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hello, i am working on a project where i crash together 2 foot copper and steel boxes in mid air (using a pneumatic colliding machine i made - yes, silly...), and i use old 16mm film to record slow motion of the collisions, and record sound using piezo film sensors and standard mikes. what i would love to know is possible ways to slow down the audio 40 - 100 times (yes, a lot) using digital means. i am NOT a sound designer, but i am trying as much as i can. i have tried phase vocoder stuff, which smears the transients and all detail, i have tried paul's extreme sound stretch (which does the same, obviously, though is more suited to extreme stretching), i have tried reel to reel, re-recording and building the stretch over several passes (which becomes quite noisy), and digital resampling (which from 192kHz ends up being quite glitchy....) i am wondering if recording 1bit 2.8MHz DSD is worth considering - giving me enough sampling speed to really play with resampling and slow time (i realise this will lower the pitch also....) or are there other options to consider??? what interests me most right now is getting the sound accurately slowed, so that i can understand what is happening and link it to the visuals of the film. i may perhaps end up editing heavily, or even foley-ing the whole thing, but at the moment i really want to understand what is happening sonically at very slow speeds - to be able to hear the smallest times and what is going on there....
in the past i have just recorded at 192kHz, slowed the sound down in Soundtrak Pro's stretcher, and then added transint peaks on top to preserve some of the punch that i imagine would be happening (!) this is a very old version of the project (two more itterations now), but you get the idea - 2 foot tall copper and steel boxes in mid air, wanting to get an accurate sonification of the collision....

[vimeo] 28642463 [/vimeo]

i hope someone has been playing with slow-down in this sort of range of 40 - 100x and has some possibilities worth exploring that they are willing to share. thanks, sean.

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Try XForm. I've had decent results, granted it takes a VERY long time to do it's processing That's only only other option I can think of. However, it's a processor which is going to add interpolation points (e.g. new sound generation). The only other option to retain transparent quality, that I can think of, is do a pitch change with no time compression, meaning the pitch is cut 50% for example, and the sound is elongated by 2x it's original. But it's only viable, in the case of how much you want to slow it down by, to have the original recording be 192kHz.

The deal is, slowing down by that much is going to create digital artifacts unless you use the latter means (and accept working with the pitch drop). Period. There's no way around that, just like how Moore's law breaks down at the atomic level. There's a limit to the forgivability factor of time stretching. There are only so many 1s and 0s, and for each time you cut the speed in half, interpolated voltage points have to be created by averaging two voltages. When averaging, it has no way of creating all of the modulated harmonics which come along with those wave forms. Hence, it has a washy and stretched-sound. Especially because in that much of a slowdown, you're creating three or more "averaged", interpolated voltage points - which in of themselves are averages of over voltages. The fidelity and quality of these interpolations exponentially decay when compounded.

Those ideas you tried sound solid (DSD may be an option, I'm just not familiar with it), and the above are some other recommendations I can provide. But unfortunately within the constructs and limitations of 'standard' digital audio formats we all work with, there is a very finite limit to forgivable time-stretching within the digital domain. And there always will be artifacting, sometimes not easily audible. The art is learning where to walk the fine line of doing the stretching you need but not beyond the point in which it reveals the problems inherent with digital stretching. But in some cases, like the speed adjustments you are trying to get, are virtually unobtainable without receiving audible artificting in return.

Unfortunately that's the best answer I can provide, both from my own experience and what I know about how digital time stretching functions. Maybe there is a 'secret sauce' I'm not familiar with, and maybe somebody knows what it is. Although from where I'm sitting, I unfortunately don't see a perfect no-strings-attached solution to the problem you are presenting.

