 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
199.66.91.93
In Reply to: RE: Thanks for the explanation, but it DOES sound self serving. . . posted by tailspn on July 18, 2017 at 13:59:48
Of course you're going to encounter some deterioration on the converted file!
But actually, at this level of resolution (either DSD256 or DXD) I'd be surprised if the deterioration IS really audible and not just a product of expectations and/or psychology. But whatever. I guess I'll just have to pay you a visit the next time I'm in Boston (a city I've missed so far in my life) - I'd love to hear those Sound Labs! ;-)
Follow Ups:
Same as occurs within an A/D converter outputting DXD instead of DSD, via the conversion within the converter. You're always welcome to visit Chris and judge for yourself :)
I apologize for cutting and pasting - this is from John Siau of Benchmark Digital:Modern PCM sigma-delta converters produce much lower error signals than 1-bit sigma-delta DSD converters. The errors in the DSD system are due to the 1-bit quantization that occurs in 1-bit sigma-delta DSD converters. Multi-bit PCM sigma-delta converters can be fully dithered and do not suffer from this un-dithered truncation. Every added bit reduces the noise signal by 6 dB. A 4-bit sigma delta converter is 24 dB quieter than a 1-bit sigma-delta DSD converter. Right from the start, 1-bit DSD signals have higher losses than multi-bit PCM signals.John wrote all this in 2015. Have things changed in the meantime?
Conversion from 1-bit DSD to multi-bit PCM is a lossless process inside the audio band. The only thing that is removed is the out-of-band noise above the Nyquist limit of the PCM system. Nothing else is lost. . .
Conversion from multi-bit PCM to 1-bit DSD is always a lossy process. The loss is due to the 1-bit truncation. This truncation introduces a very large ultrasonic error signal that makes the ultrasonic region unusable for audio.
Processing a 1-bit signal to create a 1-bit signal is also always a lossy process. A volume control is one of the simplest processes in a multi-bit PCM system, but it creates a large error signal when applied in a 1-bit DSD system. The same is true for any other 1-bit to 1-bit DSP process. The lossy part of these DSP processes is the quantization back to 1-bit. Cascaded 1-bit truncation processes can rapidly degrade the audio quality. For this reason, DSD is always processed as multi-bit PCM.
Any DSP process applied to a 1-bit signal produces a multi-bit signal. No loss of information occurs until this is quantized back to a 1-bit signal. Why incur the loss by going back to a 1-bit signal after the processing chain?
All practical DSD systems require some sort of DSP processing (gain control, mixing, filtering, etc.) and all of these processes produce multi-bit PCM results. Taking these lossless multi-bit results and adding loss by truncating them back to a 1-bit DSD signal makes absolutely no sense. DSD complicates the processing and adds unnecessary losses to the signal path. DSD does not simplify the signal path. There is absolutely no truth to the marketing hype that claims that 1-bit DSD is a simpler system than multi-bit PCM. The exact opposite is true.
Nothing has changed since 2015, and there's no such thing as a "PCM Sigma-Delta Converter". PCM is an acronym for Pulse Code(ed) Modulation, and PDM for Pulse Density Modulation, also referred to as PWM (Pulse Width Modulation).You'd have to ask John what he's talking about, but as I read it, there's two possibilities:
1- He's substituting the acronym PCM for the actual PDM (two very different formats), in which case his argument makes some sense.
2- He's conflating two independent processes and calling that a "PCM sigma-delta" converter (which coincidentally are the separate processes inside a pro A/D converter). The first process, (on the A/D side), is the Delta-Sigma modulation of the audio signal; producing either multiple parallel Pulse Density Modulation (PDM) bitstreams (multi-bit PDM, which appears to be John's inaccurately naming multi-bit PCM), or much less frequently a 1-bit PDM bitstream (DSD as with the Grimm A/D Converter). If PCM is desired/required, then these PDM bitstreams (1 to 8 typically) are then followed by low pass filtering (decimation) and conversion to parallel X-bit wide Pulse Code Modulation binary words. That latter process is most certainly a lossy process. Once decimation filtered, you can no longer reconstruct the original PDM (DSD if 1-bit) bit stream.
But so what? My original statement was, and remains that all acoustic audio recordings are made using A/D converters, and the vast majority of those A/D converters are all front ended with Delta-Sigma modulators, producing an actual DSD 1-bit bitstream (as with the Grimm A/D), or from two to eight parallel (multibit) bit streams of Pulse Density Modulation. And, any conversion to any other format for whatever reason (typically post process sweetening and/or conversion to a deliverable format) is a post process from the original A/D PDM bitstream(s) conversion, and is lossy. So, if you can archive the original A/D conversion before performing the PDM to PCM/whatever format, why not do so for future processing possibilities? And especially, if you have the option, why not do those format conversions offline using more powerful conversion algorithms with more powerful processors than are available in the A/D converter box?
