...snip
Absolutely. Thank you. I was going to stick on an input low-pass
filter with a 3dB point at about 5kHz to simplify things a bit. But,
yes, single notes.
Cheers
By the way, if you really want something which is not necessarily what
you specified in your original post, but rather you want a digital
signal which, when connected directly to a speaker, will sound like
the original audio, and which works for polyphonic music and speech as
well, then what you want is a 1-bit sigma-delta ADC. This results in
a bit stream that can sound quite good (e.g. Sony SACD or many
ordinary CD players also use a 1-bit DAC).
If you want a very simple circuit which approximates this well enough
for an intercom or phone quality of audio, then all you need is a D-
flip flop, an R-C filter and a comparator (which can be approximated
by a logic gate input if you want the cheapest system). If you have
never built one of these, you really should because it is very
surprising when you turn it on the first time with a walkman attached
to the input. Try some different resistor and capacitor values
because I have forgotten what works best. Make sure that you use the
inverting output of the flipflop for the feedback. I suggest 74HC74
devices. You could buffer the output with logic invertors to drive a
speaker. To keep the power dissipation low, put a big capacitor in
series with the speaker.
Basically, as follows (view in a fixed width font e.g. courier):
Audio in 10k 10k
*---||-/\/\/\-*----/\/\/\-,
100uF | _____ |
| | _| |
,---*--|D Q|--.
| | |
100nF--- | Q|-----Out
--- |_/_\_|
| 74HC74 |
--- Clk 1MHz
GND
By the way, you can emulate the D-flipflop using a fast
microcontroller if you want, by reading the state of one port bit (the
D input), and writing the value to another port bit (the Q output).
If you want a more fancy version, look here:
http://www.hypex.nl/technology.htm
Chris
