Sharpest crossfader cut time is coupled to buffer size

[mixxxbot]

Reported by: bronxio
Date: 2017-07-10T23:33:40Z
Status: Confirmed
Importance: Low
Launchpad Issue: lp1703475
Tags: engine
Attachments: [Scratch waveforms (with real vinyl).jpg](https://bugs.launchpad.net/bugs/1703475/+attachment/4914309/+files/Scratch waveforms (with real vinyl).jpg), [Scratch waveforms (with control vinyls).jpg](https://bugs.launchpad.net/bugs/1703475/+attachment/4914317/+files/Scratch waveforms (with control vinyls).jpg), [scratch gain comparison.JPG](https://bugs.launchpad.net/bugs/1703475/+attachment/4914336/+files/scratch gain comparison.JPG), [pitch changes@10, 20 & 50%](https://bugs.launchpad.net/bugs/1703475/+attachment/4914901/+files/pitch changes@10, 20 & 50%), [pitch changes at 10, 20 & 50 percent](https://bugs.launchpad.net/bugs/1703475/+attachment/4914902/+files/pitch changes at 10, 20 & 50 percent), [pitch changes at 10, 20 & 50 percent](https://bugs.launchpad.net/bugs/1703475/+attachment/4914903/+files/pitch changes at 10, 20 & 50 percent)

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When Mixxx controls the crossfader (internal mixer mode / MIDI), although it's set to the fastest cut in preferences, its cuts are too soft for scratching (more aggressive cuts are necessary for it, like on/off sound).

I recorded a video showing this problem, comparing stand-alone mixer mode and MIDI mode with Mixxx: https://www.youtube.com/watch?v=XXktCyWYCs8

I asked in forum if there is some way to make the crossfader more aggressive without modifying code, but they told me there isn't. It would be great to have a more aggressive option for crossfader in next versions! It would be a pleasure to test betas if you want me to do it, to check new values for the cut if someone knows where to modify that in the code and compile it for Windows (unfortunately, Mixxx can't detect my Traktor Z2 as MIDI controller at UbuntuStudio 16.04, but it does under Windows)

• Mixxx 2.0.0 Windows 64-bit (downloaded a few hours ago)
• Windows 7 SP1 64-bit (with latest updates)
• i5-2540M@2.60GHz
• Intel HD Graphics 3000 and Native Instruments Traktor Z2 (ASIO driver)

[mixxxbot]

Commented by: Be-ing
Date: 2017-07-11T00:14:34Z

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Could you try installing this and setting the crossfader to the maximum sharpness in the preferences? Is it sharp enough for you?
https://ci.appveyor.com/api/buildjobs/fapudxr335hsgpc7/artifacts/mixxx-2.1.0-alpha-pre-master-PR1311-git6321-release-x64.msi

You may want to backup your settings and library database before using the installer:
https://mixxx.org/wiki/doku.php/testing#backup_settings_and_database

Note that we currently have some pending issues with the Windows installers:
Bug #⁠1663380
Bug #⁠1662727

If you have issues using the installer linked above, try installing the MSVC 2015 runtime and using the installer again:
https://www.microsoft.com/en-us/download/details.aspx?id=48145

[mixxxbot]

Commented by: Be-ing
Date: 2017-07-11T00:23:56Z

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The Kontrol Z2 is a USB class compliant sound card and HID controller and could be used that way on any OS Mixxx runs on. Unfortunately mapping HID controllers is currently quite a bit more complicated than mapping MIDI controllers. Recently work on improving the process of mapping HID controllers has been started, but it will probably be a while before it is ready. If you are curious, you can follow the discussion here:
#1308

[mixxxbot]

Commented by: bronxio
Date: 2017-07-11T12:04:33Z

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Thanks! Unfortunately, I can't run the installer even with the MSVC 2015 installed. I have a Spanish version, but I'll make an approximate translation of the message that appears just when I execute it: "Can't open this installation pack. Please contact the application developer to verify that it's a good Windows Installer pack."

Do you want me to report it in bug #⁠1663380? It doesn't seem to be a UAC problem (I haven't turned it off and there's no privileges message)

[mixxxbot]

Commented by: Be-ing
Date: 2017-07-11T12:26:40Z

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Please report a new bug for that issue.

[mixxxbot]

Commented by: bronxio
Date: 2017-07-11T12:49:42Z

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Ok, the last installer in bug #⁠1663380 works fine for installation (but sblaisot is still working for a right unnstallation). I've tested by myself.

