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General Discussions => Truth Table => Topic started by: living sounds on December 20, 2018, 04:23:30 PM

Title: What's inside a Prism Dream AD-2 converter
Post by: living sounds on December 20, 2018, 04:23:30 PM
It's impossible to find reliable information how conversion is achieved in this unit. Does it use common sigma delta chips or is actually discrete as claimed in a few articles from usually respactable magazines? The only high end converters since the early 90s rumored to not use sigma delta modulation are the two Pacific Microsonics units and the  old db technologies / Lavry Gold model. No idea how this is practically achieved.

The Prism AD-2 has specs unobtainable with the chips availible at its inception in the late 90s. Did they maybe cleverly combine more than one chip per channel?

Any information is welcome.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: boji on December 25, 2018, 08:26:28 PM
It would be nice to see internals, but I imagine the warranty sticker on the seam of a $5k+ device holds a fair amount of power over an owner's curiosity.  :)

Title: Re: What's inside a Prism Dream AD-2 converter
Post by: Gold on December 25, 2018, 10:07:36 PM
9K+  new.  I have one but have never tried to figure out what’s going on. I know they are hand built. Still sound great.  Vintage digital.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: living sounds on December 26, 2018, 06:23:13 PM
9K+  new.  I have one but have never tried to figure out what’s going on. I know they are hand built. Still sound great.  Vintage digital.

Have you shot it out against other converters? Anything you prefer?
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: Gold on December 26, 2018, 08:25:26 PM
Have you shot it out against other converters? Anything you prefer?

I still think there is nothing better available. I haven't shot it out with anything lately. My test is how a converter sounds with tape as a source. Once audio has gone through a reconstruction filter the difference between converters is very small. Using a music  file as a source to test converters will surely lead you to think  all converters sound the same. They don't.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: living sounds on December 27, 2018, 10:48:19 AM
Thanks Paul!
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: living sounds on January 04, 2019, 09:10:18 AM
I got one and had a peek inside. I think the actual converter ICs are under a big heat sink connected via thermal grease. I won't touch anything under the hood, so the mystery will stay unsolved for now. It all appears to be mostly late 90s surface mount parts, nothing fancy. Looks really solid, industrial no-corners-cut design.

Now to test how it sounds and measures.   8)
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: boji on January 04, 2019, 03:59:58 PM
Quote
I got one and had a peek inside.

Ok you are obligated to share that peak with us, given this thread.   ;D ;D
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: living sounds on January 04, 2019, 06:36:21 PM
All right, but I've only got a cell phone foto:

https://www.dropbox.com/s/au45pc0sxq47hc9/AD-2.jpg?dl=0
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: living sounds on January 04, 2019, 06:48:50 PM
I did some tests, the performance is really remarkable, the noise floor is much lower than with any other converter I've ever used. There appears to be active DC offset  compensation that slowly removes static (or at least slow moving) offsets.

Title: Re: What's inside a Prism Dream AD-2 converter
Post by: Gold on January 04, 2019, 07:23:54 PM
The real time SRC is also one of the best sounding SRC’s there is. It took others a long time to catch up.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: boji on January 05, 2019, 03:31:15 AM
Quote
All right, but I've only got a cell phone foto:

I was mostly joking, but thank you! 
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: JohnRoberts on January 06, 2019, 10:29:00 AM
The mystery may not be revealed by visual inspection. The good performance is probably a combination of good hardware "and" good software, to slice and dice and post process the oversampled data, and/or multiple conversion data streams.

There is a tendency to think conversions are fully characterized by the codec or chip set used, but execution of that design still matters.

JR
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: living sounds on January 06, 2019, 10:59:34 AM
The mystery may not be revealed by visual inspection. The good performance is probably a combination of good hardware "and" good software, to slice and dice and post process the oversampled data, and/or multiple conversion data streams.

There is a tendency to think conversions are fully characterized by the codec or chip set used, but execution of that design still matters.

JR

The SNR is really uncanny. The evaluation boards of current chips don't get close. Maybe there are multiple gain stages combined or something along these lines.

As for distortion, here is an example of someone reducing THD in a DAC by pre-processing the signal:

https://www.diyaudio.com/forums/equipment-and-tools/328871-digital-distortion-compensation-measurement-setup-2.html#post5584029

Don't know if this would work for program material, but it's interesting.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: JohnRoberts on January 06, 2019, 11:15:33 AM
The SNR is really uncanny. The evaluation boards of current chips don't get close. Maybe there are multiple gain stages combined or something along these lines.
Not sure how extra gain stages could help, but poor gain stage execution could hurt.

