Author Topic: "Set digital gain to 1" clarification  (Read 8645 times)

salty

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"Set digital gain to 1" clarification
« on: January 28, 2013, 04:45:23 am »
Hi, I recently got back into vinyl after a long hiatus and I picked up an ART USB Phono Plus to do some digital conversions.  It came with some software but I liked Vinyl Studio better (and it's still being developed) so I bought it.  I'm using Win7.

But, even after reading every topic I could find I'm still a bit confused about recording levels.  I went through this before - when I bought the box it was incredibly distorted, I thought it was defective, then I saw the levels (with the sider set to 100) were pegged.  The manual recommends setting the slider at 4, and I'd seen elsewhere online to set it at 5.  I was using the latter because it seemed to be a good level without distorting.

Then, I got VS, and when I hit "Set digital gain to 1" it set the slider to 3.  First, I wanted to confirm that is the intended result.  Second, why 3?  Is that documented somewhere (by Microsoft) or did you determine it empirically?  Just to be pedantic - this should mean that whatever numeric value the ADC on the device is producing is exactly the numeric value received by VS and encoded in the WAV file, right?

When I have the level set to 3, the levels seem really low, like the peak absolute sample values are in the range of 22k, whereas with the level set to 5 (comparing the same album) they are right below clipping (32k), which I think makes sense since 3/5 of 32k is ~20k and I may have messed with the gain control on the device.  I could feed in a constant tone and try some experiments...

About that gain control, it is on the USB Phono+ itself, as well as a Signal/Clip light.  The manual has the (dubious?) advice "Adjust the Gain Trim while noting the Signal/Clip LED so that green is on most of the time (with music playing) and red is on rarely during signal peaks."  I say "dubious" because f it's really a "clip" LED, I wouldn't want it to be red at all.  I'm trying to adjust it so it stays green, but I do see it flash red very occasionally.

But, putting those things together, does that mean when the LED on the device flashes red, the ADC in the device is really only outputting ~20k and there's still plenty of headroom?  So, I should turn up the gain rather than messing with the slider or normalizing?

Sorry, this got way longer than I intended, just trying to understand exactly what is happening...

Paul Sanders (AlpineSoft)

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Re: "Set digital gain to 1" clarification
« Reply #1 on: January 28, 2013, 10:47:16 am »
Hi,

The slider level of 3 (which I agree sounds ridiculously low) is actually determined by Windows.  We tell Windows to set the digital gain to 1 and that's the slider level we wind up with.  Strange but true.  And yes, you're right, this means that a signal level of, say, 10000 coming from the ADC gets recorded as 10000 in your audio file.

So you should not adjust the slider after clicking the button.  Use the gain control on the device instead, guided not by the lights on the box but by the recording level indicators in VinylStudio's Check Level dialog.  This will give you the most dynamic range before clipping.  When I was recording my own collection I found a suitable setting and left it there but some people like to adjust the level for each recording.

HTH.

salty

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Re: "Set digital gain to 1" clarification
« Reply #2 on: February 02, 2013, 10:04:08 pm »
Thanks for the very quick and detailed reply.  I ran some experiments today and indeed what you said appears to be 100% correct.

I wrote a small program to output the top N sample values and their offsets so I could look for "flat" groups of samples to indicate clipping.  I kept the windows slider set at 3 and increased the gain on the USB interface until I just barely saw clipping, and verified that only happened when the sample values reached exactly 32767/-32768.

I was a bit suspicious about whether the USB interface did any sort of AGC or other nonsense to avoid clipping, but that doesn't appear to be the case.  I changed the windows slider to 2 and I saw flat groups of samples at 16580/-16581.  That's a bit odd mathematically, but in any case I think it's clear evidence that the signal was being clipped in the USB interface.

So, the previous advice I'd followed about setting the slider to 4 or 5 was costing me a lot of dynamic range.  I used to have the gain control set around +2dB; now it's properly set around +7dB.  At that setting the "clip" light is flashing red like crazy but I'm convinced that it's not actually clipping.

Thanks for clearing up that bit of misinformation floating around, and thanks again for your work on VS!

Paul Sanders (AlpineSoft)

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Re: "Set digital gain to 1" clarification
« Reply #3 on: February 02, 2013, 11:52:08 pm »
Excellent, thank you for the update.  Strange that the light is so misleading but it needs more than just a light to measure the input level I guess.  Other ART USB Phono Plus users please take note.

claretpacker

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Re: "Set digital gain to 1" clarification
« Reply #4 on: May 24, 2017, 01:31:22 pm »
Have recently purchased Vinyl Studio, I think it is great.

I am still a little confused about this subject though and levels/clipping in general.  I have posted this as a reply as I too am using the ART USB Phono Plus between my deck (Pro-ject 1-Xpression with Ortofon Red cartridge) and my PC.  In contrast to salty above, if I click set digital gain to 1, with the gain trim knob on the device set to 0, I immediately get clipping.  The slider control goes to 54.  I eventually lose the clipping by sliding the gain trim knob down to -2.  Does this sound like the correct setting.

On clipping:  Before finding this post, I have been manually adjusting the slider to about 50 with the gain trim knob set to 0.  This sees the level indicators popping into the red but not displaying the word CLIP and the clip count is zero. (In the old days of recording cassettes from vinyl I was always advised that the indicators should just flick into the red now and again and this would get the best recording level). When I move on to split the tracks I see that the tops and bottoms are levelled off and flat, not nicely rounded or all contained within the top/bottom limits.  Should I be aiming to contain the levels within the graphic?  When I play back the recordings they sound pretty cool whether on my hifi or in the car.

