"Pinging a filter" on Diva or Zebra for TR 808 bass drum replica?

[luqsuasa]

Hello,

I am learning about synth programing techniques. On my searches through Internet, I found some info about the “Pinging a Filter” technique, where you feed a “pulse hit” or “short impulse” into the audio input of a filter on the edge of self-oscillation, which should produce a percussive sound.  

This is supposedly the way used on classic drum machines for some of their sounds, for instance on the TR-808, to create its famous bass drum sound by using nearly self-resonating low pass filters (bridge T-Network) as a sound source, and then using the cutoff to set the frequency or “Tone” of the sound, and the resonance to set the “Decay” of the sound.

I know on Diva or Zebra (or even Repro), you can use the VCF with cutoff set very low or 0 and resonance almost maximum to “self-oscillate” the VCF and generate a sine wave which is, as I understand, could be the basic mechanism used by these drum machines bridge T-Network circuits, am I correct?

But, from this point, I am really confusing about how to replicate this “pulse hit” or “short impulse” and how to “feed it into the VCF input”.

If I understood correctly, this “pulse hit” or “short impulse” should be a real short audio signal feed into the VCF, therefore, I should use an OSC or VCO to create it? If so, how can then create a “short impulse” VCO? Or there is another way to replicate this “pulse hit” or “short impulse”?

Then, regarding the TR 808 bass drum “Tone” and “Decay” controls, I know If the VCF key tracking is off (0) the self-oscillating sine wave is set to a specific pitch or frequency and by moving the cutoff you can adjust this sine wave frequency, which I understand is what the “Tone” control does, Am I right?

On the other hand, I do not see, how moving the resonance control you can adjust the sine wave “Decay”, as supposedly does the TR-808 bass drum control. Honestly, I cannot understand this.

I believe I am missing something or not really understanding correctly how this TR-808 bass drum works, besides I do not have the knowledge necessary to interpret the drum machine schematics you can find on Internet, but I believe you could be able to replicate it on Zebra … don’t know if is possible on Diva.

… Any advice or help with this, please …

Thanks in advance … regards.

Luqsuasa

[justin3am]

Yes, I've done it before with Diva, should also be possible with Zebra. I find that Bazille and ACE are ideal for this kind of thing.

In Diva, I used the noise generator from the mixer section of the Triple Osc for the impusle. I routed one envelope to control the volume of the noise (with very short decay and sustain set to 0) and used the other as the main amp envelope (with a decay which accommodates the natural decay of the ringing filter). Getting the resonance balanced is more tricky in Diva, than synths like Bazille and ACE which give you more flexibility in terms of gain staging.

For percussive sounds, a very short DC offset is the best thing to use for the 'impulse'. This is also easy to do in Bazille and ACE but more difficult in synths with fixed architecture.

[cthonophonic]

In Zebra, you can create a ping using just the prefill (no input, no feedback) of a Comb filter. The Square and Saw prefills have enough low frequency content to excite a filter nicely for a kick, and if your Comb is tuned to the same frequency as the filter you're pinging, you can also use the comb feedback to "sweeten" the kick a bit.

Another way is to use a sine chirp as a ping, for instance an FMO with tuning modulated by a fast MSEG. Using this method, you can also mix in the dry chirp to get a stronger transient.

Whether or not you can just adjust the filter resonance to control decay time will depend on the properties of the filter. Sometimes the zone of transition into oscillating is very narrow, and it's hard to get satisfying intermediate decay times. Using an envelope to control resonance can be easier for this kind of thing.

Attached is a Zebra2 preset with a Comb and FMO chirp pinging one of the "MS20" filters to create a kick (not exactly a 808 replica, but in the same spirit). Controls are mapped out in the XY pads to test out different variations.

[luqsuasa]

Yes, I've done it before with Diva, should also be possible with Zebra. I find that Bazille and ACE are ideal for this kind of thing.

In Diva, I used the noise generator from the mixer section of the Triple Osc for the impusle. I routed one envelope to control the volume of the noise (with very short decay and sustain set to 0) and used the other as the main amp envelope (with a decay which accommodates the natural decay of the ringing filter). Getting the resonance balanced is more tricky in Diva, than synths like Bazille and ACE which give you more flexibility in terms of gain staging.

For percussive sounds, a very short DC offset is the best thing to use for the 'impulse'. This is also easy to do in Bazille and ACE but more difficult in synths with fixed architecture.

Thanks very much for your answer, I will try as soon as I return home.

If I understood correctly, On Diva, I should use ENV2 as modulation source for the TRIPLE VCO NOISE level or volume on the MIXER. Am I correct? … I suppose this can be done on the MODIFICATIONS Noise & Dual VCO Mix, Am I correct?

Then, I should set the VCF | LADDER on the edge of self-oscillation (more of less CUTOFF = 0 and RESONANCE around 60.00) and finally use ENV1 to shape the VCA for the kick (A =0, D short, S = 0, R =0), Did I get the idea correctly?

I am still a little bit confused, between this approach and the other general approach for the kick by using an envelope controlling the speed of pitch descent for a sine wave oscillator, for instance, setting the Diva VCF | LADDER on self-oscillation and controlling with the ENV2 (A =0, D short, S = 0, R =0)?

What is the real approach used by the real vintage analogue machines?

Thanks in advance for your clarification.

Regards.

Luqsuasa.

[luqsuasa]

In Zebra, you can create a ping using just the prefill (no input, no feedback) of a Comb filter. The Square and Saw prefills have enough low frequency content to excite a filter nicely for a kick, and if your Comb is tuned to the same frequency as the filter you're pinging, you can also use the comb feedback to "sweeten" the kick a bit.

Another way is to use a sine chirp as a ping, for instance an FMO with tuning modulated by a fast MSEG. Using this method, you can also mix in the dry chirp to get a stronger transient.

Whether or not you can just adjust the filter resonance to control decay time will depend on the properties of the filter. Sometimes the zone of transition into oscillating is very narrow, and it's hard to get satisfying intermediate decay times. Using an envelope to control resonance can be easier for this kind of thing.

Attached is a Zebra2 preset with a Comb and FMO chirp pinging one of the "MS20" filters to create a kick (not exactly a 808 replica, but in the same spirit). Controls are mapped out in the XY pads to test out different variations.

Thanks very much for the example, I will try it as soon as I return home.

Regards.

Luqsuasa.

[hakey]

Then, I should set the VCF | LADDER on the edge of self-oscillation

Try selecting the Korg type version 2 in the central HPF panel and using that at self-osc with the VCF cutoff fully open.

(IIRC, the 808 kick is a swept oscillating HPF)

[dave dove]

the tr-808 kick is a band pass filter created with a ‘bridged-t’ network
approx 12db but wider at the start
the trigger logic is quite complex and using noise is tricky as it is often inconsistent - an envelope can work well but not sure if diva can do that
the 808 also has a short sweep on it @2-5ms which gives it some attack
there’s loads of data available online on how to do it including my website...
is a good thing to learn

davr