Digital

CD-Recordable FAQ's by Andy McFadden

http://www.cdrfaq.org/faq02.html

Audio FAQ's from Digital Domain

http://www.digido.com/index.php?option=com_kb&Itemid=67

The 25 Years of the Digital Audio Compact Disc

http://www.soundfountain.com/amb/cd25years.html

user posted image

Digital amps DAC free

TI and Apogee have had pure digital amps for quite a while. Totally DAC-free, although some argue that the amplifier itself is actually a power-DAC.

http://focus.ti.com/docs/pr/pressrelease.j...?prelId=sc07161

http://www.apogeemems.com/ddx/ddx.html

Both companies have lots of app notes and datasheets available. Apogee sold their amplifier products to Sigmatel and ST makes the chips. Here's a blurb from the old Apogee website that references their amplifier products:

"The Company built upon this audio experience and developed the worlds first all-digital, high efficiency audio amplifier ICs, which it trademarked as Direct Digital Amplification or DDX®. DDX technology became the market leader and in conjunction with the Company's DDX technology licensee, STMicroelectronics, the companies developed over 25 DDX ICs that had total sales of over 35 million devices to leading consumer brands such as Sony, Sharp, Samsung, Harman Kardon, Hitachi, Kenwood, LG, Philips, RCA and Zenith. During the growth of this business, the Company won the Deloitte Technology Fast 50 award in 2004 as the fastest growing technology company in New England"

An interesting article about the CD history, from Dr. Kees A. Schouhamer Immink:

The CD Story.

An interesting article from PS Audio about Class D amps.

PWM power stage explained

QUOTE (clint e. @ September 16, 2006 07:19 am)

The Truth About Digital (Class D) Amplifiers

" Firstly I'd like to point out that "digital amps" is a misnomer. There are two categories:

Analog-controlled class D. Switching amplifiers with an analog input signal and an analog control system. Normally some degree of feedback error correction is present.
Digitally controlled class D. Amplifiers with a digitally generated control that switches a power stage. No error control is present. Those that do have an error control can be shown to be topologically equivalent to an analog-controlled class D with a DAC in front.
Both use switching power stages and have high power efficiency as their most eye-catching feature.

Why Digital… Digitally controlled class D initially delivered a success in the form of the Tact Millennium. However, by its mere existence this device proves that obtaining good performance from such a contraption is largely an analog design exercise - a very complicated and expensive one at that. After all, the distortion phenomena that stand in the way between a perfectly formed digital control signal and a perfect analog replica are inherently analog. Similarly, cheaper digital class D's (such as Sony S-Master and TI's) go on to show that at practical price and complexity levels, performance is quite abysmal (better than 0.1% THD is unusual and be sure it aint just third harmonic!)

One should ask the question: would any D/A converter designer in his right mind build a DAC using power components? Probably not. Then how about the old argument that digital-to-the-end is best? Well, I should think the D/A barrier is best put precisely where it allows the whole signal chain to perform at its best and why should we believe that this is necessarily right at the end? Quite obviously the concept of a digital class D amplifier was dreamt up by DSP folks who presumed that the signal should be kept out of the big bad analog world as long as possible, at the same time expecting the power stage, power supply and filter (all highly analog in nature) to perform flawlessly.

…or Rather, Why Not?
Analog controlled class D is quite a different story. Although most commercially available implementations are well short of this ideal, proper error control can be used to make analog class D amplifiers with performance figures giving the digital variety a run for their money, at a price well below that of even the cheapest digital class D's. They can have vanishingly low output impedance right across and beyond the audio range (which the digital ones can't!) while frequency-independent distortion (for that "zero-feedback sound") is actually easier to achieve than with digital ones.

To the Ear
So how about sound? The output filter's highish HF output impedance, when uncorrected (in amplifiers without post-filter-feedback, ie. all digital ones and many analog ones too) is responsible for the oft- quoted tube-like warmth and air. At low frequencies the filter impedance is low, resulting in a commanding, dynamic bass. Because switching amplifiers previously had a reputation for sounding harsh (due to people who hadn't heard them but presumed that switching couldn't mean anything else), about every modern entry in the field was heralded as the "first audiophile class D". Read a review about the Bel Canto Evo or the Sharp 1-bit (which is analog, btw) to see what I mean. Keep in mind that if a device sounds radically different from what you've held in high esteem previously, there's usually something fishy going on.

Unfortunately, while warm and airy is nice, it isn't all you need for real audiophile sound. I like to think audio components should sound neutral and transparent too. A frequency response that wanders 10dB off the line at 20kHz isn't conducive to neutrality. High THD isn't good for transparency, especially when it goes up with frequency. Ergo the Tact Millennium (which has flat and low THD but a nonflat frequency response) sounds transparent but not neutral, and your average transistor amp (which has a low output impedance but sharply increasing THD) sounds neutral but not transparent.

Now, the technology (if you can call a circuit with 16 transistors that) to deliver low output impedance with frequency-independent low distortion in class D exists. Built with audiophile-grade parts it'll scare the pants off any high-end amp (while I'm the designer of that circuit and thus some care reading this statement is warranted, I do have a lot of serious folk to back up my claims). Of course, having these characteristics it sounds more like other high-end amps than that it sounds different, in the same vein as that the best tube gear and the best solid state gear don't differ by miles in sound.

