TIME-DOMAIN TRANSIENT RESPONSE TESTS OF ADI-2 DAC FS & BIFROST MB DAC’s
[Edit/]
So I've learned a ton since I first posted this, less than a week ago, so I am adding edits to this original post (but preserving all the text, not removing anything) so that my errors don't mislead anyone or cause people to draw false conclusions. The main thing this first post does is capture the DSP filter response of the RME ADI-2 DAC FS, with the filter set to "SD Sharp", and the Schiit Bifrost Multibit. Later in this thread I capture the other filters.[/Edit]
Recently, I decided to take my work headphone listening setup to the next level, and get better headphones and a DAC and an amp (or a DAC / Amp combo unit) to go with them, so I started digging into head-fi and online reviews and websites again, wondering excitedly what new stuff from my favorite (or exciting new) companies had come along while I had been listening contentedly to my Sonos PORT +TIDAL lossless --> Bifrost Multibit (Gen 1) --> Asgard 2 --> Audeze LCD-2 set up.
Of course, one of the first things I did was go to Schiit’s website to see what was on offer, and was thrilled to see a Bifrost 2 with an ever better multibit implementation!
Because, I have to say, of all my hi-fi audio purchases, the Bifrost Multibit was one of the best and most important. Finally, finally, I was experiencing the kind of audio bliss I had been seeking, and even from just my Senn 595’s. To my ears, there was such a lovely detail and air and cleanness to the music, without any of the edge and digital glare I had been experiencing, to one extent or another, for years. I had sometimes heard that the DAC really didn’t make much difference and that if you thought you were hearing significant differences that you were fooling yourself (and the same for amps, provided they had enough power to avoid clipping), but after years of improving my headphones, and my sound-file source quality, and trying higher powered amplifiers, and not really feeling like there was much of a difference, I was ready to take advantage of Schiit’s 15 day return policy and give a high(er) end DAC a try to see if that was the missing piece of the puzzle in my quest.
And, well, it was. Was it ever! My Bifrost 1 Multibit was a revelation to me. I was so happy with it!
So, I was pretty excited about Bifrost 2! And I figured I’d go balanced and get the Jotunheim 2 (all to go with the Audeze LCD-XC 2021 carbon’s I had on order). Woo hoo! Yea!
While I was waiting for my Schiit order to get fulfilled, I got the LCD-XC’s, and while I did love their sound on the whole, I personally found it a bit too hot in parts of the treble to be as non-fatiguing as I would like. So then I was also thinking about how to get some parametric EQ’ing into my setup as well, as I do love the LCD-XC’s and felt they just needed a couple tweaks.
And in the process of researching all this, I ran across the Audio Science Reviews of the Schiit Multibit DAC’s, and they gave me more than a little bit of pause! How could the Bifrost Multibit measure so badly! What was going on? Had I just not ever heard a truly great DAC? Or, I don't know, something?
So I dug deeper, and ran across the RME ADI-2 DAC FS, which did measure very, very well, and, it had not only a very highly regarded DAC section, but also, a very powerful, low distortion headphone amplifier,
and a built in 5 band parametric EQ to boot! All for the same price as Bifrost 2 + Jotunheim 2. Plus, I could get it in just a couple days, instead of 6-8 weeks. So I cancelled my Schiit order, and pulled the trigger on the RME ADI-2 DAC FS.
I excitedly read the quick start section of the truly awesome manual, hooked it up, and started listening and playing with the EQ. Immediately I felt it was pretty good, at the very least, and I actually didn’t mind the interface, and I loved all the customizability. I spent a number of hours each day listening to music through it, into the LCD-XC’s, as well as some of my other headphones (but mostly the LCD-XC’s).
And . . .
For me--to my ears--it just wasn’t as good as the Bifrost. It was good, no doubt. Just not
as good. For me.
Even with EQ helping things out I still preferred the listening experience through my Schiit stack. (And no, it wasn’t due to volume differences, I don't think.)
To my ears, the ADI-2 just didn’t sound as good. It was more fatiguing, and less enjoyable, less realistic on acoustic music, flatter, and astonishingly, somehow also less detailed?!? How could that be?!? It measured
so much better than the Bifost! What was going on?
