For the purpose of measurements, we want a tool that can allow us to obtain reproducible results and good accuracy. Over the years, I've achieved reproducible results with consistently minimal inter-test variation by standardizing the way I run most measurements with the digital sources, cables, standard procedures, and types of tests I run. What I want is better accuracy - an ADC that has lower noise floor for improved resolution, doesn't add as much of its own distortions, have higher timing accuracy (eg. for jitter tests), and perhaps more features to expand the measurement quality (eg. higher sample rate to capture impulse responses more accurately, and can handle wider input levels for the range of devices tested).
Remember that over the years, I have used 2 other ADCs on a regular basis for measurements here. For a cameo, I dug out the old Creative E-MU 0404USB for the line up:
Let's see what it can do...
I. Noise Floor & Stability
The noise floor speaks highly of the device overall but perhaps specifically to the choice of the AKM AK5574 "Verita Series" AD converter first released in 2016. According to the datasheet, it is capable of a 121dB A-weighted dynamic range, increasing to 124dB with the "4-to-2" mode since it is a 4-channel part that can be summed down into 2-channels. As with the DAC side of the ADI-2 Pro FS, it can convert analogue to 768kHz PCM and DSD256.
It's always nice to have a look at the lower frequencies, particularly ~60Hz here in North America to see if any mains hum is seeping through into the ADC or DAC. To look at this with higher resolution, I've employed the use of SpectraPLUS-SC (version 184.108.40.206 at time of testing) with the ability to do up to 1M bin FFTs at 192kHz sampling rate through ASIO:
Beautiful 0-250Hz noise floor. Nice and clean which tells us that neither the Oppo DAC playing digital silence nor the RME ADI-2 Pro doing the recording is introducing 60Hz hum or its harmonics into the system. Remember folks, the RME device is powered by a switching power supply... Clearly this is not an issue given no evidence of noise across the spectrum at least to 96kHz in the plots above, nor around 60Hz.
One could check the noise floor even further out since the ADI-2 Pro can sample all the way to 768kHz. WaveSpectra can handle up to 384kHz using ASIO in realtime. Here's what the noise level out to 192kHz (384kHz sample rate, 131k FFT) looks like:
Again, nice and clean all the way to 192kHz with no gross idle tones sticking out.
Finally, even though I don't have a realtime FFT that can handle 768kHz, we can do this by recording at 32/768kHz through Sound It! Pro 8 and then run it through the FFT (65536 points) in Adobe Audition which accepts files of this bit-depth and samplerate:
Adobe Audition scanned 5 seconds of the 32/768kHz digital silence recording to come up with that noise floor. Again, nice to see the absence of any unusual noise - Raspberry Pi 3 USB to Oppo UDP-205 to RME ADI-2 Pro FS with generic USB and XLR cables.
Other than extreme oversampling in the home, research into ultrasonics, or perhaps professionals wanting to archive something at 2xDXD, I think it's highly unlikely than anyone would want/need to perform home recordings or vinyl rips at such a high samplerate!
For perspective, remember that bats with all their amazing echo-location capabilities are said to have their most sensitive hearing between 15-90kHz, maybe at the extreme, hear up to 200kHz. Marine mammals like dolphins can emit sounds up to 150kHz, also useful for echo-location. So, I think an ADC like this could be great for animal research into recording ultrasonic vocalizations and animal hearing with proper microphones like these! :-)
II. -90.3dBF 1.1025kHz non-dithered 16-bit LSB WaveformOne measurement I have not done in awhile has been the -90.3dBFS 1kHz non-dithered 16-bit LSB "waveform peep". In principle, if the device being tested is of high resolution, and our ADC is likewise of excellent quality, we should see a beautifully stable signal on both channels even at the very low -90dB amplitude zoomed in for visualization... Here's the Oppo UDP-205 (sharp filter) recorded with the ADI-2 Pro FS (sharp filter) compared to idealized performance:
For the record, the RME ADI-2 Pro FS's ADC has 4 digital filter settings to choose from: minimum phase "SD Sharp" and "SD Slow", plus linear phase "Sharp" and "Slow". At some point, I'm sure I'll use this ability to demonstrate filter ringing on the ADC side like what we've done with impulse responses on the DAC side.
