XK-System (community project) > Hardware Development

(Request for Discussion) Bit Depth and Sampling Rate

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Artur D'Assumpção:
Hi guys,

We are looking for several opinions on which should be the best Bit Depth and Sample Rate to sample the analog levels for the VCA faders and Ultimation on the SSLMixed Computer design. Please justify your reason / theory / math behind your opinion.

What we know is that the SSL used 12 bits for the bit depth and sample rate is unknown so far.

Currently we were thinking to increase the bit depth up to 13 bits to give it a little more detail so we can take advantage of nowadays circuit/chip capacities and low noise. On the other side we didn't want to risk it up to 16 bits afraid that might get the noise level up.

Regarding the sample rate, this is an ongoing doubt and debate. What could be the best sample rate for these types of applications?

Appreciate your help guys,

Artur

Artur D'Assumpção:
Hi

Regarding the bit depth, the VCA-only systems used 10 bit converters and the Ultimation systems used 12 bits.

Because of the difficulty in finding 13bit ADC chips we are now thinking on going to 12bits or 14bits chip.

We have a 10v range to deal with on the VCA send/returns:

VCA-Systems: 0-10v
Ultimation-Systems: -5v - 5V

If we use a 12 bits ADC here's the precision we get:

10V/2^12 = 0,00244V = 25 mV

If we use 14 bits ADC here's the precision we get:

10V/2^14 = 0,00061V =~ 0.6 mV

Does anyone knows or can measure what's the noise level of the VCAs? Also, does this noise increase greatly with more channels loaded?

Cheers,

Artur

waltzingbear:
that's the resolution of the control voltage. Most circuits I recall having seen have at least a low pass filter to dump the hf noise that may get in, You don't need or want anything greater than the fastest rate of change that the fade will change at, that is only potential garbage. Defining that number will give you a better handle on the potential sample rate as well. Zipper noise being the deadly outcome of the wrong choices.

Not sure what you are asking about regarding noise and loading.

Basic information is on the THAT datasheets for the 2151 for most of the circuits actually built by SSL, or 2181 for new revisions that can realistically be implemented.

Cheers,
Alan

Artur D'Assumpção:
So you think the best is to keep the 12 bit resolution (original Ultimation and +2 bits for non-ultimation)?

We're trying to get the best balance between accuracy and noise rejection.

Regarding the sampling time we were thinking on start by doing a 500 sample per second, which would cover a 250Hz frequency. This would also serve as a low pass filter around 250Hz.

This might seem to much, but how much detail do we need to cover the fastest fader move from bottom to top (or vice versa) and replicate accurately the move without degrading the VCA voltage curve so much that level changes would start to be heard (like stepped instead of linear).

Do you think it's exaggerated or too little?

Cheers,

Thanks.

Artur

waltzingbear:
I don't think!  :-)

What I suggest is to do what used to be called a literature search on the topic. There is published information on these topics (don't reinvent the wheel). Then make sure the new system meets or exceeds these specs.

In other words, I don't know, but I think it is important to know!

Alan

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