I started with the assumption that the ASR measurements just weren’t telling the whole story and that my ears were picking up on the rest of the story, so to speak. And quickly from there, I realized, that, indeed it was possible that there was more to the story. Those reviews were frequency domain measurements almost entirely. I mean, yes, OK, jitter, but what about transients? What about, say, a square wave? I felt that there was maybe something to this—or rather that it was a good place to start to investigate things at least. It was significant to me that I found the differences between these two DAC’s most apparent on acoustic music.
So, start simple. Use a tone generator program to make digitally perfect square, saw, pulse, and triangle waves and feed them into both DAC’s to see if there would be differences, and to look not on a spectrum analyzer, where you see frequency vs dB, but on an oscilloscope where you see voltage vs
time. Maybe then I could start to see the differences that I thought I was hearing? I felt it was probably a long shot, but I still wanted to do the work to find out.
But honestly, I really didn’t expect to see a lot of difference. Not anything close to the differences I did find. And I didn’t expect the ADI-2 to deviate as much as it does from the ideal of the input wave.
[Edit] The ADI-2's Short-Delay Sharp filter is what is called a minimum-phase filter, which can be thought of as a minimum-delay filter. It does distort wave-shapes that go into it, but, it's main advantages are two-fold: it has no pre-ringing ahead of an impulse or step, and it decays faster than a linear-phase filter (which does a better job of preserving wave-shape, but does have pre-ringing). The ADI-2 has many other filters, however, and if you want, you can change them--see follow up posts for more information[/Edit]. I couldn’t believe it when I first saw the ADI-2’s output, of a square wave input, on my oscilloscope! I thought for sure there was something wrong! That I was over-driving it or that the 1Mega-Ohm impedance of the scope was the problem. I had fed it a -6dbFS (6 decibels down from digital full-scale) square wave, at 96kHz sample rate, which I had thought was enough headroom, but I immediately changed that to -12dbFS. No change. Then I consulted with one of the other engineers here at Sonos, who is one of the people who designs the amplifier sections of our players, and had been involved with the line-out section of PORT. He suggested a 10kOhm load instead of going directly into the scope.
So, I changed my setup and soldered 10kOhm 1/4 watt resistors across the ends of the left and right RCA cables, and used a 1GHz, 1MegaOhm, 1pF N2795A Keysight active differential probe (so as to avoid any potential for ground loops), to measure the voltage across the resistor into the Keysight DSO404A 4GHz 20GSa/s oscilloscope.
But the results didn't change.
Apparently what I was seeing from the ADI-2 wasn’t due to overdriving or impedance mismatching or clipping. I now suspect that it isn't a
bug. I suspect it is part of a feature. I suspect that the AKM DAC chip is limiting the impulse response due to some kind of Finite Impulse Response filter (like the Parks-MacClellan filter, perhaps). But I’m getting ahead of myself. Backing up . . .
Here is what I saw, here is what I was getting from it:
To me, this looks like fairly bad amplifier ringing! (Here’s an example of an amplifier ringing like that, for reference:
PS Audio Transients 2
I increased the sample rate to 192kHz, and things got a little bit better, but not very much better:
Now I was
really curious to see how the Bifrost Multibit would fare! Would it be just as bad? Marginally better? Worse?
Turns out, it was definitely better!
[Edit/] Not actually better or worse, necessarily, just different than ADI-2's SD Sharp. But if you change the ADI-2's filter to Sharp, or SD LD you get something pretty close to the Bifrost[/Edit]
And let’s just compare that against the ADI-2 with the same voltage / divisions scale on the scope to make things fair:
The ringing on the ADI is so large it goes off scale. The ringing is about 40 percent of the entire amplitude of the square wave. And note how it is not symmetrical. It is not reversible in time, like the original signal that went in.
Not so with the Bifrost. It maintains very good time-symmetry.
[Edit/]This is just a consequence of the linear-phase nature of the filter used in the Bifrost, and the ADI-2 can use a similar filter.[/Edit] The signal could be reversed in time and be pretty much the same. There are slight differences, but this is to be expected because we are asking the output stage to have such a large skew (voltage / time) rate and then stop on a dime.
The Bifrost also maintains better frequency domain fidelity to the input signal than the ADI-2. How do I know that without showing a spectrum analyzer output (or FFT analysis—which I plan on doing, btw)?
