Input/Output Stages for Blackfin DSP

[maczrool]

I am working on an audio processor which will have an Analog Devices BF522C Blackfin DSP with built in CODEC.

I'd like to have an adjustable input and output stage via potentiometer with a range of 1-5 volts single-ended for input/output so that it can interface optimally with a wide range of devices. I'd like the input and output impedance to be constant with standard values (10K input and 100 Ohm output)with varying potentometer settings. I'm primarily interested in opamp designs for simplicity. Can anyone point me in the right direction?

Many thanks,
Stu

 

[hifimaker]

I'm not sure why you would want an attenuator on the input, unless you are expecting voltages in excess of the device. With this DSP you can digitally attenuate the signal. The Blackfin is a beautiful chip to work with. I'm building an audio server using the Blackfin BF537 running Linux.

Perhaps you could share more about what you are trying to accomplish to give better guidance.

-David

 

[maczrool]

I'm not sure why you would want an attenuator on the input, unless you are expecting voltages in excess of the device. With this DSP you can digitally attenuate the signal.

Hi David,

Exactly and yes I am aware that it can do this digitally, but this does not prevent overload obviously. The Blackfin's CODEC will only accept up to 1 volt RMS, but I might wish to use it in an automotive application where some higher end aftermarket headunits have high voltage inputs/outputs of 4 volts or more, hence the need for attenuation and for hefty voltage output as well. I would like to be able to adjust the inputs and outputs to match these signal levels.

Thanks,
Stu

 

[hifimaker]

Hi Stu,

Would you happen to have a data sheet for the BF522C? The AD site only has the data sheet for the common processor family and not specifically for the BF522C with the CODEC.

If it were me, I'd make a fixed ladder attenuator or voltage divider in front of the analog input that is either selectable via a switch or jumper or by relay from an I/O pin on the BF. Perhaps the amount of output attenuation could be incorporated into the output buffer circuit. Also on the output, digital attenuation could be applied in the DSP to increase flexibility.

-David

 

[hifimaker]

Hi Stu,

If you are making a consumer product, perhaps consider a default setting for the max anticipated input voltage. Then measure the voltage with the ADC and downward select the amount of attenuation needed programatically.

Here's an interesting article on over-voltage protection for audio in Automotive DesignLine: http://www.automotivedesignline.com/howto/196602756

-David

 

[maczrool]

Would you happen to have a data sheet for the BF522C

Not exactly. I have a datasheet for the series as well as one just for the codec but not combined. If you'd like one, let me know. The codec datasheet advocates a voltage divider using two 5.6K ohm resistors and a couple caps to attenuate 2 volts RMS to 1 v RMS.

If it were me, I'd make a fixed ladder attenuator or voltage divider in front of the analog input that is either selectable via a switch or jumper or by relay from an I/O pin on the BF. Perhaps the amount of output attenuation could be incorporated into the output buffer circuit. Also on the output, digital attenuation could be applied in the DSP to increase flexibility.

I thought about the switch idea also but I'd really prefer pots like you find on the inputs of digital recoders because of the increased adjustability. If I could dig up the schematics of an input stage of a DAT deck or something that would probably be perfect. Digital attenuation is not such an attractive option because of resolution loss. I could use a voltage divider for the input but will need a buffer circuit as you suggest with some gain.

I've really not found any reference designs for this out there because the datasheets for single-ended voltage ouput ADCs and DACs tend to suggest an RC circuit rather than an opamp based solution with which I can implement adjustable gain.

Thanks for your thoughts,
Stu

 

[hifimaker]

Hello Stu,

With so many I/O pins on this device. It would be pretty cool auto scale the input level. Self calibrating designs provide a lot of safety for the device and simplify set-up. I'd have to got back and look, but I believe AD has code available to attenuate output for the output that does not have or limits decimation for DSP volume. I have not seen the CODEC data sheet, but it probably does not have a lot of dynamic range. Especially given the noisy environment of a car, a listener may not be able to detect decimation. It could be worth experimenting with DSP based volume, since the cost is near zero to see if it meets your sonic requires. Just a thought.

Also if you have not downloaded it, try playing with VisualAudio. It's a pretty cool tool for getting a prototype running quickly and to test DSP volume, tone, and filter controls.

-David

 

[maczrool]

Thanks for all your cool ideas. The DSP definitely presents a lot of possibilities. I think though because of the wide range of applications for this device (dynamics processor) I am going to stick with a tried and true opamp input/output stage which I guess I'll just roll myself. It will probably have a unity gain opamp (buffer) at the input with a pot on a voltage divider for attenuation at the output of the opamp input to the ADC of the CODEC and a variable gain opamp circuit controlled by another pot at the output of the DAC. I'll have to breadboard it to test but I think I can come up with a workable solution.

Stu