An audio breakthrough

It would appear that there is a particular audiophile DAC with a cult following that gets rave reviews and costs over $2000, and is based on a non-audio DAC chip.

Why would they do that? Well, I think it is so they can run it “NOS” (not New Old Stock, but “non-oversampled”) and add their own “proprietary” filtering – plus it’s different from what the hoi polloi uses so it must be better. But, it would appear that someone has found a glitch, literally.

I am no expert, but I think that because this chip is a non-audio DAC, the output comes directly from a R-2R ladder, or similar. Small capacitive charges are transferred whenever the ladder switches operate, and sometimes the switches don’t all operate at the same speed. This means there is a glitch at the output whenever the DAC value changes, and it is worst when all the switches operate simultaneously i.e. when the most significant bit changes – around the mid range in other words (hmm…). Presumably there are other significant glitches at multiples of 1/4 full scale and 1/8 full scale too.

Low pass filtering the output can reduce the amplitude of the glitch at the expense of increasing the settling time. There are better techniques using a further piece of circuitry (a sample-and-hold) but, apparently, for the designers this was regarded as unacceptable for some reason (why?), and at audio frequencies still wouldn’t be as good as a typical $1 audio DAC in a mobile phone.

The evidence is all in the DAC chip’s data sheet:

I don’t know whether the glitch energy scales with the the VREF (i.e. the full scale signal amplitude), but this glitch is huge compared to the smallest signals that we might generate with the DAC.

An owner of this product now thinks he is hearing a certain harshness in the sound, and seems to have found that when reproducing a sine wave at -90dBFS, the output of the $2000 DAC contains significant glitches at the zero crossings. It would be interesting to know if there are detectable glitches at 1/4 and 1/8 full scale, too. This could be the phenomenon shown in the data sheet, or a by-product of whatever mechanism is being used, unsuccessfully, to suppress the glitches – they are rumoured to be using a combination of two DAC chips. Scrutiny of other reviews and measurements of the device seems to reveal distortion and noise figures that suggest something strange is going on – apparently.

An aspect of integrated circuit DACs is that because they are very small and constructed on a single chip, they have fantastic performance relative to themselves i.e. they remain monotonic and linear at all times. However, their absolute gain and offset may drift slightly with temperature. These temperature coefficients vary from chip to chip and can even be positive for one chip and negative for another (this appears to be the case for this particular DAC chip according to the data sheet). This means that any attempt to blend the outputs of two DAC chips externally using a combination of scaling, offsetting, inverting, mixing and interleaving would be most unlikely to succeed down at the lowest levels.

If these suppositions are correct, then this product is a great example of where the basic engineering of a basic product appears to have been sacrificed in the interests of just making something ‘different’ and supposedly ‘simpler’ – although as usual it ends up being more complicated.

[Last edited 04/05/16]

Later… series 48

Jools Holland’s programme is back on BBC2 (series 48!).

This week I was very impressed with Laura Mvula’s song Overcome which featured a beautiful descending harmony from the three backing singers.

(I saw Ms Mvula presenting a programme on Nina Simone recently. In the programme she showed that she is a ‘proper’ musician who can play classical music on the piano. I am always impressed by that).

I also enjoyed The Coral’s Miss Fortune (very Echo and the Bunnymen-esque vocal..?)

Vinyl just “for decor”

Maybe you suspected it all along, but an article in the Daily Telegraph suggests that 48% of vinyl albums haven’t been played one month after purchase, and that 7% of vinyl purchasers don’t even own a turntable.

Student Jordan Katende told the BBC: “I have vinyls in my room but it’s more for decor. I don’t actually play them. It gives me the old-school vibe. That’s what vinyl’s all about.”

The Subjectivist/Objectivist Synthesis (Audiostream’s latest article)

Audiostream has posted a new article on the Objectivist/”Subjectivist” debate.

(I have to be careful to put inverted commas around the word “subjectivist” because, as an earlier commenter pointed out quite rightly, “subjectivists” rarely justify their name; in reality they are usually claiming that their subjective experiences are a superior form of objective measurement).

In summary, the new article says “Hey, everyone’s views are valid. Let’s not too get dogmatic, and just have a beer”. I think this misses the point (or several points). For me, the biggest ones are these:

• Measurements are always incomplete, and are just part of the picture; this seems to be misunderstood by many people, who think that there is a supernatural mystery as to why supposedly good measurements don’t always correlate with good sound. I don’t think there is a mystery.
• Subjective experiences are always affected by spurious factors; if the measurements of what comes out of the speakers show substantial deviation from the signal (i.e. are ‘bad’), this, for me, trumps the subjective opinion. But as mentioned above, if the measurements are ‘good’ they may still be incomplete.
• Let’s only start worrying about the above when we’ve designed the system to avoid the obvious errors as best we can. This is neither objectivism, nor “subjectivism”, but rationalism. For reasons of tradition and ‘folk intuition’, this may be rare in the world of hi-fi.
• I would be only too keen to hear people’s subjective experiences if they are referring to something measurably out of the ordinary and arguably good; less so if they are describing differences between digital cables that, rationally, cannot be affecting the sound. Some people do it all within the same article!

