Re: Class/type of amp ?


"William Sommerwerck" <grizzledgeezer@xxxxxxxxxxx> wrote in message
news:gidggj$tkd$1@xxxxxxxxxxxxxxxxxxxxxx
Arfa, please see my other post. Most class D amplifiers are analog, not
digital..

The web in general, would seem to disagree with you on that one, William
...


http://www.pcmag.com/encyclopedia_term/0,2542,t=Class+D+amplifier&i=55417,00.asp

http://www.pcmag.com/encyclopedia_term/0,2542,t=amplifier+classes&i=55350,00.asp
http://en.wikipedia.org/wiki/Electronic_amplifier
http://www.answers.com/topic/class-d-amplifier
http://www.maxim-ic.com/glossary/index.cfm/Ac/V/ID/1063/Tm/class-D

and many many more examples. The signal in a class D amplifier is only
analogue at its input, and at the speaker terminals after the low pass
filter that removes the HF PWM component. Thus, the whole amplifier is
fundamentally digital in the way that it amplifies the signal applied to
it.

The Web is wrong. Most switching amps are analog. That is, everything
varies
continuously, rather than in quantized steps.

That is a confusing and not particularly true statement.


By the way, Arfa, you're doing something intellectually invalid -- you're
"appealing to authority", rather than thinking for yourself, or explaining
what's going on.

I am not.


For those who would like to read about the correct explanation of "analog
versus digital", please refer to the following references. (I can't find
my
college textbooks, and I don't really think any of these are very good,
because the best explanation is graphical.) Sampling is an analog process,
that involves multiplying the signal by the sampling function, which
produces a convolution in the frequency domain. NO QUANTIZATION OCCURS. If
those convinced that sampling = digitization, let them tell me what the
bit
depth is.

http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem

http://graphics.cs.ucdavis.edu/~okreylos/PhDStudies/Winter2000/SamplingTheory.html

http://www2.egr.uh.edu/~glover/applets/Sampling/Sampling.html

Here's a quote from the last reference. Note especially the third and
next-to-last sentences.

"The signals we use in the real world, such as our voices, are called
"analog" signals. To process these signals in computers, we need to
convert
the signals to "digital" form. While an analog signal is continuous in
both
time and amplitude, a digital signal is discrete in both time and
amplitude.
To convert a signal from continuous time to discrete time, a process
called
sampling is used. The value of the signal is measured at certain intervals
in time. Each measurement is referred to as a sample. (The analog signal
is
also quantized in amplitude, but that process is ignored in this
demonstration. See the Analog to Digital Conversion page for more on
that.)"

The following is directed at everyone in this group -- and is not a
rhetorical question -- why is it, that when someone _explains_ to you, in
a
fairly clear manner, why what you and millions of other people believe to
be
true, but _is not_ -- you don't believe them? Aren't you able to think for
yourselves?


Because on average, in the real world, if "millions of people" believe
something, and one does not, it is not the millions who actually *are*
wrong.



The fact that most people do not understand, and refuse to understand, the
difference between analog and digital is, to me, a little frightening,
because it touches on the willingness of human beings to believe what they
want to believe -- or worse, what "experts" tell them -- rather than the
truth.

You have to draw the line somewhere, and make a judgement as to who or what
an "expert" is. The dictionary defines an expert as someone who has special
skills and knowledge in a subject, and is an authority on that subject.
Would you consider that the man who comes to repair your boiler is an expert
? Or the man who troubleshoots problems at your local garage? I suspect,
like most people you probably would. So if you believed what these people
told you about the dangerous gas leak on your boiler, or that you needed
$300 worth of work doing on your car to make it run right again was the
truth, would that make you frighteningly stupid ?



Disclaimer: When I was a young'un, I thought that if I believed something,
it was so. In retrospect, this is ludicrous, but most people are like
that.
It was many years before I recognized this error of thinking.



I'm not really quite sure exactly what you're saying here about the class D
amplifier. The analogue input signal is not converted directly to any kind
of 'value' represented by a binary number, such as might be the case if you
ran it through a traditional A-D converter. Instead, it is run through a
comparator, with a triangle wave as the reference input. This results in
direct conversion to a PWM signal. I accept that this does not represent
'quantization' as such, so is not producing a 'truly digital' signal, but I
also do not believe that once the signal is in PWM form, it can either be
considered to be analogue any more.

The term "digital" may not be a strictly true one for this class of
amplifier, and in truth, there is no such thing as a fully digital amplifier
in the sense that you are advocating, but in the way that most people would
understand the term "analogue", it's not that, either. The reason that it
gets called digital, is because all of the power amplification is done with
devices that have only two states - on and off. And before you say that
those devices are linear ones in that they are transistors of one persuasion
or another, they are not used in that way in this type of output stage.

So, if you are amplifying a signal that has only two levels, using devices
driven to have only the two conditions of on or off, then I think that you
are stretching the imagination more by calling it an analogue process, than
you are by calling it a digital one.

I don't believe that there is a true term for what the process is, but I
also think that "digital" provides for a better understanding of what is
fundamentally going on, than calling it an analogue process as you would.

And for the record, I am perfectly capable of thinking for myself, thank
you, and I am quite happy that I understand the principles of the class D
amplifier, enough to be able to make valid contributions to any discussions
about it. For you to suggest that the entire web, including respected
manufacturers, has got it wrong, seems a little opinionated to me, and based
once again on dancing around terminology and semantics, as popularly
understood by the electronic engineering world at large.

Anyway, I'm not going to get into another of those long-winded pissing
contests with you over it. If you want to believe that it is an analogue
process, fine, go ahead at that. Right, wrong or indifferent, I will
continue to refer to it as a 'digital' amplifier, as most engineers and
manufacturers would, and indeed do ...

Arfa


.

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