In any event, the comparison image above looks great! I set the RME ADC to capture the peak input level (+13dBu) appropriate for the Oppo without clipping and just recorded the -90dB signal. Nothing was done to amplify that -90dB signal when fed into the ADC to increase SNR. The only thing done to visualize the waveform was normalization the low amplitude signal to ~85% in the Adobe Audition software. Remember, the waveform shown is of the lowest bit of a 16-bit digital signal flipping back and forth to create a 1kHz undithered "sine wave" (that looks square for obvious reasons due to the 16-bit resolution). For a DAC (Oppo) to produce that and on the other end, the ADC (RME) to capture that detail obviously meant excellent fidelity with very low noise level on both ends.
BTW, notice that unlike how Stereophile plots this test, I've created the 2 channels to be in inverse polarity so the waves don't fully overlap on top of each other. One will need to instead appreciate the symmetry of the "square box" formed by the two channels.
With these low level signals, we could set the ADI-2 Pro's settings to +4dBu reference level and capture that -90dB signal utilizing more of the dynamic range available to the ADC. The result is an even nicer looking (almost ideal) waveform:
Let's not short change the Focusrite Forte, because it's also a high-resolution capable ADC. Here's what the same waveform looks like through that device:
Not bad at all. The morphology of the peaks are not as consistent as the RME and overall not looking as nice, but certainly it's showing good channel balance with no anomalies like a DC offset that we see sometimes when looking at these -90dB waves.
III. RightMark Comparison Between RME ADI-2 Pro FS vs. Focusrite ForteNow let's do some relative comparisons between results I obtained previously with the Forte versus the RME in RightMark. While the software does have limitations and still has a number of unfortunate bugs, it does work and allow for a nice suite of tests which over the years I've found to be replicable and accurate within the limits of test hardware of course.
As I mentioned above, by default, the RME uses "SD Sharp" (SD = Short Delay = minimum phase) filter setting. This was what I kept the device at for these measurements. While not shown here, changing this to the linear phase "Sharp" filter does not change the results to any significant degree; this is to be expected since the measurement system does not introduce non-bandwidth-limited signals to the ADC to induce any kind of ringing that might be picked up. Since changing the filter made no difference, going forward with my measurements, I will leave the ADC filter to "Sharp".
The hardware measurement chain looks like this:
Device (PonoPlayer / Oppo) --> 6' RCA/XLR (for RCA, an RCA-to-XLR adaptor used with RME) --> RME ADI-2 Pro FS / Focusrite Forte --> 12' USB2.0 cable to measurement laptop (Windows 10)Obviously the PonoPlayer is its own source. For the Oppo DAC, I used my Raspberry Pi 3 B+ "Touch" through USB2.0. The Pi 3 was connected to my home ethernet system (10Gbps capable though not at this speed for the Pi 3 of course). All cables are generic, nothing expensive. As discussed and measured years ago, the high-priced audiophile cables are really quite unnecessary for high quality signal conduction. Of course, enjoy them if they look nice to you.
From now on, all RightMark testing will be done with RightMark PRO (currently version 6.4.5) and one change I've also made is to adjust the frequency response parameters to 20Hz-20kHz instead of the default of 40Hz-15kHz which is numerically reflected in the "Frequency response (multitone), dB" row in the summary chart. With high quality gear, reporting the full "20-20" spectrum just made more sense to me.
It's fun to start with this mobile device as we know that it has some idiosyncrasies we should be looking for. A few years ago, I also compared the PonoPlayer between the Forte with E-MU 0404USB.
