BlurRiver®应用之电缆选型

When it comes to AV signal switching and extension over standard copper category cabling, Semtech’s BlueRiver® products are optimized for both high-performance AV-over-IP for Software Defined Video Over Ethernet (SDVoE™) network-based applications and low cost point-to-point HDMI extension. Taking advantage of standard Ethernet technologies, typically operating at 10Gbps, BlueRiver devices support transmission of high bandwidth 4K60 video with uncompressed image quality. However, when video bandwidth does not require the full 10Gbps, BlueRiver devices can operate at the 2.5Gbps and 5Gbps Ethernet rates. The benefits at operating at these lower data rates include significantly longer cable reach over high performance category cable, or reuse of existing network cable infrastructure designed to carry lower data rate Ethernet.

标准 BaseT 技术的优势

A great deal of R&D effort, money and brainpower has gone into the development of silicon technology to support high data rate transmission up to 10Gbps over standard copper category cabling. Advanced digital signal processing techniques are employed to maintain signal integrity, compensating for signal loss and crosstalk, while providing noise rejection. By using a fully IEEE-compliant 10G Ethernet copper PHY, BlueRiver delivers a robust standards-based solution, taking full advantage of the huge investment in Ethernet technology. To learn more about the benefits of using standard Ethernet PHY technology, you can watch a demonstration here.

Standards also define the key characteristics of category cable and the expected distances when these are met. The following table shows the recommended cable type for each Ethernet data rate supported by BlueRiver to meet 100m maximum cable distance.

但客户经常问我：“为什么在解决方案中必须使用 Cat6a 电缆？”简单的回答就是“不是必须使用”！可使用任何电缆，但要对电缆的性能有合理的期望。不同类别的电缆有高低之分，应特别注意所选电缆的规格。最好选择知名品牌的电缆、遵循行业标准的安装技术并使用高质量的连接器，但正如汽车制造商常用的经典免责声明中所说，实际状况可能有所不同！

讲点物理知识

One of the major limiting factors for sending high speed data over copper cables is the skin effect, which results in increased attenuation as the frequency of the signal increases. This effect is due to signal currents flowing mostly on the surface (or skin) of a conductor at high frequencies. The skin effect is defined by the equation below, where ѡ is the angular frequency of data transmission. As a result of the skin effect, the insertion loss of a cable increases as the square root of the frequency of the signal it carries.

趋肤效应导致衰减随信号频率的增加而加大

Other factors such as impedance mismatches at the connectors also limit cable length performance at high bit rates, but beyond about 20m or so at the frequencies being discussed here, losses are dominated by the skin effect.

简而言之，数据速率越高，电缆“阻性”越大，从而导致电缆长度缩短。降低数据速率可减弱趋肤效应，就能以较低的数据速率在更长的电缆上传输。与使用细导体的电缆相比，粗铜导体的表面积更大。因此，电缆的线规规格越粗（表面积越大），覆盖性能就越好。很合理吧！

为什么规格很重要

我们来看一下使用 23 AWG 实心铜导体的 Cat6a 额定电缆的真实例子。其插入损耗规格如下表所示。

The above table clearly illustrates the increasing loss, or signal attenuation, as the frequency increases. It’s important to understand which frequencies are of interest when studying the specification of your category cable of choice. The bandwidth required at the Ethernet data rates supported by Semtech’s BlueRiver chipset, 2.5Gbps, 5Gbps and 10Gbps, are shown in the following table.

由于此电缆（Cat6a）的规格能完全满足 “在 10Gbps 时达到 100 米传输距离” 这个要求，因此导体必须至少为 23 AWG，而且在10GBaseT 工作模式下需要大约 40dB/100 米或更优的插入损耗。从 400MHz 的 dB/100 米换算为 dB/米，可计算出此电缆（Cat6a）在 2.5GBaseT 和 5GBaseT 节点上运行时预期的电缆长度性能，如下表所示。

通过该分析我们可以看出，在 10GBaseT 工作模式下传输长度达到 100 米的Cat6a电缆，在 2.5GBaseT 工作模式下的传输长度能翻倍（达到 200 米）。换句话说，如果您的视频带宽支持在更低速度的以太网节点上运行，则可以使用标准兼容电缆来获得很好的传输长度性能。

选择符合您要求的电缆

正如前面所看到的那样，以低于 10GBaseT 的速度运行时，还可以使用适合较低频率的不同类别电缆，如 Cat6 和 Cat5e。现在我们回到 Pro AV 的世界，讨论视频及其对以太网数据速率选择的影响。下表为典型的 HD 和 4K 格式，以及相对应的以太网接口和类别电缆。

HD formats still dominate in many Pro AV applications, from low-cost digital signage to corporate AV. The bandwidth required for a 1080p source running at 60 frames per second is less than 2.5Gbps, whereas a similarly sampled 4K60 video source requires over 9Gbps. If your application only demands HD video extension or distribution, then configuring the BlueRiver device to operate at 2.5GBaseT means you have the following options to consider:

• 如果电缆距离很关键，则使用 10GBaseT 额定 Cat6a 电缆并达到 200 米的距离，或

• 如果需要重新利用现有的 Cat5e 网络基础设施，则可达到 100 米

So based on the cost and performance requirements needed to meet your specific application, the choice of cabling is yours to make. Products based on the BlueRiver chipset provide the flexibility to meet a broad range of Pro AV applications, from the most demanding, performance critical AV distribution for medical operating rooms, to low-cost point-to-point AV extenders.

You can learn more about the power and flexibility of BlueRiver solutions for AV-over-IP and why it is also the ideal choice for low-cost point-to-point AV extension in the “BlueRiver Pro AV 2.0 for SDVoE” webinar. 

Semtech、Semtech 徽标和 BlueRiver 是 Semtech Corporation 或其子公司的注册商标或服务标志。SDVoE 是 SDVoE Alliance 的商标或服务标志。