PDA

View Full Version : writing 16digital pins at once (for r2r dac)



michi
01-30-2016, 02:36 PM
hi,
i'm musician and elecronic sound entusiast - i've developed in the last time a life instrument with teensy 3 & audioboard.
i will write a article about this in the next days on my blog & post it in the forum.

currently i'm interested to test also a 16bit r2r dac maybe for use with higher samplerates as 44100.

how can i write 16digital pins at once? (like 8 Bit with "PORTD = ...")

michi

PaulStoffregen
01-30-2016, 04:11 PM
Sorry, 12 bits is the most you can get, on PTC0 to PTC11. See the schematic for info about which pins correspond to the native ports.

http://www.pjrc.com/teensy/schematic.html

PTC8 to PTC11 are on the bottom side pads.

If you all you want is a higher sample rate, hacking the I2S code is definitely the easier and better path. Others have done this, so if you search I'm sure you'll find the prior conversations about hacks for 96 and 192 kHz sample rates.

High quality audio DACs will give you much lower distortion than you could possibly hope to achieve with an R-2R ladder, even with expensive precision resistors. Don't forget the transistors inside the chip act (mostly) as temperature-dependent resistors when on, with slightly different values driving high vs low, which skews the accuracy of your otherwise well matched R-2R ladder. If you make your external resistors much higher values to reduce the errors due to the internal mosfets, stray capacitance combines with those resistors to low-pass filter your signal.

You might also read some well informed opinions about the practical effects higher sample rates, particularly with ultrasonic intermodulation distortion effects. This one is my favorite. ;)

http://people.xiph.org/~xiphmont/demo/neil-young.html

michi
01-30-2016, 08:09 PM
Thanks Paul,

i will see what i do ...

... higher rates are interesting for me to reduce aliases in sound synthesis (in fm-synthesis or waveshaping for example)

on the other hand the the simple princip of the r2r-network is fascinating for me, and i want to try it out - if i found the time :-)

for my synth projects i need only one mono output (in good quality) and no input, because this my idea with the r2r.

on a new tread i will present my last instrument RA.

michi

Frank B
01-30-2016, 08:39 PM
The Audioboard can work with more than 44100 (perhaps 192K, but read the datasheet - i'm not sure)