ndiamone
600 Club - QQ All-Star
Because my interface has no 24 - only has two word lengths - 16 and 32 (IEEE)
Lou Dorren: A new CD-4 Demodulator!!! [ARCHIVE]
- Thread starter loudorren
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After reading much of the latter parts of this thread, wanted to add something I learned along the way.
The Nyquist theorem promises that a steady-state signal, sampled at more than 2X the highest frequency in the signal, will be reproduced faithfully. Key term here is "steady-state." Think of it this way: a 20kHz signal, sampled at 44.1kHz, will only be sampled at two spots in one wavelength. But there will be another wave right after that and that will be sampled, again twice, but at slightly different spots and so on. Now, if you took pictures of each of these two-spot samples and laid enough of them on top of each other, you will see the original wave appear eventually. The reconstruction filter inside a DAC effectively works like this and the combination of samples reconstructs the original signal.
What happens though if the signal is NOT steady-state, and music is not? If you research this on the web, you will come across exactly ONE study, prepared at the University of Halifax, in Canada. In their research they have tested reproduction of short bursts of frequencies, so may be a few iterations, 5 to a hundred. They have concluded that in these cases the reproduction is certainly distorted and the higher the frequency and the smaller the number of iterations, the greater the distortion. In fact if you look at the reproduced frequency bursts, the distortion is only at the beginning and the end. In the middle enough samples have accumulated so that the signal has good form. None of this would show up in ordinary ADC - DAC testing as those are carried out with steady-state tones and not frequency bursts.
Is this short duration distortion audible? I don't know myself or of any research to that end.
For purposes of the CD-4 modulation signal, only, this is not too relevant as it is in fact steady-state. The music it carries, of course, is not.
The Nyquist theorem promises that a steady-state signal, sampled at more than 2X the highest frequency in the signal, will be reproduced faithfully. Key term here is "steady-state." Think of it this way: a 20kHz signal, sampled at 44.1kHz, will only be sampled at two spots in one wavelength. But there will be another wave right after that and that will be sampled, again twice, but at slightly different spots and so on. Now, if you took pictures of each of these two-spot samples and laid enough of them on top of each other, you will see the original wave appear eventually. The reconstruction filter inside a DAC effectively works like this and the combination of samples reconstructs the original signal.
What happens though if the signal is NOT steady-state, and music is not? If you research this on the web, you will come across exactly ONE study, prepared at the University of Halifax, in Canada. In their research they have tested reproduction of short bursts of frequencies, so may be a few iterations, 5 to a hundred. They have concluded that in these cases the reproduction is certainly distorted and the higher the frequency and the smaller the number of iterations, the greater the distortion. In fact if you look at the reproduced frequency bursts, the distortion is only at the beginning and the end. In the middle enough samples have accumulated so that the signal has good form. None of this would show up in ordinary ADC - DAC testing as those are carried out with steady-state tones and not frequency bursts.
Is this short duration distortion audible? I don't know myself or of any research to that end.
For purposes of the CD-4 modulation signal, only, this is not too relevant as it is in fact steady-state. The music it carries, of course, is not.
ndiamone
600 Club - QQ All-Star
Which is exactly why analog-generated and digitally-recorded ``test noise'' (white, pink brown, etc) always sucks worse than the digitally-generated versions.
FYI:
The ANRS "curves" appear in an AES Journal document about CD-4, it should be possible
to digitally implement ANRS decoding.
Kirk Bayne
The ANRS "curves" appear in an AES Journal document about CD-4, it should be possible
to digitally implement ANRS decoding.
Kirk Bayne
FM_Quad
Member
This observation isn't complete. The lack of steady-state in music (any program material, for that matter) can be accurately described in terms of the superposition (summation) of a set of sine (or cosine) waves, each with a different amplitude and phase, relative to the others. Human hearing can only sense such waves if they are between the range of about 20 Hz and 20,000 Hz (less than that for many of us). It is those waves, which are steady-state for a period of time that determine the digitizing requirements, and must satisfy the Nyquist theorem.
