Tag Archives: S/PDIF

Dolby Digital and DTS bitstream output with HiFiBerry Digi

The HiFiBerry Digi driver is still in development. Stereo output works great even at highest sampling frequencies and bitrates (192kHz/24bit). Today we tested what happens, when we output a DTS or AC-3 bitstream through the HiFiBerry Digi. Would an external decoder recognize the signal? See yourself:

yamaha-dd yamaha-dts

It worked :-) That means there is a good chance, that DTS and AC-3 output will be supported. However, we don’t know yet if standard software like XBMC will be able to use the HiFiBerry Digi.

S/PDIF output transformers

For the HiFiBerry Digi, we tested different methods to drive the 75Ohm output: with and without output transformers. Output transformers are not used very often in consumer SPDIF outputs: they increase the costs of a device and many people do not care about it. Bad quality transformers may even make the output worse. But for HiFiBerry Digi, we want to use them – at least as an option.

Lets’s have a look at two different transformers:

spdif-unshielded spdif-shieldedThis shows signal quality of a 192kHz S/PDIF-output. The output is simply terminated by an 75 Ohm resistor. Even the simple, unshielded transformer (left picture) works ok. The jitter is much lower than our tests with optical links. However, with the second transformer (right picture), the edges of the signal look really good. The bandwidth of the second transformer is higher.

Let’s have a closer look at the signal edge:

risetimeRise time is 2.4ns. Using the general equation for the bandwidth BW = 0.35 / Tr, we can calculate the bandwidth as about 150MHz. That’s much more than needed, but that’s good for us.

SPDIF: 192kHz/24bit over an optical link

I did some tests with the HiFiBerry Digi prototype today. As expected, everything worked without problems with 44.1/48 and 96kHz. A test with my EMU-0404 showed exactly, what I expected: no digital data could be received over the optical link at 192kHz. It’s not a problem, that’s just what the interface in this device is specified for.

Have a look at this oscilloscope picture. You see, that the signal frequency is 12.27MHz.

spdif-192

But then I did another test with the optical input of my iMac (an old model from 2009). And I was really surprise to see, that it received correct data even at the highest bit rate! That means, the receiver in this Mac is capable to receive SPDIF inputs of more than 10MHz – nice.

What are your experiences with the optical input on your DAC? I’m interested which DACs support this.

HiFIBerry Digi prototyping

hifiberry-digi-developmentToday I started with the first HiFiBerry Digi tests. Unfortunately the PCB had some major problems. But with some bent pins and additional cables I was able to get the board running. I was also able to output sound already via the optical SPDIF output.  That means, the development can go on. I will start developing the Linux driver for the board now. It is a bit more complex than the driver for the HiFiBerry Mini, because it needs I2C communication to configure the chipset correctly. However, I don’t expect major problems here. Stay tuned!

HiFiBerry PCB design finished

hifiberry-designToday, the design of the HiFiBerry sound card PCB for the Raspberry Pi has been finalized. Expect a prototype in the next weeks. The HiFiBerry will not only provide high quality audio output for the Raspberry Pi, but also a stereo input. Due to the limited space, we will not have PCM inputs and output jacks, but only a pin header. If you want to use PCM input or output, you can simply add some cables to the board connecting to external connectors.

Update 1: The picture does not show the final version. I realized (very late), that the chip is in a SSOP package and fixed this.

Update 2: We’re testing the prototype now.