示例：信号生成

信号合成

以双音多频 (DTMF) 信号为例。这些信号常用于按键式电话机拨号和远程控制功能。DTMF 信号由一个高频和一个低频正弦信号组成。常见音调表如下所示。

数字电话键盘		高音调组
数字电话键盘		1209 Hz	1336 Hz	1477 Hz	1633 Hz
低	697 Hz	1	2	3	A
音调	770 Hz	4	5	6	B
音调	852 Hz	7	8	9	C
组	941 Hz	*	0	#	D

例如，要生成数字 5 按钮的音调，请将“低音调组”中的 770 Hz 信号与“高音调组”中的 1336 Hz 信号混合。

在该示例中，使用 PTC Mathcad 生成 # 音调。

1. 定义通道数、采样率和分辨率。

2. 设置持续时间和数据点数。

3. 计算时间间隔并生成两个数组，其中一个为索引数组，另一个为时间数组。

4. 设置信号幅度并选择生成 # 音调所需的两个频率。

5. 使用 sin 函数生成信号，并绘制该信号。

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6. 使用 WRITEWAV 函数将信号用指定的参数写入到 WAV 文件中。

如果指定的位分辨率为 1-8，则会将数据以无符号字节数据的形式写入到文件中。无符号字节数据的取值范围是 0-256 (28)。如果位分辨率为 9-16，则将字数据 (双字节) 写入到文件中。字数据的取值范围是 –32768 至 +32767 (215)。

7. 尝试更改 DTMF 信号的频率，然后播放声音文件。该声音与按键式电话机的声音类似。

频率指数倾斜

音频测试中的另一个有用的信号为频率指数倾斜信号。

1. 定义频率信号的指数倾斜。

2. 定义采样率、分辨率和通道数。

3. 定义新信号的以下参数。

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4. 使用 sin 函数生成信号，然后使用 length 函数求出其长度。

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5. 使用 WRITEWAV 将信号写入新文件。

6. 绘制信号图像。

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