Virtual Loopback Audio Driver for PC DSP

[CopperTop]

Originally Posted by phofman

All envy24-based cards with spdif input can do that physically - they can switch the clock from internal to external - the clock recovered from the spdif input stream. Of course the proprietary driver can be buggy. But it should work at least in linux.

I am not sure about usb soundcards with spdif input since the input can behave independent of the outputs, depending on the usb receiver chip. Most inputs run isochronous asynchronous (which is a must for spdif input), most outputs run isochronous adaptive - two different clock domains.

Hi phofman

That isn't quite what I meant. As I understand it, this thread exists because people need to route audio from any source (SPDIF, CD, Windows media player, Spotify, Youtube) via DSP-based processing and finally out to their speakers, and it isn't immediately obvious that this is possible without resorting to two sound cards linked by SPDIF (jitter, re-sampling), or 'virtual audio cables' (re-sampling).

From my limited experience it is possible to do exactly what they need using only a single sound card and nothing else, although it isn't possible with all sound cards. I'd like to know which cards make it possible, and which don't.

The problem, as far as I can tell, is merely that the default routing of most sound cards is to connect any incoming stream internally to the analogue outputs. The Creative Audigy, for example, seems to do this unavoidably when you're using the Creative drivers. However, the open source Kx Project drivers allow you to turn off this internal routing. Unfortunately the Audigy always re-samples internally to 48kHz, reputedly not particularly well - although it sounds OK.

The more up-market Creative X-Fi can work at a variety of sample rates in 'bit perfect' mode, and its re-sampling (should that be necessary) is supposedly very, very good anyway. The card's standard drivers allow you to connect any input to any output with a sort of matrix arrangement. You can also turn off any internal routing - which is what you need for DSP processing.

Once you have turned off the sound card's internal routing, the setup for bit-perfect grief-free active crossover (in Windows at least) is as follows:

Set the Control Panel->Sounds and Audio Devices->Audio->Sound playback->Default Device to be the sound card in question. From now on, any standard media player will route its audio to the sound card's 'Wave' input, and you can also take in SPDIF or analogue line in if you want.

Set your DSP application's source to be the same sound card's input, and select whichever input you want ('Wave', SPDIF etc.) if you have the choice, or the simply the sound card's mixer as the source.

Set your DSP application's destination(s) to be the sound card's analogue outputs (or SPDIF).

You can now process any input that the sound card is capable of handling, and send your processed audio to the same sound card's outputs, locked to the same sample rate.

I must admit, this is one of situations where I'm slightly baffled as to why anyone would consider any other arrangement than this 'perfect' one, but I am also aware of the fact that some cards won't let you do it without wasting at least two of the outputs due to internal routing.

So which cards will let you do this?

[GDO]

Originally Posted by CopperTop

So which cards will let you do this?

Most studio grade cards have an ADAT, or Toslink, or SPDIF outputs that can easily be sacrificed for loopback purposes. That what i do with an RME Fireface.

My only worry with sound cards is that whatever their performance in digital domain, the solutions they implement in the analogue domain are done on the cheap, especially for low cost units ( and sadly mine, though not cheap, also plays in that league...).

Audio is basically analogue, and that question is too often overlooked by most digital stuff designers affected by the bit perfect syndrome.

[CopperTop]

Originally Posted by GDO

Most studio grade cards have ADAT outputs that can easily be sacrificed for loopback purposes. That what i do with an RME Fireface.

Very interesting. So is this done via some sort of 'applet' supplied with the card? Couldn't you simply turn off the link to the output altogether, or do you always have have at least one input->output connection selected?

(You couldn't include a screenshot of the relevant bit of the 'applet' if there is one? Thanks)

[GDO]

This applet is called Totalmix. It's a very nice routing/mixing utility that's bundled with all RME cards and which represent a big part of the value of their stuff.

How To Use Total Mix FX Loopback feature for capturing Windows Audio into Pro Tools 9 - YouTube

Used with a vst host makes a multiway dsp machine ( drc + over for instance) quite easy to setup.

