5 Jun
2005
5 Jun
'05
10:54 p.m.
For a small application I am developing, I need to display waveforms
(read-only) from audio data loaded into my application using libsndfile.
My question is about displaying a waveform in a GUI window -- I have
searched around the net looking for general algorithms but haven't seen
anything that describes this. I haven't tried coding this yet, so I am
asking if someone can describe the basic algorithm or point me in the
right direction to a book, web site, the source file in Ardour or
whatever. Even just a high-level description of the basic algorithm
would be good -- I am thinking somewhere along the lines that I am
taking the values of samples and drawing line segments from point to point.
Thanks in advance!
-- Brett
6 Jun
6 Jun
5:13 a.m.
On Monday 06 June 2005 01:05, Them wrote:
For a small application I am developing, I need to
display waveforms
(read-only) from audio data loaded into my application using libsndfile.
As I recently found out, this can be very messy :)
The basic algorithm is this:
Each horizontal pixel represents n samples. Usually n is pretty big.
Of those n samples you take the min and the max, and then you draw a vertical
line between them.
Obviously this has to be scaled to your vertical resolution and vertically
translated to the right position on your canvas.
If n<=1 you can choose to draw lines between the points, or just show the
points.
What is often done is to generate a peakfile, containing the min and max for a
set chunksize.
This is usefull if you have to redraw the waveform often.
Where it gets messy is if you want to zoom, display parts of the file, rapidly
scroll etc.etc. Then you need to set up clever caching and deal with rounding
problems.
In ardour the is ardour/gtk_ardour/canvas-waveview.c that does the clever
caching and libs/ardour/source.cc read_peaks() that generates the peaks.
The code in ardour may be to complex for your application because it is made
to show any part of an audiofile at any position, where you may move an
audioregion to a position "in between" screen pixels, where the region starts
at any frame and the frames_per_pixel of the display is not an integer.
Gerard
8:04 a.m.
On Mon, Jun 06, 2005 at 09:17:54AM +0000, vanDongen/Gilcher wrote:
As I recently found out, this can be very messy :)
Indeed :-)
The basic algorithm is this:
Each horizontal pixel represents n samples. Usually n is pretty big.
Of those n samples you take the min and the max, and then you draw a vertical
line between them.
Obviously this has to be scaled to your vertical resolution and vertically
translated to the right position on your canvas.
If n<=1 you can choose to draw lines between the points, or just show the
points.
The most difficult situation is for n nearly equal to one.
But even this is not really correct if you want precision.
Consider an Fs/4 sine with samples -0.9 -0.9 +0.9 +0.9 -0.9 -0.9 +0.9 etc.
Now what is the (peak) amplitude of this sine ? Not 0.9, but something like
1.27. If you send this signal through a unity gain filter that produces 45
degrees of phase shift, you will get samples at +/- 1.27, and your DAC will
clip.
So even a simle digital peak meter requires interpolation of peaks to
be accurate. I don't know any in the linux world that do this.
--
FA
8:10 a.m.
On Monday 06 June 2005 10:04, Alfons Adriaensen wrote:
The most difficult situation is for n nearly equal to
one.
But even this is not really correct if you want precision.
Consider an Fs/4 sine with samples -0.9 -0.9 +0.9 +0.9 -0.9 -0.9 +0.9 etc.
Now what is the (peak) amplitude of this sine ? Not 0.9, but something like
1.27. If you send this signal through a unity gain filter that produces 45
degrees of phase shift, you will get samples at +/- 1.27, and your DAC will
clip.
So even a simle digital peak meter requires interpolation of peaks to
be accurate. I don't know any in the linux world that do this.
Good points.
What kind of interpolation is required to visualize the DAC output of a
sampled waveform?
Gerard
8:38 a.m.
On Mon, Jun 06, 2005 at 12:14:41PM +0000, vanDongen/Gilcher wrote:
What kind of interpolation is required to visualize
the DAC output of a
sampled waveform?
