Volume Control Function - Just dBFS?

[prout]

I am running a Roland Quad Capture ADC to USB into Audacity for recording. My mic volumes are preset to about max -3dBu up-line from a tube preamp. The Quad Capture preamps are set at minimum (unity gain). The Audacity volume control is not grayed out and is adjustable (I leave it at 100%). I don't understand what it is adjusting. The input to Audacity is already digital. I hope and can't believe that it is yet another DAC-ADC in the chain.

Can any one explain?

Edit: Changed "Quad Four" to "Quad-Capture"

[DVDdoug]

The Audacity volume control is not grayed out and is adjustable (I leave it at 100%). I don't understand what it is adjusting. The input to Audacity is already digital. I hope and can't believe that it is yet another DAC-ADC in the chain.

I'm pretty sure it's digital since I don't think it's controlling the analog gain in your interface. There is no additional DAC or ADC. I'd leave the Audacity recording volume at 100% and use the analog controls on you preamp or on your interface.

My mic volumes are preset to about max -3dBu up-line from a tube preamp

Unless you've got a calibrated 'pro" setup, there's no defined relationship between dBu and dBFS. And of course, if you are calibrated you can't touch the level/gain controls on the interface/ADC. The main thing is to make sure you not clipping your ADC.

[prout]

My concern is that I am already controlling the dBFS output of the Roland ADC, so I thought that the Audacity volume control would (should) have no effect on the incoming data stream via USB and be grayed out since it makes no sense to have a redundant control. I do leave it at 100%. My preamp output is set so that 0VU=+4dBu=-3dBFS on the Roland which should match, in a perfect world -3dBFS on the Audacity meter.

[Gale Andrews]

The Audacity volume control has the same effect that the Windows volume slider for the device does.

Some or most USB ports on Windows actually add gain, so the control is potentially needed.


Gale

[prout]

Changing the 'gain' of a digital audio datastream is actually changing the resolution of the digitized waveform (i.e., bit depth). Is this done in Audacity before or after the conversion to 32 bit float? If before, it would be unwise for anyone to record at 16 bit depth with the Audacity volume control at less than 100%, since the s/n ratio will decrease significantly, possibly into audible range.

[steve]

Is this done in Audacity before or after the conversion to 32 bit float?

It's not done in Audacity at all. As Gale indicated, the slider is just an alias for the Windows volume slider.

It's difficult to give a definitive answers / settings, because the exact behaviour depends on the computer sound system, and that is different depending on the operating system, the audio 'host', the sound system settings, the sound card hardware and the sound card drivers. For example, I have an old SoundBlaster card in which it is impossible to completely bypass the DSP chip when playing back through the front speakers, so for the most accurate D/A conversion it is necessary to use the rear speaker outputs - that's not a general rule, it's a quirk that's specific to one particular range of SB sound cards.

[Gale Andrews]

Is this done in Audacity before or after the conversion to 32 bit float?

It's not done in Audacity at all. As Gale indicated, the slider is just an alias for the Windows volume slider.

In other words, moving either slider makes the same change in the other.

Many USB ports will apply significant positive gain with the Windows/Audacity recording level sliders at maximum, less if the slider is turned down. Whether your device can control this is open to debate.

It's difficult to give a definitive answers / settings, because the exact behaviour depends on the computer sound system, and that is different depending on the operating system, the audio 'host', the sound system settings, the sound card hardware and the sound card drivers.

Audacity captures audio initially as 32-bit float samples, converting to that format if the samples are not received as such, irrespective of Audacity's Default Sample Format.

Unless you are using Windows WASAPI host in Exclusive Mode, Windows itself will already have converted your samples to 32-bit float before Audacity gets them.


Gale

[prout]

I am using WASAPI in exclusive mode. The Roland ADC output level in dBFS and the Audacity data stream input level in dBFS match precisely when the 'volume control' is set at 100%. This is what I expect to see if Windows has no control over the bit depth of the data stream.

To my way of thinking, neither Windows or Audacity should have any control over the bit depth of the data stream when receiving it, any more than either of them does with an imported data file, and the control should be disabled. For example, if the live data stream contains clipped segments, lowering the 'volume control' will not remove the clipping, nor should it. Once the file (live or pre-recorded) is received by Audacity, then, of course, the effects added (gain, panning, fading, etc.) will affect the bit depth, relative to 32 bits. That's OK, and expected.

I assume from your comments that the 'volume control' works in 32 bits, even though my exclusive data stream is in 24 bits and upconverted before being displayed.

Thanks guys, this is helping me.

[DVDdoug]

To my way of thinking, neither Windows or Audacity should have any control over the bit depth of the data stream when receiving it, any more than kHz either of them does with an imported data file, and the control should be disabled.

Windows drivers make everything compatible... For example, you can play a 196kHz, 32-bit floating-point file on any-old cheap soundcard that only runs at 44.1kHz 16-bits. It's the same philosophy that allows any Windows-compatible printer or video display to work with any Windows application and images of any resolution.

Pros often use ASIO hardware+drivers+applications which are less flexible and there is no resampling (and maybe lower latency). Audacity (as compiled & distributed) doesn't support ASIO, and it doesn't appear that Roland offers ASIO drivers for your interface.*

WASAPI in exclusive mode is similar to ASIO but I'm not 100% sure that it prevents resampling.





* There is something called ASIO4ALL that replaces part of the "driver stack" and makes your regular Windows hardware "look like" an ASIO device so it works with an ASIO applicatinon. That won't help here because Audacity is not an ASIO application.

[Gale Andrews]

WASAPI in exclusive mode is similar to ASIO but I'm not 100% sure that it prevents resampling.

Why exactly are you not 100% sure? Windows should not be applying explicit sample rate or bit depth conversions.


Gale