DSD: digital, or is it analog? (Just my rambling)

[Wimek]

I agree with @Paul and many of you that DSD recordings sound great. The Octave Records recordings sound great, but also the first album of Dire Straits sounds so much better than the remaster CD from 1996. The details and placement of the instruments is just great on the Mofi SACD! I know, the mastering play the biggest roll, but my point is that so many SACDs just sound really great.

Today I saw YouTube move from about 5 year ago where a YouTuber was ranting that Mofi apparently captures the original master in DSD and he claims that they are making the vinyl records from that DSD recording. His biggest problems with that was that Mofi is not telling that, while he admitted that those Mofi LPs sound great.

This made me thinking, you could consider DSD to be an analog signal…

I started to make the following comparison in my head…FM is frequency modulation, AM is amplitude modulation, DSD is pulse density modulation. All three are decoded in an analog way to audio. From these three, DSD conversion to audio is the simplest: just a low pass filter will work fine.

If FM and AM radio is considered analog, then could DSD also be considered “analog”?

DSD is digital in storage/transport, but more like a pulse-modulated analog signal in behaviour.

1 - you can see the wave shape when you show a DSD signal on a scope

2 - The DSD stream is not “numbers” you can add/multiply.

3 - Physically, it is indeed a kind of pulse waveform.

4 - When a DSD signal is fed though a simple low pass filter produces an analog signal

Off course you can call DSD a sequence of “digital bits” or “a modulated analog pulse train”. Personally I see DSD as the latter

I’m curious in your opinions

[Rushton]

Yes, I think you are right on target in your thinking. The biggest challenge so many people have in coming to grips with DSD (PDM) is that they struggle to break away from the PCM paradigm of “sampling.”

Even ChatGPT, in an otherwise spot on discussion of DSD, gets sidetracked by saying “DSD uses a 1-bit signal sampled at a very high frequency (2.8 MHz for standard DSD64, up to 11.2 MHz or more for DSD256/512)…”

To say it is “sampled” is not correct. As Tom Caulfield explained to me in an email:

That’s where everyone gets sidetracked; trying to equate DSD to PCM and signal sampling. There are no samples in PDM, nor is the resulting bitstream composed of samples, which implies successive amplitude values. It’s the simple modulation of a carrier; like in radio transmission. In PDM/DSD’s case, a squarewave which we conveniently call “bits”, rather than modulating a sine wave as in radio. A more accurate and less misleading term in place of sampling (or sampled) is clocked. That’s much truer to the modulation process.

[Wimek]

Is very nice to hear that my thoughts and reasoning was in the right direction. After your supporting reaction a the quote from the email of Tom Caulfield, I’m now fully convinced that DSD is an analog signal. Modulated, but analog. The good thing is that a DSD pulse train can be stored on digital media and that’s probably one of the reasons they used it for the SACDs.

I’ve been digging a bit deeper. The following Wikipedia page tries to explain DSD: Direct Stream Digital. I like and generally trust Wikipedia, but in my opinion in that article they’re wrong in stating that DSD is a digital format. When I keep on reading in the linked article Delta Sigma Modulation it gets complicated because the delta sigma modulation converter seams to be an ADC… still wondering whether a simple frequency modulator that modulates the audio signal with a 2.8224 GHz carrier would do the job.

I’m afraid I still have to dig deeper

[RonP]

The thoughts of a guy who is no expert on this subject, but DSD is a digital format. It uses a 1 bit stream at a very high sample rate, attempting to emulate the analog sine wave. But it’s digital.

[Rushton]

@Wimek

I’m now fully convinced that DSD is an analog signal. Modulated, but analog. The good thing is that a DSD pulse train can be stored on digital media and that’s probably one of the reasons they used it for the SACDs.

Tom Caulfield would agree. He tells me regularly, “DSD is analog” and “there is no sampling.” Of course, most people refuse to accept this because they are imbedded in the PCM terminology of samples, sample rates, and word lengths. He has told me numerous times that he failed to really understand PDM until he completely erased his embedded PCM terminology and way of conceptualizing what is going on. I admit that I don’t understand it. I’m neither an engineer or a mathematician. But, I have faith in what people like Jussi Laako (Signalyst) and Andreas Koch (Playback Designs, and earlier Sony) have to say about PDM. They are consistent with what Tom says, just more abstruse.

The closest I got to beginning to get my head wrapped around this is from the conversation Tom and I posted in Positive Feedback some years ago: An Interview with NativeDSD’s Mastering Engineer Tom Caulfield

[Audioholic]

I tried listening again to my system with the understanding that my DSD MkI DAC is an alalog device and not digital, but it still sounds the same. Lucky for me I like how it sounds.

[RonP]

Excellent interview. But with the (Tom’s) following explanation, I’m further convinced that analog is analog and DSD in analog-like.
"DSD, or the more correct non-marketing term Pulse Density Modulation or PDM, is not a digital value representation system like PCM. It’s a modulated carrier system, like radio transmission, only using a bit clock (square wave) as the carrier instead of a high frequency sine wave. In fact, DSD/PDM is an analog-like signal (due to the very large difference in the carrier frequency band versus the audio frequency band).

DSD/PDM is not digital in the form most people understand and perceive the word “digital” because there are no expressed digital values in DSD/PDM as there are in PCM. Only changing 1’s and 0’s bit densities proportional to the modulating audio signal level."

[andrewc]

In as far as DSD is made up of discrete digits which can be copied perfectly with no errors, DSD is absolutely digital. “Digital” is not synonymous with PCM.

[Wimek]

Yes, you’re right, I haven’t got extended knowledge about DSD/PDM. I just try to wrap my head around it and was hoping you guys could help me a bit. In that, I’m not disappointed: thanks @Rushton . However, I do understand electronic engineering and software development. I just thought that PDM looks more like FM and doesn’t seam to have anything in common with PCM (besides the fact that both can be written and read from digital media).

[Wimek]

So now you agree with my line of thought?

[dchang05]

If DSD is analog, is there any digital format existed?

[Rushton]

My thought: this is a rabbit hole not worth traveling down, OTHER THAN to understand that DSD/PDM is a very different animal than PCM and that there is no sampling involved, no decimation. To that extent, like Wimek, I think of it as an analog-like medium. But in my mind I still leave it on the digital-side of what we think about when talking generally about digital versus analog media. It is certainly not analog tape nor an LP, but it has more similarity than people suspect. I have to remind myself regularly that my long-embedded paradigms about quantization, samples, word lengths, etc., just don’t apply in this realm.

[Wimek]

I learned that the main difference between them is: In PCM: From the binary numbers samples are calculated and from these samples the analog waveform is recreated. In PDM the pulses don’t have a numerical value and the analog waveform is recreated by filtering out the 2.8 MHz carrier.

I agree that this could be an endless debate because for PDM you can find arguments for both camps (analog vs. Digital). I think we could conclude that PDM and thus DSD can be considered both.