This depends mostly on the maximum frequency you want to display or
measure accurately, and on the level of accuracy required.
You can get a long way by just detecting those points in your waveform
where the 'continuous' signal would exceed nearby samples, and try to
approximate the real peak value using e.g. cubic interpolation.
I'm working on a scope program as a companion to JAAA. The current
baseline is an 8 point convolution based interpolation to 4 Fs, and
linear from there on. The precision is needed mostly for finding the
trigger points accurately.
--
FA
1:40 p.m.
vanDongen/Gilcher wrote:
As I recently found out, this can be very messy :)
The basic algorithm is this:
Each horizontal pixel represents n samples. Usually n is pretty big.
Of those n samples you take the min and the max, and then you draw a vertical
line between them.
Obviously this has to be scaled to your vertical resolution and vertically
translated to the right position on your canvas.
If n<=1 you can choose to draw lines between the points, or just show the
points.
What is often done is to generate a peakfile, containing the min and max for a
set chunksize.
This is usefull if you have to redraw the waveform often.
Where it gets messy is if you want to zoom, display parts of the file, rapidly
scroll etc.etc. Then you need to set up clever caching and deal with rounding
problems.
In ardour the is ardour/gtk_ardour/canvas-waveview.c that does the clever
caching and libs/ardour/source.cc read_peaks() that generates the peaks.
Actually this may be the case, since the tool is going to be used for
analyzing recorded dialog, and zooming may be required.
The code in ardour may be to complex for your
application because it is made
to show any part of an audiofile at any position, where you may move an
audioregion to a position "in between" screen pixels, where the region starts
at any frame and the frames_per_pixel of the display is not an integer.
Yeah, my needs are much simpler than what Ardour... you can't do
anything with the displayed waveform beyond scroll and zoom (and maybe
basic scrubbing capability).
Thanks for your input.
-- Brett
7:11 a.m.
Hi Brett,
Brett wrote:
For a small application I am developing, I need to
display waveforms
(read-only) from audio data loaded into my application using libsndfile.
I personally recommend you to look at the Specimen source code, at
http://www.gazuga.net
The file src/gui/waveform.c is an almost standalone Gtk widget, that you should
be able to reuse with minimal efforts.
I'm not related in any way to Specimen, but I spent much time looking for
reusable code, and the gui code of this application is very nice and clean in my
opinion.
--
og
1:41 p.m.
Olivier Guilyardi wrote:
Ok, thanks, I'll take a look. I'm actually doing this in FLTK, but may
be able to create a custom widget based on the one for Specimen.
For a small
application I am developing, I need to display waveforms
(read-only) from audio data loaded into my application using libsndfile.
I personally recommend you to look at the Specimen source code, at
http://www.gazuga.net
The file src/gui/waveform.c is an almost standalone Gtk widget, that you
should be able to reuse with minimal efforts.
I'm not related in any way to Specimen, but I spent much time looking
for reusable code, and the gui code of this application is very nice and
clean in my opinion. 
2:05 p.m.
On 6/6/05, Them <idragosani(a)chapelperilous.net> wrote:
Ok, thanks, I'll take a look. I'm actually
doing this in FLTK, but may
be able to create a custom widget based on the one for Specimen.
Are you aware of this Waveform viewer widget in FLTK?
http://sparked.zadzmo.org/software/fltk/
-Richard
2:37 p.m.
Richard Spindler wrote:
On 6/6/05, Them <idragosani(a)chapelperilous.net>
wrote:
No! I did not! That's cool! Thanks!
Ok, thanks, I'll take a look. I'm
actually doing this in FLTK, but may
be able to create a custom widget based on the one for Specimen.
Are you aware of this Waveform viewer widget in FLTK?
http://sparked.zadzmo.org/software/fltk/
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9 comments
5 participants
participants (5)
-
Alfons Adriaensen -
Olivier Guilyardi -
Richard Spindler -
Them -
vanDongen/Gilcher