G'day all!
I have an Arduino Duemilanove hanging around spare at the moment and thought I might try a few audio interfacing projects. I'm just wondering what sort of sampling frequency I can achieve using a single analog input and applying some simple algorithms on chip, then reporting using a few digital outputs tied to LEDs.
I'd like to sample in at ~44.1 kHz if possible.
For reference the first thing I want to try is a simple guitar tuner.
I don't think you can sample that fast at full resolution. The ATMega168 can only sample at 15 ksps at its full resolution.
Having said that, you should be able to get a suitable sample rate to get a functioning guitar tuner. 44.1 kHz is most likely a fair bit faster than you will need given that the fundamental of the high E string an a guitar is around 330 Hz.
It takes about 100 us (0.0001 s) to read an analog input, so the maximum reading rate is about 10,000 times a second.
http://arduino.cc/en/Reference/AnalogRead
Rob.
Google for 'AVR guitar tuner', there are a couple projects out there that do this already, and they seem to be able to do it without too much trouble with the speed of the AVR.
If you use an analog comparator (either the internal one in the AVR or an external opamp one) that turns the analog input into a square wave, you can sample oscillations at much higher speeds. While this isn't true audio sampling, for building a guitar tuner it's often all you need since all your code would be doing anyway would be counting zero crossings per unit time.
There are a number of ADCs available that are serial, I2S is NXP's standard based on I2C. They allow you to pretty easily pull in analog even at much higher speeds. This link should get you to an NXP part that is designed for audio: UDA1361TS
Free samples are your friend :)
First, for your particular application, you really only need 1 kHz or so sampling rate, assuming you're tuning the fundamental frequency and not one of the inharmonic partials...
Anyway, as for the maximum possible sampling rate, the Arduino manual says:
It takes about 100 microseconds (0.0001 s) to read an analog input, so the maximum reading rate is about 10,000 times a second.
This would imply 10 kHz sampling frequency is the max. However. You can get higher sampling rates by accessing the ADC registers directly. The Arduino Realtime Audio Processing page uses two channels at 15 kHz, for instance. So the 10 kHz max is only while using the built-in AnalogRead() function, because it has a lot of overhead.
The ADC is optimized for best operation with a clock speed of between 50 kHz and 200 kHz:
By default, the successive approximation circuitry requires an input clock frequency [ADC clock] between 50 kHz and 200 kHz to get maximum resolution.
Since an ADC conversion takes 13 clock cycles, this would be a sampling rate of 4 kHz to 15 kHz. According to AVR120: Characterization and Calibration of the ADC on an AVR:
For optimum performance, the ADC clock should not exceed 200 kHz. However, frequencies up to 1 MHz do not reduce the ADC resolution significantly.
Operating the ADC with frequencies greater than 1 MHz is not characterized.
1 MHz clock frequency = 77 kHz sampling frequency, so that's the realistic max.
The forum thread Faster Analog Read? has more about this.
The on-chip converter will work for this application as others have pointed out, but you should really look into using an external ADC. This will save you a lot of trouble, and free your micro to sample over SPI or I2C at much, mich higher data rates, with less noise from the micro's clock, and with greater precision than using the internal ADC. If you want more resolution and/or a higher data rate, then use something like the LTC1867 , which will let you sample at up to 175kHz (Although you can clock it however quickly you want to) and then read out the 24-bit data at up to 20MHz over SPI. See what a real ADC can do? :) With that kind of power (And a 24- or 32-bit DSP), you can compress and store your audio, filter it, modulate it, play it back... the possibilities are endless.