In over sampled conversions noise shaping can shift wide band noise energy out of the <20kHz bandwidth. I don't know what/if the trade-offs are from gaming this (shifting in band noise away too?). Caveat this is just a WAG. 
Quote
As for distortion, here is an example of someone reducing THD in a DAC by pre-processing the signal:

https://www.diyaudio.com/forums/equipment-and-tools/328871-digital-distortion-compensation-measurement-setup-2.html#post5584029

Don't know if this would work for program material, but it's interesting.
Pre-distortion is relatively old technology for correcting known nonlinearities. I am not sure how this applies to digital conversions. (I followed the link but did not find that specific discussion) 

The actual distortion from better modern conversion technology seems acceptably low. Since distortion is often measured as THD+N, it seems like noise floor matters.


JR
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: living sounds on January 06, 2019, 11:48:19 AM
Not sure how extra gain stages could help, but poor gain stage execution could hurt.

Maybe gain stage is the wrong term. What I meant was something along the lines of two converters in parallel, one optimized for lower and one for higher amplitude signals, and the post-processing selecting which feed is used for the output depending on amplitude. Just speculating here, I don't know enough to judge the viability of my idea.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: JohnRoberts on January 06, 2019, 12:07:58 PM
Maybe gain stage is the wrong term. What I meant was something along the lines of two converters in parallel, one optimized for lower and one for higher amplitude signals, and the post-processing selecting which feed is used for the output depending on amplitude. Just speculating here, I don't know enough to judge the viability of my idea.
Funny, I was going to mention that several days ago but decided to not clutter up the thread while somebody might still present objective information.

I looked into that decades ago because it seems like an inexpensive way to extend dynamic resolution (level shift audio N dB between two convertors then seamlessly combine the digital output to buy extra bits of resolution. What isn't obvious is that the upper convertor linearity has to far exceed it standard linearity (for that number of bits) so the combined digital output is monotonic.   

Imagine if one LSB of the top convertor represents 4 bits or more (24dB) in the lower convertor.  This nonlinearity some 20dB above the quantization floor, is not likely to sound very bad but as usual no free lunch. There may be other gotcha's but this was enough for me to abandon it a couple decades ago.  :(

===

Bench performance could be gamed with transparent gain scaling (before/after). So bench tests need wiggle all the bits. Gain scaling would show up as noise floor modulation.

JR   

 
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: Gold on January 06, 2019, 01:51:22 PM
Maybe gain stage is the wrong term. What I meant was something along the lines of two converters in parallel, one optimized for lower and one for higher amplitude signals, and the post-processing selecting which feed is used for the output depending on amplitude. Just speculating here, I don't know enough to judge the viability of my idea.

I remember hearing that there are four discrete converters per channel in parallel.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: living sounds on January 06, 2019, 02:08:40 PM
I remember hearing that there are four discrete converters per channel in parallel.

"Discrete" as in resistor ladders (or other single components) - or do you mean 4 AD ICs per channel?
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: JohnRoberts on January 06, 2019, 04:14:11 PM
I remember hearing that there are four discrete converters per channel in parallel.
That works too but there are quickly diminishing returns ... Adding two convertors together buys you 1/2 bit(?). Signal adds coherently, noise incoherently so +3dB net gain in S/N... 4 convertors double again so one full bit improvement... +6dB in not nothing and now not as impractical as back when.

JR
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: Gold on January 06, 2019, 04:24:24 PM
"Discrete" as in resistor ladders (or other single components) - or do you mean 4 AD ICs per channel?

I'm not sure but I know there were no AD IC's that gave anywhere near that performance in the early 90's.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: living sounds on January 06, 2019, 05:32:50 PM
I'm not sure but I know there were no AD IC's that gave anywhere near that performance in the early 90's.

Late 90s, but still, yes. The best chips today are not as good as the AD-2.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: iampoor1 on January 07, 2019, 01:17:26 AM
I wonder if they rolled a custom A/D converter using an FPGA? That might be what's under the heatsink....Might explain the "discrete" aspect too IE: They designed it out of discrete gates in the FPGA  ;D

Title: Re: What's inside a Prism Dream AD-2 converter
Post by: living sounds on January 07, 2019, 05:24:46 AM
I wonder if they rolled a custom A/D converter using an FPGA? That might be what's under the heatsink....Might explain the "discrete" aspect too IE: They designed it out of discrete gates in the FPGA  ;D

There's not much room under these heatsinks. The ICs are really small.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: mhelin on January 07, 2019, 10:07:24 AM
Currently AKM's AK5578EN (8 ch A/D) for an example can be configured to parallel it's all eight converters for 130 dB SNR (112 dB THD). You will still need two chips for 2-channel conversion to get that SNR.

https://www.akm.com/akm/en/product/datasheet1/?partno=AK5578EN
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: living sounds on January 07, 2019, 10:25:09 AM
Currently AKM's AK5578EN (8 ch A/D) for an example can be configured to parallel it's all eight converters for 130 dB SNR (112 dB THD). You will still need two chips for 2-channel conversion to get that SNR.

https://www.akm.com/akm/en/product/datasheet1/?partno=AK5578EN

Yes, today you can get even better performance from a SAR chip. I'm still waiting for a commercial product for pro audio.

