On 7/20/15 7:49 PM, Nigel Redmon wrote:
To add to Robert’s comment on discrete-time analog…
The only thing special about digital sampling is that it’s stable (those
digital numbers can be pretty durable—the analog samples don’t hold up so well)
and convenient for computation. But the digital audio samples are just a
representation of the analog audio that’s been pulse amplitude modulated. (I
never worked with BBDs or CCDs, and suspect there’s some zero-order hold
involved in practical implementations,
there's gotta be *some* voltage at the output at all times. doubt that
it's return to zero, so ZOH makes the most sense.
but it doesn’t matter as long as that’s compensated for—digital to analog
converters have also dealt with the same sort of issue. Still, the basis is
that those samples in the BBD/CCD represent impulses, momentary snapshots.)
Just as with the digital versions, in the analog versions you have a lowpass
filter to ensure the input spectrum remains below half the sample rate, and on
the output you have a filter to get rid of the aliased images, created by the
modulation process.
In the early days of digital delays, the analog delays had some advantages that
are probably not obvious to someone coming from today’s knowledge. For
instance, today we’d make a delay with a constant sample rate, and use a
software LFO and an interpolated delay line to make a flanger. But back then,
computation was difficult and costly, so it was done the same way that the
analog delays did it: variable sample rate and vary the clock frequency with a
hardware LFO. The advantage of digital was better fidelity, but the analog
delays could sweep over a much wider range. Digital memory wasn’t so fast back
then, and super-fast A/Ds were huge bucks (I worked for a group in a company in
the late ‘70s that made a 40 MHz 8-bit A/D chip that was $800 in low
quantities, and they sold ‘em as fast as they could make ‘em).
geepers. that's fast. around 1979-80, i did a DSP project with a
MC6809 and a 12-bit DAC that i double-up and used with a comparator to
be a successive approximation ADC. in those days the DAC was $40 and we
didn't wanna spend money getting an ADC. the sampling rate was
something like 10 Hz (it was a medical application and the signal was
very slow.)
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r b-j r...@audioimagination.com
"Imagination is more important than knowledge."
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