[boris81]

Originally Posted by CopperTop

Hi phofman

That isn't quite what I meant. As I understand it, this thread exists because people need to route audio from any source (SPDIF, CD, Windows media player, Spotify, Youtube) via DSP-based processing and finally out to their speakers, and it isn't immediately obvious that this is possible without resorting to two sound cards linked by SPDIF (jitter, re-sampling), or 'virtual audio cables' (re-sampling).

From my limited experience it is possible to do exactly what they need using only a single sound card and nothing else, although it isn't possible with all sound cards. I'd like to know which cards make it possible, and which don't.

The problem, as far as I can tell, is merely that the default routing of most sound cards is to connect any incoming stream internally to the analogue outputs. The Creative Audigy, for example, seems to do this unavoidably when you're using the Creative drivers. However, the open source Kx Project drivers allow you to turn off this internal routing. Unfortunately the Audigy always re-samples internally to 48kHz, reputedly not particularly well - although it sounds OK.

The more up-market Creative X-Fi can work at a variety of sample rates in 'bit perfect' mode, and its re-sampling (should that be necessary) is supposedly very, very good anyway. The card's standard drivers allow you to connect any input to any output with a sort of matrix arrangement. You can also turn off any internal routing - which is what you need for DSP processing.

Once you have turned off the sound card's internal routing, the setup for bit-perfect grief-free active crossover (in Windows at least) is as follows:

Set the Control Panel->Sounds and Audio Devices->Audio->Sound playback->Default Device to be the sound card in question. From now on, any standard media player will route its audio to the sound card's 'Wave' input, and you can also take in SPDIF or analogue line in if you want.

Set your DSP application's source to be the same sound card's input, and select whichever input you want ('Wave', SPDIF etc.) if you have the choice, or the simply the sound card's mixer as the source.

Set your DSP application's destination(s) to be the sound card's analogue outputs (or SPDIF).

You can now process any input that the sound card is capable of handling, and send your processed audio to the same sound card's outputs, locked to the same sample rate.

I must admit, this is one of situations where I'm slightly baffled as to why anyone would consider any other arrangement than this 'perfect' one, but I am also aware of the fact that some cards won't let you do it without wasting at least two of the outputs due to internal routing.

So which cards will let you do this?

Thanks for the detailed write-up!
It's exactly what I had in mind. I have the software loopback driver idea because as you mentioned it's very hard to guess which sound cards allow to configure their hardware for internal routing. Usually music production sound cards provide some routing control but these often cost >$300.

It would be great if we can generate a list of sound cards that can be used in this fashion. I'm not opposed to using 2 sound cards or 2 PCs as long as costs are kept down and the signal path is well understood. An interesting option for example can be utilizing an onboard sound in addition to a hi-fi sound card. It would be interesting to investigate if the onboard sound driver can provide a loopback function without audible artifacts.

BTW, I'm very familiar with the Creative Audigy2 and I must say your description of the signal flow is precisely correct. With the KX-Project drivers one can actually do DSP in hardware eliminating the need for VST plug-ins. The only issue with it is being stuck with 16/48.

[boris81]

Is anyone successfully using any FREE vst host software?
I tried a few but none of them work well. The most common problem is either crashing or not recognizing plug-ins with more than 2 channels.

[GDO]

This one works pretty good in my set up, though i have had some crashes:

VSTHost

Though i use Console, which is not free, often crashes, and limited to 96khz, but i like better the gui.

PD: I have never had a crash playing music. Always after Console being On and idle during some ( rather long) time. No idea why.

[boris81]

I don't think that VSTHost supports plugins with more than 2 channels unless I'm doing something wrong. Crossover plugins for example usually accept 2 channels and output 4. I can't route all the output channels in VSTHost.

[GDO]

You can indeed use as many inputs and outputs that are available on your soundcard but you have to make the routing using the menu Engine Configuration ( Asign Input Channels / Asign Output Channels).
I have been testing this with ConvolverVst with a config file for the xover.

But as i said the gui in Console is much more intuitive.

[CopperTop]

Originally Posted by GDO

This applet is called Totalmix. It's a very nice routing/mixing utility that's bundled with all RME cards and which represent a big part of the value of their stuff.

How To Use Total Mix FX Loopback feature for capturing Windows Audio into Pro Tools 9 - YouTube

Used with a vst host makes a multiway dsp machine ( drc + over for instance) quite easy to setup.

Thanks. Looks like a very comprehensive solution.