I am starting a new thread to discuss the measurements and [single-]blind A/B listening tests of the Diretta protocol, as a follow-up to the original thread:
I divided it into sections, so that they can be easily referenced in the discussions. There will be two main sections: measurements and listening tests. This post presents the former, with the latter to come in a later post when completed.
I would like this to be a fact-based, technical discussion of Diretta. If you are interested in a purely subjective take of the subject, please see the original thread.
0. Setup
This is the diagram of the setup used. It is based on @David_Snyderâs instructions, which you will find in the original thread.
The host box on the left is connected to LAN through a USB Ethernet dongle and is visible to Roon server through RAAT protocol. The dongle is necessary to create an additional Ethernet port on the host, as the onboard port is used to communicate with the target box on the right through Diretta protocol.
In a normal, Diretta-only setup, the red path is missing and the DAC connects to one of the USB 2 ports on the target box (the âDiretta ONâ connection in the diagram). The signal path in this case is:
Roon â [RAAT] â host â [Diretta] â target â USB DAC.
The red path was used to be able to make measurements with Diretta protocol out of the picture. In that case, the DAC connects to one of the USB 2 ports on the host box (the âDiretta OFFâ connection in the diagram), and the signal path is the usual Roon playback path:
Roon â [RAAT] â host â USB DAC.
(There was no USB switch used in the setup; switching was done by simply unplugging the DAC from one of the USB ports and plugging it into the other.)
The DAC used is a Topping D70.
1. Measurements
This section is further divided into two: power rail measurements for both 5V and 3.3V rails and DAC output measurements.
In all images, the diagrams on the left show the target box - i.e. with Diretta - and the ones on the right the host box - i.e. with RAAT only.
1.1 Power Rails
1.1.1 Power Rails Time Domain
These diagrams show the waveform of the power rails (yellow for 5V, blue for 3.3V) over a 17-or-so seconds at a time. I used AC coupling here so I can amplify and put both on the same screen. One time division is 10s, one voltage division is 50mV.
Diagram above was captured when nothing was playing. Diretta is definitely quieter here, with an amplitude of the noise is roughly half of that of the host for both rails. This is not exactly a fair comparison though, as host boxâs USB3 port connected to the Ethernet dongle was working all the time, while there was no USB (and barely any network) activity on the target box. Still, spikes of activity can still be seen on the target. The 5V spikes are as high as the ones on the host (roughly 150mV pp), and the ones on the 3.3V rail, although smaller, appear quite random.
Same as the previous one, but when silence (more exactly, 32-bit dither) was played. The red horizontal bar at the top of the diagrams shows the playback interval, roughly one minute. We can see the noise on the target box rails âcome aliveâ during streaming, while thereâs not much change in the noise pattern on the host. The noise amplitudes on the target are now almost the same as on the host, although this is still not a fair comparison, since thereâs one USB port working on the target and two USB ports on the host. Most interestingly, the spikes on both 5V and 3.3V rails on the target seem random to me.
(You can see that Roon is streaming silence for about 5 seconds after playback is stopped. Thatâs the time after which the signal indicator light âturns offâ.)
Same as the previous one, but when death metal is played instead of silence. (I used the title track from Fractal Generatorâs Macrocosmos album. That is loud!) I was curious if the streamed data had any impact on the noise. It doesnât. I think itâs plenty apparent here that the bits that are streamed cannot be called âmusicâ in any shape or form; they may sometimes be more ones or more zeroes, but the physical layers involved in transmission work just the same.
In the capture above, I zoomed 10x on the time axis to get a more detailed noise waveform. The division is now 1s and the span is about 17s. The noise on the target still looks quite random to me. The big spikes appear to happen more than one minute in between.
Apart from the slightly smaller amplitude of the noise on the target relative to the host - which is most probably due to one less USB port being active - I donât really see anything that sets Diretta apart from RAAT. Let me know if I am missing something.
(I will perform another measurement of a RAAT-only box using only the onboard Ethernet port and with only one active USB, to see if it brings the noise to a level comparable to the one on the target box.)
1.1.2 Power Rails Frequency Domain
The following images show the spectrum of the power rail noise. I used DC coupling for these, to make sure I didnât miss any low-frequency noise.
The image above shows the spectrum of the 5V rail noise between zero and about 106Hz. One horizontal division is 6.25Hz. As usual, target box is on the left; top graphs are captured during silence playback and the bottom ones during death metal. I am not seeing any significant upward spikes here, or any difference between the two boxes.
This image shows the 5V noise spectrum between about 15.5kHz and 20kHz. One horizontal division is 250Hz and the center of the frequency axis is at 17.75kHz. Iâm not seeing any relevant spikes or differences here either.
This one captures the full spectrum between zero and 22.5kHz, so it fully contains the audible spectrum. One horizontal division is 1.25kHz and the center of the frequency axis is at 11.25kHz. Looks the same to me.
The following 3 images show the same thing as the previous 3, but for the 3.3V rail. Let me know if you can spot any differences between the rails or between target and host.
This concludes the power rails measurements.
1.2 DAC Output
Now comes the interesting part: measuring the actual output of the DAC! At the end of the day, this is what we are actually listening to - if of course we ignore all the artifacts of the analog chain. Again, the left side in each image is through Diretta and the right side through RAAT only.
The image above shows the output of the DAC when silence (32-bit dither) is played. This shows the DAC at its most silent; I think the zero-detect feature of the chip is muting the output.
I think this is the best proof that Direttaâs rationale for a two-box-solution falls flat on its face. Every single component of the Topping D70 DAC is under one single box, including the power supply. This means that thereâs a 110V/60Hz âsignalâ entering the box at all times. Itâs a very high voltage at a very audible frequency. Still, the only indications thereâs a PSU inside are the tiny peaks at 60Hz, 120Hz and 180Hz. The highest is somewhere around -150dB! If the PSU noise is suppressed to this degree, what do you think happens with EMI/RFI noise measured in millivolts and extending into higher frequencies? The fact that everything is in one single box doesnât seem to be of any practical consequence.
This are the output spectrums for a 1kHz sine. They look the same to me, and the THD+N are the same for Diretta and RAAT.
This is the 2-sine intermodulation test signal. Again, I see no notable differences.
Same for the multitone test signal.
Same for the jitter test signal. All the artifacts are tucked well below -120dB in both cases.
1.3. Conclusion
According to measurements, the Diretta myth seems BUSTED.
Let me know if you have any comments or measuring suggestions, and stand by for the listening test results.
) at that same 16bit 44.1kS/s.
