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Topic: All DAC use PDM, even for PCM? (Read 4971 times) previous topic - next topic
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All DAC use PDM, even for PCM?

Audiophiles confuse me again. In the 1990s Philips and Technics used 1 bit or 4 bit DAC designs, and IIRC this was because it was easier to build than 16 bit DACs. Now I hear even today, an external DAC for $20k uses this design. Are native 16 or 24 bit DACs still hard to build?

I mean, a DAC should just make out of the 1 and 0 an analog audio signal which can be played on my crappy notebook speakers and I'm sure, the ones Apple are putting in their devices are just fine for this job. I'm also glad that the ridiculous pricing of these audiophile devices are preventing me from buying one.

I'm just very confused. Maybe someone knows a good source or explaination how this works. Because I'm interested in how it works.

Re: All DAC use PDM, even for PCM?

Reply #1
To put a few things into perspective:

1. Classic R2R multibit DACs actually were used in CD players well into the '90s. This was because 1-bit DACs at the time still had some hurdles to overcome.

First of all, all kinds of DACs have a penchant for outputting ultrasonic garbage that may upset the following stages (read: cause intermodulation distortion). R2R DACs are famous for their glitches, but 1-bit delta-sigma with heavy noise shaping is especially bad. Extensive filtering and/or fast-slewing opamps are required to cope with this. If you were ever wondering why sample circuits liked to use OPA2604s as I/V at the time (despite them not being very good performers on typical +/-12 V), these were a fast-slewing FET-input type, read quite good RF immunity.

In addition, pure 1-bit converters also tend to be terribly sensitive to clock jitter, and only the introduction of synchronous switched-capacitor filtering in the early-mid '90s (and subsequent advances in the technology) made them robust to varying degrees. Jitter could otherwise fold back into the audio band and degrade SNR among other things.

2. Multi-multibit delta-sigma converters were pretty much the last "next big thing" in audio converters. These employ technology that switches between multiple few-bit converters (maybe 4 bit, generally imperfect) in such a way that turns their quantization noise into random noise and linearizes the whole shebang. Their main advantage next to stability is higher inherent converter SNR, so you need less noise shaping to achieve the same output SNR, making them either more output-stage-friendly or achieving higher SNR than possible otherwise. The entire success of Wolfson Micro was built on converters like that.

As you might gather from the above, discussing DAC technology in isolation is of limited use. Whether a DAC performs well ultimately comes down to implementation - the ODAC shows that you can wring good performance from a part that's only moderately high-strung, while any number of China eBay DACs with fancy chips but barely cracking the 100 dB DR demonstrate the opposite.

Re: All DAC use PDM, even for PCM?

Reply #2
Quote
I mean, a DAC should just make out of the 1 and 0 an analog audio signal
It's the analog side that's tricky.   The digital should be accurately converted to analog with minimal noise. 

A 1-count difference is very-very small, even at 16-bits.    With 16-bits you can have numbers between −32,768 and +32,767 and a 1-count difference/change is too small to show-up on a regular voltmeter/multimeter.  

That said, I've only heard a defect or difference from a "DAC" once.   I once had a soundcard that made noise from the speakers when the hard drive was accessed.    But that was noise getting into the analog electronics so I don't blame the DAC chip.

Even the DAC built into my 1st CD player that I bought almost 40 years ago sounds "just fine" to me.

I don't worry how it's made and I don't own a stand-alone DAC.

Quote
Audiophiles confuse me again.
The audiophile community is mostly nuts and the crazy ones often "don't believe in" blind listening tests or measurements.



Re: All DAC use PDM, even for PCM?

Reply #3
Are native 16 or 24 bit DACs still hard to build?
The simple answer to that is "yes".

The digital should be accurately converted to analog with minimal noise.
Should be, but that is difficult to achieve which is why fancy delta-sigma schemes are used to hide the problems.

Let's consider a 16-bit DAC implemented as literally that.  The input value 0 to 65535 (or -32768 to +32767 if you like) is used to generate one of 65536 preset voltages to present to the output pin.  Typically this will be done with an R/2R ladder (look it up).

Each step needs to have the same voltage difference as the last one, and an ideal ladder with its input nodes given precisely the same voltages works fine in theory, until you account for component tolerances.  If the most significant bit switches the output voltage by 32767 steps instead of 32768, that produces a error in the input-output transfer function, ie a non-linearity.  Preventing a one-bit error in the transfer function requires precision of 0.003% in the values of the resistors and the voltages feeding them.  If you want a 24-bit DAC with a linear (monotonic) transfer function, you're talking precisions 256 times better than that (0.00001%).
It's your privilege to disagree, but that doesn't make you right and me wrong.

Re: All DAC use PDM, even for PCM?

