Direct Digital Amplifiers

12-11-12: Willland
I am not sure if this is exactly what you are referring to ....

technically, a "direct digital amplifier" is a class-D amplifier that can accept, at its inputs, a digital pulse code modulated (PCM) data stream. Accepting a (digital) PCM data stream (which is the digital music stream) rather than accepting an analog version of the music stream is the real innovation of "direct digital amplifiers". Hence the "direct" in the name - you can directly connect the digital output of your computer or CD transport to this sort of amplifier & it will drive your loudspeakers. Internal to the direct digital amplifier, the PCM data stream is kept digital all the way & it is finally converted to analog at the output of the class-D output stage by the analog reconstruction filter/low-pass filter just before it drives the loudspeaker. The amplifier is still a class-D amplifier.
Today's class-D amplifiers accept an analog version (digital data converted to analog with an on-board or out-board DAC) of the music stream. The class-D amp converts this to a digital pulse wave modulated (PWM) signal & this is once again converted to analog by the reconstruction filter/low-pass filter just before it drives the loudspeaker.
Direct digital amplifiers do away with the initial conversion of the digital music stream to analog.
And, yes, like OP indicated, there are few direct digital amplifier products in the market today.
Hope that this helps some.

Bombaywalla, Class D amp doesn't convert anything to digital. It converts analog voltage to analog duty cycle of square wave and back to analog voltage thru filtering....

you are right, Kijanki. I was trying to be simplistic in the interest of explaining the concept.
OTOH, Kijanki, a 2-level signal which is going from logic0 to logic1 could be considered a (very simple) digital (or binary) signal. Such a signal is both analog & digital. It has infinite resolution like an analog signal & at the same time it is discrete since it transitions from logic0<-->logic1 with no other state in between.
So, I do not think that I confused anything here. Both your comments & mine are correct.
But, yes, it's not a tradition digital signal like the one that is quantized with finite # of bits.

Bombaywalla, Adjusted analog quantity is duty cycle. For instance, when you adjust oscillator's frequency by turning knob it doesn't matter if output signal is square, triangle or sinewave - your adjustment is still analog. The fact that voltage jumps between two level's doesn't make it digital if adjusted quantity is duty cycle. Also, amplifier doesn't make any use of these levels other than converting to output voltage by taking average value (filtering).

what you wrote about osc freq adjustment being analog is correct but.........
when it comes to a class-D amplifier, it is well-known & well-documented that this amplifier is a discrete-time system. You cannot analyze a class-D amplifier using pure analog techniques. The switcher portion of the class-D amplifier is treated as a digital/discrete-time system & discrete-time + analog analysis techniques are jointly used to analyze the entire system.
Pick up any book on SMPS & you will see what I mean. Basics of DSP that you would learn from a text like Rabiner & Gold are used in the analysis of class-D/SMPS systems.
Even tho' the switcher's main function is PWM (which might be a analog quantity of duty cycle variation), this part of the class-D amplifier IS digital. Make no mistake about it. What I'm gathering from your comments is that I do not think that you have ever analyzed or designed a SMPS or class-D amplifier hence you are so vehement with your comments.
Open up a SMPS text book & read a bit of the analysis of the system. You'll quickly find out that it's a discrete-time system - part digital & part analog.

Distorted electric guitar (square wave) dosn't make it digital. FM radio is not digital in spite of voltage moving between two levels.

these were bad examples. Of course, a distorted electric guitar wave is not digital 'cuz there are stable operating points at the flat tops, flat bottoms & all points connecting the flat top to the flat bottom. This is way different from the class-D switcher output that is stable at only 2 points: logic 1 or logic 0 & nothing in between.
And, FM is phase modulation - a very analog concept. Today it is digitized in HD radio for more noise immune over-the-air transmission. So, while the over-the-air transmission is digital or discrete-time, FM radio is still analog.
we understand this.......