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help with fm synthesis

Discussion in 'Recording' started by dgcrumley, Jul 7, 2005.

  1. dgcrumley

    dgcrumley Guest

    I've been reading these forums for quite some time, but this is my first post. I hate for my first post to be one asking for help, but I'm stuck. I wasn't quite sure what forum to put this in, so please feel free to move it into one that might be more appropriate.

    The project I'm working on involves creating several 1.5khz and 7khz tones with constant amplitude and their frequency modulating over time .

    For example, once such tone would be a combination of a 1.5khz tone increasing to 2khz at 200ms, then decreasing to 1.5khz at 300ms and back to 1.5khz at 1s. This would be mixed with a 7khz tone going through the same increases/decreases to create one tone but delayed 100ms. This would then be looped and sent to a transducer that would turn the tone into a heart beat to be monitored by a doctor. The purpose of this project is to train doctors on taking patients' pulses.

    The client has created these before on some sort of synthesizer or other piece of equipment that I don't have access to. He has given me crude graphical images of the frequency change over time, it basically looks like an amplitude envelope.

    I'm trying to create these tones using simple synthesis in Sound Forge. Does this sound like the best way to go about doing this? Any suggestions as to another program to use or if actually using some sort of synth would be more appropriate? Going into this, I thought it would be exremely easy, but the way the information has been presented to me has made things difficult. Thanks for any help and suggestions.

    David
     
  2. pr0gr4m

    pr0gr4m Well-Known Member

    Off hand I can't think of any synth that has the ability to do this exactly the way you want. Most of them have a frequency knob or setting but it isn't separated out into the exact frequency number. it's usually a range from low to high or 0 to 127. Some good old analog or modulars might be best for this but you would need something to monitor the frequency so you would be able to set it properly.

    I would think that there has to be an easier way to trigger the transducer using electrical pulses or something like that rather than using audio. But anyway...

    I don't know anything about the simple synthesis portion of SoundForge but you should be able to use it to put the tones together.

    What you need is some sort of Tone generator where the frequency can be set to a specific frequency and modulated over a specific time. I don't know where you would get something like that.

    Anyway, once you have that, you set it to play back your tones at the frequencies you need, first the one that is 1.5-2-1.5. Record it into SoundForge. Then record the one for 7-7.5-7, and then merge the two. Once you have a complete section, you can either loop playback or copy the section and append it to itself numerous times to give you one long audio track that repeats.

    Good Luck!
     
  3. dgcrumley

    dgcrumley Guest

    Thanks for your suggestions. That's pretty much what I was thinking and I think I have it all figured out, but there's one thing I forgot to mention in my first post.

    When talking to the client, he kept saying the carrier signal would be a normal sine wave but the modulation would be single sided(absolute sine). In sound forge, the only way to do this is to have an operator modulate the carrier signal which is what he doesn't want done. By creating the simple tones in soundforge with the freq. changing over time, will the modulation be "single-sided"? This was the part that was really confusing and I agree, sending audio to the transducer seems to be a really complicated way of going about this but the customer is always right I guess. Thanks again-
     
  4. David French

    David French Well-Known Member

    This can be done with tools like CSound and Max/MSP. They are ultimately powerful, and ultimately difficult to learn. If you want to know more, ask.
     
  5. dgcrumley

    dgcrumley Guest

    Thanks David. So what I need to do can not be done in Sound Forge? I'm downloading the demo of MAX/MSP as I type this. I'll be sure to post any questions that I have. Thanks again--
     
  6. dgcrumley

    dgcrumley Guest

    Ok, I've been searching for tutorials and demos for MAX/Msp, but haven't found much geared toward what I need to do. Since I'm short on time, do you have any suggestions on where to start? Thanks so much-
     
  7. David French

    David French Well-Known Member

    I'm not that familiar with Max/MSP. I am more familiar with CSound. If you read the first four tutorials on this site, you should be able to perform your task. After you read them, the following will make more sense. You will create two oscilators and have the output of one modulating the frequency of another (FM). You will have an envelope function controlling the frequency of the modulator. After you study the tutorials, I don't think it will be too difficult.
     
  8. redbort

    redbort Active Member

    I`ve learned CSound myself to quite some degree and have never found practical use for it in the studio outside of making nice patterned sounds for myself. Does anyone here actually use CSound in the studio?

    something about FM that has been plaguing me for a while is that if you`re modulating the frequency by a fluctuating mulitplyer, aren`t you getting all the intermediate frequencies (an infinte amount) and how is it that the numeric calcuations practicaly work out for dictating the absolute harmonics you creating since there are infinte amounts? jixxy?
     

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