On Sun, 07 Jul 2013 13:16:28 -0400
Owen Williams <owilliams-opwvymzfgylytjvyw6y...@public.gmane.org> wrote:
> On Sun, 2013-07-07 at 18:26 +0200, petah wrote:
> > Yes; the specific problem I'm trying to solve is interfacing my
> > "homegrown" CDJ timecode program with Mixxx, "xwax-style". If I could
> > pass a high resolution time position to Mixxx's internals then tempo,
> > pitch bend, FF/REW, cue recall, etc, are all implicitly handled with
> > one single message and the driver will be simple. I'd just need to
> > loopback from my app to Javascript using a MIDI SysEx or HID message
> > (or a MIDI "bundle").
>
> Ah ok now your questions make more sense.
>
> Just having a time position isn't enough. If you were to pass time
> positions but not rate values,
Ok. By "rate" do you mean "tempo"? I use terms as they appear on my CDJs;
"tempo" being a lasting rate change (until DJ moves the tempo slider again) and
"pitch bend" being a temporary rate/tempo variation for beat matching, say.
I can compute tempo fairly reliably; I store rates in a ring buffer and wait
<N> ms until the buffer is "stable" before considering a tempo change so other
commands don't interfere, especially pitch bend. Pitch bend is a little dicier
since I need to detect a sort of curly rate change. And yes, all of the above
use lots of constants, i.e. are heavily hardcoded.
> Mixxx wouldn't know to scale the audio
> and you'd just hear horrific noise as the track reseeks constantly.
Sure, you can't reset the audio position at every frame.
> Seek operations should never be used during playback except for loop
> boundaries, cues, and other actual seeks.
So if the DJ is cueing up a new track and searching for a breakdown, say, while
monitoring on headphones, "seeking" by changing the play position is a no-no?
I can detect FF/REW/cue loop pretty reliably with RedBook correction, at least
on CDJ100/1000.
> Trying to match up mixxx's
> position with a sample-accurate playback position from CDJ is a recipe
> for disaster and headache.
Right, sample-accurate is probably impossible ("famous last words":) my
smallest timestamp is 105 frames/mono samples.
> CDJ doesn't really drift so that's a non-issue.
I meant that the time position in Mixxx and the one on the CDJ display lose any
relationship; the CDJ will just jump around.
> What you want is exactly what Mixxx already has in its vinyl control
> API, which is rate and position. There is a lot of ugly hard-coding of
> vinyl control, but it should be possible to create an interface with
> your javascript.
Ok, which messages would that be in the mixxxcontrols wiki page? Or am I
looking at the wrong doc?
> > This would be much more complex, less reliable and because the CDJ
> > track position will drift from Mixxx's track position could even hit a
> > CD track boundaries at which point it's stuck.
>
> yes, this is a known problem, and I wrote a lot of code to crate an "end
> of timecode" mode (CONST mode) so that playback wouldn't stop when this
> happens. On commercial Serato, if you use CDJ and your tracks are
> longer than 15 minutes, they just stop!
That happened to me too in the first iterations. I use 4 x 15-min tracks by
default but can generate single-track CDs too, though by losing some timestamp
resolution. It's not a perfect solution but swapping CDs is fast enough.
> look instead at vinylcontrolxwax.cpp. It has CD-specific code which
> makes it more precise than regular vinyl.
Ok just took a quick glance; it looks a lot like my own hackery. Do you suggest
I add my timecode format in there? I'm worried we'll get a worst of both worlds.
Would it make sense to have a base abstract class with only the interfaces that
Mixxx needs, plus a factory method so I can instantiate my derived class & hide
all my internal hacks in it?
thx!
-- p
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