more formatting

Author: joachimheintz

book/01-a-digital-audio.md

@@ -177,8 +177,10 @@ Aliasing
 
 Here is a graphical representation of aliasing.
 
+<figure>
 <img src="../resources/images/01-a-aliasing-1.png" width="50%">
-<small><i>Aliasing (red) of a high frequency (blue)</i></small>
+<figcaption>Aliasing (red) of a high frequency (blue)</figcaption>
+</figure>
 
 The sinusoidal waveform in blue is being sampled at the vertical black
 lines. The line that joins the red circles together is the captured
@@ -187,8 +189,10 @@ waveform express different frequencies.
 
 Here is another example:
 
-![](../resources/images/01-a-aliasing-2.png){:width=50%}
-<small><i>Aliasing of a 30 kHz sine at 40 kHz sample rate</i></small>
+<figure>
+<img src="../resources/images/01-a-aliasing-2.png" width="50%">
+<figcaption>Aliasing of a 30 kHz sine at 40 kHz sample rate</figcaption>
+</figure>
 
 We can see that if the sample rate is 40,000 there is no problem with
 sampling a signal that is 10KHz. On the other hand, in the second
@@ -311,8 +315,10 @@ plot the correct amplitude of each sample. We can see in the diagram
 that some vertical lines stop above or below the real signal. For a
 signal with lower amplitude the distortion would even be stronger.\
 
-![](../resources/images/01-a-bitdepth.png)
-<small><i>Wrong amplitude values due to insufficient bit depth resolution</i></small>
+<figure>
+<img src="../resources/images/01-a-bitdepth.png">
+<figcaption>Wrong amplitude values due to insufficient bit depth resolution</figcaption>
+</figure>
 
 The standard resolution for CDs is 16 bit, which allows for 65536
 different possible amplitude levels, 32767 either side of the zero axis.