Creating a tone speedup locked to a Timeshift speedup

[sleadley]

I have a job that requires that we have a speed ramp on a piece of music (say from 25 fps to 50 fps over a couple of seconds), there is also a tone on a second track that will drive the ARRI camera speed (this can be a ramp of 50Hz-100Hz) but it needs to track the speed ramp of the Music. I propose using Pitch n' Time Pro for the ramp but how do you think this might be achieved in practice.
I notice that if I generate a 50Hz Sine wave in Pro Tools that it does not accurately line up (at least on my LE system) with a 25 fps grid, which it should. This might make zero crossings and selections of time less accurate as well.
But really all I want is a tone that will chase the ramp of the Music speed up. Any ideas how best to achieve this?
thanks in advance
Simon L.

[FlorianE]

Hi,
if the visual must speed up exactly like the audio: wouldn't you have to ramp down the ARRI control frequency?

Florian

[Postman]

What do you mean "in practice"? Do you mean this will be a recurring challenge?

If it were me I'd just break out an old sampler. MIDI allows very smooth gradations in speed, around 16000 steps which should be very smooth, so just about anything is possible if the sampler is up to the task. You can "play" the pitch in realtime if you want, or draw controller lines and curves manually for precision. My ancient K2500 can handle multiple sounds and channels easily, all with the exact same pitch/time relationship (or different relationships, depending on how a patch is defined.) If you need exact percentages or fps changes, a little math is required to determine cents vs/ freq, but the info is easy and available online.

Pitch n Time is great but for my tastes, nothing beats the capability of old school for this, once you get past the hurdle of setup. I will say, though, the last time I had to do something this way I really did not want to be bothered and put it off as long as I could. In the end, the process was so easy and efficient I wondered why I was concerned in the first place.

As to 25 hz not lining up with frame boundaries, I'm not sure why that is. I read somewhere that nearly all digital oscillators are slightly off frequency, because it is so much easier to use math that gets pretty close to the right frequency but with errors. It does not surprise me at all that Signal Generator is not quite perfect. But the difference should be small. Or, you could plug PAL into a line level analog input and, with lots of eq, generate your own that will be near perfect. We used to do this with NTSC all the time. A heavy audio matching transformer connected between works even better, the iron stores plenty of energy and "smears" the video signal into a useable waveform. With some simple low-pass the waveform is very usable for servo motor control. But now I'm reallly off topic.