diff --git a/1000-hours/sounds-of-american-english/2.5.1-pitch.md b/1000-hours/sounds-of-american-english/2.5.1-pitch.md
index d0df33bb..981aa2db 100644
--- a/1000-hours/sounds-of-american-english/2.5.1-pitch.md
+++ b/1000-hours/sounds-of-american-english/2.5.1-pitch.md
@@ -12,6 +12,10 @@ In this sentence, the single syllable words *Yet* and *life*, the first syllable
 
 <video controls width="720"> <source src="/videos/yet-it-is-a-fact-of-life.mp4" type="video/mp4"></source>Your browser does not support the video tag. </video>
 
+以上视频来自于 2023 年 9 月我自己写的第一个 Demo（[ToBI Player](https://github.com/xiaolai/tobiplayer)）—— 现在的 [Enjoy App](https://1000h.org/enjoy-app/) 就是从它开始后继组织人员开发的。在 Enjoy App 中，你同样可以看到每句话的音频波谱（waveform）以及高低（音高）起伏（声调，即[下一章](2.5.2-tone)的讲解）……
+
+The video above is from a demo I wrote myself in September 2023 ([ToBI Player](https://github.com/xiaolai/tobiplayer)). This demo eventually led to the development of the current [Enjoy App](https://1000h.org/enjoy-app/) with a team. In the Enjoy App, you can also see the waveform of each sentence and its pitch fluctuations (intonation, which will be explained in [the next chapter](2.5.2-tone))...
+
 如果你可以唱出 $\frac{4}{4}$ <span class="pho">| 3 2 1 - |</span> —— 当然，这三个音符其实任何人都可以唱出来……
 
 If you can sing $\frac{4}{4}$ | 3 2 1 - | - indeed, anyone could sing these three notes...