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Controlling Potentiometers from a PC

Hi,

I have an analog audio circuit which contains a number of pots. I'd
like to be able to control these pots from a PC using a program I have
written in C++.

So for example, I'd like to tell my program to adjust the volume from
min to max over the course of 10 seconds and receive the exact same
audio result as I would get it I was to adjust the volume knob with my
hand. In other words, I don't want any static or glitch induced into
the audio output.

I have looked at digital potentiometers but it seems to me that they
are not suitable for my application since they adjust the potential in
a stepping manner. Any comments on this?

So I'm wondering if the best option is to output an smooth analog
signal using a dac and then have this signal either control a
potentiometer or else create the potential by some other means. Any
comments on this? What devices might I look at?

Thanks for your help,

Barry.
 
M

Martin Griffith

Jan 1, 1970
0
On Tue, 4 Nov 2008 01:05:35 -0800 (PST), in sci.electronics.design
Hi,

I have an analog audio circuit which contains a number of pots. I'd
like to be able to control these pots from a PC using a program I have
written in C++.

So for example, I'd like to tell my program to adjust the volume from
min to max over the course of 10 seconds and receive the exact same
audio result as I would get it I was to adjust the volume knob with my
hand. In other words, I don't want any static or glitch induced into
the audio output.

I have looked at digital potentiometers but it seems to me that they
are not suitable for my application since they adjust the potential in
a stepping manner. Any comments on this?

So I'm wondering if the best option is to output an smooth analog
signal using a dac and then have this signal either control a
potentiometer or else create the potential by some other means. Any
comments on this? What devices might I look at?

Thanks for your help,

Barry.
http://www.thatcorp.com/Blackmer_IC_Voltage-Controlled_Amplifiers.html

martin
 
E

Eeyore

Jan 1, 1970
0
Hi,

I have an analog audio circuit which contains a number of pots. I'd
like to be able to control these pots from a PC using a program I have
written in C++.

So for example, I'd like to tell my program to adjust the volume from
min to max over the course of 10 seconds and receive the exact same
audio result as I would get it I was to adjust the volume knob with my
hand. In other words, I don't want any static or glitch induced into
the audio output.

I have looked at digital potentiometers but it seems to me that they
are not suitable for my application since they adjust the potential in
a stepping manner. Any comments on this?

So I'm wondering if the best option is to output an smooth analog
signal using a dac and then have this signal either control a
potentiometer or else create the potential by some other means. Any
comments on this? What devices might I look at?

Look at the recent threads below on MDAC like devices such as TI's
PGA2310.

"Anyone here familiar with so-called 'zipper noise' in audio
remote/automation level control applications ?

zippers and rabits

Does turning a volume knob create sidebands?"

Also consider motorised pots from the likes of ALPS.

Graham
 
M

MooseFET

Jan 1, 1970
0
Hi,

I have an analog audio circuit which contains a number of pots. I'd
like to be able to control these pots from a PC using a program I have
written in C++.

Well there's half your problem right there. C ick poo ick poo!
So for example, I'd like to tell my program to adjust the volume from
min to max over the course of 10 seconds and receive the exact same
audio result as I would get it I was to adjust the volume knob with my
hand. In other words, I don't want any static or glitch induced into
the audio output.

Does the adjustment need to be able to hit exact values or can we be
only roughly right?

I have looked at digital potentiometers but it seems to me that they
are not suitable for my application since they adjust the potential in
a stepping manner. Any comments on this?

If the steps are small enough, a step is just fine. Digital pots
usually have too few steps. A DAC may be a better option.

So I'm wondering if the best option is to output an smooth analog
signal using a dac and then have this signal either control a
potentiometer or else create the potential by some other means. Any
comments on this? What devices might I look at?

A Gilbert cell or just a single JFET or BJT can be used to control the
gain by multiplying two analog signals. You can buy volume control
chips that are basically Gilbert cells. For all of these you need a
DAC to make an analog voltage to control them. The signal from the
DAC needs to be filtered to clean it up.

Since you are into writing software, why have the circuit at all. Use
the sound card in your PC and software to do all of the work.
 
V

Vladimir Vassilevsky

Jan 1, 1970
0
MooseFET said:
If the steps are small enough, a step is just fine. Digital pots
usually have too few steps. A DAC may be a better option.

The big problem with the digital POTs is that they are actually
RC-RC-RC-RC... This RC of the high and the variable order comes into
play. Another problem is that the THD of the digital pots is
substantial. For those two reasons, I wouldn't recommend using digital
pots for audio.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
 
V

Vladimir Vassilevsky

Jan 1, 1970
0
Jim said:
Your comments are appropriate only when the digital pot is improperly
loaded. I've rolled my own for use as RF attenuators up to several
hundred MHz.

I've even made them with dB steps ;-)

One dB is a large step. I did tunable narrowband filters with the
off-the-shelf digital pots; it was quite tricky to compensate for the
variable RC while maintaining the reasonable SNR and THD.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
 
E

Eeyore

Jan 1, 1970
0
Paul Hovnanian P.E. said:
We're all assuming that these pots are plain old gain (volume) controls.
As others have suggested, amplifiers with voltage controlled gain might
be the way to go. You can select from a wider range of DACs to give you
the step size you require.

If these pots are for tone controls, or other effects, the proper analog
circuitry will have to be developed to do voltage to whatever. Not
knowing what kind of 'audio circuit' you are starting with, consider the
fact that it might be simpler to throw the whole thing out and design
something optimized for digital control.

Motorized pots are a nice option in that the operator can see (and in
some cases manually tweak) familiar looking controls. Some sound
technicians like the old analog controls because the knob (or slider) is
both the control plus a quick visual indication of the status of their
sound board.

Very much so.

A reasonable alternative is an encoder with a ring of leds. ALPS ? even make
one (somewhat expensively) ready to go.

Graham
 
M

MooseFET

Jan 1, 1970
0
The big problem with the digital POTs is that they are actually
RC-RC-RC-RC...  This RC of the high and the variable order comes into
play. Another problem is that the THD of the digital pots is
substantial. For those two reasons, I wouldn't recommend using digital
pots for audio.

As you switch a digital pot, there is also a step because of the gain
change not happening at zero voltage. These tend to jump out at you.
 
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