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标题: USB传播的奥秘（二）：USB传播中影响信号的因素 [打印本页]

作者: 52sherlock 时间: 2019-8-13 23:43
标题: USB传播的奥秘（二）：USB传播中影响信号的因素
本帖最后由 52sherlock 于 2019-8-13 23:51 编辑

同样是来源于英文网站，来源于uptone 和sonore 的主设计师John Swenson设计USB产品的思路。
https://www.monoandstereo.com/2015/10/uptone-audio-usb-regen-review.html

To explain how the UpTone Audio USB REGEN works and why it is so effective with such a wide range of USB input DACs, we first need to define some technical terms and some problems inherent in USB audio interfaces----hopefully in not-too-technical language:

PHY: PHY is an abbreviation for the electronics that interface to the physical bus. PHYs exist in most every type of data interface (Ethernet, FireWire, optical, etc.) A USB PHY serves two primary functions: to convert the analog voltages used on the Data-plus and Data-minus wires into a digital format normal logic can understand, and convert the high speed one-bit-at-a-time serial data stream into a slower parallel set of wires to be sent to the USB protocol engine (XMOS processor, FPGA with USB core, etc.). The lowly PHY chip is actually a tremendously noisy and complicated device containing several PLLs and clocking at various phases----and there is no such thing as an optimized-for-audio PHY. The PHY part of a DAC's USB is highly susceptible to the condition of the USB signal, its "Signal Integrity" (SI).

SIGNAL INTEGRITY: A high-speed USB signal runs at 480 mega bits per second, which is fairly high. SI is comprised of the rise/fall times of the signal edges, amplitude of the signal, noise sitting on top of the signal and jitter of the edges. Variations in any or all of these can decrease the SI. The computer determines this initially, and then it can get significantly degraded by running through cables and connectors.

The decrease in SI can be so large that it becomes difficult for the PHY to determine the actual bits. Thus the PHY contains several methods used to pre-process the analog signals in order to make it easier to determine the bits. When the SI is very good, the PHY can turn off the pre-processing steps and easily determine the bits. As the SI degrades the PHY turns on different parts of the pre-processing as needed. Each of these steps takes a fair amount of power to operate, thus creating noise on the power and ground planes. The more processing the PHY needs to use to determine the bits, the more noise is generated. Thus part of the packet noise is directly related to the signal integrity of the incoming signal. The higher the SI, the lower the noise.

PACKET NOISE: In a DAC the data packets coming in on the USB bus are not continuous----there is significant time in-between each packet. Thus the processing of these packets produces noise on the power supply and ground plane that come in bursts, and we refer to this as "packet noise". Since the rate of USB packets is 8KHz there are strong components of this noise in the audio band. This noise can cause jitter in clock oscillators, re-clocking flops, and DAC chips. It can also go directly into noise on the output of DAC chips.

Part of this noise is determined by the USB protocol engine (chip after the PHY) and is going to be constant for a particular DAC.

POWER DELIVERY NETWORK (PDN): In order for a power supply to properly respond to instantaneous load variations, it needs to have a low impedance over a very broad range of frequencies. For digital audio this is from low Hz to hundreds of Mhz range. The entire supply flow from mains AC to board layout and capacitors on the board play a role in getting this right. The process of frequency optimizing the PDN is something that is done in expensive high-speed network equipment, but is almost never done in consumer products, especially audio equipment. (And our experience with the REGEN points to this being quite important for digital audio.)

Okay, now that some definitions and issues have been set forth, let's look at how the UpTone Audio USB REGEN addresses them.

The lower the signal integrity (SI), the harder the PHY has to work, which produces greater packet noise. If the SI is very good, the packet noise from the PHY is less than that from the protocol engine. As the SI degrades the packet noise from the PHY can dominate.

Again, the packet noise consists of two parts: noise from the USB protocol engine and from the USB PHY. The protocol engine noise does not depend on the input signal quality, just the data, so its impact is always going to be the same no matter what is done with the input. The PHY is the part that actually connects to the electrical signals on the bus, ITS contribution to packet noise IS dependent on the quality of the input signal.

It is very important to keep in mind that all this is what happens INSIDE the DAC by its own operation, it is NOT noise on the USB bus that is somehow getting into the DAC as is commonly thought.