But the AD-2 came to market in 1998.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: JohnRoberts on January 07, 2019, 10:39:10 AM
Currently AKM's AK5578EN (8 ch A/D) for an example can be configured to parallel it's all eight converters for 130 dB SNR (112 dB THD). You will still need two chips for 2-channel conversion to get that SNR.

https://www.akm.com/akm/en/product/datasheet1/?partno=AK5578EN
Yup... 3dB for every doubling so 1-2 is 3dB, 2-4 is 6db, 4-8 is 9dB... not nothing but they would need to go to 16 for the next 3 dB.

JR

[edit] another strategy available to increase resolution as convertors get faster and faster is more over sampling.  If the convertor is fast enough that you can capture multiple samples within a single output sample interval, those samples can be averaged together to improve resolution following a similar rule (3 dB per doubling). I have used this inside digital meters with modest A/D resolution. Since I didn't care about aliasing (in a meter data stream) I combined together multiple samples at lower than sampling theory wants for frequency integrity to realize more meter resolution.  [/edit]
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: Juanpcdiy on March 06, 2019, 04:37:50 PM
crap because it does Not sound better than others....

the New Prism Atlas are much better.

other interesting are the Lavry Gold, very interesting, has temperature controlled self calibrating resistor network, its a true resistor network AD converter...
but the clock is crap, has FIFO a memory buffer used as clock source, with 60ns / 80ns jitter. "too much."
also has too much latency, and warm up time. 15~30minutes.
the lavry blue bad, the black are ok.

i have apogee rosetta800 192k, the AD is very natural, dynamic, but the DA feels weird, lacks bass, and sounds less dynamic...
sometimes i want to modify it, some times i just want to sell.

the latest Focusrite RED 8Pre are very interesting they have dual parallel paths, to phase add signal, and phase cancel distortion...
because analog signal is balanced before AD converters, or After DA...

see differential signal & twisted pair technology..

The UA Apollo, classic 5532 sound, Not my thing/taste.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: gyraf on March 07, 2019, 02:04:23 AM
Juan,

This thread is about technology, not about what we think "is crap" or "is interesting" - that sort of discussion is much more interesting to take to gearslutz or somewhere like that, where majority of users would buy into the idea that everyone can differentiate between modern converters (in a blind test)..

Jakob E.

Title: Re: What's inside a Prism Dream AD-2 converter
Post by: living sounds on April 21, 2019, 05:13:58 PM
I think there is a (two or more) stage system involved. With a high frequency (18k) test tone input I can see distortion products in the midrange rise linearly while increasing the test tone amplitude. At a certain point of increase, the distortion suddenly drops. This is predictable and reproduceable.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: ChrioN on July 31, 2019, 09:39:40 PM
...where majority of users would buy into the idea that everyone can differentiate between modern converters (in a blind test)..

Jakob E.

Right?! I can't even tell if that electric bass went thru a blue stripe or if it was a blackface, in the mix. Neither spot if the vocals did or didn't go thru the pultec set on "bypass". sh*tty damn ears.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: e.oelberg on September 01, 2019, 02:41:23 PM
btw the new sound devices Mix Pre 3 II have an astonishing 142 dB dynamic range  https://www.sounddevices.com/mixpre/ (https://www.sounddevices.com/mixpre/)
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: mhelin on September 02, 2019, 03:26:14 AM
"The mixpre3 uses the smaller 6 channel AK5576EN in a 6-to-3 mode giving it better dynamic range.  "
https://jwsoundgroup.net/index.php?/topic/33053-zoom-f6-a-32bit-recorder/&page=2
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: e.oelberg on September 03, 2019, 06:39:29 AM
Thanks For the Info
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: cyrano on September 03, 2019, 07:09:00 AM
"The mixpre3 uses the smaller 6 channel AK5576EN in a 6-to-3 mode giving it better dynamic range.  "
https://jwsoundgroup.net/index.php?/topic/33053-zoom-f6-a-32bit-recorder/&page=2

Zoom has a seemingly similar 32-bit setup in one of their new portable recorders. The Sound Devices one is patented. Wonder what's original enough to get a patent for it. And who they'll hit with it, of course.
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: JohnRoberts on September 03, 2019, 09:17:25 AM
Zoom has a seemingly similar 32-bit setup in one of their new portable recorders. The Sound Devices one is patented. Wonder what's original enough to get a patent for it. And who they'll hit with it, of course.
Patents are published so you should be able to read what they claim...

JR
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: cyrano on September 03, 2019, 11:28:18 AM
This kind of patent is usually heavily obfuscated, if it is already publicly available at all...
Title: Re: What's inside a Prism Dream AD-2 converter
Post by: JohnRoberts on September 03, 2019, 01:07:34 PM
This kind of patent is usually heavily obfuscated, if it is already publicly available at all...
By law the patent is supposed to show the "preferred embodiment" or best way. While I have never heard of this happening, obvious obfuscation could be used to deny an application from ever winning a patent.

That said companies are motivated to not give away the entire farm, but they "must" describe adequately how the invention works.

 JR