That's difficult to answer. Some months back, I did some tests in which I want to get a feeling about the amount of jitter of the Teensy spdif-output.
I can share the results although they don't exactly answer your question.
First some background on what I wanted to find out:
I use 4 Teensys 4.0 as multichannel processor in my surround system. My speakers are digital active speakers and they are connected via coax spdif cable to the Teensys.
I was therefore curious about jitter of the Teensy spdif output signal. Maybe my dIY solution introduces lots of jitter and totally degrades the sound quality.
I don't have the equipment to measure the jitter of the spdif signal directly and if I could it would be difficult for me to asses its influence on the signal.
Also, the ASRC of the speakers removes the jitter from the Teensy to some degree and I was more interesseted in the remaining jitter that gets baked into the signal at the
resampling. I also don't have access to the resampled I2S signal inside my speakers, but I had an unused board with an Cirrus Logic CS8422 ASRC lying around and another Teensy 4.1.
So I decided to build the following setup:
- One of the Teensy 4.0 outputs a J-test signal via coax spdif output.
- This spdif output is connected to Cirrus Logic CS8422 board.
- The CS8422 resamples the signal and is connected to the Teensy 4.1 via I2S. (The Teensy 4.1 is I2S master)
- The Teensy 4.1 sends the resampled signal to the laptop via usb serial, were I can have a look at its spectrum.
I assume that both Teensy have the same amount of jitter and the result shows the effect of the jitter of both Teensys combined. My reasoning was that the dsp and ASRC of
my active speakers probably perform equal or better that the quick and dirty setup and if the distortion of the received J-test signal is reasonable low, I don't have to worry about jitter.
I first used the CS8422 hardware resampler instead of the software resampler because I thought that it would provide more comparable results to the resampler of my speakers.
But out of curiosity, I also connected the spdif output of the T4 to the spdif input to the T4.1 and used the software resampler. That's why I think the test is also interesting for
you.
Here is the result when the Teensy software resampler is used:
In my setup the Teensy run at 48kHz instead of 44.1kHz.
The blue spectrum is the resampled j-test signal from the T4.1. The FFT was applied to 96000 samples and a Hann window was used.
The orange spectrum is the resampled signal without any jitter. I used a python implementation of the resampling algorithm that runs on the Teensy. According to
to the receiving T 4.1 the average input frequency at the test was 48000.04 kHz. So I resampled the j-test signal from 48000.04 kHz to 48000 kHz in python as a reference without
jitter.
The following to images show the result of the hardware resampler. The orange spectrum is again the result of the python resampling.