Reply #4
Are native 16 or 24 bit DACs still hard to build?
The simple answer to that is "yes".

A 1kohm impedance at 20 KHz bandwidth has half a microvolt of thermal noise at room temperature, so short of cryogenic cooling or really low sampling rates a full 24 bits is more like impossible.  The random motion of electrons is too noisy for that. 

16 effective bits isn't that hard though, parts that will do > 16 bits are mass produced by the billions, even if many are ultimately paired up with analog electronics that are less sensitive. 

Re: All DAC use PDM, even for PCM?

Reply #5
16 effective bits isn't that hard though, parts that will do > 16 bits are mass produced by the billions
OP wants "native 16 bit" though. I'm not sure what this means.

Re: All DAC use PDM, even for PCM?

Reply #6
The audiophile community is mostly nuts and the crazy ones often "don't believe in" blind listening tests or measurements.
Of course, HA is also part of the audiophile community. It's a shame we've let a few marketers and scammers discredit an entire community.

Just saying.
Quis custodiet ipsos custodes?

Re: All DAC use PDM, even for PCM?

Reply #7
OP wants "native 16 bit" though. I'm not sure what this means.

I don't want a 16 bit PCM DAC, my question is just how are DAC really designed and does it matter if I just want to listen to music?

Of course, HA is also part of the audiophile community. It's a shame we've let a few marketers and scammers discredit an entire community.

I kinda disagree, for me it seems that most of the audiophile world doesn't understand or doesn't want to understand how digital audio works. I a while ago, audiophiles were confused when they found out that Apple would dare to use AAC for AirPlay instead of Lossless.even though AirPlay allows different codecs to be used.

But in the end, this digital data needs to be converted into a analog signal, which can drive a speaker so I can hear it. And I remember a Philips CD player I owned in the late 1990s which said it was a 1 bit DAC or I also had a Technics with MASH which was (as i found out later) a 4 bit DAC.

This is why I joined HA, wanted to know how this works. Of course I forget these things and search is also not always working.

P.S. Also why I prefer to watch Fantano

Re: All DAC use PDM, even for PCM?

Reply #8
And I remember a Philips CD player I owned in the late 1990s which said it was a 1 bit DAC or I also had a Technics with MASH which was (as i found out later) a 4 bit DAC.
That isn't really the whole truth.

When specifying a DAC, there are two main performance metrics: bandwidth and signal-to-noise ratio. If you say you want a 16-bit DAC, that doesn't really specify those two metrics. I can only assume you want a DAC of which the signal-to-noise ratio is better than what is offered by a 16-bit digital signal. As for bandwidth, I can only assume you mean 0 to 20kHz. If you want to play music for dogs, a higher bandwidth would be needed, which impacts DAC design.

Now, what is meant by that 1-bit or 4-bit MASH design, is that the DAC consists of a first stage that outputs a 1-bit or 4-bit signal. But that is only the first stage, not the final product. The point is, the are different ways to get the performance you want. You can use a very expensive first stage that is able to output 16-bit directly (I believe these are called non-oversampling DACs or NOS DACs) or you can use a first stage that outputs a few bits and run that at a very high sampling rate. This process is called delta-sigma modulation and is often referred to as 'oversampling'.

If all you care about is performance (signal-to-noise ratio), the best chips available seem to use the latter method, those are often called multibit delta-sigma DACs. See for example this datasheet. That is a very high performing DAC chip, with a THD+N of -115dB, which uses a multibit delta-sigma modulator. That thing is able to run in single-bit mode (DSD) but then it's performance will be slightly worse.

In the end, it doesn't really matter what is inside the device, as long as it does its job well enough. These chips are more then capable of not being the weakest link in the chain: what is often forgotten is that performance of analog-to-digital converters (ADCs) is not able to reach the levels DACs are. And often these figures are well beyond the capabilities of human hearing anyway.
Music: sounds arranged such that they construct feelings.

Re: All DAC use PDM, even for PCM?

Reply #9
OP wants "native 16 bit" though. I'm not sure what this means.

I don't want a 16 bit PCM DAC, my question is just how are DAC really designed and does it matter if I just want to listen to music?

They're designed similarly to any other computer chip.  Engineers lay out a lot of transistors, do timing analysis, etc then send it to a fab.  You don't have to care how they're made anymore than you have to care how your computer's CPU is made, but it can be interesting. 

 

Re: All DAC use PDM, even for PCM?

Reply #10
[In the end, it doesn't really matter what is inside the device, as long as it does its job well enough. These chips are more then capable of not being the weakest link in the chain: what is often forgotten is that performance of analog-to-digital converters (ADCs) is not able to reach the levels DACs are. And often these figures are well beyond the capabilities of human hearing anyway.

Mics are even worse than ADCs still which is why the audiophilia circle is so laughably silly.