Seems self evident to me. But on the other hand, I don't have products to sell, or customers to convince.
Edits: 07/19/17 07/19/17 07/19/17
But, as I read it, I think that your differences with what John wrote seem to lie in what each of you thinks is lost when going from DSD to PCM.
When John says:Conversion from 1-bit DSD to multi-bit PCM is a lossless process inside the audio band. The only thing that is removed is the out-of-band noise above the Nyquist limit of the PCM system. Nothing else is lost. . .And you say:
Conversion from multi-bit PCM to 1-bit DSD is always a lossy process. The loss is due to the 1-bit truncation. This truncation introduces a very large ultrasonic error signal that makes the ultrasonic region unusable for audio.If PCM is desired/required, then these PDM bitstreams (1 to 8 typically) are then followed by low pass filtering (decimation) and conversion to parallel X-bit wide Pulse Code Modulation binary words. That latter process is most certainly a lossy process. Once decimation filtered, you can no longer reconstruct the original PDM (DSD if 1-bit) bit stream.
So. . . as I read this, I conclude that you're definitely removing SOMETHING in going from DSD to PCM. But what is it? Is it the out-of-band noise and nothing else, as John seems to suggest? I mean, in a way, you guys are kind of saying the same thing. Right?
Chris, you're asking a great question that has been discussed with many thousands of words. I'll give you the short answer, and you can research the underlying discussions. I'd suggest if you're interested, the many postings at Computer Audiophile. Do a search on Miska, the provider of HQ Player, particularly on the DAC Forum.
The short answer to your concluding question is yes, the decimation filter has to eliminate the out of band noise, and in doing so, injects phase artifacts (unfortunitly required by the steepness of the filter) that render the resulting PCM words not convertible back to the original PDM/DSD bitstream(s). Therefore, it's a lossy process.
It's the effects of the necessary decimation filter required in the DSD > PCM conversion to preclude folding back into the audio band any energy above half the sampling rate of the converted PCM result. Since the vast majority of energy in any PDM (DSD) bitstream lies above the audio band, being almost all the energy approaching the bit rate frequency, the decimation filter has to be very steep approaching the upper frequency of the audio band of interest. The decimation filter shaping, with its subsequent phase distortions and nonlinearities (necessary to achieve the required filtering shape), make the reconstruction process impossible back to the original bitstream. Therefore, it's lossy, and in my experience quite perceivable.
A PDM (DSD if 1-bit) bitstream is almost entirely uncorrelated white noise. It's a useable A/D converting format through the process of noise shifting (an activity of the Delta-Sigma Modulator), where the uncorrelated noise is shifted to above the audio band, then filtered out when the audio content is retrieved in playing. Since PDM/DSD is already an analog bitstream, it simply requires low pass filtering to retrieve the audio content (there's many ways to do this, which I won't go into here, but the filter has to be either very steep (not good) if the audio is to be retrieved at the original bit rate, or the bit rate upconverted to a much higher bit rate, and a gentler shaped filter employed).
But for digital post production processing, since PDM/DSD is an analog representation of changing input analog signal levels, it contains no digital values ala PCM. It only contains analog signal level changes represented by the density of bits in the bit stream. Since no values are represented (actually, the density of bits is directly proportional to percent of modulation, which in turn CAN be translated to absolute signal level values in the conversion to PCM), the bit stream must be converted to a value based format (PCM) to be further digitally processed.
So, back to my original point; if as a recording producer you can archive the A/D converters original front end Delta-Sigma modulation bitstream(s), in the form of a 1-bit DSD bit stream(s) at the original recorded bit rate, why not do that? Then you can convert to whatever required/desired 2nd generation format meeting the needs/desires of the producing engineer/label, and have as backup the original PDM/DSD content for future possibilities.
2L's Morten Lindberg, as well as Bert van der Wolf of Northstar/Turtle/Challenge Records (among others) say they prefer the sound quality of DXD, and so record in that format (they say). That's their prerogative and choice, but it is a 2nd generation conversion.
I'm not sure I agree with everything you're setting forth in your post, but I don't want to get into a multi-day discussion of PCM vs DSD - that argument has been going on for a couple of decades now! ;-)
BTW, I've seen some of those discussions with Miska, but I'm sure there are many more which would be useful to follow.
Thanks again!
| FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
||||||||||||||
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
