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发表于 2019-12-31
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从标准看看我们对于AES3 的连接误区
SREXACT
SREXACT 台灣日昇電子 專業舞台燈光音頻線纜連接器 整體連接方案提供商
以下内容转自知名台湾线缆生产商台湾日升电子的公众号SREXACT，作者是黄英哲。阿哲老师是台湾知名音响工程师，从事音响行业近20年，对行业有很多独到的见解。该公众号上很多关于线材和连接的技术干货均出自他的手笔。
看点:
AES3 居然不用在意信号的冷热端
AES3 可以传多远?
正文开始
在音频行业中相关标准的制定者基本上有几大组织:
TIA ITU 主要基于通信需要做标准建立
AES 以专业音频为主
EBU 以电视/电影配套的音频标准为主
IEC 国际电工委员会
那谈到数字音频标准最最重要的标准应该就是 AES3了
这个AES3标准在我们连接行业当中的指导标准被写在
AES3-4-2009(reaffirmed 2019)
用于数字音频的AES标准—数字输入输出接口-
两信道串行传输格式线性表示的数字音频数据-第4部分:物理和电气
AES standard for digital audio — Digitalinput-output interfacing —
Serial transmission format for two-channellinearly represented digital audio data
Part 4: Physical and electrical
(Multi-part revision of AES3-2003,incorporating Amendments 5 & 6)
关于连接被写在附件C（规范性文件）平衡传输当中
下面我们就带着大家浏览一些重点吧！
这张图上面我们可以看到几个亮点:
１． 发送端必须接地而接收端是虚线可以选择性的连接
２． 传输线是对绞线
３．发送端线接收端都是110 Ω
NOTE The electrical parameters of the interface are based on those defined in ITU-T
recommendation V.11 which allow transmission of balanced-voltage digital signals over cables
up to a few hundred meters in length.
注释提到了: 这样的传输方式允许在长达几百米的电缆上传输平衡电压数字信号。
所以我们测试线缆AES3传输能力在低电容材料上曾经验证过400M的传输
是没有问题的,甚至中间有数个接头
下面这是报告的一些部分
C.1.2 Equalisation
Equalization may be used at the receiver.
There shall be no equalization before transmission.
The frequency range used to qualify the interface electrical parameters is dependent on the maximum data rate
supported. The upper frequency is 128 times the maximum frame rate (about 6 MHz for 48 kHz).
C.1.2均衡器环节
最后的这一句是重点＂必须保证能适应48 kHz取样频率下6 MHz 的传输带宽＂
C.1.3 Cable
The interconnecting cable shall be balanced with a nominal characteristic impedance of 110 Ω at frequencies
from 100 kHz to 128 times the maximum frame rate.
The cable shall be one of the following types:
• Shielded (screened) cable
• unshielded (unscreened) twisted pair (UTP) structured wiring (Category 5 or better, see
ISO/IEC 11801). see note 5.
• Shielded (screened) twisted pair (STP) structured wiring (see ISO/IEC 11801).
The same cable type shall be used throughout any single interface connection, including patch leads.
C.1.3电缆Cable 环节提到了:
互连电缆的额定阻抗为110Ω
可用频率为100 kHz 至最大播放速率的128倍。也就是上面提到的6 MHz
电缆可以是以下类型之一:
•屏蔽(屏蔽)电缆
•非屏蔽(非屏蔽)双绞线(UTP)结构布线(5类或以上,见ISO/IEC 11801)。见注5。
•屏蔽(屏蔽)双绞线(STP)结构布线(见ISO/IEC 11801)。
在任何单一接口连接中,应使用相同类型的电缆,包括接插线。
那这边需要解说一下为甚么可以用网线呢?
因为网线的特性阻抗是110Ω 又有很好的对绞密度
但是作为流动演出还有复杂案场为了降低被干扰与可靠性
使用专用的软质有屏蔽 AES数字线就有其必要了
NOTE 1 Holding closer tolerances for the characteristic impedance of the cable, and for the
driving and terminating impedances, can increase the cable lengths for reliable transmission
and for higher data rates.
