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Old 8th January 2004, 04:39 AM   #1
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Default class D - digital qualities?

from a debate on another forum about "digital amplifers".

i say the output stage has the qualities of a digital system - the transistors ideally are acting a switches. i realize the input to a class D amplifer is an analog signal, and the ouput is an analog signal, but there seems to be a A/D and D/A conversion.

the other argument is that class D isn't digital becasue switching doesn't make it a digital system.

a side argument is that binary systems may or may not be digital systems. and that PWM is not digital.

any help?
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Old 8th January 2004, 07:00 AM   #2
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Good point Chris,

“Digital” implies “quantisized”. That means that only a number of fixed signal levels exist. There is nothing between that fixed levels. With an analog-in -> analog-out class-D amp, which just contains a PW modulator, that is for sure NOT the case. Although the voltage signal levels has fixed values they are modulated continuously in the time domain without quantization. What counts is the area of the pulse: height x time, and that is varied “analog" if you can say so.

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Old 8th January 2004, 07:39 AM   #3
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If you are tolerant you can call a power-DAC like the TacT millennium (and it's "younger brothers" from TI) a digital amplifier because they make use of dicrete-voltage and discrete-time signal representation.
The average class-d amp however isn't digital even they are called like that in the advertisements.

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Old 8th January 2004, 10:45 AM   #4
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Would quantization add quality?

Sorry i couldn't help noticing the extra word 'qualities' in the topic of this thread.
That word might save this thread from becoming just another academic discussion of what is the definition of digital or not, and probably later what is the definition of Class D and other classes. It usually goes that way...... even if nobody ever really have agreed on these definitions in here .. Only endless discussions.
I think we have more than 100 of threads with this content here on diyaudio.com, so why start another one?

However i find the real topic of this thread very interesting, if you mean Class D qualities vs. Digital qualities.
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Old 8th January 2004, 11:18 AM   #5
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Don’t see what you want to say Lars, but this is what’s in the first posting"

“i say the output stage has the qualities of a digital system - the transistors ideally are acting a switches. i realize the input to a class D amplifier is an analog signal, and the ouput is an analog signal, but there seems to be a A/D and D/A conversion.”

“Qualities” in this context can be replaced with “properties” IMHO. So what's the topic here IYHO? There is certainly a difference between a “powerdac” and a class-D amp sec. Also there are different class-D topologies concerning modulation schemes. Can’t we discuss the topology differences and its audible effects also?

I’ve heard the Tripath approach and yes that sounds amazingly good to me.

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Old 8th January 2004, 11:44 AM   #6
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Quote:
Would quantization add quality?
BTW: also a self oscillating class-d amplifier quantizizes the signal !

Quantisation does of course not add quality.

Regarding the term digital:
I once learned that you talk of digital signal representation when it is represented by discrete values at discrete time samples.

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Charles
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