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In Reply to: Re: These guys agree with Alex for their new CD player . . . posted by J North on October 10, 2003 at 12:03:46:
all class D (or "switching") amplifiers are Analog. "D" stands not for digital!
They may feature a digital input that accepts PCM, but this is always converted to analog ( yes... a dac inside) before being converted to PWM. And PWM is Not a digital signal.The overruling feature for a signal to be digital is that it contains BITS at a certain Sample Rate, and those Bits form a Word with a specific length; and this word contains specific data, stored as such into this word according to some complex mathematical structure. PWM is just a square wave that is modulated in the time domain by the analog signal that comes out of the (internal) DAC, with the help of a comparator (kind of fast opamp in open loop configuration).
For DSD this is different, this is directly converted to PWM because their composition schemes are somewhat similar. Still: after this conversion the signal is essentially Analog.
This all, however, does not disqualify class D in the least: the big bang of Class D is imminent and beware: it sounds better than anybody is willing to admit - if applied properly, which nobody has done so far (commercially - at least).
A note about HF noise: No such thing affects data in the all-digital audio chain, until the signal reaches the final amp stage. Here the signal amplitudes are so large and the HF is so far out of audible territory that it don't reach your ears.
Kevin of Solar is deceptive...talking about digital amps that are immune to jitter but can be too accurate. Are you kidding?
No digital: No jitter? Wrong! Jitter is just "timing errors".
The 350 kHz to 600kHz square wave that is the carrier for the audio signal must not be allowed to vary beyond what is dictated by the audio signal, because variations in time will automatically translate into a lower frequency that is put on top of the audio signal; and from this point on (if it will pass the output filter) it has become an undistinguishable PART of the audio signal.Who is going to separate these two? There is as of yet no system capable of determining which part of an analog audio signal is distortion and which is the undistorted music part. Would be fancy, just taking the distortion out of your music, at the end of the chain! Would take a hell of a library to store all possible sounds, number them all, and have them available for comparison at lightspeed...
Class D has its own set of distortions, and it will take time to iron them out. But they are fewer, and they are easier to understand.
No more hassle with thermal compression, negative feedback that is always too late, output transistors that loose all their current gain when emitter currents reach over just a couple of ampères, or open loop bandwidths of just a couple of kHz introducing horrible phase distortions.As Ric sez: the revolution has begun.
I say: Beware of "poyama people" that want to block progress because they just can't handle it. Or don't understand it.
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Follow Ups
- There is no such thing as Digital Amps : - Marconi 14:10:43 10/11/03 (1)
- Good post - thanks (nt) - Metralla 19:04:17 10/11/03 (0)
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