Audio In

My Shruthi synth only outputs audio routed through the audio in, when the Shruthi is triggered via a MIDI sequencer or keyboard, is this normal?

I had a vague idea the unit would allow external audio to be processed through the unit in stand-alone fashion…

Also, when all the Oscillators are set to none, I still get a click whilst pressing a key on my keyboard, or when being triggered via a sequencer…

Note* none of this distracts from my love of this synth, I am purely curious! :slight_smile:

Happy New Year

> I had a vague idea the unit would allow external audio to be processed through the unit in stand-alone fashion……

Of course it can do this!

> My Shruthi synth only outputs audio routed through the audio in, when the Shruthi is triggered via a MIDI sequencer or keyboard, is this normal?

Yes. What about checking what the manual has to say about the external audio input?

“”“
Line-level, mono audio input. Note that the external audio signal flows through the VCF and the VCA – you won’t hear anything until a note or sequence is played by the Shruthi to “open” the VCA.
”""

So… The signal from the VCF goes into a VCA. And you won’t hear anything unless the VCA is open. And how do you open the VCA? By default it’s controlled by the envelope, and you need to play a note to open the envelope.

But you can also get the VCA constantly open by routing the “offset” modulation source to “VCA” in the modulation matrix. Patches 124 … 128 are actually good examples.

Which means that you can not only get filtering effects on the signal, but also amplitude effects like tremolo on the external signal…

Super! Thank you :slight_smile:

Ah, I see>>

(5: Line-level, mono audio input. Note that the external audio signal flows through the VCF and the VCA – you won’t hear anything until a note or sequence is played by the Shruthi to “open” the VCA.)

Yes, reading the manual is always useful :smiley:

There are times when you might want to use an external oscillator, so it is useful for the external sound to be controlled by the AMP and Filter envelopes.

1 Like

Line-level, mono audio input. Note that the external audio signal flows through the VCF and the VCA – you won’t hear anything until a note or sequence is played by the Shruthi to “open” the VCA.

Does it:

  • runs first through the VCA stage and then through the VCF or
  • runs first through VCF and then VCA stage

?

Greetings,
rio

The external audio signal (as well as the internal osc’s) flows through the VCF and then through the VCA.

ok. Is there a way to bypass the cpu-driven VCA for external signals? So that the external input can go directly through the VCF and OP?

Why would you want that? You would not get a line level signal whithout the vca.
By the way: it‘s a completely analogue signal path. The vca gets just a control voltage from the cpu.

The idea is to loop the external audio thru the VCF, regardless of the VCA setting of a preset.

Greetings,
Rio

Just look at the last presets (124 to 128 IIRC) – in the mod matrix there is an offset routed to the VCA. As a result, the VCA is always opened.

Thanks, I understood that so far.

I wanted to know more whether it is possible (on the hardware side) to bypass the VCA, so that external signals are independent from the VCA can go through the VCF…
it’s just a question of whether it would be possible in general?

Best regards,
rio

You can install a switch to select whether you want the final output to be the pre-VCA (straight out of VCF) or post-VCA signal. Here’s how it would look on the SMR4 mkII filter board.

From your message, it seems that you want the external signal to flow through the VCF but not the VCA; but the oscillators signal to flow through the VCF and the VCA. This is not possible: both signals will be mixed together at the VCF output, it’s not possible to “unmix” them and route only one component of the mixture to the VCA.

Wow - Impressive. I have to look at that in detail. Thx so far.

Best regards,
Rio

Hey pichenettes,

I just looked over there for a moment, but do I understand that correctly, that the “pre-VCA” mode bypasses the TL072 completely - thus audio signal goes out directly from the VCF? is the signal not too weak? Btw: Any Shruthi VCA settings will be ineffective, right?

maybe I’m just thinking wrong…and you can still explain that to me. Thank you.

Best regards,
rio

No, it’s not too weak, it’s about 4Vpp.

I know the A in VCA implies that somehow the signal is amplified, but it’s amplification with a gain < 1. Voltage-Controlled Attenuator would be more accurate.

To avoid noise, the signal flowing through a filter should always be at near-maximum amplitude.

Yes.

Would a simple SPDT do the same and electronically ok, which can switch between 5V+ (from board) and the I_GAIN connected behind the IC3 (in your schematic above)?

Greeings,
rio

If you do that you’ll cause a short between +5V and -5V + 2 diode drops, and your LM13700 will release smoke!

If you want a hardware switch that opens the VCA, you can use an SPDT switch that routes either +5V or the VCA> CV signal from the control board to the VCA> pin of the filter board.

yes you mean I have to connect either +5V or VCA CV to that CV Gain connection, right? Therefore I need to cut that VCA CV connection on the controlboard (which must be connected now to e.g. right pin of SPDT). The middle pin of SPDT goes to VCA pin of the board-connector then. It’s all more theory at moment :wink:

And the +5V can be taken directly from controlboard to the left pin of SPDT, right? Did you mean it makes a difference to take the 5V+ from controlboard instead filterboard therefore? (The controlboard is fed by the filterboard)

Greetings,
rio

Yes.

Yes.

No, it doesn’t make a difference.