 and multiple) due to main oscillator - this causes lot of jitter in output I2S signals. Second of all, Rpi cannot output MCLK (master clock) and although this clock is beyond I2S bus specification , is often needed by modern DAC chips. Basic idea is to run Rpi as SLAVE , exactly as shown here: Transmitter = Rpi Receiver = I2S HAT If we take high quality oscillators, all mentioned problems will disappear. But this idea requires kernel modification (HAT has to know the value of the actual sampling frequency). This whole project is divided into four subsections - hardware, VHDL code, OpenOCD program and GNU/Linux kernel modification.

High quality Raspberry Pi I2S HAT - part 2 - hardware

Schematic diagram You can download pdf version here . Schematic is very simple: Design consists of three main parts: Q1, Q2 oscillators 45/49Mhz, U4 - Xilinx CPLD - main logic and reclocker circuit build on U5, U6 and U7. Each part has its own LDO (U1, U2, U3). You can use many IC's in SOT23 package, also there is footprint on the PCB for bypass cap. In the prototype I'm using LP5907. X1 is the input of the external power supply. In case of an external supply, JP3 should be open. There is no need for a separate jtag port, as we will program Xilinx directly through Raspberry Pi gpio. PCB Assembled boards Here with ordinary oscillators and xc9572xl And here with CCHD-957 from Crystek and xc9536xl Here is the BOM: C1 47uF/10V SMB tantal. C2, C5, C18 2.2uF/25V 1206 X7R C3, C4, C19 10uF/16V 1206 X7R C6, C7, C8, C9,

High quality Raspberry Pi I2S HAT - part 6 - conclusion

Now you have clean I2S signals, without using USB, SPDIF etc. :) This HAT was tested with different DAC's, oscillators and worked perfectly. On this picture with PCM5102 board (new post is coming):) P.S. I’ve made a small batch of this PCB and for those who wish to buy one, please do not hesitate to contact me via email.

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