Re: Iterating through audio data
Re: Iterating through audio data
- Subject: Re: Iterating through audio data
- From: James Chandler Jr <email@hidden>
- Date: Thu, 26 May 2011 18:12:06 -0400
- Importance: Normal
Thanks Brian
That sounds like an excellent idea to use the sample-rate converter to upsample
and draw audio zoomed in at less than 1 sample per pixel.
I don't know how many samples that a sample-rate converter could downsample
"batch" before the graphic becomes too full of errors to be useful.
For instance, to display a 3 minute, 44.1K song within 1000 horizontal pixels
(180 ms per pixel)-- One would resample from a rate of 44100.0 down to a rate of
5.56. The resampled output Nyquist would be 2.78 Hz and the audio window would
show a blank line. All details of interest would have been brickwall-filtered.
In some cases even a small amount of downsampling might give unexpected results.
For instance, on a dainty finger cymbal, if we downsample even by a factor of
two, then the graphic representation would be horribly wrong. The downsampling
would have knocked out too many high harmonics to remain an accurate display.
Thanks
James Chandler Jr.
-----Original Message-----
From: Brian Willoughby
Sent: Thursday, May 26, 2011 5:17 PM
To: email@hidden Audio
Subject: Re: Iterating through audio data
Note that Apple's AudioConverter can produce this smoothed waveform,
provided to set the output sample rate to match the screen pixels.
When zoomed out and viewing multiple samples per pixel, the
AudioConverter would be downsampling. When zoomed in and stretching
samples out across a several-pixel gap, the AudioConverter would be
upsampling. In either case, the output data from the converter
should be a very accurate representation of the audio signal.
Brian Willoughby
Sound Consulting
On May 26, 2011, at 06:02, James Chandler Jr wrote:
When the user finally zooms in so close that there are fewer than 1 samples
per pixel, then you have to start drawing "connect the dots" waveform lines
interpolating a smooth line connecting the sample dots.
There are various ways to smooth the line connecting the dots. Some methods
are faster and some methods are more accurate. The most ideal would be a
method which draws on-screen as similar as possible to what the audio ought
to look like after it has passed thru an audio output D/A converter. The best
view would look like the band limited continuous signal being output from the
soundcard D/A converter.
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