The description of successive samples being sampled at different points in the cycle, resulting in "filling in" the waveform is illustrative, but not necessary. If you were sampling at 48 KHz, for example, and the material consisted of a 24 KHz sine wave, (assuming both frequencies were exact) it would sample the same two points of the cycle over and over again. However, part of the "decoding" process is a low pass filter which would reconstruct a 24 KHz sine wave out of those two points.
That example is useful, being the worst case. It does show one limitation of the process that I alluded to in a previous post. The sampling and reconstruction process introduces some amplitude changes that are frequency dependent. It is based on the formula sin(x)/x. They are most pronounced near the nyquist frequency and diminish as the program content goes lower in frequency. In this case, if the phase was just right, and the frequencies exact, the amplitude could be zero! However this is the ONLY case that is that extreme. Any frequency less than the Nyquist frequency exhibits significantly less amplitude loss.
This is one (of several) reasons why oversampling is useful. IMO 44.1 KHz is marginal if one expects to accurately reconstruct frequencies up to 20 KHz. Sin(x)/x variations are noticable below 20 KHz. OTOH if the upper limit expected is 15 KHz, then 44.1 KHz isn't as bad. Note also that it was chosen as an "odd" frequency. The worst frequencies for sin(x)/x errors are ones that multiply integrally to the Nyquist frequency. Using an "odd" sampling rate makes it less likely that a real world signal would be on one of those frequencies. 48 KHz gives significanatly more margin, and although not "odd", is high enough to have minimal impact in the audible range. (Of course, higher rates like 96 KHz) move the sin(x)/x problem further out. Note that in the example I gave, the 24 KHz signal, that could totally disappear under the right conditions, is well above 20 KHz. That means it does not contribute to any audible experience. It also means that it is probably going to be filtered out before digitization any way, so is of no concern.
Another method that is sometimes used in processing audio signals is the Fourier transform (in one of several forms). The Fourier transform also has some limitations of its own related to the OP. It transforms blocks of information (in time) between a time domain signal (the waveform we hear) and its frequency domain equivalent (the sum of cosine waves that I mentioned above). That transformation is only exact if there is an integer relationship between the block of time being sampled (which is related to sample rate) and the frequencies in it. In most real world cases, there is not such a relationship. When that happens, "leakage" occurs, which means that energy is "observed" in frequency bins that didn't really have any. Leakage is minimized by shaping the signal with a filter, called a window. Several mathematical windows have been created each of which has a different set of trade offs. But the bottom line is that a Fourier transform does introduce some of the artifacts the OP mentioned due to the lack of steady-state in the signal. Personally I avoid using the Fourier transform in working with audio signals, because of these limitations, although there are some things that can be done with it that are difficult or impossible without.
But just digitizing an analog (music) signal, if done properly, using a high enough sampling rate, and proper filters, is capable of creating a signal that is indistinguishable from the original, to the human ear. The problem is doing it properly. Especially in the early days (such as when the CD was invented), the necessary hardware, if available at all, was expensive. Extra data storage is required. In the commercial (especially consumer) world there is a strong temptation to cut corners. How much can I shave before the customer notices the difference? How many customers will notice the difference? How many will be willing to pay the difference, not to notice the difference? So we end up with things like MP3 that are clearly not accurate, but good enough for most people most of the time. This is no different than, say, the trade-offs in designing a turntable. How much rumble is acceptable? How much wow and flutter are caused by the (lack of) accuracy in punching the center hole in the disk? etc.
Four Rules
Member
Hi folks,
after having struggled through the 48 odd pages of this thread I was wondering, if this technical discourse is getting us any nearer to a new CD-4 demodulator? Obviously Lou Dorren seems to be not well enough to pursue this project any further. Basically, what a shame should he fail or be unable to conclude his plans. I nevertheless hope he is well/regenerating/improving, but after having waited for years I still uphold my order but fear at no avail!
after having struggled through the 48 odd pages of this thread I was wondering, if this technical discourse is getting us any nearer to a new CD-4 demodulator? Obviously Lou Dorren seems to be not well enough to pursue this project any further. Basically, what a shame should he fail or be unable to conclude his plans. I nevertheless hope he is well/regenerating/improving, but after having waited for years I still uphold my order but fear at no avail!
