同軸S/PDIF的雜訊

聽過好幾次一手消息，提到用同軸S/PDIF會受到其他電器干擾，
leo-friday開關燈時，偶爾會斷音，葉sir也碰過，論壇上也有人提過；
SH謝先生則是瓦斯爐點火時會斷音。

前者或許是電源波動造成，後者就該是電磁波造成，
許多人總說光纖太冷，同軸比較暖，
以我的經驗判斷，通常這種用語就表示，後者有失真。

那失真怎麼來的？其一或許就是電源/電磁場雜訊。
想想，若是開關燈、瓦斯爐點火會造成嚴重的干擾，那麼充斥於周遭的各式各樣電磁雜訊干擾必然也不時影響同軸訊號，
當然，除非很嚴重，才會造成資料錯誤而斷音，
一般狀況下，影響的會是jitter！
jitter越高，聽來越糊，太高就會覺得吵了。

順道提一下，Wikipedia上關於S/PDIF的限制（ http://en.wikipedia.org/wiki/S/PDIF#Limitations ）

The receiver does not control the data rate, so it must avoid bit slip by synchronising its conversion with the source clock. This means that S/PDIF cannot fully decouple the final signal from influence by the analogue characteristics of the source or the interconnect, even though the digital audio data can normally be transmitted without loss. The source clock may carry inherent jitter or wander, and noise or distortion introduced in the data cable may further influence the process of clock recovery.^[5][6][7] If the DAC does not have a stable clock reference then noise will be introduced into the resulting analogue signal. However, receivers can implement various strategies which limit this influence.^[7][8]

TOSLINK cables do not work well (and may even suffer permanent damage) if tightly bent or squashed by, say, a misplaced foot. Their high light-signal attenuation limits their effective range to 6.1 metres (20 ft) or so. On the other hand, TOSLINK cables are not susceptible to ground loops and RF interference, as coaxial cables are.^[9]