Well, what we are seeing with the ADI-2 DAC, is definitely not the Fourier components that fall within the 1/2 sample-rate bandwith (or even just the audio bandwith), leaving out all of the higher-order ones. A square wave has only odd harmonics of the sine-wave function. So we would have 1kHz, 3kHz, 5, kHz, 7kHz, 9kHz, 11kHz, 13kHz, 15kHz, 17kHz, and 19kHz, at least. That is 10 partials, which should make a wave that looks very much like the Bifrost’s output, and not very much like the ADI-2’s. If you check out this
Wolfram Mathworld link, you can see a square wave get approximated by more and more partials, shown in different colors on the graph. There are only five partials shown, so 10 would get you a lot closer to an ideal square wave than what is show in the last sum of partials here, but you can already see where it is going, and it’s not going towards the ADI-2’s output. It’s going towards the Bifrost’s.
So, if the Bifrost Multibit is this good at 96kHz, what about at 192kHz? See for yourself:
Look at that thing. Other than a little bit of overshoot at the leading edge and a finite slope on the rising and falling traces, it’s nearly a perfect square wave!
What about going down in sample rate? Well, here are images of ADI-2 and Bifrost at 48kHz and 44.1kHz:
Again we see the same differences. With the Bifrost, there is a lot less flatness on top and bottom, but it’s still doing a pretty good job of approximating a square wave. The ADI-2, on the other hand, still looks like a badly ringing amplifier stage that was fed a square wave, and the ringing lasts longer, and is at a lower frequency.
So, let’s up the ante here and push things even more. Let’s do a 12kHz square wave at 96kHz sample rate into both and see what happens. That’s only 8 samples per entire waveform. Not much to go on! How well will the reconstruction filters work on both DAC’s? Let’s see:
What is going on with the ADI-2 now? That’s not even symmetrical about ground! It’s not even symmetrical flipped around it’s midline! It’s off-scale on the bottom and not on top, and looks very different bottom vs top.
The Bifrost, on the other hand, seems to me to be doing a very good job of intelligently filling in the gaps between the 8 points it was given. It comes up quickly, goes through the first one at the top (and keeps going a bit beyond), then turns around and heads down through the 2nd one at the top, up through the 3rd, then down through the fourth, all the way to the first point (fifth total) on the bottom, and similarly up and down through the others, finishing off by going up through the final fourth bottom (eighth total) point, and starting the waveform again.
This is what Schiit means, I think, when they say that the original samples are “retained”. The reconstruction filter works around and through those points to do the best interpolation it can do, optimizing not just for the frequency domain but also for the time domain.
And certainly the ADI-2 is not preserving the original samples here. If it were, it wouldn’t be so asymmetrical. Here it is again with the whole waveform on the screen:
I honestly have no idea what it’s doing here or why. Perhaps someone who has studied all the various filters out there can tell us? But whatever it’s doing, it’s not being faithful to the original 8 samples per waveform it was fed.
Let’s relax things just a little bit and see what happens with a 1kHz pulse waveform at 10 percent duty cycle. So, basically a 10kHz-width pulse up, baseline, 10kHz width pulse down, back to baseline, repeating 1,000 times per second:
So, thankfully, the ADI-2 is doing something sane at least, even if there is the same ringing we saw before, but again, the Bifrost is being more faithful to the signal that went in. There's not much ringing, there is more evenness on top of the pulses, and tighter control.
OK, enough of things that are square. Let’s move on to something a lot easier, we hope, for a DAC to handle. A triangle wave:
Both DAC’s are doing a nice job here, but you can see that the ADI-2 looks cleaner. And well, it is. This DAC has less noise than the instrumentation setup I was using! It’s a stellar piece of engineering, and a lot of thought went into its design and build. Here is the inherent noise of the ADI-2 as seen when connected directly to the oscilloscope:
It’s like a mV or so, and that is probably just due to noise pickup on the RCA cables. Awesome performance! How does the Bifrost fare on this? Well, not great. You can already see from the width of the line on the triangle that it has more noise, but this really shows you how much:
About 5-10 times more noise. And this is into the 1Meg load of the oscilloscope. Things are worse against 10kOhm, and when trying to reproduce an actual signal. Here is both DAC’s fed a -48dbFS 1kHz sine wave:
Clean and clear showing by the ADI-2, but look at all that noise in the Bifrost! You can still clearly see (and hear) a 1kHz sine wave, but there’s a lot of noise around it. Not great. But clearly not the whole story!
So, again, this is another reason why I suspect that the performance of the ADI-2 against square waves and pulses is due to the design of the whole digital-to-analog conversion system and not due to some failing of the ability of the components or circuits. This is a fabulous piece of kit! So I suspect, honestly, that it is baked into the sigma-delta AKM chip, and couldn’t be taken out even if RME tried.