The Upshot

Digitally controlled class D: dead end street.
Analog controlled class D: definitely the future, although you shouldn't expect it to flatten competition from traditional solid- state and tube amps by a tremendous margin.
In the very high-end segment the three are bound to coexist for a very long time. In mainstream gear, class D is certain to take over the scene completely, although one serious problem remains: building a good class D amp is an order of magnitude tougher than a linear amp, and the knowledge required is much more diverse. It may take long before each large company has at least one knowledgeable designer. It won't stop them from putting class D based products on the market, but until then and unless they buy completed amplifier modules from specialist vendors (which eastern companies rarely do, they'd rather commit harakiri than having to swallow their pride), they will be putting out seriously substandard products for years to come. "

"Quote from Bruno Putzeys, Chief Engineer Class D Audio at Philips Digital Systems Labs"

QUOTE

Class-D audio power amplifiers (APAs) were first introduced nearly 50 years ago. Since then, they have been used sparingly in a relatively small number of applications with limited bandwidth, such as public address systems and telephony equipment. This will soon change as a new class of integrated Class-D APAs make their way into such mainstream applications as portable computers, battery-operated music systems, wireless communication devices, and other compact low-power systems.

In fact, they were originally called Class S, what was originally called class D was quite different, I can't remember what, or re-find the web page I got this from.

As for that paper you presented, let me note somethings which are yet to be explained,
what is yet to be explained is why analogue types are easier to operate open loop than digital ones, I know that digital types have problems with A/D converters in the feedback loop, which is one of their drawbacks, but how does being digital make it harder to achieve that zero-feedback sound. There is also a distinction between class AD (can be half-bridge), which is always on (apart for the deadtime) and only the percentages of the polarities are modulated and class BD (always full bridge), which is not only on or off but the polarity of the pulses is also modulated by the polarity of the modulation signal, as far as I know, this is better for audio performance, its and advantages, one is that the output pulse widths approach zero at low levels, unlike class AD, where they approach 50%, which considerably reduces demands on the LC filter. I realise that there are other advantages too, because the pulses are only one polarity at a time, there is no need for harmonic distorting dead time. Also, for digtial applications, the same pulse widths can be used for samples with 0 as the most significant bit (MSB), which corrospond the negative polarities, and samples with 1 as MSB, which corrospond to positive values. This reduces the required Master Clock (MCK) rate to switching frequency (SF) (this is what that determines the resolution of a digitally generated PWM signal) ratio required to achieve a given resolution. Contrary to the claims of the paper, I would like to see further improvements in digital class D, I hope that when class D does take over, they will all be digital, and all Class BD (except maybe, for SACD playback, because Class AD can amplify DSD directly). On a final note about this, if it is actually class AD that was originally class S, than Tripath's class T designation can be perceived as a logical next class up.
Nevertheless, I have come up with another idea, class CD, nothing to do with either the compact disc or 400 in Roman Numerals, the idea is to combine class BD with class G style rail switching, the idea is to further advantages offered by Class BD, this idea is just to use one rail and lower levels and on/off switching, and at higher levels, use the rail switching, modulating the percentages of two adjacent power rails.

QUOTE (Myrtone @ December 20, 2007 04:13 am)

.....
Nevertheless, I have come up with another idea, class CD, nothing to do with either the compact disc or 400 in Roman Numerals, the idea is to combine class BD with class G style rail switching, the idea is to further advantages offered by Class BD, this idea is just to use one rail and lower levels and on/off switching, and at higher levels, use the rail switching, modulating the percentages of two adjacent power rails.

So, your main concern is more output power?

From what i know class BD modulation permits filter-free operation in class D power amps, and don't you think class G are a little outdated and imho have poor audio quality...?!

...or are you talking about car audio amps?

Could you please be more specific?

Well, I was being specific, my idea was not an actual class G amplifier, but just to add rail switching (which Glass G uses), so that we can have PWM with more than three levels, class AD is limited to two levels, class BD can only have three, or two plus 0, or one positive, zero, and one negative. If we have more than three levels, then we can reduce the magnitude of the HF components. While I did mention Class G style rail switching, my intentions are different, class G uses rail switching to limit the voltage drop across the valve or transistors that amplify the sound, wheras in my case, I want to have more than three levels, for various reasons (as mentioned above).

Tanx for the input.

Let's see if i understand your idea...

From the use of the three-level two main advantages can be obtained: more resolution and increased efficiency. Right?
However, there is an important disadvantage: higher cross-over distortion for signals with small amplitude, than the obtained with two level PWM output voltage.

Therefore, to overcome this disadvantage the control/modulation method you're mention, uses three-level PWM output voltage for signals with large amplitude and two-level for signals with small amplitude ?

It seems a great idea, more output voltage and increased harmonic spectrum. soundt/thumbup.gif

QUOTE (clint e. @ December 22, 2007 02:03 pm)

Tanx for the input.

Let's see if i understand your idea...

You might be confusing Class BD with Class B, they are quite different, even a well designed Class B suffers from crossover distortion because of inherent non-liearities involved in switching from positive to negative, though bear in mind that a class B does deals directly with a continous time signal, Class D deal with pulse trains, even class AD does also suffer from croosover distortion, but at the zero crossing rate of the pluse train, this translates into high order harmonic distortion in the base sideband* (the signal modualting the square wave and what is left after the LC filter). With Class BD, the dead time delay is elimentated, note that each zero crossing in the base sideband translates into a two pluses of opposite polarity separated by a gap. I don't know that zero corssing distortion is really and issue in Class BD (as opposed to class

, and if it is, that would be less so.

*I have been wondering if much of this distortion is actualy aharmonic, whenever the switching rate is aharmonicaly related to the base sideband.

Re:higher cross-over distortion for signals with small amplitude

Do you mean lower level relative to the pulse carrier? With my proposal, the pluse carrier has a lower and more constant ampitude relative to the modulating signal, so maybe again, zero crossing disortion will be less of an issue.

Do you have any prototype of this amp that you'd like to share with us all, or you have it only on paper?
The reason i'm asking this, is because i find your ideas very pragmatic and interesting.

Laser Pickup For Every CD Players

http://www.microelectronic.co.yu/download/CD-pikup.pdf