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Old 22nd August 2011, 07:40 AM   #141
borges is offline borges  Norway
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Hi guys,

the perhaps easiest way to get galvanic isolation in my latest hardware is to insert optocouplers between the USB-I2S module and the Analog Board it sits on.

To everyone who is new to this project: AB-1.1 is split in two parts. One is a digital module with all the hard-to-solder stuff. It has connectors for USB, I2S and lots of other IO.

The other board is purely for analog things; clocks, power, DAC and audio connectors. We encourage analog hackers to toy with the analog board. Optocouplers and non-VBUS PSU are just some out of many options.


Cheers,
BÝrge
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Old 22nd August 2011, 07:53 AM   #142
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Hi all,

1. Isolation - there are no cheap and effectively USB isolation at HiGH speed (480MBPS). Isolation of the i2s signal may be possible. Isolation of the MCLK at 24 or so MHz is also difficult. You will NOT match the <1 ps jitter of the MCLK with our design. All the power supply onboard (digital to uC, digital to ES9023, analog to ES9023, XO etc can be supplied by external psu, eg LifePO4 or Silas shunt selectable via jumpers. We have considerable experience with USB noise (RF and AF) with our companion project sdr-widget where we run an AK5394A ADC with quiescent noise level of < -120 dBm WITH USB power.

So have a listening test of our AB1.1 powered by external power first - u will be surprised by the sound quality - at a cost an order of magnitude cheaper :-)

Having said all these, it may be worthwhile to try the widget with a high quality HIGH speed USB hub, which can block most of the noise from the PC.

2. 7.1 channel 192/24 widget - yes we are designing one.

Alex
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Old 22nd August 2011, 08:02 AM   #143
borges is offline borges  Norway
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Hi guys,

I agree with Alex, isolating the MCLK is not good jitter-wise. The crystal oscillator and its power supply are IMHO the _most_ important analog component. The analog core of clock, DAC and filters must be made together without any noise sources.

But the digital core with MCU and USB interface may very well be isolated from the analog core. One opto isolator option could send MCLK in one direction and remaining I2S lines the other way.

In our AB-1.1 design the DAC is MCLK master and the MCU is MCLK slave and I2S master.



BÝrge
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Old 22nd August 2011, 08:07 AM   #144
cviller is offline cviller  Denmark
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Quote:
Originally Posted by borges View Post
In our AB-1.1 design the DAC is MCLK master and the MCU is MCLK slave and I2S master.
So is it correctly understood that the AB clock is "pulling" the correct bit stream out of the computer through the MCU? What happens if the clock is "too fast" on the AB, does it increase pitch of the music?
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Old 22nd August 2011, 08:17 AM   #145
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Quote:
Originally Posted by alexlee188 View Post
Hi all,

1. Isolation - there are no cheap and effectively USB isolation at HiGH speed (480MBPS). Isolation of the i2s signal may be possible. Isolation of the MCLK at 24 or so MHz is also difficult. You will NOT match the <1 ps jitter of the MCLK with our design. All the power supply onboard (digital to uC, digital to ES9023, analog to ES9023, XO etc can be supplied by external psu, eg LifePO4 or Silas shunt selectable via jumpers. We have considerable experience with USB noise (RF and AF) with our companion project sdr-widget where we run an AK5394A ADC with quiescent noise level of < -120 dBm WITH USB power.

So have a listening test of our AB1.1 powered by external power first - u will be surprised by the sound quality - at a cost an order of magnitude cheaper :-)

Having said all these, it may be worthwhile to try the widget with a high quality HIGH speed USB hub, which can block most of the noise from the PC.

2. 7.1 channel 192/24 widget - yes we are designing one.

Alex
Hi Alex.

Sounds promising.

You should have the guy called Raytech ( even though he's a bit involved in the Exa project) comparing your device against his modded EXA2UI. Just to see where you're at in the real world.

From my rather limited background I havn't seen any interface which could live up its promises. Perhaps you got the first which does it.

Meanwhile most of the folks around here (after years of following this or that marketing claim) know that (noise) measurements and or buzzwords like aynchronous USB won't tell us everything.
Those little hidden secrets, that nobody is talking about, are usually the limiting factors, which will make the difference.


Anyhow. I'm more than interested to have a look at your device - especially
if it comes to multichannel I2S. Let see when that'll be available.

Cheers
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Old 22nd August 2011, 08:20 AM   #146
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Hi guys.

To the folks invloved in the project.

You might want to add a reference link to your project homepage in your signature.

THX.
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Old 22nd August 2011, 08:23 AM   #147
borges is offline borges  Norway
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Hi,

That's "YES" on both questions! This is the essence of asynchronous USB.

But the rating of the oscillators on AB-1.1 is quite good. Jitter-wise it is superior to the USB bus. Long-term stability wise it is probably much better as well. So if anything, I belive having the AB "pull" the clock as you say gives steadier pitch.