As expected, noise floor lower and dynamic range higher with the RME.
One of the obvious idiosyncrasies with the PonoPlayer is the Ayre "listen" minimum phase, slow roll-off filter employed which at 20kHz causes a ~4.5dB dip demonstrated using both the RME and Forte. Notice also the concordance in the calculated results for distortion measures for THD and IMD. While similar, we see the RME numbers being lower than the Forte, likely due to the improved accuracy of the device overall contributing less distortion.
Some graphs to consider...
|16/44.1 graphs. Both the RME and Forte demonstrate the early frequency roll-off from the Ayre "listen" filter.|
|24/96 graphs. RME able to measure the frequency response a bit further, lower noise levels overall.|
|Grrr... RightMark Pro still has issues with graphing at 192kHz so crosstalk and IMD+N sweeps not shown. Notice for the noise level that both the RME and Forte are picking up noise with the PonoPlayer at ~80kHz.|
B. Oppo UDP-205
Although I did grab some results for the TEAC UD-501 which has been a reference for me since 2013, these days, the Oppo UDP-205 and its internal ES9038Pro DAC is my reference for accurate conversion. Looking around, it's clear that for technical accuracy, it's going to be tough to beat the resolution of what this Oppo can do, even down to the intersample overload protection of the digital filters which is not the case with all ES9038Pro-based DACs. It seems the Oppo engineers have taken some extra effort in this regard.
Here are some results from the Oppo UDP-205's XLR balanced output, with digital filter set to "Linear Phase Fast":
Like with the PonoPlayer measurements above, we're seeing consistently better noise level and remarkably lower harmonic and intermodulation distortion results through the RME indicating a more accurate ADC that's not adding as much distortion to the signal.
With the lower noise floor, we're seeing a fantastic ~20-bit measured resolution with the Oppo UDP-205 used as a USB DAC!
This time around with the same 6' balanced XLR cable with no adaptors used, the results are better with the RME compared to the Focusrite Forte across the board including stereo crosstalk.
As mentioned 2 weeks ago, RME has improved the smoothness of frequency response with the recent firmware updates. The ADI-2 Pro FS ADC is clearly capable of significantly flatter and markedly extended frequency response than the Forte as witnessed with the 192kHz measurement.
|Reduced ripple with new firmware on ADI-2 Pro FS.|
IV. SpectraPLUS THD+N & IMDUsing the SpectraPLUS software, we can do a simple 1kHz measurement of THD(+N) in high resolution. Here is the Raspberry Pi 3 B+ USB --> Oppo UDP-205 XLR --> ADI-2 Pro FS measured with 96kHz bandwidth (ie. ADC operating at 192kHz, only 20Hz to 20kHz shown in graphs) with a 0dBFS 1kHz tone (click on image for enlarged view) sourced at 16/44 and 24/48:
We can see that the results from the two stereo channels show good consistency. Looking at the 24-bit results, there's a THD distortion factor of around 0.00015% for both channels or -116dB distortion attenuation, and THD+N of around 0.00023% or less than -112dB. SINAD (signal-to-noise and distortion) likewise of over 112dB; excellent.
Here's a peek at a standard IMD test using SpectraPLUS (again, analogue output from the Oppo, bandwidth of 96kHz, ADC running at 192kHz) with 16/44 and 24/48 signals as source:
Note that this is using a test tone set of 250Hz and 8020Hz with a 4:1 amplitude ratio (-12dB) for some variation. The RightMark test above uses the standard SMPTERP120-1994 signals at 60Hz and 7kHz when measuring IMD. Again we see that the two stereo channels are the same, both reporting around 0.00005% IMD (a tiny -126dB) - essentially no intermodulation products (this is not IMD+N, so noise not accounted for in the measurement) with the 24-bit signal.
As usual, 16-bit audio presents no problem to modern DAC or ADC hardware as it doesn't strain the limits; nonetheless I think a 16/44 signal is worth reporting on due to the vast amount of music in this bit-depth and sample rate.
