这30元钱花的真值，10年发烧最值的一次。

最好的数字就是7

目前系统已经稳定了一年多，很满意。一直在考虑下一步升级就是选择合适的同轴线替换m scaler那两条原机的同轴线，但一直没有拿定主意。喜欢aq的线，但一直没有找到原装带bnc 头的线，需要用到rca到 bnc的转接头，势必会有负面的影响。无意中看到，有朋友说同轴线需要1.5米以上，否则会有信号反射导致jitter。我就在网上找了一家专门做视频bnc线的商机，试试效果。30元一对，还是铜镀银双屏蔽（真是便宜，不知道卖上千的线的商家，看到这个价格是不是有点惭愧）。我就买了一对1.5m，一对2m的。拿回家一试，提升真的是太大了，特别是2m的线，完全没有想到这个是30元的一对线带来的效果。下次准备试试更长的，但理论上太长会有信号衰减，也会带来jitter，所以还得不断尝试。为了弄清楚原因，在网上看了很多文章，终于有一篇讲的非常透彻，我直接转发给大家，免得我翻译的不好。如果大家需要我翻译可以给我说。发个30一对的图片，价格真实。
---------------------------------

Many of you may have heard or read that it is beneficial to use at least a 1.5m length digital cable from your Transport to your DAC. There are actually technical reasons for this, but the requirement also depends on the behavior of the signal from the S/PDIF digital output on your Transport. It turns-out that the jitter on the digital signal can increase if the cable is too short, and the increased jitter can cause the audio to have "halos" or be out of focus.

S/PDIF (or Sony/Philips Digital Interface) is a digital signaling standard specified at 75 ohms characteristic impedance and terminated on both ends. This means that the source driver (in the Transport) must have an output impedance of 75 ohms and the receiver (in the DAC) must have a parallel resistive termination of 75 ohms. If these terminations are both set to 75 ohms, then ideally the signal will propagate from the Transport to the DAC and no reflections will occur on the transmission-line that connects the Transport to the DAC, assuming that all components of the transmission-line are also 75 ohms. The transmission-line components (excluding the driver, receiver and terminations) include:

• The traces on the Transport circuit board that connect to the driver chip
• The wiring to the output connector
• The output connector jack and plug (BNC or RCA)
• The digital cable
• The input connector jack and plug at the DAC input (BNC or RCA)
• The wiring to the circuit board
• The traces on the DAC circuit board that connect to the receiver chip
  
  

Any of these individual components may cause a reflection on the transmission line if they are not 75 ohms characteristic impedance. This is why it is insufficient to replace the RCA connectors with BNC’s without addressing the wiring and circuit-board traces that are not 75 ohms characteristic impedance. I have never seen impedance control on any Transport or DAC circuit board. Occasionally, the wiring from the circuit board to the connector is impedance-controlled, but this is the exception, not the rule.

Characteristic impedance is not something that is measurable with a volt-ohmmeter. It is an AC characteristic that is defined as: sqrt(L/C), where L is the inductance per unit length and the C is the capacitance per unit length of the transmission-line. If the transmission-line is properly matched and terminated, transmission of digital signals can take place over very long distances without reflections; only losses are a factor. Losses can also have an impact on jitter, and this is why some 75 ohm digital cables sound better than others, even in well-matched impedance-controlled systems—but that is another paper, so back to the question of digital cable length.

When the signal is launched into the transmission-line from the Transport, it is essentially a voltage square-wave, consisting of rising edges and falling edges. These edges are no more than transitions of voltage from about –250mV to +250mV, the rising edge transitioning from minus voltage to plus voltage and the falling edge transitioning from plus voltage to minus voltage. When an edge transitions, it can be described as having a rise-time or fall-time. This is the time it takes for the signal to transition from 10% to 90% of the entire voltage swing. The rise-time is important because this is what causes reflections on the transmission-line. If the rise-time were very, very slow, say 50 nanoseconds, then there would be no reflections on the transmission-line unless it was extremely long. Alternately, if the rise-time were less than 1 nanosecond, reflections would occur at every boundary, such as the connection from the circuit board to the wires that go to the connector.

