XK-System (community project) > Hardware Development

(Request for Discussion) Bit Depth and Sampling Rate

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Artur D'Assumpção:
Hi Alan,

Well, actually I've trying to look for info on what's the appropriate sample rate for these kinds of applications (faders/VCAs), but it's not easy to find something so specific.
 
I was yesterday discussing this topic with Steve (retrocores) and common sense says we are going over board even with 250 samples per second.

If we go to the visual word (which is really different than the audio), 25 frames per second is enough for our eye to see a linear/fluid flow of the image, instead of stepped. My gut feeling says we should be far from those values, maybe 50 samples per second. This would cover a max 25 Hz frequency on the fader movement, which is a lot if you think about it.

Anyways, since this is a voltage, in one case that drives a motor (ultimation) on the other only drives the VCA (non-ultimation), I think I will try to search some literature on what's the advisable sampling for voltages or analog motors or something.... that should give me an idea of what's the ball park we should be on.

Cheers,

Artur

waltzingbear:
here are a few of the papers I found on the AES website.  moving faders is a good search term.  The ones below inclues Flying Faders, GML, Allison VCA, BBC  and one from Bruce Swedien.

there are other, less obvious resources out there, I think old RE/P's have good info in them, but not as seachable. A lot of the stuff you are looking for is from the 70's and 80's

joining AES reduces the online cost to $5 per paper. If you have a good technical college library available they can often be had thru interlibrary loan for free. 

Flying Faders: A Case Study in Product Development
http://www.aes.org/e-lib/browse.cfm?elib=5692

Design Considerations for an Economic Retrofitable VCA Fader Automation System
http://www.aes.org/e-lib/browse.cfm?elib=4765


The Design and Application of the Necam Computer Aided Mixdown System
http://www.aes.org/e-lib/browse.cfm?elib=3114


A Practical Approach to Recording Studio Automation
http://www.aes.org/e-lib/browse.cfm?elib=1753

Implementation and Evaluation of Autonomous Multi-Track Fader Control
http://www.aes.org/e-lib/browse.cfm?elib=16226

Ergonomic Aspects of Assisted Mixdown Systems
http://www.aes.org/e-lib/browse.cfm?elib=2231


The Evolution of Recording Studio Remix Automation
http://www.aes.org/e-lib/browse.cfm?elib=2231

Artur D'Assumpção:
Hi Alan,

I will dig into these articles!

I spend the afternoon reading about the anti-aliasing and reconstruction filters, to apply on the ADC/DAC circuits.

This will be a great addition to validate the design! Thanks so much!! ;)

Cheers,

Artur

waltzingbear:
as the Fs on this should be fairly low, the filtering could be kept simple and still attenuate things enough to keep from aliasing problems.

or the Fs can be higher and the analog HF pass band restricted enough that simple 2nd order filters would provide reconstruction with enough slew rate to make appropriate fades.

Hopefully the data in the articles noted will give you the time constants needed for good fades.
I think I have read most of those articles, but it was probably 30 years ago.

Cheers
Alan

Artur D'Assumpção:
Hi Alan,

Yeah, Steve (retrocores) is figuring out the best filter combination. What's curious is that on the SSL implementation they had a 13kHz lowpass filter, which is much higher than I expected. Makes me wonder why...

I doubt they would be sampling at higher rates, specially because they had only 1 ADC/DAC channel that they multiplexed for all channels.

I'm trying hard to find someone that has a subscription to AES Library so I can ask them to download the articles or if I could buy at the discounted rate. Currently I don't have any access to the articles.

Thanks for all the tips, they are invaluable for our work!

Cheers,

Artur

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