Another nice measurement that can be done but not shown here is SpectraPLUS's THD+N vs. Frequency sweep which is automated by the software. I'll remember to show that when we measure the ADI-2 Pro FS's DAC output quality.
To end off this segment, here's the THD+N comparison using a -5dBFS 1kHz signal on the ADI-2 Pro FS compared to the Focusrite Forte. Distortion with a -5dBFS signal is probably more in line with actual music levels and the Oppo's +13dBu output level overwhelmed the Focusrite's maximum XLR input amount of +12dBu...
Because of the fact that the Forte cannot handle the full output level of the Oppo UDP-205, I'm about 1dB short of being able to achieve -5dB peak to reflect the Oppo's true output (without some kind of attenuator of course). In any event, the result from the RME ADI-2 Pro is about a 10dB improvement in SINAD over the Focusrite Forte; about the same amount of improvement in the THD(+N) between the two devices as well. Remember that because my ADC was set to 192kHz sample rate for the 96kHz bandwidth used in the calculations, some of the difference between the RME and Focusrite in THD+N is likely the ultrasonic noise shown in the comparison between the devices as per part I above.
V. Jitter ContributionsSpeaking of distortions contributed by the ADC, we can also consider whether the ADC could be adding some jitter into the J-Test.
We already know that the Oppo UDP-205 has excellent jitter performance especially over the asynchronous USB interface. Using the 1M-point FFT from SpectraPLUS again, this is what the 16-bit J-Test looks like with the Forte compared to the ADI-2 Pro FS (same input levels into both ADCs):
And here is the 24-bit J-Test using SpectraPLUS, again comparing the result from the Focusrite Forte and the RME ADI-2 Pro using the same analogue input signal from the Oppo UDP-205:
In both instances, we can easily see the superior lower noise floor achieved with the RME. We can also better appreciate that there are much fewer sidebands than the tracings obtained with the Focusrite Forte. Note that for the 16-bit J-Test, the sidebands are all below the level of the odd harmonic low-frequency LSB square wave. With the RME J-Test result, we can make out the presence of what looks like a Gaussian bandlimited random component ("skirt") at the base of the primary signal along with some low frequency periodic sidebands rather than a mass of sidebands with the Focusrite - especially noticeable with the 16-bit test.
Remember that jitter anomalies in the frequency domain are more evident at higher input frequencies. Over the years, to strain the system even more, I've used the 24-bit (24/48) J-Test accelerated to 96kHz. I've rarely shown this test with the Focusrite Forte because the noise floor around that primary signal of 28kHz isn't as clean as I would like. So, to drive home how much better the RME is, notice how this test is not a problem with the ADI-2 Pro FS!
Impressive how well controlled noise and potential jitter is with the Oppo UDP-205 using the USB input as shown with the RME. With the primary signal at -6dBFS, even the strongest sidebands (particularly proximal to the primary signal thus easily masked even if one could hear them!) are at -130dB. Interesting that with the primary signal at both 12kHz and 24kHz, the sidebands are of the same amplitude and general morphology. I've seen Amir on Audio Science Review comment that this finding in the Oppo UDP-205 is reflective of reference voltage modulation rather than actual jitter since jitter should significantly increase in amplitude with frequency. BTW, you can also compare what I have here with his results using the Audio Precision APx555 (remember, that's a >$20k machine; nope, don't think I'll need that :-). Secrets of Home Theater and High Fidelity likewise published some UDP-205 results with the AP2700-series device but with older Oppo firmware which likely had an effect on the output results.