The Involve guys should get... er... involved in this project, get the design from Mr. Dorren, make it a reality and split the profits.
dlaloum
Member
Just thought I would bump this thread and ask if there is any news of Lou's demod phono stage?
The Quadfather
1K Club - QQ Shooting Star
We have not heard from Lou in quite awhile. Last time we did was after a shorter previous pause in contact. Health issues were cited. If he passed away, would anyone there know to let us know? We just don't know. It seems that CD-4 shall remain completely a thing of the past.
I believe Lou is still alive as he is still selling his record cleaner kit on ebay. I bought one earlier this year. His response and shipping were "a wee bit slow", but it did come and the customs label was signed with Lou's name. Good product, by the way. Until I hear definitely, I will assume he is alive and encumbered with life and all that entails.
Wow, it's been two years since his last activity. I hope he hasn't given up on the project. The economical climate has changed a little for the better and even I would be interested in this project again. When I was unemployed, a $700 "toy" was a bit much. But now, I could probably afford a little extravagance.
Owen Smith
2K Club - QQ Super Nova
I'd buy one if it actually happens, and I'm a man with no turntable. But that can be resolved any time.
The Quadfather
1K Club - QQ Shooting Star
If Lou is indeed still around, he needs to let us know if he has given up on the CD-4 project, or if he still intends to produce it. It's been many years since we first heard of this. It's time for answers.
A while back the fine people at Involve Audio were said to be talking with Lou Dorren about finishing his project for/with him, but no announcement.
The Quadfather
1K Club - QQ Shooting Star
That's interesting. I hope it happens. He supposedly had a prototype to send to Jon for evaluation. I guess Jon never got it. I wonder if he was having technical problems with his design. It was supposed to work like an FM radio receiver. Interesting concept. I don't think anyone tried it back in the day. No PLL, just limiters and a discriminator.
Monthes or a year ago I have asked the Invole people, if they could help by finishing the demodulaor. They have been agreed. Then I told this Lou, that they have a modern and automastic production line and it would easy to build 100 demodulators. The< need of corse the circuit diagrams .But I have got no a answer from Lou. Years before I have helped to retipping his 2 Grado pick ups for Shibata. He told me (e-mail) that the Grados are working fantastic. Sometimes ago I have told hi, that I would stay before his front door to see, if there is a rela protoype. He said, I would be welcome and could see and test all. But from Germany it is a "little" too far away. Meanwhile the story is the craziest in the hifi-world.
Dietrich
Dietrich
I think Lou had good intentions, but life come along and sends you off into other directions.
It's was something he really wanted to do, and looked forward to doing, but with his work obligations, home life, and medical issues, it just was too much. He never took any money and never took advantage of anyone, unlike other people doing other things, so we'll just have to pretty much give up on the idea and wish him well.
The few times I spoke with him on the phone (he called me, I never called him), he seemed very enthusiastic and upbeat. I hope he is doing well and continues his work with his record company and other projects. He has never come out and said this project is dead, but time will tell.
It's was something he really wanted to do, and looked forward to doing, but with his work obligations, home life, and medical issues, it just was too much. He never took any money and never took advantage of anyone, unlike other people doing other things, so we'll just have to pretty much give up on the idea and wish him well.
The few times I spoke with him on the phone (he called me, I never called him), he seemed very enthusiastic and upbeat. I hope he is doing well and continues his work with his record company and other projects. He has never come out and said this project is dead, but time will tell.
The Quadfather
1K Club - QQ Shooting Star
I guess I'll wait and see. That's all we really can do. If another party gets involved, (no pun intended), it will probably drive the cost up. I can't afford it now anyway. Perhaps when things get better, he'll finish it, and I can buy it then. Perhaps.
