Real hardware filters emulation.... how to know what the filter was?

[Parduz]

Hello.
My question is about back-engeenering my old Ensoniq SQ1 (a sampled-based synth) so i can "emulate" some of my most loved patches with some virtual rompler.

The most difficult thing right now are to find how to "copy" the filters.
There's nowhere an info about what they was, and i'm not even a newbie in this field. But i see that a lot VSTs have filters sorded (or listed) by "Dbs", or by some "type"... i mean, saying "lowpass" seems that's not enough
So i'm asking:
having my SQ1 working again here at my side, how could i "back-engineer" his filters to know the "kind" they was?
I may attach a scopemeter and an frequency analyzer and save pitures of what a square wave becomes for every 10 "steps" of the cutoff parameter, but then?

[Ingonator]

Hello.
My question is about back-engeenering my old Ensoniq SQ1 (a sampled-based synth) so i can "emulate" some of my most loved patches with some virtual rompler.

The most difficult thing right now are to find how to "copy" the filters.
There's nowhere an info about what they was, and i'm not even a newbie in this field. But i see that a lot VSTs have filters sorded (or listed) by "Dbs", or by some "type"... i mean, saying "lowpass" seems that's not enough
So i'm asking:
having my SQ1 working again here at my side, how could i "back-engineer" his filters to know the "kind" they was?
I may attach a scopemeter and an frequency analyzer and save pitures of what a square wave becomes for every 10 "steps" of the cutoff parameter, but then?

Hi,

never had a SQ-1 myself ut i found this:
http://www.synthark.org/Ensoniq/SQ-1_Plus.html

Quote:

Serial low pass and multimode (low or high pass) filters share 4 poles (i.e. 2+2 or 3+1)

Looks like a dual serial filter with eiter LPF + LPF or LPF + HPF.
If i understand the detzails about the Poles correctly it uses either a dual 12dB LPF + 12dB LPF or a 18dB LPF + 6dB HPF combination (with 1 Pole = 6 dB/oct).

As it seems to be a digital filter it seems to be more difficult to find a comparable one. Most filter emulations available usually seem to be based on analog filters.


UPDATE:
While typing this and searching with Google i just found this:
http://www.kvraudio.com/forum/viewtopic.php?p=5967998

The signal flow diagram seems to show the possible filter combinations:
1.) 3-Pole LPF + 1-Pole LPF (= 18dB LPF + 6dB LPF)
2.) 2-Pole LPF + 2-Pole LPF (= 12dB LPF + 12dB LPF)
3.) 3-Pole LPF + 1-Pole HPF (= 18dB LPF + 6dB HPF)
4.) 2-Pole LPF + 2-Pole HPF (= 12dB LPF + 12dB HPF)


So actually ro replicate the SQ1 you would need a serial dual filter with 12 dB + 18 dB LPF for the first and 6dB LPF, 12dB LPF, 6dB HPF and 12 dB HPF for the second filter.
If i see it correctly the filter do not contain resonance.

The closest Plugin i could currently see from my own collection should be Tone2 Electra2 which could also play wavetables and samples (and has tons of additional filters and other features).

KV331 Audio Synthmaster 2.6 could maybe do it too (has a dual filter for each layer). The "analog" LPF and HPF filter modes there have a freely adjustable slope so you could also get results like 6dB and 18dB (the Digital filter have only 12dB and 24dB). You could also try to use the "Slope" knob in the analog filters to manually adjust the filters to the sound you want.


UPDATE:
Just noticed that the other thread with the signal flow diagram was from you (the OP) so maybe my posts above were not really helpful...

I am also not sure if you are up to use an existing software synth or want to develop your own one.

[VariKusBrainZ]

CEM 3379

some info and schematic
http://www.synthtech.com/cem/c3378pdf.pdf

[Parduz]

Thanks for your fast reply
Yep, i know that post, it's mine as well.

While you were writing, i was using my poor skills to post the manual page about the filters, which i'm trying to attach to this post.

SQ1 Filters small.png

My ignorant question is:
is that's all? I mean: is the db value (the slope angle) the only thing i need to know (other than what cutoff = 70 means in Hz for my SQ1) to obtain the "same sound" starting from the same wave? (quotes are 'cause i know that the hardware D/A converter still "adds" to the final sound, i'm just talking about the principle, here).

Thanks again

[Ingonator]

Of course another option could be to analyze a WAV file recording of the different SQ1 filter mode in a Signal Analyzer.
A 24dB or 4-Pole filter means that for e.g. a LPF in the right part of the filter (where the signal in the spectrum goes down fastly) within one octave the ampltude goes down by 24dB.
A free Spectrum analyzer like e.g. Voxengo SPAN could display the notes and the amllitude for the current mouse position in the display. This way it should be possible to caculate/measure the slope of the filters.

Anyway my guess is that there should be the same result as mentioned in my previous post (which is based on the signnal flow diagram).

There are many different sounding filters with the same slope (e.g. 12dB/2 Poles) so just having the slope will not give a perfect emulation of the filter.

[Ingonator]

Thanks for your fast reply
Yep, i know that post, it's mine as well.

While you were writing, i was using my poor skills to post the manual page about the filters, which i'm trying to attach to this post.

SQ1 Filters small.png

My ignorant question is:
is that's all? I mean: is the db value (the slope angle) the only thing i need to know (other than what cutoff = 70 means in Hz for my SQ1) to obtain the "same sound" starting from the same wave? (quotes are 'cause i know that the hardware D/A converter still "adds" to the final sound, i'm just talking about the principle, here).

Thanks again

So it is still a series of two filters and a LPF + HPF combination will usually resulti in a Bandpass filter.
Anyway this combination is more flexible then a dedicated bandpass filter with a single Cutoff control.

[Ingonator]

My ignorant question is:
is that's all? I mean: is the db value (the slope angle) the only thing i need to know (other than what cutoff = 70 means in Hz for my SQ1) to obtain the "same sound" starting from the same wave? (quotes are 'cause i know that the hardware D/A converter still "adds" to the final sound, i'm just talking about the principle, here).

Thanks again

As already mentioned just knowing the dB value will not enable you to get the same sound in another synth.
There are LOTS of synths with a 12dB or 24 dB filter and each f those could sound differently.

Also the parameter range for the Cutoff is not comparable in different synths.

If you really want to copy the sounds in another synth the best you could do is to adjust the settings in the other synth "by ear" and coompare to the desired sound. If you have done that for a few patches maybe you could find a "rule" for tranferring the parameters without having to do a ditect compariosn.
Another problem is that usally the Cutoff parameres do not behave linear. For example the knob at a middle position (e.g. 50 from 100) could mean 1000 Hz while at 100 it is 18000 Hz or more).

[deastman]

The answer was already given. It used a Curtis Electronics CEM 3379 filter chip. Follow the link to read all about it. Curtis chips were used in most of the synths from that era, although the 3379 is not one of the better ones.

[Ingonator]

The answer was already given. It used a Curtis Electronics CEM 3379 filter chip. Follow the link to read all about it. Curtis chips were used in most of the synths from that era, although the 3379 is not one of the better ones.

The CEM 3379 VCF chip was used in the ESQ-1 and SQ-80 but the OP asked for the SQ-1 which should be a digital filter. The SQ-1 seemed to be released in 1992.

As you could see from the psted above the SQ-1 uses a dual multimode filter.
Opposing to analog filters for digital filters there is usually no detailed information available and teh original DSP code credeatd for old hardware synths is maybe not very helpful for replicating it in a VST, at least compared to having the detailed schematics/circuits of an analog filter..