CEM3379 vs my big mess of OTAs and op amps
  • Comparison here
    (link removed, see below)

    Note that I did not aim at emulating the CEM3379 in particular. I tried to get something clean, and I think the main difference lies in the resonance… My filter is very gentle with it (it doesn’t distort much even in self-oscillation), and even if there’s no temperature compensation, the tracking is quite OK. With the Shruti-1 and CEM3379 I could never get tracking right just by digital adjustments… So I don’t recall for example coming close to results like this with the Shruti-1: http://soundcloud.com/mutable-instruments/move-this – nor do I remember getting something like this with my ESQ-1 anyway… It really makes me visualize the CEM3379 resonance as a big blob of clipped sines sparkling here and there, not as a “glove” on top of the sound

  • the big mess has a high pitched whiny sound. I think I like the CEM3379 sound better....on this particular comparison.
  • so you’d need tempco resistors to do that, eh? Thinking about omitting them, or making them optional? This is another one of those terrible to source parts…

  • @11hz Robot. Really? I can’t hear this and this is a big problem. I don’t see much of it on a scope either.

    @smrl. I plan to make them optional. It looks quite stable without them but I have to investigate a bit more by seeing what happens with big changes in the temperature…

  • @11hz Robot. Oh my. I see something on a FFT analyzer but absolutely can’t hear it. This is a bit dramatic because this would mean my hearing is worse than I thought and I should maybe stop doing this kind of stuff

  • The high pitched noise is pretty noticeable, I'm afraid :( It's at that kind of frequency that people find harder to hear as they get older, although I've got tinnitus which makes me particularly sensitive to high frequencies I should be too old to hear.

    Please don't stop, though!
  • i can hear it too, but maybe my hearing is actually better than i thought?
    but don't stop it!
  • I have deleted the samples. I was kind of happy with them, but now that there’s this looming bug, this changes the deal…

    I can’t help but feeling devastated about this… I knew I was limited in the high frequencies, with a cutoff at around 12kHz from previous tests post hospital (eardrum polyps gone bad), but I did not realize it got worse over time. This nasty stuff is at ~9kHz, not 15… I’m absolutely bummed at the idea of being unable to judge audio quality questions.

    Hopefully for this one it might be only a problem with the wrong caps for the DC/DC converter (I’m using a new one more robust to higher voltages), or it might be some self-oscillation in the resonance loop (though it doesn’t appear to be tuned to cutoff), or an op-amp playing unstable. I don’t think there’s some digital noise at this frequency, that sharp. It’s probably only a matter of putting a cap at the right spot.

    From now on I’ll have to work with this in mind… Maybe buy a scope with built-in FFT analyzer :( :(

  • My condolences go out to you :(

    I found it to be fairly noticeable as well, and I don’t believe I have any hearing defects, beyond the damage of the years of abuse i’ve put my ears thru…

  • Really sorry abou this too! I really hope this doesn't get too much in the way between you and the things you love to do...
  • Sorry to hear about your not being able to hear some frequencies, but I'm with the others, definitely don't stop creating synths. If your hearing gets worse we'll all be racer X and do the speed racer- racer X thing except with hearing instead of vision.
  • I’ll try to do a debugging/troubleshooting session tonight without a spectrum analyzer and see how hard it is to work this way. What I’ve realized is that:

    • a soundcard is NOT reliable measurement equipment (though in this particular case, it shows up in the waveform)
    • there’s a lot of crap in the “cheap measurement equipment” space, and it is a waste of time and money
    • you can’t do very good things without good measurement equipment
    • good measurement equipment is expensive

    I’ve invested in a good second-hand scope (tektronix 2216) which proved to be awesomely useful, but I can’t get it to do FFT, and given its age I suspect it doesn’t (though it has digital storage). I think it’ll better if I upgrade it. If you know where I can get some good stuff at a reasonable price, let me know… Or I’ll get a new one if I manage to get the Shruthi-1 out (without the whining/hissing) and make a profit from it.

  • why not record it at a high sample rate and bitrate and look at it with a wave editor with FFT

  • @altitude: soundcards have a bunch of filters in the signal path that may remove precious hints as to what happens. + with a soundcard, it’s not easy to make a measurement (you don’t know at which voltage the ADC is referenced, you don’t know what kind of pre-amplification there’s on the signal path) – and I think knowing the voltage of some stuff can help you in figuring out where it comes from. Also they are referenced to ground, unless the editor can do some pseudo-differential stuff by subtracting the channels… And without the right piece of software that can be manipulated like a scope with only one hand, while keeping a hand messing with the circuit, this is all going to be a real pain to use… There’s “Mac the Scope”, i’ll give it a try, otherwise I’ll have to code my own stuff and I am not too keen on spending time doing that…

  • If u really need a good oscillo, you know where i’m working dude and can come doing ur test ;)

  • Last night I heard it and thought it was part of the recording and not the actual synth signal.
    Well, then yes, I plainly hear the high pitch as well.
    My hearing range is solid, but I have really crappy eustachian tube problems so my range comes and goes over the day; this is depending on the weather, environment, how much I yawn, etc . . .

    Either way, the big mess filter seems softer and less "steppy" than the Curtis filter, despite said high pitch noise.
    I'm actually more of a fan of the Big Mess, but that's subjective on what type of music you're approaching.
    I tend to lean towards gentler . . .
  • I managed to hear it faintly by playing a very very low note (so all the masking effects are gone) and cranking the volume up. It’s there, awfully disturbing. I’ve tracked it to the current source for the VCA – I don’t know if those are oscillations coming from the op-amp, from my envelope source or possibly, from weird interactions with the OTA...

  • Hi all,

    I decided to join in over here, it's too exciting to watch silently:)

    Pichenettes, much empathy about the hearing, I can only imagine the worry that would put in my mind!

    But I really think you are far too gifted at this to consider not doing it.

    In terms of workrate, things seem to be accomplished around here faster than I can even follow- faster than anyone else, even synth gurus and experienced companies.

    But forget that- the sound!!
    The reason I'm here are the amazing sound demos. There is such character and quality there.
    And to me your first stab at a filter sounded markedly better than a sought-after vintage chip - truly incredible.
    (The whine sounded like just an anomoly, very seperate to the tasty synth that was going on)

    So whatever you are doing, and however your ears are going, your decisions about the sound are clearly very good ones. Please keep trusting those imperfect ears of yours, as we all do the same! :)

    Adam Jones
    Sydney, Australia

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