Typical stock Transports have around 25 nanosecond rise-times. The primary concern for the manufacturer is to pass FCC regulations for emissions and electromagnetic interference, as well as making the interface reliable. When the regulatory testing is done, they attach very inexpensive, inferior cables and measure the emissions. To insure that the manufacturer passes these tests, they take a number of precautions. One is designing-in the slower than necessary 25 nanosecond rise-time. Another is the insertion of various filters in the Transport to eliminate high frequencies from the signal. As a result of these choices, there is a hazard created in using too short a digital cable. It is a result of the slow rise-time.

The slow rise-time has an advantage as well. Discontinuities in characteristic impedance, such as circuit board traces, funky wiring and RCA connectors will have less effect on the signal integrity or voltage "shape." Because the transitions are slow, reflections are not as high in amplitude and therefore have less effect on the jitter. However, the penalty is paid at the receiver chip where the slow edge causes uncertainty of when the transition actually took place. Jitter is created by the receiver chip when it inaccurately senses the slow transition.

When a transition is launched into the transmission line, it takes a period of time to propagate or transit to the other end. This propagation time is somewhat slower than the speed of light, usually around 2 nanoseconds per foot, but can be longer depending on the dielectrics used in the digital cable. When the transition reaches the end of the transmission line (in the DAC), a reflection can occur that propagates back to the driver in the Transport. Small reflections can occur in even well matched systems. When the reflection reaches the driver, it can again be reflected back towards the DAC. This ping-pong effect can sustain itself for several bounces depending on the losses in the cable. It is not unusual to see 3-5 of these reflections before they finally decay away, particularly when using the best digital cables, which are usually low-loss.

So, how does this affect the jitter? When the first reflection comes back to the DAC, if the transition already in process at the receiver has not completed, the reflection voltage will superimpose itself on the transition voltage, causing the transition to shift in time. The DAC will sample the transition in this time-shifted state and there you have jitter. Let’s look at a numerical example:

If the rise-time is 25 nanoseconds and the cable length is 3 feet, then the propagation time is about 6 nanoseconds. Once the transition has arrived at the receiver, the reflection propagates back to the driver (6 nanoseconds) and then the driver reflects this back to the receiver (6 nanoseconds) = 12 nanoseconds. So, as seen at the receiver, 12 nanoseconds after the 25 nanosecond transition started, we have a reflection superimposing on the transition. This is right about the time that the receiver will try to sample the transition, right around 0 volts DC. Not good. Now if the cable had been 1.5 meters, the reflection would have arrived 18 nanoseconds after the 25 nanosecond transition started at the receiver. This is much better because the receiver has likely already sampled the transition by this time.

The other obvious solution is to make the Transport S/PDIF driver faster. If the transition took only 10 nanoseconds to complete rather than 25, the reflection would arrive at 6 nanoseconds with a half-meter cable, and the reflection with a 1-meter cable would arrive after the transition had completed. Faster transitions also cause the receiver to switch more predictably, which reduces jitter even more. As mentioned above, stock component manufacturers are reluctant to provide these fast transition times because it may make them fail FCC tests, particularly with inexpensive "freebie" cables.

This is where the digital modder can make a huge difference. Speeding-up the transitions and maintaining a good impedance match is not something that novice DIYers can easily accomplish. It requires specialized tools and measurement techniques to insure that the impedance is properly matched. It is not simply a matter of soldering in 75 ohm Caddock resistors. These types of tweaks are best left to qualified modders. Once the impedance is accurately matched and the transition times decreased, the Transport will become less sensitive to different digital cables, shorter cables can be used with confidence and the result is cleaner audio with better focus, definition and clarity.

Steve Nugent
Empirical Audio
www.empiricalaudio.com

云南的白药

ms

云南的白药

你们中的许多人可能听说过或读到，从运输到DAC使用至少1.5米长的数字电缆是有益的。这实际上有技术原因，但要求也取决于运输上S/PDIF数字输出的信号的行为。事实证明，如果电缆太短，数字信号上的抖动可能会增加，而增加的抖动可能会导致音频有“晕”或失焦。