[Edit/] THIS IS TOTALLY WRONG! Not only could RME take this out, they did--you can take it out with the turn of a knob, even to the point of almost entirely taking out the output reconstruction filter, if you want (called "NOS"). Apologies for my ignorance of basic DSP![/Edit]. I doubt that Schiit would go to all the trouble to take a DAC chip that is not meant for audio at all and design all the support circuits and stages around it to make it fulfil that role if they could have just used an off-the-shelf AKM chip!
In any case, let’s get back to transient response testing. How about a sawtooth wave? Let’s see how the two DAC’s will fare!
Ouch! Again, I have to ask, what is going on with the ADI-2?
The midpoint is offset by almost four tenths of a volt, and it’s definitely not symmetrical and has got significant ringing going on. Bifrost, by contrast, has significantly less ringing, is symmetrical, and is centered on 0 volts across the RCA outputs. Pretty darn good showing! Even better than with the square wave, I would say.
Finally, let’s make sure that both DAC’s give us a sine wave output when fed a square wave at 1/4 the sample rate frequency. So both here are at 48kHz sample rate, and are being fed a 12kHz “square” wave.
Ignore the voltage scale, I took one capture at -6dbFS and the other at -12dbFS and I had the volume turned down on the ADI-2—this was early on, and was a sanity check—but what you can see is that both are giving us sine waves despite being fed a square wave.
One way to explain that is to go back to the Fourier analysis and look at the partials, and see that 12kHz is the fundamental so the first overtone is 36kHz, which is greater than 1/2 the sample rate (thus would violate the Nyquist criteria and be folded back into the spectrum at a lower frequency than the fundmental), and so it is truncated as are all higher order harmonics.
But, this is kind of a misguided way to look at this, honestly. The DAC does not know what kind of wave was behind the samples that it is getting! It only sees the samples.
And when you have only
four samples per waveform, what kind of information do you have, really? With that little information, you can’t differentiate between a square, a sine, or a triangle wave! So the best thing for the DAC to do is to reconstruct a sine wave, so as not to introduce any nasty higher order harmonics that weren’t there in the original music. Better to leave out something, than to create something that was never there.
This is the whole reason for the reconstruction / interpolation filter of a DAC (well, that and quantization noise). It’s because, honestly, our sample rates are probably too low! It’s easy to claim that our hearing kind of sucks up that high and there’s no loss of information, but I do not believe that that is a proven fact, for a number of reasons.
But either way you fall on that topic, right now, for better or worse, we’re stuck with most of our music being at 44.1 or 48 or, if we’re lucky, 96kHz, so we need our DAC’s to be absolutely as faithful to the samples that we
do have as possible, while at the same time intelligently interpolating between them so as not to introduce terrible sounding and unmusical harmonics.
And from what I've seen so far, it is my opinion that the Schiit combined time-and-frequency domain filter seems to be doing a really great job at this. And that, I suspect, is the reason why the Bifrost Multibit sounds so good to my ears. Yes, the THD is worse than the ADI-2. Yes, the jitter is worse. Yes, the noise floor is worse. But all that is fairly picayune compared to what is better, to what it is getting
right, which is huge.
[Edit/]The Schiit proprietary filter seems to be doing pretty much what a linear phase filter does, and what the ADI-2 "sharp" filter does, so at this point I can't say that it is special nor that it is doing a better job than the ADI-2 with the sharp filter in place. I'm in the process of listening to the ADI-2 with the other filters, and will update this first post, but at the moment, I think I do like it better with "sharp" instead of "SD Sharp". Not sure how it will stack up, for me personally, against the Bifrost Multibit, but I will continue to listen[/Edit]
OK, well I have a lot more to do around this whole subject, and I plan on measuring some more DAC’s, and on learning more about DAC’s and giving all this a lot more thought, but I was excited to share what I have so far.
And, excited to learn. If this has already been done; if anyone can shed more light on this; if anyone can suggest next steps to take; please do chime in! Constructive feedback is welcome! And I haven’t spent much time (or really any time) here over the past five years or so, and I didn’t even do a lot of searching (sorry) before posting this thread. So if I’m missing stuff, please pardon my ignorance.
OK, I will leave it here for now. I hope some readers find this interesting! Cheers!
