[Gambas-user] Managing soundtracks, WAVs

[Jean-Yves F. Barbier]

Rolf-Werner Eilert a écrit :
> Hi Jean-Yves,
> 
> you're right:
> 
>> ...
>>> At this time, as far as my own needs are concerned, it would be ok if it 
>>> had a "set to zero position" button and "start". The tricky thing would 
>>> be following the signals from the projector and keeping the soundtrack 
>>> synchronised to them. If there was a playback speed option, it would 
>>> make things somewhat easier. Without it, you will have to correct the 
>>> position from time to time, resulting in little "jumps" or "stutter" of 
>>> the sound.
>> This is quite a nonsense unless you have another track that indicates where
>> is the next possible jump (usually @ the beginning of a music measure) - in
>> case it is music, for speech there's no solution.
> 
> It's ugly. But I think, most projectors run so evenly that in 99 % of 
> the cases the sound will play synchronously anyway. Maybe a very slight 
> correction after 10 or 15 minutes would be necessary.

Not at all: as the bip that launch next dia is ON the sound track, there
can't be any drift.

> However, there's one thing I've discussed with other folks in a film 
> forum. They proposed to take the film with a video camera first and then 
> make a soundtrack for it within one of the usual video cutting programs. 
> This way you would have it basically synchronous.

YES: THIS is THE idea :) this way you just have to determine the window
in which you're allowed to go further, knowing that at the end of this window,
if you haven't pressed 'next' it will be automatically done.

> The trick would then be to have this resulting soundtrack run 
> synchronously with the projector when the film is shown "in real". Just 
> imagine, you have every frame on the video and the soundtrack fitting to 
> it, but the projector might not have the correct speed but could vary in 
> speed by a few frames. The art in it would be analysing on the fly how 
> much THIS very machine differs from the speed of the soundtrack and 
> adapting its playback speed so it plays synchronously after a while.

The regular way to do it is to have the projector correctly adjusted to its
nominal speed(s) by regularly sending it to a specialist.
However, it is very easy to get a square signal from the projector, integrate
it, and make an sound lock loop (through a PLL, exactly alike radio frequency
generators), the only parameter added is: what is my non-audible timing speed 
I can use to vary sound, so people won't ear the drift catch; this has a specific
name in PLL but I didn't touch it for years, so I don't remember it.
 
> And as you put it, it must be so smoothly that the audience will nearly 
> not hear it.

Yes: earing is one of the most precise "tool" when you speak about frequency
accuracy (obviously :D), so the drifting shall be so slow that id don't ring
a bell, saying: "hey, you inside - the moron making the conference is tricking
ya about sound speed!"
It is easy to realize that if you use 2 turntables with drift possibility:
sometimes, in order to make a good transition between two musics with different
beats, you'll be obliged to:
raise/reduce next speed || raise/reduce current speed || take inverted actions on both
depending on the time you'll take to do so, it can be non-noticeable, disagreable, 
or terrible.

>>> Many years ago, in the time of i386 machines, I programmed a similar 
>>> thing under DOS with PowerBasic, but as far as I remember, I never found 
>>> an easy way of reading the impulses, and of course I missed an app for 
>> Use the parallel port, or on some machines (if it already exists) the swi 
>> switch/connector.
> 
> Good idea :-) But which switch/connector do you mean?

I don't know it it already exists but there was such a connector (at least; sometimes
a physical switch) that raises an SWI (SoftWare Interrupt), this way you just have to
have a small launcher @ SWI address to interract [well, that's not so easy, because
usually SWI also need an 8 bits address present on the bus @ SWI trigger time].
You have to make research because this info comes from quite a bit of time ago.

The other way is to build a special addon card with an embedded DSP, which will be
able to solve all @ once (but at a cost that is non-trivial.)

JY
-- 
Beware of low-flying butterflies.