What should we be listening for?

As you may have seen, I built my own audio system because I had never before heard a system which took the seemingly obvious steps of using large sealed woofers, time alignment, DSP crossovers, driver correction etc., and I wanted to know what it sounded like. I also wanted to experiment with various aspects of crossover design (although I ended up doing less of this than I expected), and to understand what is important versus what is myth. Maybe DEQX, Kii, B&O or Meridian could have sold me something that sounded good, but it would have been an expensive black box that I couldn’t tinker with.

What have I learned from listening to my DIY system? I think the following:

There is a superficial ‘hi-fi’ sound that is achieved by many conventional systems – and I have owned some of these systems. The frequency response is balanced. Harmonic distortion is low – it sounds ‘clean’ at moderate volumes – it has bass and top end in seemingly generous amounts. It is certainly ‘stereo’ as you can clearly hear different things coming from the left, right and middle. If you start to turn up the volume, it does begin to sound somewhat ‘loud’ and ragged, and it can also feel as though the sound is being extruded through an opening that’s slightly too small. After a long listening session at high volumes your ears feel quite ‘sore’, but you are reasonably satisfied with the sound. This, presumably, is how recordings must really sound – you certainly can’t put your finger on anything that isn’t a reasonable facsimile of what it is supposed to be.

And then there are other, more specialised systems which cost much more to buy, and take us into the realms of audiophilia. They’re often impressive to look at, but I think they can sometimes sacrifice “high fidelity” in order to indulge their creator’s interest in a particular material or ‘retro’ technology. You may disagree.

There should be the possibility of a system that just implements the obvious pragmatic steps necessary to get the recorded waveform out of the speakers reasonably accurately (however we define that). There aren’t many of these about, it seems to me. I have heard only one such system – the one I put together using cheap off-the-shelf parts and DSP ‘glue’ – and I find its sound to be different and, if I may say so, better than conventional systems. Here’s what I think it sounds like:

The first thing that strikes you is that although it is ‘clean’, ‘sweeter’ and less ‘edgy’ than the conventional system, it also has ‘flavour’ and ‘body’ – it sounds just like real music. If it’s a double bass being plucked, you hear the fingers releasing the string, and the sound hits you in the chest; if it’s a hi-hat cymbal being struck, you hear the stick meeting the cymbal in delicate detail; if the sound is being made by something heavy and solid, or wooden and hollow, you picture the object making the sound. A part of the dynamism of the sound is how quickly it stops, as well as how quickly it starts. Bass is ‘real’ and not just a ‘rumble generator’; there is no arbitrary limit on how low the bass can go – not that you analyse the sound in those terms. It is simply ‘real’.

Next, you notice the imaging, the clear separation of the instruments, and the acoustics. The person singing is in front of you, located at some position in space between or behind the speakers – you feel as if you could reach forward and touch them. If it’s a live acoustic recording, they are in an acoustic space, and you are there too. There is separation between the singer and other instruments spread around the space – if that is how they were recorded. In a studio recording, maybe the vocalist is in a smaller space than your listening room, and you picture them close to the mic in a booth, perhaps, singing towards you. Or if the sound is coming from within a cool, stone cathedral, you picture the cathedral extending beyond the walls of your room. This is definitely the ‘party trick’ of stereo – a compelling, coherent acoustic space that appears as if by magic in thin air.

Then, you realise that you can listen to the recording at its intended volume. The ‘natural’ volume setting for each recording is usually quite apparent, but the system doesn’t mind what volume you set it at. The copious, dynamic bass means that you are not tempted to turn up the level excessively to compensate for a missing part of the spectrum. Your ears don’t ring afterwards, and even after listening for long periods at what would normally be considered high volume, they don’t feel sore. And there is a physical element to loud, natural, dynamic music that generates an excitement all its own.

It doesn’t need training or experience to appreciate these aspects of the sound, but at the same time there is just so much more to hear in the recording. You become more engaged with it; involvement rather than passively observing a superficially pleasant sound wafting over you.

Thinking about it some more, it is obvious that what I am describing is the sound of the recording, not the system. Some would describe this as a “neutral” system, but the mistake would then be to say that the resulting sound is neutral; I think recordings are astonishing if only we get to hear them without an intervening interpretation.