docs(rock4d): note that the speaker amplifier is not populated

The ROCK 4D interface list mentions 'Speaker Reserved Pads' and the
headphone page lists the related signals, but nothing states that the
amplifier ICs, speaker connectors and their peripheral components are
left unpopulated (NC) in production. Users therefore may try to wire a
speaker to the reserved pads and get no sound.

Add a note to both the Chinese and English headphone pages explaining
the reserved amplifier circuit is unpopulated and listing the supported
ways to use an external speaker.

Author: tzz-radxa

docs/rock4/rock4d/hardware-use/headphone.md

@@ -97,6 +97,16 @@ sudo aplay -Dhw:1,0 /tmp/tmp.wav
 详细接口规格参考下载专区的 [硬件设计 : 原理图](../download)
 :::
 
+:::note
+ROCK 4D 出厂默认**不焊接扬声器功放**。下表中的信号来自主板上预留的扬声器功放电路：功放芯片、喇叭座及其外围元件（磁珠、滤波电容等）出厂均未贴（NC），因此「扬声器预留焊盘」**无法直接驱动扬声器发声**。
+
+如需外接扬声器，可参考以下方式：
+
+① 通过 3.5mm 耳机插孔连接有源音箱（耳机插孔可直接驱动 32Ω 耳机）；
+② 外接功放模块，从预留焊盘取出 `AUD_LINEOUT_L` / `AUD_LINEOUT_R` 模拟音频信号；
+③ 自行补齐预留的功放电路（功放芯片、`VCC_SPK_AMP` 供电、`SPK_CTL_H` 使能及输出滤波元件）后，再连接扬声器。
+:::
+
 | 左声道 Pin# | 左声道 Name   | 右声道 Pin# | 右声道 Name   |
 | :---------: | :------------ | :---------: | :------------ |
 |      1      | SPK_CTL_H     |      1      | SPK_CTL_H     |

i18n/en/docusaurus-plugin-content-docs/current/rock4/rock4d/hardware-use/headphone.md

@@ -97,6 +97,16 @@ sudo aplay -Dhw:1,0 /tmp/tmp.wav
 For detailed interface specifications, please refer to the [Hardware Design: Schematics](../download) in the download section.
 :::
 
+:::note
+The ROCK 4D **ships without the speaker amplifier populated**. The signals listed below come from the reserved speaker amplifier circuit on the board: the amplifier ICs, the speaker connectors and their peripheral components (ferrite beads, filter capacitors, etc.) are left unpopulated (NC) in production. Therefore the "Speaker Reserved Pads" **cannot drive a speaker directly**.
+
+To use an external speaker, you can:
+
+① Connect powered speakers to the 3.5mm headphone jack (the jack can directly drive 32Ω headphones);
+② Use an external amplifier module, taking the `AUD_LINEOUT_L` / `AUD_LINEOUT_R` analog audio signals from the reserved pads;
+③ Populate the reserved amplifier circuit yourself (amplifier ICs, `VCC_SPK_AMP` supply, `SPK_CTL_H` enable and output filter components), then connect the speaker.
+:::
+
 | Left Channel Pin# | Left Channel Name | Right Channel Pin# | Right Channel Name |
 | :---------------: | :---------------- | :----------------: | :----------------- |
 |         1         | SPK_CTL_H         |         1          | SPK_CTL_H          |