audio processing

[mischl]

hello all

i'm looking for a small solution which can control an audio signal as following :

- volume
- equalizer
- limiter

i would like to do that with a pic. but i don't know which one is fine for that. smust it be a dspic?

thanks for any help

[mister_e]

no need for a DSPIC.. TI and few other company already do some sounds DSP easy to use with I2C or SPI interface. Usually those chip are use in home theater sound system..

For the limiter.. no big deal. Use a PIC analog in, read the signal, compare to your own treshold, then play with the volume.

[mischl]

hi mister_e

thanks a lot for the answer

can you tell me some types from TI? this are only eq or also volume control include?

and i understood right for the limiter: read in the audio signal parallel to the TI (i think the audio signal must go trough the dsp) and then control the volume via I2C or SPI on the DSP?
or another setup?

thanks for helping a starter again

[mister_e]

http://focus.ti.com/paramsearch/docs...s&familyId=378
look for TAS3002 and TAS3004. Yup not as easy as ABC but i already used them and they gives decent results. TAS3002 will probably be the smartest choice. Of course you can find many other from aother company... google search dude

For the limiter, you're right. You read the signal in parrallel with the Audio Processor then you play with the volume VIA I2C bus. TAS3002 include a compression/expansion module but i can't comment on that section as i didn't used it.

Depending your skills, it willn't be easy to do, but really interesting project you have on hand now. Good luck!

[Sebastian]

Hi,

I just found your thread while searching for information on a TAS3002. I've some experience with microcontrollers (80C552, PIC) and know how to use an I2C bus.
My next project will be a midi controlled distortion unit (for guitar) and I'd like to use the TAS3002 for that.
However, I've got no idea of these "strange" filter coefficients that I need for the biquad filters (I need a 4 or 5 band parametric eq). Can you help me with some formulas or other calculation tools?

Thank you
Sebastian

Sorry for my English, I'm German.

[mister_e]

Yeah Biquads are a bit hard to figure out. TI have a software called ALE (Automatic Loudspeaker Equalisation) you sould aksed them and play a little bit with. It should be enough to give you some hints.

Now, do you really need all feature of the TAS3002? DRC, Mixer, Bass, Treble, Biquads and such?

If you just need a simple equaliser you should jump to the TDA7416.

But TAS3002 is a reaaaaallly cheap'n great Audio DSP

[Sebastian]

Hi,
Thanks for your fast answer. The TDA7416 really seems to be an alternative, at least it might be much easier to set up the equalizer. I don't need all the features of the TAS3002, but I couldn't find a distributor for the TDA, yet.

All in all the TDA should be the better choice, as the controller doesn't have to calculate these filtercoefficients... and that is the main problem.

Thanks for helping me

Sebastian

[Sebastian]

I've just asked TI for the ALE software.. let's see if they gonna send it to me....

The TDA is quite good but I cannot realize a full parametric eq with it..

Sebastian

[mister_e]

i agree, but doing all the biquads maths within the PIC

Maybe there's some alternative. I didn't spent too much time with the TDA7416 as now, neither the datasheet. It fitted perfectly for a previous customer request so i used the minimum... Eq and Spectrum.. not much.

I'm pretty sure they're something else 'round before playing with DSPics. For a single ended project i guess you don't want to waste too much of your time on it

[Sebastian]

Yes, that's another main problem.. the maths. I hope to get the ALE software to see if it is possible just to store some coefficients and not calculate them within the pic. However (I think), all parameters change (all 5) just by changing one value (for example the gain) of a filter.. so every coefficient has to calculated, not only one.

Hm... I don't mind to spend some time working with the TAS3002 as it seams to be a very interesting IC and still easier than a "real" DSP, but first I should know how difficult it is and whether it is possible with a pic at all... I have a free C-compiler, but it can only manage 16 bit variables.. so thats the first border.. difficult as a 4.20 data format is needed.


Sebastian

[mister_e]

sure it is possible with a PIC. If you plan to do the 24-32 bit maths.. i would suggest to use a 18F PIC and it's hardware multiplier. Have a look to Microchip, there's certainely some interesting app note and code template for those maths

TAS3002 is a really nice chip. Audio quality is respectable... well for that price it's excellent. For sure nothing beat the pure analog stuff but you'll never be able to fit all TAS feature in a respectable PCB size using only analog stuff

For purist like me digital sound processing is not an option.. sound quality is too much altered... but that's easy, portable and 99% of the people are satisfied Anyway, how many people 'round the world says that MP3/WMA sounds good? Not me... but portable.

[Sebastian]

Hey.. a new problem. My free compiler does not support any PIC18Fxx, only 12Fxxx and 16Fxxx.
http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt
Do you know if these formulas work for the biquad filters (for example bandpass)?
This would be a start.. if I don't get the ALE program. It doesn't seem to be too hard for a little pic (16F876), at least I will give it a try.

Sebastian

[mister_e]

As i remind you can enter your own coefficient in ALE and plot a graph of.

look at that, i think it could work... untested
http://www.gennum.com/audio/hip/soft...uad_filter.htm

[mister_e]

About a free C compiler, you may try the Microchip C18 student version.
http://www.microchip.com/stellent/id...&part=SW006011

i hate the microchip compiler but it's working. In another hand if you buy it... it will by far cheaper than Hi-Tech C.

CCS do one too. I just had a look to their website... seems to grow since last time i went there.

[Sebastian]

Thank you, the Excel sheet gives me a first impression of the calculations.
TI answered me.. "If you are a student, as you mentioned, I will like to inform you that we do not support students here at Texas Instruments." blabla....

Before trying the Microship compiler I'm going to test the 16F876 for it's maths capabilities.

regards,
Sebastian

[Sebastian]

Hm, I've tried to calculate the coefficients for a low pass filter with these formulas http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt

Im comparison to the excel sheet I had to set in the "center frequenzy" and not the -3db frequenzy. So when calculating with 2000Hz (2100 in the sheet) and Q=1, I got the following results (41000Hz sample rate) for a low pass filter:

formulas from the link:
b0 = 0.0256695991
b1 = 0.0513391983
b2 = 0.0256695991
a1 = -1.8973216034
a2 = 0.3674095060


Excel sheet:
b0 = 0,026936651
b1 = 0,053873303
b2 = 0,026936651
a1 = -1,343504439
a2 = 0,451251044

I think the values are quite similar, however, I don't know if it's correct..
The next step will be the pic calculation...

regards
Sebastian

[Sebastian]

Hi,

I've now tested the free C compiler... but as it is limited to 1kByte it is useless to me. One sin or cos 24bit floating point calculation and it's over. So I will try the Microchip Compiler.

regards
Sebastian

[Sebastian]

UPDATE:

I changed to an ATMEGA32 with AVR GCC. Floating point calculations and output to a LCD is no problem... thanks to some libraries

regards
Sebastian