Analog Boost on Access Virus B, C and TI

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JustinJ
KVRist
Topic Starter
38 posts since 31 Oct, 2017

Post Tue May 24, 2022 2:14 am

I'm currently trying to work out what the Analog Boost DSP does on Access Virus synths. Two controls, 'boost' and 'tune'. I'm guessing 'boost' is the amount applied and 'tune' is the frequency that the effect centres around.

As far as I can tell it's a pure EQ effect, no saturation etc as no harmonics are generated if you feed it an oscillator sine wave. It does indeed boost lower frequencies but if you increase 'tune' it also starts to lower higher frequencies.

My first guess was a simple tilt filter - two shelves. But that's not it. My next guess is a high shelf combined with a 6db low pass filter that only gets introduced as 'tune' increases. Seems like the Virus' would use a simpler method though.

Another possibility is to apply a 6db low pass filter at 'tune' frequency and then add that back to the signal by amount 'boost'. Hmmm.

Anyone here know what's actually being applied?

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mystran
KVRAF
7052 posts since 12 Feb, 2006 from Helsinki, Finland

Post Tue May 24, 2022 2:58 am

If you have a Virus around, perhaps try building a patch that just sweeps a sine-wave slowly from low to high frequencies (ie. a chirp), then record that and look at the envelope. Another possibility is to use something like a low (basically as low as you can get; you're trying to pretend you're recording a step-response) frequency saw-wave (hopefully predictable 6dB/octave shape) and look at the results in a spectrum analyzer (some of which can compensate for the slope, so you'll ideally get a straight horizontal line where no EQ is applied).

Once you have a plot (of one type or another) of what is going on, it's easier to guess how it's implemented. In general though, there is no real difference (except possibly some difference in parameterization) in something like a 1-pole shelving response whether you implement it as a 1st order shelving filter or a low-pass added to the dry signal. When it comes to linear filters, it's really just the poles and zeroes that determine the response and the actual implementation usually doesn't matter.

ps. You could also try asking adam (the author of Viper viewtopic.php?t=470486). I believe Viper tries to be more or less 1:1 copy of a Virus and the author takes the details seriously, but might not notice this thread?
Preferred pronouns would be "it/it" because according to this country, I'm a piece of human trash.

JustinJ
KVRist
Topic Starter
38 posts since 31 Oct, 2017

Post Tue May 24, 2022 3:52 am

mystran wrote: Tue May 24, 2022 2:58 am ps. You could also try asking adam (the author of Viper viewtopic.php?t=470486). I believe Viper tries to be more or less 1:1 copy of a Virus and the author takes the details seriously, but might not notice this thread?
I've been using the DSP56300 to run a Virus C emu to check it out. I little less detailed than your approach - been using noise and modifying the parameters to see what's happening.

I've already been in touch with Adam. He recommended performing a one pole low pass and adding that back to the signal multiplied by the 'boost' amount. Works nicely for bass boosting but I didn't spot the high frequency drop as boost increased. That got me thinking that there may be a low pass applied gradually as 'tune' increases.

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mystran
KVRAF
7052 posts since 12 Feb, 2006 from Helsinki, Finland

Post Tue May 24, 2022 4:36 am

JustinJ wrote: Tue May 24, 2022 3:52 am
mystran wrote: Tue May 24, 2022 2:58 am ps. You could also try asking adam (the author of Viper viewtopic.php?t=470486). I believe Viper tries to be more or less 1:1 copy of a Virus and the author takes the details seriously, but might not notice this thread?
I've been using the DSP56300 to run a Virus C emu to check it out. I little less detailed than your approach - been using noise and modifying the parameters to see what's happening.
Well, broadband noise (white, pink, etc) theoretically works fine as well, but the advantage of either sine-sweeps or some other predictable signal like a low-frequency saw/pulse wave (sine-sweep is better if you're willing to process the results; low-frequency saw is great for quick interactive observations of the spectrum in a live analyzer) is that you get a much cleaner readings as you don't need to worry about variance.
I've already been in touch with Adam. He recommended performing a one pole low pass and adding that back to the signal multiplied by the 'boost' amount. Works nicely for bass boosting but I didn't spot the high frequency drop as boost increased. That got me thinking that there may be a low pass applied gradually as 'tune' increases.
More likely the overall gain just goes down to compensate at high boost so the volume doesn't increase too much. If you're implementing this sort of thing by mixing a one-pole low-pass signal with the dry signal, then you can get this sort of "decrease in high frequencies" simply doing a weighted average (eg. at minimum you take dry only, at maximum you take 50-50 .. something like that). If you wanted more maximum boost with only a slight decrease in high frequencies, you could do this by increasing the gain of the low-pass signal before you do the mix.
Preferred pronouns would be "it/it" because according to this country, I'm a piece of human trash.

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Markus Krause
KVRian
1388 posts since 2 Jul, 2018

Post Tue May 24, 2022 4:45 am

No sure here if it is just a simple EQ with bass boost. It could also be some kind of dynamic EQ where the boost depends on the current overall volume. Some car-radios do this e.g. Sony's XPlod devices.

In this case you must do measurements with several volume-levels/ADSR-sustain-levels.
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JustinJ
KVRist
Topic Starter
38 posts since 31 Oct, 2017

Post Wed May 25, 2022 8:22 am

mystran, ah ok, yes, taking a weighted average would filter high frequencies. Adam also thought that there could just be an overall volume lowering but it sure does look like a low pass to me.

I'm pretty sure whatever they did is going to be pretty simple. More experimentation required...

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lb24569
KVRist
52 posts since 12 Mar, 2020

Post Wed Jun 15, 2022 5:44 pm

Head over to the DSP56300 discord where they have reverse engeineered a lot of the virus code and you will get your answer.

https://discord.gg/nQnQsTYr
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NAD
KVRian
1359 posts since 28 Jan, 2004

Post Fri Jun 24, 2022 2:47 am

I think it's just a low shelf boost with volume compensation.
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