Friday 27 February 2015

DSD vs Class-D

There was a post the other day on the Audio Review web site by Brent Butterworth.  In it, he laments how on one hand audiophiles are falling over themselves swooning over DSD, while at the same time Class-D amplifiers receive short shrift.  After all, he tells us, DSD and Class-D are the same thing.  Reading between the lines, this is all some kind of giant marketing faux-pas.

Well, sorry, but DSD and Class-D are most definitely not the same thing.  In his post he suggests “Put bigger transistors, a bigger power supply, and larger filter components on the end of a DSD DAC …. and what have you got?  A Class-D Amp.”.  Well, if life really were as simple as that, Class-D amplifiers might indeed sound great - at least better than they do for now.  But it is not at all the same thing.  Let me explain why.

I have already explained elsewhere how DSD DACs function, and what they have to do to sound as good as they do.  But, in order that this post can stand on its own two feet I am going to go over the key points once more.

DSD is a 1-bit binary bitstream running at a frequency of 2.8224MHz.  In order to convert it to analog, all we have to do is pass the bitstream itself directly through an analog low-pass filter, and the result is music.  It really is as simple as that.  Except, that is, if you want the ultimate in sound quality.  In that case, what you find is that the filters required for that task are not as benign as you might like.  They tend to be similar to the anti-aliasing filters required to be inserted into the signal path prior to 16/44.1 PCM encoding, and tend to endow the sound quality with many of the same characteristics.  To get around that, virtually all DSD DACs up-convert the incoming DSD to a ‘variant format’ of DSD with a higher sample rate, and quite often with an increased bit depth of 3-5 bits.  It is that ‘variant format’ of DSD which is passed into the low-pass filter during analog reconstruction.

With the ‘variant format’ of DSD, we can specify a filter whose characteristics are not so aggressive, and which has, as a result, better sound quality.  It was always so, even with the first SACD players introduced to the market some 15 years ago.

But what does it mean in practice, to pass a digital bitstream through an analog filter?  This is easiest to describe when we confine ourselves to a pure 1-bit bitstream.  The waveform is a pseudo-square wave, which is either at its maximum when the bitstream reads ‘1’, or at its minimum when the bitstream reads ‘0’.  In a DAC chip, those maxima are of the order of +1 Volt and the minima of -1 Volt.  So in a DSD DAC we would be generating a pseudo-square wave whose voltage varies rapidly between plus and minus one Volt.

The differences between a DAC and a Power Amplifier are twofold.  A DAC is a low-power device whose output is of the order of a few hundred milliwatts, with a maximum swing voltage of the order of a volt.  By contrast, a Power Amplifier is a high power device, whose output is of the order of hundreds of Watts, with a maximum voltage swing of many tens of volts.  This places some significant demands on the circuit whose job it is to feed the appropriate pseudo-square wave into the analog reconstruction filter.  Instead of switching the voltage between plus and minus one volt it now has to switch between plus and minus a hundred volts (give or take).  And instead of those voltages carrying a few hundred milliamps, they must now carry a few Amperes of current.

What happens when you start switching a 100-volt line carrying several amps of current on and off at a frequency of several MHz?  The answer is that you generate massive quantities of radio-frequency energy.  In fact, the chances are good that you will jam everybody’s radio for a distance of several blocks.  That can get you into a lot of trouble.  But even if you could fix that particular problem - which you can, at some cost - you still require some pretty sophisticated (read expensive) components to switch that kind of signal at that kind of frequency.  Frequency is the big problem here.  The higher the frequency, the worse the problem gets.  So to answer Brent Butterworth’s question “Put bigger transistors, a bigger power supply, and larger filter components on the end of a DSD DAC …. and what have you got?”  The answer is: “A radio station.”.

So how does a Class-D amplifier work, then?  The answer is that they grab the bull by the horns and instead of moving the frequency UP, they move it DOWN.  Using a sigma-delta modulator a Class-D amplifier remodulates the incoming signal to a lower sample rate than that of DSD, but to preserve the integrity of the signal it uses a bit depth of more than 1-bit.  I don’t want to get too technical at this point, but how it does that takes it into a different realm.  It increases the effective bit depth not by using it to encode the intensity of the pulse in the waveform, but instead the width of the pulse.  In effect it encodes the output of the sigma-data modulator not in with “Pulse Density Modulation” representation (which is what DSD is) but with a “Pulse Width Modulation” (PWM) representation.

By using this PWM approach at a much lower frequency, usually in the high hundreds of kHz, it means that the switching can be accomplished using affordable components, and we have less of a problem with RF emissions.  But we still have the analog filtration to do, and this remains an area of concern for ultimate sound quality.  The other issue is whether or not the PWM switching can maintain the linearity and distortion performance necessary for high-end audio applications, while still delivering the amount of current that loudspeakers typically consume.  It is this area which is ripest for exploiting with intelligent and innovative engineering solutions.

Today, Class-D amplifier technology is making serious in-roads.  For non-audiophile applications they are beginning to rule the roost.  Even in the audiophile sphere, a number of quality Class-D amplifiers are now on the market, and while there are notable exceptions which do deliver seriously impressive sound quality (I’m looking at you, Devialet!), they remain largely confined to the low-to-mid range price/performance tier.  But note that even for serious audiophile applications, Class-D amplifiers have totally revolutionized powered subwoofer technology.  All things considered, at the present rate of progress, I can see Class-D dominating even the high-end market before too long.  But not for the reasons Butterworth suggests.

All that said, we’re not there yet.  And don’t forget that, even today, the market is still very much alive with vacuum tube power amplifiers.