Measurements of THD+N/Jitter using an external PC to expand the DSP capability.

[Tranquility Bass]

Someone emailed me recently after I posted the results of routing digital audio through an external PC for the purposes of expanding the DSP capability of the Ultimate-Preamp and asked whether or not using a PC adds any additional noise, distortion or jitter to the audio stream ? Well I said in theory it shouldn't due to the isolation and transparency provided by this interface but there is only one way to find out and that is to measure it.

So using the same J-test methodolgy that we used for the normal jitter tests on the various inputs to the preamp we can now apply the same test methodology whilst routing the audio through an external Dell PC used in our lab and see how it affects the jitter and noise. So we setup Audioweaver on a PC to process audio from the Preamp at 192kHz in a direct input-to-output connection as in the following:-



Then we measured the jitter from the output of the Preamp using the same J-test methodology as we used on earlier tests of jitter.



The screenshot below is the same as above but with an expanded frequency scale. As can be seen the spectrum is as clean as it is with internal DSP processing and any artifacts are way below the threshold of hearing !



We did the same for THD+Noise.



As can be seen in the test results above the PC is essentially transparent to the audio stream and does not add any noise, distortion or jitter to the incoming audio stream thanks to the hardware of the Ultimate Preamplifier Plus. This is contrary to the reputation of PC's as a noisy and hostile environment due to high-speed digital design and a switching power supply not optimized for low noise. However, the UPP mitigates these issues with ease without the use of re-clockers, network switches, fancy USB cables, add-on FIFO's etc, which would not improve anything or could indeed make things worse simply because our hardware is designed right in the first place and the above measurements are proof of that



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