Is it possible to plot a spectrogram of a wav file in just red color?

I know how to plot a spectrogram in different formats like jet, grayscale hsv etc. But I was wondering if its possible to plot a spectrogram of a wav file in just red colour or in green colour or in blue colour.

I have attached a screenshot to show an example. I made this by saving the original spectrogram in png format and converting it into Red, green and blue colour using usual image processing technique.

[KALYAN ACHARJYA]

Can you share the wav file and conversion code?

[sangeet sagar]

You can take any wav file and plot its spectrogram. After plotting the spectrogram save the file as .png format. like:

h=imagesc(t,f,S_log);
colormap('jet');
saveas(h,'spectro.png');

I did this using STFT. Now write:

img = imread('spectro.png'); % Read image
red = img(:,:,1); % Red channel
red=imshow(just_red);

Answer 1

[Adam Danz]

Couldn't you just create your own colormap that varies along one RGB column while the other two columns have a constant color such as black? Here's an example using the red axis.

colorMap = [linspace(0,1,256)', zeros(256,2)]
colormap(colorMap);

[sangeet sagar]

Thanks a lot. This works. Can you please tell me what should I write similarly as you wrote above for green and blue colour?

[Adam Danz]

Look at the value of colorMap (my variable above). it's a [256-by-3] matrix. The columns are the portion of [Red, Green, Blue] of each row. This is standard "RGB" color formatting. Note that the Green and Blue columns are all 0 (dark) while the red varies. I varied the red using linespace(). You can do the same for green and blue. I'll show you how to do it with Green and you can figure out how to do it with blue.

Start with all zeros

colorMap = zeros(256, 0);

Vary the green column.

colorMap(:,2) = [linspace(0,1,256)', zeros(256,2)];

[sangeet sagar]

Thanks for your explanation. It is really helpful and I figured out how to do the blue one. but I realised now that I dont need to find the spectrogram using red colour or green. I just need to save the original spectrogram in png format and then convert it to red green and blue colour and plot it.

Here I am finding it difficult to add the Y axis limits. I tried writing

set(gca,'YTickLabel',{8000,6000,4000,2000,0});

but it's gets repeated after 0. How I wish to scale the Y-

axis limits I have written in the screenshot with red colour.

[Adam Danz]

In general, whenever you set the 'YTickLabel' you should also set the 'YTick' (same with x axis). 'YTickLabel' tells matlab to use the current y ticks but label them differently. If you have 5 ticks but 3 labels, then the labels will start to repeat [1 2 3 1 2].

To fix this, if your tick labels are the same as your ticks (ie, tick 6000 has the label 6000), then just set the YTick instead of the YTickLabel. They should also be monotonically increasing.

set(gca,'YTick',[0, 2000, 4000, 6000, 8000]);

If you want to flip the direction of the y axis

set(gca, 'YDir', 'normal')   % or 'reverse'

[sangeet sagar]

Sorry but this does not seem working. I wrote the following

im_yellow=imshow(just_red);
axis on;
set(gca,'YTick',[0, 2000, 4000, 6000, 8000]);

But Y axis becomes blank.

[Adam Danz]

What are the y tick values prior to changing them? Any chance to get a screenshot?

[sangeet sagar]

I wrote the following and it is working awsome.

set(gca,'YTickLabel',{'8000','6000','4000','2000','0'},...
    'YTick',[1 175 350 525 730],...
    'XTickLabel',{'0.5','1','1.5'},...
    'XTick',[266.67 533.34 800],...
    'Layer','top',...
    'TickDir','out',...
    'YDir','reverse',...
    'DataAspectRatio',[1 1 1]);

[Adam Danz]

Oh, I see. So your ytick values aren't the same as your yticklabels (which always worries me a little). Glad you worked it out!

[sangeet sagar]

Yes exactly. YtickLabels denote what you wish to label on the ytick which denotes the location. I manually found out the location.