NOTE 2 Closer tolerances for the balance of the driving impedance, the terminating
impedance, and for the cable itself can reduce both electromagnetic susceptibility and
emissions.
NOTE 3 Using cable having lower loss at higher frequencies can improve the reliability of
transmission for greater distances and higher data rates.
NOTE 4 Care should be taken in design of the interface to provide adequate balance on the
twisted pair within the Category 5 cable. Using RJ45 connectors, conventionally wired, current
practice favours the use of pins 4 and 5 for AES3 signals (separating them from ATM signals
on the same cable, for example). Pins 3 and 6 are the preferred second pair. For full
protection, the interface may have to withstand power voltages specified to support network
equipment, and the use of transformers and blocking capacitors on the AES3 interface is
strongly recommended.
NOTE 5 UTP cable has been shown to offer transmission up to 400 metres overall
unequalised, or 800 metres equalised, at 48 kHz frame rate. (See AES preprint 3783)
注释部分对于上面的条文做了更精确的说明:
对电缆如果能有极小的公差以保证特性阻抗跟发送接收端匹配
就可以增加电缆长度,以实现可靠传输和更高的数据速率。
选用损耗较低的电缆可以提高传输的可靠性,以便较高频率下使用获得更大的距离和更高的数据速率。
这两者笔者深有所感
以3070100176 6.0mm/3070100098 4.8mm线缆来说
紧密的对绞特性阻抗约在90Ω 极低的电容 可以传输200M无虞
但这个200M的中长期劣化程度是很低的!!
因为 176使用的是实心的XLPE形变量小
但若是选用的是发泡类型的线缆例如:3070101988
那初期传输距离与性能非常好
但是在中长期辗压挤压之后劣化就相对明显
也就是上面讲的＂公差＂明显了实用距离会陡降
这也就是为何很多AES线缆为何在骨架填充上倾向选用PE条的原因
例如我司3070100180 就是为了给传输线对撑起空间
避免遭遇挤压时导致大量的损坏
注4 则提醒用户必须把网线的＂线对＂用起来用对!
标准中首选建议插脚4和5 次之是插脚3和6
标准中也对于设备端网口接口提出了一定的要求
至于注5 提到UTP电缆可以支持400米的无均衡器传输
这点写得比较不严谨
忽略了:
１．一般网线是中空的对于对绞控制是缺省的可能导致对绞松开
２．网线的导体粗细多样目前CAT5E UTP 市面上大多贩卖的是
26AWG 这种使用0.4mm 铜丝的产品截面积仅得 0.12mm2对比早期的
24AWG 使用0.51mm 铜丝的产品截面积0.2mm2
这意味着截面积减少造成了电阻升高 也就是插入损耗大
这就可能导致传输能力不如标准上所标称的距离
（图片来自于网络）
C.2 Line driver characteristics
C.2.1 Output impedance
The line driver shall have a balanced output with an internal impedance of 110 ù with a tolerance of 20 %, at
frequencies from 0,1 MHz to 128 times the maximum frame rate when measured at the output terminals.
C.2.2 Signal amplitude
The signal amplitude shall lie between 2 V and 7 V peak-to-peak, when measured across a 110-Ω resistor
connected to the output terminals, without any interconnecting cable present.
NOTE a typical value is 4 V ± 10 %.
C.3 Line receiver characteristics
C.3.2 Maximum input signals
The receiver shall correctly interpret the data when connected directly to a line driver working between the extreme voltage limits specified in C.2.2.
NOTE The AES3-1985 specification for line driver signal amplitude was 10 V peak to peak
maximum.
C.3.5 Common-mode rejection
There shall be no data errors introduced by the presence of a common-mode signal of up to 7 V peak at frequencies from DC to 20 kHz.
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