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Author Topic: software generator loopback  (Read 7466 times)

Alfredo Prada

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software generator loopback
« on: November 02, 2011, 05:32:05 am »

Hello, everyone

Thanks for starting this measurement forum. my subject is that when using the internal generator loopback (Smaart 7 / Systune) the delay times are not accurate and my understanding is because (in part) of the clock on the input and output of some cards not being synchronized, if this is true which sound cards would be time synchronous and is this enough to fix the delay inaccuracy? I know Systune will soon release a sound card to work with their internal generator and Smaart 7 is working on one also.
I am using the USB pre2 and I know this is not time synchronous and I would like to be able to use the 2 mic inputs and use the internal loopback without the delay issue.

thanks for any advice
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Mac Kerr

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Re: software generator loopback
« Reply #1 on: November 02, 2011, 11:27:46 am »

Hello, everyone

Thanks for starting this measurement forum. my subject is that when using the internal generator loopback (Smaart 7 / Systune) the delay times are not accurate and my understanding is because (in part) of the clock on the input and output of some cards not being synchronized, if this is true which sound cards would be time synchronous and is this enough to fix the delay inaccuracy? I know Systune will soon release a sound card to work with their internal generator and Smaart 7 is working on one also.
I am using the USB pre2 and I know this is not time synchronous and I would like to be able to use the 2 mic inputs and use the internal loopback without the delay issue.

thanks for any advice

Why is the added latency a problem? It doesn't matter as far as your measurements are concerned, as long as the 2 streams being compared are in time with each other.

For finding the delay time between mains and delay speakers you are only concerned with the difference between the two measurements, not the absolute number that shows up as delay for one system or the other. The difference will be the same whether you use an external reference in for lower latency, or the loopback which incurs higher latency.

Both transfer function and finding speaker delay offsets will be the same either way.

Mac
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Doug Fowler

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Re: software generator loopback
« Reply #2 on: November 02, 2011, 02:12:59 pm »

Why is the added latency a problem? It doesn't matter as far as your measurements are concerned, as long as the 2 streams being compared are in time with each other.

For finding the delay time between mains and delay speakers you are only concerned with the difference between the two measurements, not the absolute number that shows up as delay for one system or the other. The difference will be the same whether you use an external reference in for lower latency, or the loopback which incurs higher latency.

Both transfer function and finding speaker delay offsets will be the same either way.

Mac


Mac -

In this case, it becomes an apples to oranges comparison.

I don't know how other programs handle it but SysTune monitors the stream internally when using the "Out" setting as the reference channel.  The problem as I understand it is Windows applies an arbitrarily latency as well, and it can change each time the system is rebooted.

This leads to timing errors, not such a huge deal with unwindowed magnitude measurements but causes problems when using phase, or in some cases can cause windowing errors.  It's not recommended in SysTune unless using the EASERA Gateway or the new AUBION x.8, which use a digital loopback scheme to get around this vexing issue.

If it were merely added equal latency to both measurement and reference streams, then yes, no big deal but it's not the case here.

Moral of story: use a Y-Cable to implement the reference channel :-)
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Mac Kerr

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Re: software generator loopback
« Reply #3 on: November 02, 2011, 03:01:56 pm »

Mac -

In this case, it becomes an apples to oranges comparison.

I don't know how other programs handle it but SysTune monitors the stream internally when using the "Out" setting as the reference channel.  The problem as I understand it is Windows applies an arbitrarily latency as well, and it can change each time the system is rebooted.

This leads to timing errors, not such a huge deal with unwindowed magnitude measurements but causes problems when using phase, or in some cases can cause windowing errors.  It's not recommended in SysTune unless using the EASERA Gateway or the new AUBION x.8, which use a digital loopback scheme to get around this vexing issue.

If it were merely added equal latency to both measurement and reference streams, then yes, no big deal but it's not the case here.

Moral of story: use a Y-Cable to implement the reference channel :-)

Doug,

As long as the latency is the same for all streams being analyzed how are the results changed? AFAIK the measured channel needs an offset to match in time with the reference, so any added latency in the reference due to the loopback vs external input should be added to the measured signal when the impulse response is matched.

If the latency is arbitrary it might effect your ability to compare 2 measurements done at different times, but surely it is the same throughout any given measurement session.