S/PDIF（或索尼/飞利浦数字接口）是一种数字信号标准，指定为75欧姆特征阻抗，并在两端端终止。这意味着源驱动器（在传输中）必须具有75欧姆的输出阻抗，接收器（在DAC中）必须具有75欧姆的并行电阻端接。如果这些终止都设置为75欧姆，那么理想情况下，信号将从传输传播到DAC，并且在连接传输到DAC的传输线上不会发生反射，假设传输线的所有组件也是75欧姆。传输线组件（不包括驱动器、接收器和终端）包括：

连接到驱动器芯片的传输电路板上的痕迹

输出连接器的接线

输出连接器插孔和插头（BNC或RCA）

数字电缆

DAC输入处的输入连接器插孔和插头（BNC或RCA）

电路板的接线

连接到接收器芯片的DAC电路板上的痕迹

如果这些单个组件不是75欧姆的特征阻抗，它们都可能在传输线上引起反射。这就是为什么在不解决不是75欧姆特征阻抗的布线和电路板痕迹的情况下，用BNC替换RCA连接器是不够的。我从未见过任何运输或DAC电路板上的阻抗控制。偶尔，从电路板到连接器的布线是阻抗控制的，但这是例外，而不是规则。

特征阻抗不是用伏特欧姆计可以测量的。它是一种交流特性，定义为：sqrt（L/C），其中L是每单位长度的电感，C是传输线每单位长度的电容。如果传输线正确匹配和终止，数字信号的传输可以在非常长的距离上进行，没有反射；只有损失是一个因素。损失也可能对抖动产生影响，这就是为什么一些75欧姆的数字电缆比其他电缆听起来更好，即使在匹配的阻抗控制系统中也是如此——但这是另一篇论文，所以回到数字电缆长度问题上来。

当信号从传输器发射到传输线时，它本质上是一个电压方波，由上升边缘和下降边缘组成。这些边缘只不过是电压从约-250mV到+250mV的过渡，上升边缘从负电压过渡到正电压，下降边缘从正电压过渡到负电压。当边缘过渡时，它可以被描述为具有上升时间或下降时间。这是信号从整个电压摆动的10%过渡到90%所需的时间。上升时间很重要，因为这是导致传输线反射的原因。如果上升时间非常非常慢，比如50纳秒，那么除非传输线非常长，否则就不会有反射。或者，如果上升时间小于1纳秒，每个边界都会发生反射，例如从电路板到连接器的电线的连接。

典型的股票运输有大约25纳秒的上升时间。制造商主要关心的是通过FCC关于排放和电磁干扰的法规，以及使接口可靠。当监管测试完成时，他们会连接非常便宜的劣质电缆，并测量排放。为了确保制造商通过这些测试，他们采取了一些预防措施。一个是在比必要时间慢的25纳秒上升时间进行设计。另一个是在传输中插入各种滤波器，以消除信号中的高频。由于这些选择，使用太短的数字电缆会产生危险。这是上升时间缓慢的结果。

缓慢的上升时间也有优势。特性阻抗的不连续性，如电路板痕迹、时髦的布线和RCA连接器，对信号完整性或电压“形状”的影响较小。由于过渡缓慢，反射的振幅没有那么高，因此对抖动的影响较小。然而，罚款是在接收器芯片上支付的，那里的缓慢边缘导致过渡实际发生的不确定性。当接收器芯片不准确地感知缓慢的过渡时，就会产生抖动。

当过渡进入输电线路时，需要一段时间才能传播或传输到另一端。这种传播时间比光速慢一些，通常约为每英尺2纳秒，但可能更长，具体取决于数字电缆中使用的电介质。当过渡到达传输线的末端（在DAC中）时，可能会发生反射，在传输中传播回驾驶员。即使在匹配良好的系统中，也会发生小反射。当反射到达驾驶员时，它可以再次反射回DAC。这种乒乓球效应可以持续几次反弹，这取决于电缆的损失。在这些反射最终衰变之前，看到3-5个这些反射并不罕见，特别是在使用最好的数字电缆时，这些电缆通常是低损耗的。

那么，这如何影响抖动呢？当第一次反射回到DAC时，如果接收器上已经进行的过渡尚未完成，反射电压将叠加在过渡电压上，导致过渡及时移动。DAC将采样这种时移状态下的过渡，那里有抖动。让我们看一个数字示例：