Thanks

Paul Sanders (AlpineSoft)

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Re: "Set digital gain to 1" clarification
« Reply #5 on: May 24, 2017, 05:37:56 pm »
Hi,

If you adjust the slider downwards after clicking the button you are just hiding the clipping from the software.  It's still happening but VinylStudio can no longer detect it.  Better to leave the slider where VinylStudio puts it and know the truth.

The point is that the clipping is happening inside the ART, and adjusting the slider doesn't change this.  You could take this up with them (although there's nothing they can do either, other than suggest that you fit a lower output cartridge to your turntable).

salty

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Re: "Set digital gain to 1" clarification
« Reply #6 on: May 24, 2017, 06:34:26 pm »
Usually when you clip with digital (flattened peaks as you observed) it sounds pretty nasty, although if you're not clipping that badly it might not be noticeable.  With analog tape the distortion is much more gradual.

What Paul said about "hiding" the clipping by turning down the slide is definitely true (which is what I was experimenting with in my post from 2 Feb 2013).  The ADC inside the box puts out a 16-bit signed value, and the slider makes Windows multiply that value by some scaling factor (which can be greater or less than 1).  If you turn the slider down, then the ADC might be putting out a max/min-value sample (which is what VS detects as a clip, I believe) but since it's being scaled down VS cannot tell.

"Set gain to 1" is supposed to set that factor to 1, so Windows doesn't change the incoming sample values at all.  Why they don't just expose the actual number to the user rather than showing some arbitrary slider value, I don't know.  I wonder if something has changed between Windows 7 and 10 (assuming you're using 10?) so that what used to be shown as "3" is now "54".  Let me check when I get home; I'm not sure I've actually ripped any vinyl since I upgraded.

Once that is done, then you just need to adjust the gain on the ART to prevent clipping - as I noted, the "signal" light on the ART will occasionally blink red even if it's not actually clipping; I shoot for a peak somewhere around 90 as reported by VS.  The -2 does sound a bit odd; I also use a 2M Red cart and IIRC I'm almost always on the positive side, sometimes as high as +6, some albums are closer to 0 although I don't recall going negative.  What does the light on the box look like when you're at -2?  Let me play around when I get home.

Paul Sanders (AlpineSoft)

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Re: "Set digital gain to 1" clarification
« Reply #7 on: May 24, 2017, 06:47:21 pm »
> I wonder if something has changed between Windows 7 and 10 (assuming you're using 10?) so that what used to be shown as "3" is now "54".

Yes, I'm sure that's what's happened.  And I was not aware that the ART has gain values lower than 0, sorry.  A good thing, given what is happening.

salty

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Re: "Set digital gain to 1" clarification
« Reply #8 on: May 24, 2017, 06:56:12 pm »
Yeah, the device itself has an analog gain knob - it definitely makes it a lot more usable.  Although, given the SNR of vinyl I often wonder why I don't just record at a low level and then normalize.  Trying to hit ~90% means I end up having to re-record a lot of album sides because there's some peak later on.

http://artproaudio.com/images/products/usb_phono_plus-ps/usb_phono_plus-ps_angle_lg.jpg

salty

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Re: "Set digital gain to 1" clarification
« Reply #9 on: May 25, 2017, 04:10:38 am »
Well, I can confirm that in Win10 pressing the "set digital gain to 1" button also sets the slider to 54.

It occurred to me there is a far easier way to empirically determine that is the correct value (rather than writing a program to look at sample values like I did before). Knock the slider down to 53 and crank the knob on the ART all the way while playing something (loud).  This guarantees the ADC is clipping, but you'll see VS doesn't show anything higher than 98, because windows is reducing the amplitude. Put the slider back on 54 and it will show CLIP.  So, 54 is definitely the correct value.

Maybe I was wrong about where I set the gain knob, though - the album I'm currently listening to (Fugazi - 7 Songs) I have it set right at 0 (maybe even a tad negative) and it's made it up to 96.  The light does glow red on the peaks.

claretpacker

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Re: "Set digital gain to 1" clarification
« Reply #10 on: May 25, 2017, 02:21:41 pm »
Hi, thanks for the replies, it is all a little clearer now.  I am indeed using Windows 10, that seems to explain 54 vs 3 then, it seemed hugely different, I'm pleased my setup is "normal"!

So I have set the digital gain to 1 (level 54); I have set the gain knob on the ART to about -1.5 (the slightest of adjustments more positive sends everything into the red, lights and level indicator). I didn't see any red lights on the ART box and the level in VS maxed out at 99!  I don't know if it makes any difference but yesterday I was looking at Hall&Oates which was a bit laid back, today was David Bowie, Aladdin Sane, more rocky.

Having recorded the album, going into the split tracks screen everything was contained with no flattening out of the signal.  I'm assuming that would be correct. Should I assume this would be the correct setting to move forwards with?

Thanks

Paul Sanders (AlpineSoft)

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Re: "Set digital gain to 1" clarification
« Reply #11 on: May 26, 2017, 12:39:03 pm »
In my opinion, 99 is a bit on the high side.  Better to allow a little more headroom, say 80.