BÝrge

Quote:
Originally Posted by cviller View Post
So is it correctly understood that the AB clock is "pulling" the correct bit stream out of the computer through the MCU? What happens if the clock is "too fast" on the AB, does it increase pitch of the music?
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Old 22nd August 2011, 08:30 AM   #148
borges is offline borges  Norway
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We actually try to accomodate the people who are keen on sorting out the hidden secrets (myself included as an old analog hack). The goal is to let the USB-I2S module provide bit-correct digital audio on its own.

Then on the analog side of the fence please try your favourite discrete shunt regulators, class-A output stages, non-oversampling DACs, passive IVCs or indeed any topic on this forum :-)

When hunting for the hidden secrets, I hope it comes as a relief that the analog side controls the clock.

Cheers,
BÝrge

Quote:
Originally Posted by soundcheck View Post

Meanwhile most of the folks around here (after years of following this or that marketing claim) know that (noise) measurements and or buzzwords like aynchronous USB won't tell us everything.
Those little hidden secrets, that nobody is talking about, are usually the limiting factors, which will make the difference.

Anyhow. I'm more than interested to have a look at your device - especially
if it comes to multichannel I2S. Let see when that'll be available.

Cheers
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Old 22nd August 2011, 08:32 AM   #149
cviller is offline cviller  Denmark
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Great, just wanted to be sure. In my mind this is a better approach than to do FIFO reclocking or ASRC.

Does anyone have any experience with how this affect the sound timing when watching videos? Are some players automatically adjusting the rate to accommodate any slight pitch variations between "computer time" and "usb sound card time"?
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Old 22nd August 2011, 09:28 AM   #150
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Quote:
Originally Posted by borges View Post
We actually try to accomodate the people who are keen on sorting out the hidden secrets (myself included as an old analog hack). The goal is to let the USB-I2S module provide bit-correct digital audio on its own.

Then on the analog side of the fence please try your favourite discrete shunt regulators, class-A output stages, non-oversampling DACs, passive IVCs or indeed any topic on this forum :-)

When hunting for the hidden secrets, I hope it comes as a relief that the analog side controls the clock.

Cheers,
BÝrge

To me above thinking incorporates a big mistake.

Jitter sources must be avoided as far upstream as possible. Jitter, noise, intermodulations, power variations asf accumulate to a big mess the
further downstream you go.
Leaving it up to downstream devices to get the mess fixed later on is IMO the wrong thinking. The further downstream you go the more complex it gets to clean the mess up.
If your interface just offers a naked I2S you'll blow all the mess into the downstream DAC. That wouldn't work if I just hookup a TP Sabre or similar. The I2S signals you provide need to be
as perfect as possible.


Not any usb device I'm aware of, I'm honestly not sure if there's any, has managed to decouple from the incoming USB stream related mess.
It's been proven a million times that optimzations on the PC side (my key area of involvement), which is also an example for upstream improvements mentioned above btw, do have an impact
even on asynchrounous USB DACs.
Those PC related optimizations (I've been doing it for years on Linux environments / SB Touch) relate to getting the load down on the PC side, giving USB stream related processes high exlusivity, improving power supplies, running realtime kernels on the PC side asf asf...

The key issue is and was that the USB interfaces were not able to get rid of those little variations (logical and physical), which go beyond that bit-transparancy discussion ( which used to be a hot topic 4-5- years ago).
Bit-transparency is the least of all problems nowadays. It used to be much higher on the priority list. If you ask me, people had to find something that could be blamed for the bad sound quality we all experienced on earlier USB interfaces. Luckily somebody invented "audiophile" PC setups and players, to get around the still existing USB audio interface limitations. No. I'm not talking midfi stuff like EMU or similar being affected.
I'm talking about mega bucks audiophile USB audio interfaces such as Wavelength, Ayre you name it, which respond to those kind of optimizations. All those manufaturers still use Amarra or similar to present their interfaces for a good reason. Next time at RMAF you should ask those manufacturers to use standard iTunes - which is also bit-transparent - just to see what's actually in charge for the sound quality.
Audiophile PC setups do obviously have a much higher impact on lower quality devices.

Anyhow:

If you guys tell me that applications like Pure Music or Audiophilio on a MAC, Fidelizer+JRiver+Wasapi/Wasapi Event Style or XXHighend on a W7 or any other well known PC sided optimizations (I could even try your interface on my modded Squeezebox Touch which runs a standard Alsa driver with asynch USB being supported btw.) won't make a difference compared to normal audio applications and PC setups on your design, I'll place an order immediatly to check that out !!!

Let me know if you think it's worth to hand in an order. I'd love to.

Cheers

Last edited by soundcheck; 22nd August 2011 at 09:56 AM.
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