[sdiy] DECISION TIME: need a Ucontroller/ARM/DSP chip that will process audio

[Dan Snazelle]

Wow

This is valuable information.



You should write a book on AVR audio!


I will take your advice and work on passing audio.




Thanks

Sent from my iPhone

On Mar 3, 2012, at 10:33 PM, Olivier Gillet <ol.gillet at gmail.com> wrote:

>> 1. 10 bit adc
> 
> 3 of which are rubbish if you don't take extra care. Check the
> datasheet - if you want high quality conversions you need to put the
> chip to sleep during conversions to avoid noise from the other
> peripherals - or at least make sure you don't toggle GPIOs while a
> conversion is taking place. Proper decoupling + use of a dedicated
> external voltage reference can also help!
> 
> Or go with an external ADC or codec.
> 
>> 2. 8 bit dac which means errors converting floats to 0-255
> 
> Which 8-bit DAC are you using? You mention floats. Don't use them for
> anything, they are emulated so incredibly slow. This applies to many
> of the MCUs used here, even PICs, STM / LPC - no floating point unit
> so only operations on integer types are efficient!
> 
>> I do know that chips like the pt2399 require serious input filtering....maybe i need to implement that on the avr as well
> 
> Which sample rate are you working at? To avoid aliasing problems, a
> filter attenuating all frequencies above half the sample rate should
> be present on the input.
> 
> Also check for clock jitter - writing to the DAC at unequal time
> intervals makes everything sound ugly. On ATMegas the right way of
> structuring audio output is to have a timer ISR and write samples from
> there (generated in situ or pulled from a FIFO). You'll have to put
> the code doing the write to the DAC at the very beginning of the ISR
> to make sure that the code path from the beginning of the ISR to the
> actual write to the DAC always executes in the same amount of time.
> That's where the issue of "predictable" timing discussed in earlier
> posts can arise...
> 
> In some older post you mentioned doing stuff in the main code loop:
> 
> void loop () {
>  // Read some inputs
>  // Write a sample
> }
> 
> Don't do that, you won't get a regular sample clock!
> 
> Don't use arduino's analogRead (it's blocking!). Learn how the ADC
> work on the ATMega, how to configure it...
> 
> First thing to do would be to have a simple "pass-through" project -
> just take the input and write it to the output, as cleanly and
> accurately as possible. Once you get that right, move to effects.
> Don't get distracted by anything unless you don't get this basic thing
> right.
> 
> Olivier
>