Want to get really extreme with these J-Test measurement? How about we accelerate the 24-bit J-Test even further to 192kHz? Here's the analogue output of the Oppo fed by my Raspberry Pi 3 B+ "Touch":
Again, a beautifully clean noise floor from which the -6dBFS 48kHz primary signal rises! Notice that the graph includes +/-20kHz on either side to look for any significant symmetrical sidebands. Remember that there is a 1kHz 24th bit square wave buried under the noise floor. Needless to say, humans would not be able to hear such a high primary frequency... It's really just a "torture test" for the digital audio gear and as you can see, the Oppo DAC fed into the RME ADC results in an amazingly clean signal. It's also a reminder as to how well a modern asynchronous USB interface is able to maintain jitter-free performance these days even when fed by an inexpensive (<$40) Raspberry Pi 3 single board computer! Make sure to let that sink in when hearing talk about jitter-this-and-that with people claiming that multi-thousand-dollar server and player systems often without even a DAC component can "improve" audible jitter performance. [Remember to have a listen to the jitter simulation yourself and consider whether you really think jitter is even worth worrying about!]
VI. Conclusion...The RME ADI-2 Pro FS performed wonderfully as an ADC!
Comparing the ADC performance of the Focusrite Forte vs. RME ADI-2 Pro FS is important as a way to make sure the upgrade is going in the expected direction for the measurements I intend to use the hardware for. The ADI-2 Pro ADC is capable of very low noise performance that is unhindered by hum with a noise floor that remains stable over hours of operation. It is obviously capable of accurate conversion even of very low input levels such as demonstrated with the undithered -90dB 16th bit of a 16/44 signal.
Furthermore, using this ADC, I can reach down to around 20-bits of dynamic range as calculated in the RightMark tests; a nice improvement from the previous hardware with at least an extra half bit of resolution in the 20-20kHz audible spectrum. More importantly, the measured distortion levels are also significantly lower suggesting minimal distortion added by the ADC itself; realistically achieving a THD+N level better than -112dB operating at 192kHz using the stock power supply plugged into my usual power conditioner (Belkin PureAV PF60, see some measurements here using the old Gen1 hardware - maybe I'll look at this again with the better gear ;-) where I also plug in my audio system. It would be interesting trying the RME battery powered. The improvements shown in the J-Test over the Focusrite Forte likewise are substantial and suggests a significant upgrade in the accuracy of time-domain performance provided by the RME's ADC clock as well as freedom from noise intrusions. All this befitting of a professional level device.
At this point I haven't even started using higher sample rates like 384 and 768kHz for measurements but they're available. Unfortunately the SpectraPLUS-SC software currently cannot go beyond 192kHz... Hmmm, SpectraPLUS-SC developers, how about opening up the software all the way to 768kHz? As more ADCs are released with this extended sample rate, it would be nice to provide the option with a commensurate increase in the maximum FFT size to 4M points...
Despite the clear superiority of the RME, one convenient feature of the Focusrite Forte is that it can be completely USB bus-powered. As such, there might come the occasion where I would use the Forte simply for the convenience if/when I do some testing away from home for example.
With that, let's get going with using the "Generation 3" measurement system :-).
Next time, we'll start looking at the characteristics and quality of the RME ADI-2 Pro FS's own DAC output...
Autumn's here. Hope you're all enjoy the music!
Addendum - September 11, 2018:
I was reviewing the results and realized that for the SpectraPLUS data, although I had shown only the 20Hz - 20kHz FFTs, in fact the software was calculating THD(+N) and IMD for the full bandwidth that the ADC was operating at. Since I had the RME ADI-2 Pro FS and Focusrite Forte operating at 192kHz, the results, especially the THD+N would have incorporated the ultrasonic noise from the ADCs into the calculations up to the 96kHz bandwidth!
Typically when we see manufacturers publish THD+N specs, they're using a lower bandwidth like 22kHz and the number will be better looking than what I've got here (all else being equal like number of FFT bins used).
I'll do another post looking at the RME compared to Focusrite with more typical bandwidths while operating the ADC at 44.1kHz or 48kHz to correspond with the signals used to demonstrate and for comparison with what's typically seen in published results.