http://en.wikipedia.org/wiki/XLR_connector
查了一下国外的网站，有一个说插头的。没说线，呵呵，但说了接地。贴过来大家研究一下，我烂英语翻腾了一下。凑合看，不对指出来。本来我觉得三芯线也是屏蔽层的接法更灵活，不管设备内部外壳和一脚连不连，都不会产生地环路，所以理论上应该比二芯的产生交流声及干扰的可能性低很多。接法也就是1地2正3负屏蔽接壳。
Some audio equipment manufacturers reverse the use of pin 2 (properly the normal input) and pin 3 (inverting input). This reflects their own previous usage before any standard existed. Pin 1 is always ground, and many connectors connect it internally to the connector shell or case.
一些音频设备制造商反向使用2脚（正确接法为正向输入）和3脚（反向输入）。这只反应了在一个标准产生前他们的用法。1脚总是接地的，而且许多连接器把它和内部的机壳相接。
Although covered in AES48-2005 and in AES54-3-xxxx, there is still some disagreement on the best way to handle the usage of pin 1 at both ends of a cable, particularly with respect to the cable shield, the connector's shell, signal ground, and a third cable conductor connected to pin 1, which may (or may not) be connected to the shield. The main controversy is whether the shell of the connector should be connected to pin 1 or the shield, or left floating. AES standards mentioned above recommend that shells of cable-mounted connectors should never be connected to pin 1 or the shield, because inadvertent contact of the shell with another grounded surface while in use can create unwanted current paths for fault current, potentially causing hum and other noise. On the other hand, equipment containing active circuitry should always have pin 1 connected to the conductive enclosure of the equipment as close as possible to the point where the signal enters the enclosure. The argument centers around the radio frequency shielding provided by the shell of the connector, which may be reduced if it is left floating. An alternative solution is to connect the shell to pin 1 and the shield through a small value capacitor, providing RF shielding but allowing very little audio-frequency current to flow. This capability can be built into a fixed jack or a cable terminated with XLR connectors.
虽然AES。。。。和AES。。。标准包含了这个，但在在线的两端的1脚的最佳用法上仍有一些不同见解，尤其是线的屏蔽层，连接器（插头）的外壳，信号地和第三导线连接到插头一脚，应该（或不应该）连接到外壳。主要的争论在于是否将外壳接到一脚或屏蔽层或悬空。AES标准提及上述线的屏蔽层总不能接到一脚或是屏蔽层，因为外壳和另一个地的非故意连接，在用法上将产生不想要的电流通路，会潜在地产生哼声和其它噪声。另一方面，主动设备总是有一脚就近连接到信号进入设备的导电外壳点。这个观点主要是因为连接器的外壳能够对无线电频率提供屏蔽，一个可行的解决办法是在一脚和屏蔽层之间接一小电容进行RF（射频）屏蔽，但允许非常小的音频电流流动。这个性能可以用在固定插头或线的终端用XLE插头的场合。
查了一下国外的网站，有一个说插头的。没说线，呵呵，但说了接地。贴过来大家研究一下，我烂英语翻腾了一下。凑合看，不对指出来。本来我觉得三芯线也是屏蔽层的接法更灵活，不管设备内部外壳和一脚连不连，都不会产生地环路，所以理论上应该比二芯的产生交流声及干扰的可能性低很多。接法也就是1地2正3负屏蔽接壳。
Some audio equipment manufacturers reverse the use of pin 2 (properly the normal input) and pin 3 (inverting input). This reflects their own previous usage before any standard existed. Pin 1 is always ground, and many connectors connect it internally to the connector shell or case.
一些音频设备制造商反向使用2脚（正确接法为正向输入）和3脚（反向输入）。这只反应了在一个标准产生前他们的用法。1脚总是接地的，而且许多连接器把它和内部的机壳相接。
Although covered in AES48-2005 and in AES54-3-xxxx, there is still some disagreement on the best way to handle the usage of pin 1 at both ends of a cable, particularly with respect to the cable shield, the connector's shell, signal ground, and a third cable conductor connected to pin 1, which may (or may not) be connected to the shield. The main controversy is whether the shell of the connector should be connected to pin 1 or the shield, or left floating. AES standards mentioned above recommend that shells of cable-mounted connectors should never be connected to pin 1 or the shield, because inadvertent contact of the shell with another grounded surface while in use can create unwanted current paths for fault current, potentially causing hum and other noise. On the other hand, equipment containing active circuitry should always have pin 1 connected to the conductive enclosure of the equipment as close as possible to the point where the signal enters the enclosure. The argument centers around the radio frequency shielding provided by the shell of the connector, which may be reduced if it is left floating. An alternative solution is to connect the shell to pin 1 and the shield through a small value capacitor, providing RF shielding but allowing very little audio-frequency current to flow. This capability can be built into a fixed jack or a cable terminated with XLR connectors.
虽然AES。。。。和AES。。。标准包含了这个，但在在线的两端的1脚的最佳用法上仍有一些不同见解，尤其是线的屏蔽层，连接器（插头）的外壳，信号地和第三导线连接到插头一脚，应该（或不应该）连接到外壳。主要的争论在于是否将外壳接到一脚或屏蔽层或悬空。AES标准提及上述线的屏蔽层总不能接到一脚或是屏蔽层，因为外壳和另一个地的非故意连接，在用法上将产生不想要的电流通路，会潜在地产生哼声和其它噪声。另一方面，主动设备总是有一脚就近连接到信号进入设备的导电外壳点。这个观点主要是因为连接器的外壳能够对无线电频率提供屏蔽，一个可行的解决办法是在一脚和屏蔽层之间接一小电容进行RF（射频）屏蔽，但允许非常小的音频电流流动。这个性能可以用在固定插头或线的终端用XLE插头的场合。