  • Copied and pasted for later reference. – Utopia Jan 23 '12 at 5:19
  • thanks greatly - i will try the xform plugin and see how it goes.... i think you are right - there is no special sauce - the slowdown has its own taste and i must learn to use it well in a recipe and make something good out of it.... at the moment i think i want something very scientifically acurate, so that i can at least understand what is going on sonically in those smallest times, before going and fiddling something up to "make it sound right".... thanks! – sean Jan 23 '12 at 23:44
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Try:

paulstretch

or

photosounder

  • Woah, Photosounder, I must get that ;) Been looking for something Win-compatible that's along the lines of Metasynth... Although I would just like Metasynth PERIOD heh. – Stavrosound Jan 23 '12 at 7:08
  • plus one on paulstretch. that's my default for this application. – Rene Jan 23 '12 at 23:19
  • i have used paulstretch as i mentioned, but it really stretches out all the transients to a big wash of sound. and thanks - photosounder is damn pretty! will have a look, but it seems like another vocoder based stretcher.... when i recorded the sound on an open reel, and slowed it down to 40 times slower (three runs through a 3-speed machine, sdding a lot of noise) all the transients were intact - super bassy, noisy, and boomy, but the transient attack was still quite strong. and using the same audio file as i used in paulstretch, it sounded absolutely completely different. – sean Jan 23 '12 at 23:47
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Sorry to sort of hijack this thread by asking a question within question! But is it possible to record at a high sample rate, say 192kHz or even up to 1bit 2.8MHz DSD (granted I dont know much about DSD!), and then play it back at a 44.1kHz or 48kHz for example to slow it down? In a similar way to high speed cameras that record at 1000's of FPS and then being play the footage back at around 24-30 FPS?

  • yes you can, the effect will be as you described, thou imo 192khz played back at 44khz most of the times is already too slow...and i really don't understand why this ain't enough for OP. – Linas Jan 23 '12 at 10:41
  • Ah I see! But does it work that 192 played at 44.1 is 4.3ish times slower? Because 192 ÷ 44.1 = 4.3ish. Or am I just doing the wrong maths? – RockTheKasper Jan 23 '12 at 15:05
  • 192k played at 48k is 1/4 speed.... he wants 1/100 which would be inaudible in pitch, or smeared tones if timestretched – user49 Jan 23 '12 at 19:01
  • absolutely - i was wondering about this too - and the 5.6MHz DSD rate when slowed down 100 times would be 56kHz sampling - fine enough to resolve without too many articfacts? i realise that it would occupy a very low frequency range - the range of the recording is about 500Hz - 8kHz, which when shifted would be a range from 5Hz - 80Hz. maybe i could just analyze this, for my own understanding, maybe i could pitch shift it, maybe i could use this knowledge to edit up a mix that made sense of it all in an audible spectrum, or just use 15Hz - 80Hz as the main content and add higher stretches – sean Jan 23 '12 at 23:55
  • on top....? at the moment it is about finding other possibilities. the time stretches i have been using are crap, and really i want to understand the sonic content in any way possible - through hearing it or seeing slowed spectrograms of it - i want to understand it at the time of 100x slower. after that i will have towork out how to make something to present it, whether it is the original untouched, something edited up from bits, or a combination of both... I WOULD SO LOVE TO KNOW ABOUT CONVERTING THE SAMPLE RATE DOWN FROM DSD 5.6MHZ - surely there is some way of playing it slow.... – sean Jan 23 '12 at 23:58
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Photosounder probably comes the closet to doing what you want to do. Mostly if you fiddle with the parameters for precision/resolution in the configuration file (see manual). Also see this http://www.youtube.com/watch?v=zTi_HwdlJ20

PaulStretch is nice but not as precise in time, even when slowing things just 8 times it sounds like a smear, and most other time-stretching algorithms aren't suited to such ratios so they sound too unnatural.

  • thanks - i will try photosounder again. it is quite beautiful and has helped me "see" what is really going on in terms of transients over time, so perhaps it can help with the actual slowdown too... – sean Jan 24 '12 at 0:03
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This does a good job of slowing things down.

http://www.csounds.com/cecilia/

  • cecilia looks good - downloading it now and will have a play over the next few days - thanks!!! seems like it was generated for the same sort of purpose - really fucking around with the sound, rather than changing the speed of your guitar to match the drum track.... appreciated! – sean Jan 24 '12 at 0:02

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