Whazon, a S/PDIF switch built around WM8805

[nattawa]

A S/PDIF switch can be useful when we have more digital audio sources than the number of inputs available on a DAC.

I wanted to do this a long while ago and finally managed to get at it recently.

This little project makes use of the internal 8-channel selector offered by WM8805 S/PDIF receiver, and was designed to select one out of up to 8 S/PDIF feeds under the monitor and control of an Arduino Nano micro controller board.

Unlike a usual switch-and-buffer type of device, WM8805 re-clocks its on-chip S/PDIF transmitter and does not pass the incoming jitter on to the output, therefore upkeeps the S/PDIF signal quality in terms of jitter reduction/rejection. That is why so my choice was.

Whazon’s operation is simple, it keeps polling the 8 inputs in turn and stops at the first one its PLL manages to lock up to. A duplicated, de-jittered S/PDIF signal is then sent out with the S/PDIF transmitter, available in coaxial and optical form.

Whatzon receives remote control via an inexpensive Bluetooth-serial I/O tranceiver, model HC-05, paired with a smart phone running a Bluetooth terminal app.

The channel polling described above can be enabled/disabled with the phone. Forced channel change to desired channel, and next higher channel are also easily implemented.

An I2S copy of the selected input signal is also sent out at WM8805’s bidirectional digital audio interface for future project expansion. It is however not being put to use at present, except that a buffered copy of the LRCK is sent to the Arduino for exact sample rate detection (that WM8805 is not fully capable of).

The PCB is a compact 10cm * 10cm double sided design, with a break-away optical I/O section. Whazon accepts 4 coaxial (BNC) and 4 optical inputs with current PCB design. It is possible to replace the optical input receivers in the current breakaway design with BNC connectors to make all channels BNC input. The coupling and termination circuit layout around the WM8805 works with both types for that 4 channels.

Whazon is powered by a 5Vdc cell phone charger through a micro USB port that serves as a programming port for the Arduino as well when connected to a PC that runs Arduino development system.

Successful so far: input channel polling, PLL locking, exact sample rate detection -- tested good with 44.1, 48, 88.2, and 96KHz signals, haven't tested 32KHz, 176.4KHz, and 192KHz yet as I don't have such sources.

Being worked on: an OLED screen display that shows current input channel and sample rate --- Done.

In the plan: A button push or a remote that forces switching to the next input and starting the polling/channel rotation --- Remote done.

The PCB layout screen shot is of top side of Rev.2 design to be sent to a PCB house soon.

There is a youtube video in post #5 below that shows this project in action.

[Gusser]

While your project has more capabilities, there is a much simpler way to make an SPDIF switch.

SPDIF is basically the same bandwidth and electrical parameters and NTSC analog video. 1volt into 75ohms 6mhz.

So any plain old analog video switch will work. Maxim makes many video crosspoint chips with built in cable drivers. TI does a s well. With a MAX453 you can make an 8x1 switch with just 1 IC and a few resistors. No need for transformer coupling. Most of these switch IC's are DC coupled although AC coupling would work just fine as well.

If using an OEM professional video switch you just need to be sure it doesn't have sync tip clamping enabled if so equipped. As we know SPDIF does not have a sync pulse and the clamp may chew up the data stream

[nattawa]

I agree that there can be many and simple ways of switching SPDIF feeds. Whazon is perhaps more of a SPDIF receiver than a mere switch. An I2S interface makes it possible to hook up with other DIY projects in the forum that also come with I2S I/O, an OLED display makes it a toy of a little more fun. A bit of automation and remote control make it an practicable device in a stereo equipment rack too -- living room worthy was one of the objectives. Coupling transformers may help cut down ground loops loops among but can always be opted out when not needed.

[nattawa]

The OLED display is up and running

Channel polling in action

All Channels Down

Channel 4 has something

CH4 is good

[nattawa]

I finally managed the most tedious part of the project, making an enclosure. Now it's in the system and doing its deed.

In Action Video

A few pictures inside and back.

Remote control is via a Bluetooth serial tranceiver module HC-05 paird with a smartphone running a Bluetooth Terminal app.

[raptorlightning]

In my searches for a source switch I realized I would like to switch signals as early as possible to avoid analog degradation. This meant switching in the digital domain, and the common denominator for digital audio is S/PDIF. It is easy to find HDMI audio extractors that have S/PDIF outputs these days, and most devices are configurable for PCM output. I couldn't find any acceptable designs or products, besides nattawa's design, that focuses around high quality switching/jitter cleaning of S/PDIF signals. The WM8805 is really a great chip and this design recognizes all available, realistic sampling rates. Nattawa's microcontroller code intelligently handles the quirks of the WM8805, allowing perfect software control over the switching and sample rates. I've modified the design a bit to simplify the architecture and created my own version of his design that works excellently.





If you have multiple digital sources and need a way to switch between them, I highly recommend this project.

[gemby]

Originally Posted by nattawa

I finally managed the most tedious part of the project, making an enclosure. Now it's in the system and doing its deed.

In Action Video

A few pictures inside and back.

Remote control is via a Bluetooth serial tranceiver module HC-05 paird with a smartphone running a Bluetooth Terminal app.

Hello nattawa, is it possible to share schematics, pcb files and arduino sketch? I am in process of designing preamp with dac and would like to have switch for digital imputs.

[nattawa]

Hi gemby, yes, I'm more than happy to share the design. I'll make ready a file package and come back to you in a couple days.

[gemby]

Originally Posted by nattawa

Hi gemby, yes, I'm more than happy to share the design. I'll make ready a file package and come back to you in a couple days.

Tnx, i appreciate it ;-)

[nattawa]

Check your PM, gemby