Okay guys and gals, a rare "mid-week" update on DSD testing, a "Part II" follow-up to the previous article.
As I mentioned as an update in that article, I have a suspicion that the Topping DX3 Pro might not be using "DSD Direct" in its playback. However, I do have the RME ADI-2 Pro FS R Black Edition here as well. As you might know, I've been using the RME line of ADC/DACs for awhile now. These are certainly some very well thought out and high performance units with a ton of customization options.
Two of these options are shown in the screen in the picture above - "DSD Direct" and "DSD Filter". The "DSD Direct" is either OFF/ON, and "DSD Filter" either 50kHz/150kHz and is only activated when "DSD Direct" is ON, in recognition of course that DSD noise shaping does create significant ultrasonic content that in general probably should be filtered out; how much filtering is up to you.
Here's a snip from the AKM AK4493 datasheet:
Notice the highlighted items referring to the "DSDD" (DSD Direct) bit which allows the DSD data to be sent either thru the "Normal Path" to the DATT (Digital Attenuator) which implements volume control through a multibit mechanism then back to a delta-sigma modulator to the "switched capacitor filter" (SCF) from which the analogue output is generated.
The alternate pathway is through the "DSD Direct" path which bypasses the internal DATT and delta-sigma modulator and sends the signal directly to the SCF. Wonderful that the RME ADI-2 allows us to try this out and hear the effect for ourselves, and run measurements as well.
Okay, let's get to it! I'll try not to type too much text and let the data speak for itself.
I. Effect of the "DSD Direct" and 50/150kHz filter in RightMark
As mentioned, there are 3 possible setting:
- A. DSD Direct ON + 150kHz Filter
- B. DSD Direct ON + 50kHz Filter
- C. DSD Direct OFF (50/150kHz switch greyed out)
A. First, let's just go wide open with "DSD Direct" ON and 150kHz filter, with 32/384 bandwidth test signal. This is "as direct" as one can get with these SoX-DSD converted test files on the RME:
Notice we have DSD64, DSD128, and DSD256 here. Notice the high frequency spikes in the SoX-DSD encoded signal for DSD64 and DSD128. For DSD64, don't expect accurate frequency response above 50kHz and for DSD128, stick with <100kHz.
At this high 384kHz samplerate and with high levels of DSD noise, I've found that the "Noise level" measurement and graphs can be inaccurate. Better to show the noise floor with the "Dynamic Range" plot which includes the -60dBFS 1kHz tone and aligns the noise floor appropriately.
As much as I would like to always use 384kHz, sometimes I run into issues and RightMark is much more stable at 96kHz. So if we run the test at various DSD rates using a 32/96 test signal, we get:
As you can see, I've included a 24/96 PCM measurement on the right. Obviously at 96kHz, the bandwidth is limited to 48kHz. Practically, this is probably more than good enough for human hearing although it would be nice to be aware of all that higher frequency content.
Note that with the sampling rate reduced to 96kHz, I can look at the "Noise level" graph, plotted accurately.
B. Time to switch the filter down to 50kHz... Here's what "DSD Direct" ON, filter at 50kHz measures like in 32/384:
In general, the numbers look better than the "DSD Direct" ON + 150kHz filter results above. Thanks to lower ultrasonic noise affecting the results either in the RightMark analysis or perhaps aliasing into the lower audio spectrum. Here are the graphs:
We can definitely see the effect of the filter as it smooths out the roll-off with DSD64 and DSD128 particularly, as well as in the shape of the ultrasonic "mountain" with DSD64 and DSD128. Again, I'm showing the "Dynamic Range" graph because the "Noise Floor" graph is inaccurate at 384kHz (likely as suggested by Bennet/Dtmer Hk in the comments link last time).
Here are the results at 96kHz and we can show the actual "Noise level" graph, comparing with 24/96 PCM.
Results look good and personally I would have no issues sticking with this 50kHz filter and sparing the amplifier from trying to reproduce too much ultrasonic content.
C. Finally, the last of the 3 options. "DSD Direct" OFF (50/150kHz filter option inactive):
We're in the "Normal Path" (multibit) processing now with volume control turned on - I've still got the DAC set at 100% volume for the measurements of course. Numerically, noise level looks good. And likewise low distortion.
Looks like the "Normal Path" has resulted in filtering of the DSD64 and DSD128 streams, similar in fact if not identical to the 50kHz filter above.
From an objective perspective at least as it applies to RightMark's measurement algorithm using the AK4493 DAC chip, the multibit "Normal Path" performs very well. It appears to have even marginally better noise level and lower distortion than when we turn on "DSD Direct". Of course, subjectively one might listen and come up with a different impression!
II. 1/10 Decade Multitone 32 and DSD
So what have we learned?
1. Regarding RightMark (I'm using version 6.4.5 Pro), let me just say it's a real pain measuring DSD at the 384kHz setting. There are unpredictable crashes, sometimes graphs don't plot out properly, and I suspect there's some kind of memory hole such that I have to close and restart once awhile otherwise it can hang or refuse to capture the data. At 384kHz, the "Noise Level" graph is also inaccurate so I've resorted to showing the "Dynamic Range" graph so the noise floor lines up properly with the -60dB tone.
Testing with SoX-DSD converted 96kHz signals is comparatively much more stable in RightMark whether in PCM or DSD. Arguably, so long as the results at 96kHz (48kHz bandwidth) look good, I don't think human ears will complain even though it would still be good to make sure the noise higher up isn't extreme.
Despite the bugs, RightMark is convenient as a simple, quick test battery. With a bit of experience, it's not hard to get consistent results. I've E-mailed the author with the hope of future updates and bug fixes but this probably will not be coming any time soon - 2018 was the last update! :-(
If anyone knows of an alternative, easily available software that can do what RightMark does, let me know.
2. The RME ADI-2 Pro FS line of devices (R Black Edition here) IMO remains a fantastic "audio toolbox" to explore features and tweaks like these DSD options. I know it takes some getting used to with the plethora of knobs, buttons, options and features with its heritage in the professional audio world. It's not hard once you get used to it and once you dial in the settings, there's probably no need to fool around too much.
As you can see, with the RME, there's the ability to dissociate "DSD Direct" and the ultrasonic filter (50/150kHz) to map out the effects. In the future, I can look at the results from other DACs and compare their graphs with the ones above to see if it looks like the device is passing along the DSD raw data or if there's low-pass filtering applied. "DSD Direct" playback using my SoX-DSD conversion of RightMark tests will result in those spikes in the 384kHz DSD64 and DSD128 frequency response graphs along with the noise patterns in the "Dynamic Range" / "Noise level" graphs.
If we now look back at the Topping DX3 Pro's performance from Part I:
Clearly, the DX3 Pro is not using "DSD Direct" + 150kHz filter given the smoothness of the DSD64 and DSD128 frequency response curves. At the very least, this is "DSD Direct" + 50kHz filtering, or likely the "Normal Path" ("DSD Direct" OFF). I'm still quite impressed by this little DAC's ability to maintain low noise, high dynamic range, low distortion as an unbalanced output device! I suspect the dual-AK4493 architecture has a large part to play in this level of performance. This DAC configuration paired with balanced outputs could be very impressive (did Topping ever made such a version?). Alas, AKM DACs are getting rarer these days on account of their factory fire in October 2020.