[sdiy] New to list - and DSP development

[Kenneth Elhardt]

Ingo Debus writes:
>>MIDI is 31.25 kBaud. The external clock (1MHz in most cases) is required
because this baud rate cannot be generated by dividing down the internal
clock.<<

I did notice the divide down problem wouldn't allow for it.  I've never
understood why the MIDI people didn't use 38.4 kBaud which is a standard
rate.


Magnus Danielson writes:
>>The Mac serial ports have a _very_ nice feature in that you can supply an
external clock signal to it.<<

Yeah, I picked up on that from one of those earlier posts.  It's just a bit
misleading because there is sometimes a dialog box that pops up where the
user picks the speed from .5 to 2 MHz.  Fools the user into thinking he is
setting the speed.


Richard Wentk writes:
>>No, you still need to understand what Paul was saying.
If you try to synthesize a 440Hz saw wave with a sampling rate of 44.1KHz,
you can easily hear the upper harmonics folding back into the audio range.
So you can't use use procedural synthesis (i.e. using a counter for your
ramp, or looking up the ramp wave from a ROM) but to have to do some more
complicated tricks instead.<<

You're right, I didn't understand.  Thought he was saying I might be sending
data to the DAC faster than it can handle it so some gets dropped.

A couple of things about what you say.  First, what I'm programming now is a
form of additive synthesis that only uses sine waves.  As pitches rise I'll
drop the higher harmonics.

Second, I think it's how samples line up with very high wave frequencies
that can drive the sound back down into the audio range, in other words
aliasing, as it can happen with sine waves that have no harmonics.  I don't
think it's the harmonics of a lower frequency wave.  So I don't think that's
a problem at 440Hz.  I think it happens at considerably higher frequencies.
Some form of lowpass filtering or added jitter may help solve that problem.
I also thought audio DACs or external circuitry provided some form of
lowpass filtering to help eliminate the problem.  Many times I've had to
generate pure digital waveforms or swept sine waves going from 20Hz to 20KHz
in Cool Edit and I haven't heard a problem.  I assumed that was taken care
of in hardware.  But if I'm wrong I hope somebody points out the pitfalls.

-Elhardt