555-based FM synthesis

[ion_nine]

Hello all, I have posed a long, long time ago about my concept of cascade synthesis. I have been researching non-stop since then, and am implementing the concept into an FM synthesizer based on the LM555 to produce square and triangle wave form. I plan to key the oscillator using resistors (notes) and capacitors (octaves) which are selected by an Arduino unit.

The basic concept is (in a brief nutshell) the modulation frequency is generated by a complex yet percussive pulse stream which is produced as a result of six individual, variable frequency pulse generators being passed through a "cascade" of user-definable gates. This is further processed through a PLL, and the comparator's PWM output is then converted into an analog signal and applied to the 555's CV input (this is all beside the point).

I know that 555 is less than desirable as a musical instrument. However, from what I understand this is primarily because the CV input is inverse, however, I will not be using CV for keying, but rather for frequency modulation (and even if I were, the arduino would permit any type of CV)

What I am wondering is this: does the LM555 have any other issues aside from the it's difficulty to use in a 1v/octave synthesizer and relative difficulty to produce anything but square waves?

Aside from the cascade method itself (which I am pretty confident in and the explination above is pretty brief) do you see any major issues with what I am trying to accomplish? Will the 555 "fail" at FM synthesis (I am assuming tot since it can be implemented as an RF FM transmitter)? Is there any kind of other concerns which I need to know for spending time and money on the components?

I have to get going now, will post schematics if requested...

[Danno Gee Ray]

Please post schematics.

[slugger]

[ion_nine]

Hopefully below you will find an attachment of my VCO circuit. It is adapted from:

http://www.all-electric.com/schematic/eticircuits/555-triangle-with-independent-slopes.htm

Let me know what you think Basically the way it works is the resistors and capacitors function like a digitally controlled incremental pots, the IC's which are connected to these banks are counters. With a "NOTE ON" message, the counters are reset, and the arduino selects the correct resistor and capacitor for the frequency requested using the digital output. I used trim pots rather than digital potentiometers to 1) simplify the arduino program and 2) to avoid tuning problems since you have fixed resolution (any error would be multiplied in the other octaves). I know that digital pots would have advantages, but, I also like screwdrivers.

The "tune" pin header is connected to a potentiometer which allows for an arbitrary detune.

---

VCO.pdf
Description:		Download
Filename:	VCO.pdf
Filesize:	31.09 KB
Downloaded:	217 Time(s)

---

[slugger]

[JovianPyx]

A true sawtooth waveform is possible with a 555 timer by using a current source to charge the capacitor. It is the current source that causes the ramp to be linear. In your schematic, current is supplied to the capacitor through a resistor, this will produce an exponential charge curve and not a linear ramp. See the PAiA Fatman schematic for a 555 oscillator that generates a sawtooth with a linear current source. That VCO can be FM'd in the classical sense.

As for FM, I'm not sure that what you're doing is really FM, it sounds more like synch. This may or may not give you sounds you appreciate, but as I said, it may be a misnomer to refer to it as "FM".

True FM is accomplished by using a voltage controlled oscillator (where the oscillator pitch is controlled by a DC voltage) and adding an audio rate AC signal to the DC pitch voltage. This is usually done with a linear VCO, that is, a VCO that uses a linear pitch CV (as opposed to an expo VCO). With FM, the FM signal acts on all parts of the waveform, not just affecting the reset time. As such, FM imparts harmonics onto the output. FM is usually performed using a voltage controlled sinewave oscillator. The reason for that (a la DX7) was to give the audio impression of having a lowpass filter when there is none.

My own attempt to do FM with a 555 (7555 actually, the CMOS version is better because it doesn't crowbar the supply while pin 3 switches states) used the 555's CV (or FM) input. The problem with this is that it works only for FM inputs that are lower in frequency than the operating frequency of the oscillator. This is because the FM (or CV) input controls only the threshhold at which the timer resets and does nothing to change the ramp contour as real FM would. When I tried to use higher frequencies than LFO, it just sounded like crap.

If you use a current source to charge your capacitor, you will not need the capacitor bank for octaves (nor the resistor array for semitones) because you should be able to generate 5 octaves worth of linear CV using the arduino and a DAC in a manner that is similar to the way it is done in the FatMan. The FatMan does this with an _eight_ bit DAC and it is quite accurate. Using the current source will also allow you to do true FM.

I'll be interested to hear some audio samples when you get it together.

[slugger]

[pemdasi]

It's definately possible to make some FMish noise using 555s. Right now on my breadboard I have two 555s setup in astable mode, one of them acting as an LFO, the other as a audio range osc. I took the fake saw output on the threshhold pin of the LFO 555 and passed it to the CV pin of the other osc using a diode in between. The output of both oscs are going to a 2-bit R2R ladder. Nice radiophonic type sounds are coming from it.

This is a sound example, minus the r2r bit: 555 FM Example