Mac
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Arthur Skudra

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Re: software generator loopback
« Reply #4 on: November 07, 2011, 03:33:52 pm »

my subject is that when using the internal generator loopback (Smaart 7 / Systune) the delay times are not accurate and my understanding is because (in part) of the clock on the input and output of some cards not being synchronized, if this is true which sound cards would be time synchronous and is this enough to fix the delay inaccuracy?
It's not an inaccuracy, it's just added propagation delay by using the software loopback with your interface.  Nothing more than that, nothing to be concerned about.  Back in the good old days of external loopback, you had a couple ms of propagation delay going through the interface, with the software loopback its an additional 40 to 60 ms of delay, depending on the interface used, driver software, and operating system.  But does it really matter?  No, just a tradeoff in what you're comfortable with.  Maybe in a future version of the software an offset can be put in the delay to make people feel better?

I am using the USB pre2 and I know this is not time synchronous and I would like to be able to use the 2 mic inputs and use the internal loopback without the delay issue.
Using the USBPre2's built in loopback feature you lose one input on the interface.  Essentially it's like an external loopback, without the cable.  If you want to use 2 mics with your USBPre2, then you should use the software loopback feature in Smaart 7.3.  The new USBPre2 is time synchronous and will work fine with the Smaart software loopback function, it's the older/original USBPre that isn't and will give you erroneous results with the software loopback.
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Alfredo Prada

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Re: software generator loopback
« Reply #5 on: November 10, 2011, 07:52:05 am »

Thank you Arthur and everyone else for your replies, I will start using the USBpre2 with 2 mics and see how it works.

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Dan Richardson

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Re: software generator loopback
« Reply #6 on: November 13, 2011, 10:08:00 am »

The new USBPre2 is time synchronous

That's not my experience, except with their ASIO drivers.
Unfortunately for me, the ASIO drivers crash EASERA Systune.
Actually, every ASIO driver I have crashes EASERA Systune.
Fortunately for me, the USBPre2 loopback mode seems to get me
everything I need using XP's native DirectSound drivers.

Just starting on this analysis stuff after 30 years in sound. It's interesting.
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Ivan Beaver

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Re: software generator loopback
« Reply #7 on: November 13, 2011, 10:50:21 am »

That's not my experience, except with their ASIO drivers.
Unfortunately for me, the ASIO drivers crash EASERA Systune.
Actually, every ASIO driver I have crashes EASERA Systune.
Fortunately for me, the USBPre2 loopback mode seems to get me
everything I need using XP's native DirectSound drivers.

Just starting on this analysis stuff after 30 years in sound. It's interesting.
I use the Motu 8 pre and it works just fine on Systune.
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Arthur Skudra

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Re: software generator loopback
« Reply #8 on: November 14, 2011, 01:28:20 pm »

That's not my experience, except with their ASIO drivers.
Unfortunately for me, the ASIO drivers crash EASERA Systune.
Actually, every ASIO driver I have crashes EASERA Systune.
Fortunately for me, the USBPre2 loopback mode seems to get me
everything I need using XP's native DirectSound drivers.

Just starting on this analysis stuff after 30 years in sound. It's interesting.
I'm not sure about EASERA Systune and the specific requirements of that program, however the OP is talking about using the USBPre2 with Smaart, and I have confirmed with the folk at Rational Acoustics that it indeed does work and is synchronous for their purposes.  Maybe you have some conflicts on your computer with Sound Device's ASIO drivers?  I'm a Mac guy myself, I guess that with Windows, anything is possible!   :P
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Dan Richardson

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Re: software generator loopback
« Reply #9 on: February 28, 2012, 03:29:04 pm »

I'm not sure about EASERA Systune and the specific requirements of that program, however the OP is talking about using the USBPre2 with Smaart, and I have confirmed with the folk at Rational Acoustics that it indeed does work and is synchronous for their purposes.  Maybe you have some conflicts on your computer with Sound Device's ASIO drivers?  I'm a Mac guy myself, I guess that with Windows, anything is possible!   :P

Now that I'm a SMAART owner, I've hit it myself, and found the solution thanks to Adam at Rational. With the internal generator and a USBPre2, you must use the ASIO drivers for it to be synchronous. Other Windows drivers toss out a different delay time on each playback.
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