如果上升时间为25纳秒，电缆长度为3英尺，则传播时间约为6纳秒。一旦过渡到达接收器，反射会传播回驱动器（6纳秒），然后驱动器将其反射回接收器（6纳秒）= 12纳秒。因此，从接收器上看，在25纳秒的跃迁开始后12纳秒，我们有一个反射叠加在跃迁上。这是接收器尝试采样过渡的时间，大约在0伏直流电。不好。现在，如果电缆是1.5米，反射将在接收器开始25纳秒的过渡后达到18纳秒。这要好得多，因为接收器此时可能已经对过渡进行了采样。

另一个明显的解决方案是使运输S/PDIF驱动程序更快。如果过渡只需要10纳秒，而不是25秒才能完成，反射将使用半米电缆以6纳秒到达，使用1米电缆的反射将在过渡完成后到达。更快的过渡还会使接收器更可预测地切换，从而进一步减少抖动。如上所述，库存组件制造商不愿意提供这些快速过渡时间，因为这可能会使他们无法通过FCC测试，特别是使用廉价的“免费”电缆。

这就是数字改装机可以产生巨大变化的地方。加快过渡和保持良好的阻抗匹配不是新手DIYers可以轻松完成的事情。它需要专门的工具和测量技术来确保阻抗正确匹配。这不仅仅是在75欧姆卡多克电阻中焊接的问题。这些类型的调整最好留给合格的改装者。一旦阻抗准确匹配，过渡时间缩短，传输对不同数字电缆的敏感度将降低，可以放心地使用较短的电缆，结果是更清晰的音频，具有更好的焦点、清晰度和清晰度。

史蒂夫·纽金特

云南的白药

给做了个机翻

最好的数字就是7

云南的白药 发表于 2024-1-20 19:36
给做了个机翻

哈哈，多谢！

云南的白药

最好的数字就是7 发表于 2024-1-20 19:42
哈哈，多谢！

目前是TT2叠MS吗？非常完美漂亮的叠放组合，比Dave叠MS好看

最好的数字就是7

云南的白药 发表于 2024-1-20 19:44
目前是TT2叠MS吗？非常完美漂亮的叠放组合，比Dave叠MS好看

哈哈，毕竟设计就是和tt2一套的。chord的产品都是艺术品，非常漂亮。

周大壮壮

这个史蒂夫是做什么的

最好的数字就是7

周大壮壮 发表于 2024-1-20 22:28
这个史蒂夫是做什么的

Steve Nugent is President of Empirical Audio ([color=rgb(var(--google-purple-700))]www.empiricalaudio.com)

jcboy007

iis线长短影响大吗

line916

BNC接头比RCA好？同样1.5M 75欧姆？

最好的数字就是7

jcboy007 发表于 2024-1-21 00:46
iis线长短影响大吗

还没有研究过，抱歉。

最好的数字就是7

line916 发表于 2024-1-21 09:11
BNC接头比RCA好？同样1.5M 75欧姆？

不是bnc比rca的好。是m scaler 与 tt2之间用的是bnc接口，所以不想转换了。

zhang8673123

外行的问一句:信号传输不都是75Ω阻抗吗？怎么选50Ω了？

电脑小痴3

说说这2米bnc同轴线与你之前线的听感差异

jieruibeibei

bnc的同轴本来就是很棒的，你甚至可以试试用两个rca转bnc然后用一根时钟线接到普通同轴，但要注意阻抗

hk1959

好文，。受益了！同问为什么买的50欧？不是75欧的？，私信一个店家连接，也想试试看。

最好的数字就是7

zhang8673123 发表于 2024-1-21 10:19
外行的问一句:信号传输不都是75Ω阻抗吗？怎么选50Ω了？

是的，通常信号传输用75欧没错。但有些朋友喜欢50欧的，这种线用在信号发送的比较多，也是想试试。

最好的数字就是7

hk1959 发表于 2024-1-21 10:56
好文，。受益了！同问为什么买的50欧？不是75欧的？，私信一个店家连接，也想试试看。

是的，通常信号传输用75欧没错。但有些朋友喜欢50欧的，这种线用在信号发送的比较多，也是想试试。

最好的数字就是7

hk1959 发表于 2024-1-21 10:56
好文，。受益了！同问为什么买的50欧？不是75欧的？，私信一个店家连接，也想试试看。

链接已PM