At this point there aren't any DACs that have been specifically optimizing their USB inputs for SI and impedance match, it's too new as a specific concept to design to. But the best DACs do optimize this to some degree, whether by trial and listening or as a by- product of optimizing for something else.

无脑谷歌翻译下，

为了解释UpTone Audio USB REGEN如何工作以及为什么它如此有效地使用如此广泛的USB输入DAC，我们首先需要定义一些技术术语和USB音频接口固有的一些问题----希望不要太多 - 技术语言：

PHY：PHY是电子设备物理总线接口的缩写。PHY存在于大多数类型的数据接口（以太网，FireWire，光纤等）中.USB PHY有两个主要功能：将Data-plus（D+）和Data-minus（D-）线路上使用的模拟电压转换为数字格式，还有将高速一比特串行数据流转换为慢速并行数据组，以发送到USB协议引擎（XMOS处理器、带有USB核心的FPGA等）。低PHY芯片实际上是一个非常嘈杂和复杂的设备，包含多个PLL并在各个阶段进行时钟控制----并且没有针对音频优化的PHY。解码器 USB部分的PHY非常容易受USB信号条件的影响，即“信号完整性”（SI）。

信号完整性（SI）：高速USB信号以每秒480兆比特的速度运行，相当高。SI由信号边沿的上升/下降时间、信号的幅度、位于信号顶部的噪声和边缘的抖动组成。任何或所有这些的变化可以降低SI。计算机是决定SI的首要因素，然后通过数据线和接插件运行会显着降低SI。

SI的减小可能很大，以至于PHY难以确定实际真实的bit。因此，PHY包含几种用于预处理模拟信号的方法，以便更容易确定bit。当SI非常好时，PHY可以关闭预处理步骤并轻松确定bit。随着SI降级，PHY根据需要打开预处理的不同部分。这些步骤中的每一步都需要相当大的功率才能工作，从而在电源和接地层上产生噪声。PHY需要用于确定bit的处理越多，产生的噪声就越多。因此，分组噪声的一部分与输入信号的信号完整性直接相关。SI越高，噪声越低。

数据包噪声：在DAC中，进入USB总线的数据包不是连续的----每个数据包之间存在大量时间。因此，这些分组的处理在电源和接地平面上产生突发的噪声，我们将其称为“分组噪声”。由于USB数据包的速率为8KHz，因此音频频段中存在强噪声。这种噪声会导致时钟振荡器，重新计时触发器和DAC芯片的抖动。它也可以直接转换为DAC芯片输出的噪声。

部分噪声由USB协议引擎（PHY之后的芯片）决定，并且对于特定的DAC将保持不变。

电力输送网络（PDN）：为了使电源能够正确响应瞬时负载变化，它需要在很宽的频率范围内具有低阻抗。对于数字音频，这是从低Hz到数百Mhz范围。从电源AC到电路板布局以及电路板上的电容器的整个电源流程都可以实现这一目标。频率优化PDN的过程是在昂贵的高速网络设备中完成的，但几乎从未在消费产品中完成，尤其是音频设备。（我们对REGEN的经验表明，这对数字音频非常重要。）

好的，现在已经阐述了一些定义和问题，让我们来看看UpTone Audio USB REGEN如何解决它们。

信号完整性（SI）越低，PHY必须工作越困难，这会产生更大的分组噪声。如果SI非常好，则来自PHY的分组噪声小于来自协议引擎的分组噪声。随着SI降级，来自PHY的分组噪声可能占主导地位。

同样，数据包噪声由两部分组成：来自USB协议引擎和USB PHY的噪声。协议引擎噪声不依赖于输入信号质量，而仅取决于数据，因此无论输入如何处理，其影响总是相同的。PHY是实际连接到总线上的电信号的部分，ITS对分组噪声的贡献取决于输入信号的质量。

重要的是要记住，所有这些都是DAC内部通过其自身的操作发生的事情，USB总线上的噪声不会像通常认为的那样以某种方式进入DAC。

此时，没有任何DAC专门针对SI和阻抗匹配优化其USB输入，这对于设计来说是一个特定的概念。但是最好的DAC确实在某种程度上优化了这一点，无论是通过试听，还是作为优化其他东西的副产品。

作者: alex_to 时间: 2019-8-14 00:28
http://erji.net/forum.php?mod=vi ... =2149112&extra=

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