[mixxxbot]

Commented by: bronxio
Date: 2017-07-12T17:00:06Z

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Ok, I finally could install that alpha you gave me. I set again the crossfader to the fastest cut and it sounds a little better, but I would need more. Could it be? The ideal is like a switch (on/off sound, with no curve). The only need to make a curve is for a technical thing: avoiding "clicks" ("ugly" wave cuts done out of 0dB, specially dangerous when cutting bass, where it's more probable).

[mixxxbot]

Commented by: daschuer
Date: 2017-07-12T21:20:37Z

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Can you explain your use case?
Which audio buffer size do you use?

With the current master (kTransformMax = 1000.0)
In the GUI, we have already go from -inf to 0 dB in a single step. The same is true for a single precision Midi Slider. On a 14 bit Midi slider we have -6 dB after 12 Steps and -0.01 dB after 100 Steps This is 0.6 % of the slider = 0.6 mm on a 10 cm Slider.
My RMX2 controller has a 14 bit xfader of 4 cm and I cannot manage to set a value between - inf and -0.01 dB.

With Be's latest changes (kTransformMax = 10000.0), we have 10 Steps to -0.01 dB that is 0.06 % of the slider.

[mixxxbot]

Commented by: bronxio
Date: 2017-07-12T22:29:52Z

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I'm not sure if I understood right the first question, so I'll answer it a little widely.

I have a Traktor Z2 (but it could be, for example, a Denon X600 or X1600, Traktor capable, but without software included). The crossfader is a mini Innofader (included in the Z2). I want to use a MixVibes control vinyl for scratching and, without another sound card, I can do it with Mixxx only in internal mixer mode (MIDI/HID). Scratching, beyond the called "baby scratch" (that hasn't any cut in its procedure), needs a crossfader that cuts the sound (not fading). Cuts could be so fast (scratch DJ's use cut-capable crossfaders since 90's at least), you can check it in any scratching video, like in this one: https://www.youtube.com/watch?v=y8b6TiXYAFQ And there is already commercial DJ software that make crossfaders cut-capable in internal mixer mode (Serato DJ, Traktor Scratch Pro, Cross, etc.). Nowadays scratch DJ's use them a lot since several years ago (because DVS).

The buffer size is set to 21.3 ms.

As I saw when testing (I looked for the cut point in the crossfader), I think this is not actually a movement distance problem in the crossfader, but in speed fading (cutting or "switching on") the sound by software when the crossfader sends the value for the change. The distance works, as I could see, exactly the same as in external mixer mode. Can't make Mixxx faster for this? I can test it with less steps than 10 (if possible) anyway.

[mixxxbot]

Commented by: Be-ing
Date: 2017-07-12T22:52:55Z

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I think this is not actually a movement distance problem in the crossfader, but in speed fading (cutting or "switching on") the sound by software when the crossfader sends the value for the change.

...

Here is your problem:

The buffer size is set to 21.3 ms.

That is a bit high for scratching. Your buffer should be as small as your system can handle reliably without audible glitches.

[mixxxbot]

Commented by: daschuer
Date: 2017-07-13T06:32:12Z

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I the video in the initial post, do you use a time coded vinyl? I assume the buffer size is 21.3 ms I'm both cases?
I will analyze the recorded sound a and try to compare to compare the ramps. Maybe we can reduce the ramp internally for big buffers in the scratching case.t

An other idea, could be a mapping issue.
Mixxx suffers the issue that the we engine is not synced with midi. So it might happen, that it receives a LSB midi message first. Which leads to a 42.6 ms fade.
I will check that.
Please post the link and js file of the used mapping.

[mixxxbot]

Commented by: daschuer
Date: 2017-07-13T08:23:26Z

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One unrelated question:
Did you notice a difference between vinyl scratching and time code scratching regarding gain by tempo changes? I have done a pull request that is trying to fix that:
#602
Does it has any value for you?

[mixxxbot]

Commented by: Be-ing
Date: 2017-07-13T14:01:12Z

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An other idea, could be a mapping issue.
Mixxx suffers the issue that the we engine is not synced with midi. So it might happen, that it receives a LSB midi message first. Which leads to a 42.6 ms fade.

If Ambrosio is using the Z2 with MIDI, then he is using it through Native Instruments' proprietary driver that translates HID to MIDI. I do not know whether that uses 14-bit MIDI. It is plausible the driver may introduce some extra latency.