Digital audiocontrol suggestions and opinions is seeked!

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Hello!

I'm would like to build sort of a digitally controlled mixer (or more to speak I only need 2 aux sends+ returns). Is has to be able to change "fast" (50-70ms would be ok I guess) when the programs is changed. I have picked an Atmega8 for the purpose for a starting point. I would like to hear if someone has experiences with theese things? I has got to be pro quality (so to speak).

Have anyone tried to use the PWM output + some low pass filters and transistors eg.? (the sound have to be analog all the way through the path). I have the PWM up and running on the Atmega8.

What about digital potmeters? Motor controlled pots? (I have som of thoose bu they are rather slow).

Hope for some ideas!

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I guess we need more info :
How many stereo/mono input channels ?
Do you need master L/R outputs ( think so )
Sound controls per channel ? Parametric or graphic EQ's ? Hardware PWM on the AVRs is available but only up to 4 channels , afaik. Software PWMs can have more channels but is limited in PWM frequency which you probably want to keep out of audiable range.

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When you say "mixer", you mean "audio mixer"? There are several kinds of mixers.

What is the control based on? Time? Signal amplitude? Operator input?

If it IS audio, what sort of bandwidth and distortion levels are required? What amplitude levels?

PWM is probably not very useful for this, if you need any sort of quality audio (assuming that it IS audio). Also, the internal ADC will not be adequate because of the low sample rate. With 2 channels, you can only get 7,5KHz sample rate per channel at 10 bits resolution, and that is good for an absolute maximum of 3.75KHz bandwidth.

Jim

 

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Isn't this more of a job for op-amp's configured as summing junctions? Or are you planning to do that anyway and use the mcu as channel select/amplitude control?

--greg
Still learning, don't shout at me, educate me.
Starting the fire is easy; the hardest part is learning how to keep the flame!

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Quote:
I has got to be pro quality

I don't really understand your overall project, but:

If you need low noise with the digital level switching then I would recommend using an audio chip designed for this purpose.

I recall an old project I did, a VHF repeater, which used a Basic Stamp as the processor and an analog Devices "digital pot" to adjust each transmission's audio gain, (software controlled AGC loop).

It worked, but you could hear the circuit ramp up and ramp down at the start of each transmission where the audio level was outside the window.

Turned out to be not such a great design... But one always knew which repeater they were on :cry:

JC

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Is this a digitally controlled analog mixer? Like you want to send a digital volume level that gets sent to a dac driving a vca volume control? Or is the audio 44100 Hz 16 bit samples, and you actually have to do 16bit x 16bit multiplies by whatever the volume multiplier is? That will need a lot more mips and a 16bit processor. Or 32 bit. Like an arm.

Imagecraft compiler user

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XMOS devices are rather good at high-end audio:

http://www.xmos.com/applications...

Leon Heller G1HSM

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I used the good old LM1036 in an older bassamp project and apart from its low level handling ( no more than 300mV RMS ) it did a good job. It is controlled by 4 voltages ( Volume, Balance, Bass, Treble) which can be generated by a e.g. TDA8444 DAC through TWI/I2C bus.
My latest bassamp is designed around a TAS3208 DSP from TI and it is controlled by a Mega328 equipped with a DOGM LCD. You can play with it using the TAS3208-EVM-LC which cost around 90$ at the TI Store. All parameters are controlled by TWI/I2C.
http://www.schoeldgen.de/bassali...

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Hey Thanks for the many answers! (sorry for the late reply I though I had checked the "notify me when a reply is posted", which I apparently had not). Looking forward to go through all the answers later today:)

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mschoeldgen wrote:
I guess we need more info :
How many stereo/mono input channels ?
Do you need master L/R outputs ( think so )
Sound controls per channel ? Parametric or graphic EQ's ? Hardware PWM on the AVRs is available but only up to 4 channels , afaik. Software PWMs can have more channels but is limited in PWM frequency which you probably want to keep out of audiable range.

Specifications:

Mono in, stereo out for the mixer as well as the FX sends/return. Only the controls on the picture attached. The sound has to stay analog all the way through, no DSP.
It does not have to be able to L/R pan the stereo so the same MCU control for the effect return L/R should be able to be used. The easiest would be to use the 4 PWM's on the atmega8, as I need some display (maybe LEDs will do) to and probably a rotary encoder to program it with. I also need an EEPROM to make it remember presets after that power has been shut off, I guess the 512Bytes internal would just do.

Quote:
...PWM frequency which you probably want to keep out of audiable range....

This is exactly what I want to know something about. If I keep it over eg. 24KHz and use it with a transistor/optoFET eg. then should I be safe? Or can it cause some strange unwanted effects that way? would I have to make a low pass filter, or at least put a cap on it?

Attachment(s): 

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gregsmithcts wrote:
....Or are you planning to do that anyway and use the mcu as channel select/amplitude control?

Yes! Sorry I can see now I should have made it more clear from the start :)

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By the way it is for guitar/guitar fx control (guess that explains the mono in, stereo out better). Again if the PWM could work it would be preferable as it would simplify the design mostly..

(I just see it only has 3 PWM channels in the datasheet though)..

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I wouldn't use the PWMs to control analog voltages. I've experimented a lot with and its really hard to supress the PWM frequency while keeping the controls responsive and fast (and keeping the genuine guitar sound) . I'd use either a VCA ( like the above mentioned LM1036 or similar ) with a MCU controlled DAC or switch to a DSP. DSPs are very versatile but harder to come by , VCA are cheaper but not so flexible. My Bassamp ( link above ) made that evolution and the difference is great. The DSP now does real Biamping with 7-band EQs per channel and everything is completely controllable and stored in the Mega. Sound is cool and after all its still much cheaper than any soultion you can buy.