Why USB Cables Can Make a Difference。为何USB线有差异。

音乐之贼

David Salz, Wireworld

Are USB cables really just 1's and 0's traveling down a wire? If not then what? Digital signals are actually square waves that are detected by the receiver when they pass a voltage threshold. Any variation in the shape or timing of these waveforms can cause errors in these signals.

How sending a file of music is different than that of an image to print? It is just data after all. When a file is saved or sent to a printer, the error correction system in the receiving device can ask the sending device to resend the file until it confirms that the file is completely correct.

Streamed music signals do not benefit from this resend function and therefore error correction systems cannot replace missing or corrupted data.

What is happening inside a USB cable that could impact the sound or information being transmitted? The normal losses of cables will round and tilt the signal waveforms, creating data errors that cause audible losses, tonal coloration and distortion. Those losses and colorations can easily be heard when standard USB cables are compared to reference adapters made from USB plug contacts soldered back-to-back. Designing a USB cable for minimal loss and maximum waveform fidelity is the only way to minimize these losses.

Separate power and signal? Computer power supplies are notoriously noisy and that noise can compromise the fidelity of USB audio. Most USB cables do not isolate the noisy power conductor from the signal conductors, so noise can mix with the signal and it can also enter the DAC through the 5 volt USB power conductor. Therefore, the 5 volt conductor needs to be isolated from the signal and filtered of noise or replaced by a separate quiet power source.

Noise? Noise can also be generated from static charge interaction within the cable. This triboelectric noise adds roughness to the signal waveforms. That roughness creates additional timing errors (jitter) in the signal. Reducing this noise enables cables to sound closer to the purity of the reference direct connection.

Marcin Ostapowicz, JPLAY

1's and 0's are logical interpretations of voltage signals. USB signals are transmitted using differential signaling: for USB 2.0 signal levels are −10–10mV for logical low and 360–440mV for logical high level. So basically we are dealing with a very sensitive electrical (analog!) signal. High data rates of USB 2.0 result in jitter and attenuation becoming a major problem (higher frequencies attenuate faster than lower frequencies over distance. Additionally, higher frequencies are more susceptible to jitter). Degradation in the signal can be seen as it travels from a transmitter (PC) to a receiver (USB input in DAC). Reflections and clock inaccuracies cause jitter which closes the so called eye-pattern and makes the signal less readable. Music playback is a real-time process where accurate timing is everything, so it is extremely important to minimize jitter. We use these techniques in our cables to maintain signal integrity:

1) Reduce the capacitive reactance and inductive reactance to make the signal impedance reduce more accurately.

2) Considering the skin effect we only use very slim conductors.

3) We use triple shielding to prevent EMI. CPU speeds in PCs are up to few Gigahertz. It will emit a lot of electromagnetic radiation that's within the frequency range of USB signal.

In our top USB cable we also introduced a dual-lead design that separates the USB data lines from the power line. Each line has a separate USB type A connector with the dual lines terminated with a single USB type B connector. That plus double-shielded solid aluminum connectors results in better sound quality and a lower noise floor.

感兴趣的自己去看。

https://positive-feedback.com/audio-discourse/why-usb-cables-can-make-a-difference/

Jedibosin

Wireworld的David Salz
USB电缆真的只有1根和0根沿电线传输吗？如果没有，那该怎么办？数字信号实际上是方波，当它们超过电压阈值时，它们就会被接收器检测到。这些波形的形状或时序的任何变化都可能导致这些信号的错误。
发送音乐文件与要打印的图像有何不同？毕竟只是数据。保存文件或将文件发送到打印机后，接收设备中的纠错系统可以要求发送设备重新发送文件，直到确认文件完全正确为止。
流音乐信号无法从此重发功能中受益，因此纠错系统无法替换丢失或损坏的数据。
USB电缆内部发生了什么事情，可能会影响正在传输的声音或信息？电缆的正常损耗将使信号波形变圆和倾斜，从而产生数据错误，从而导致听觉上的损失，音调着色和失真。将标准USB电缆与由背对背焊接的USB插头触点制成的参考适配器进行比较时，这些损失和颜色很容易听到。设计USB电缆以最大程度地减少损耗和最大波形保真度是最小化这些损耗的唯一方法。
电源和信号分开？众所周知，计算机电源很吵，而且噪音会损害USB音频的保真度。大多数USB电缆不会将嘈杂的电源导体与信号导体隔离开，因此噪声会与信号混合，并且还会通过5伏USB电源导体进入DAC。因此，需要将5伏导体与信号隔离并过滤掉噪声，或用单独的安静电源代替。
噪声？电缆内的静电荷相互作用也可能产生噪声。摩擦电噪声会增加信号波形的粗糙度。这种粗糙度会在信号中产生额外的时序误差（抖动）。降低此噪声可使电缆听起来更接近参考直接连接的纯度。

Marcin Ostapowicz，JPLAY
1和0是电压信号的逻辑解释。USB信号使用差分信号传输：对于USB 2.0，逻辑低电平为-10-10mV，逻辑高电平为360-440mV。因此，基本上，我们正在处理非常敏感的电（模拟）信号。USB 2.0的高数据速率导致抖动和衰减成为一个主要问题（较高的频率在整个距离上的衰减快于较低频率的衰减。此外，较高的频率更容易受到抖动的影响）。从发射器（PC）到接收器（DAC中的USB输入），可以看到信号的衰减。反射和时钟的不准确会引起抖动，从而使所谓的眼图关闭，并使信号的可读性降低。音乐播放是一个实时过程，其中准确的时间就是一切，因此，最小化抖动非常重要。我们在电缆中使用以下技术来保持信号完整性：
1）降低容抗和感抗，使信号阻抗更准确地降低。
2）考虑到集肤效应，我们仅使用非常细的导体。
3）我们使用三重屏蔽以防止EMI。PC中的CPU速度高达几千兆赫。它会发出很多USB信号频率范围内的电磁辐射。
在我们的顶级USB电缆中，我们还引入了双引线设计，该设计将USB数据线与电源线分开。每条线都有一个单独的USB A型连接器，双线以一个USB B型连接器端接。加上双屏蔽实心铝制连接器可带来更好的音质和更低的本底噪声。
兴趣的自己去看。
https://positive-feedback.com/audio-discourse/why-usb-cables-can-make-a-difference/