Nighteye wrote:
Sorry that I read from an Chinese forum & extracted
"回覆(1): good digital cables recommended?
那種數位傳輸介面好
數位音響設備中，數位信號在傳輸轉換過程中會引發時基誤差（Jitter），而時基誤差是導致數位音響音質不良的重要原因。所以數位音響設備傳輸介面性能的好壞，應以引起時基誤差大小為衡量標準，
數位音響設備的數位信號傳輸介面有三種類型，各有優缺點，而以75Ω同軸式數位傳輸最完美，由此而產生的時基誤差最小，其中BNC插頭座的表現又優於RCA 插頭座，110Ω平衡式AES/EBU卡農插頭座雖有可靠性高的優點，但工作頻帶較窄，時基誤差率較高（均為BNC的10倍）。光纖中的TA&T 玻璃光纖雖是理想的數位傳輸方式，但它的發射器和接收器卻是產生時基誤差的元兇（約為BNC的20倍）。"
Below is my translation (forward my poor wording) & wish to discuss.
The first paragraph describes jitter & which affects the signal quality most. Thus, the quality of transmittion interfaces should be based on jitter.
75Ω is the perfect among three interfaces since its jitter is the smallest, BNC connector is better than RCA. 110Ω AES balanced plug is reliable but with limited bandwidth (??), much jitter (??), (& which is 10 times of BNC). Those optical TA&T is an idealistic interface as its transmitter & receiver are the root cause of jitter (20 times of BNC).
From this article, seems I need to give up the AES in DA10 & switch to coaxial with BNC.
My advice: don't believe everything you read.
The statement: "75Ω is the perfect among three interfaces since its jitter is the smallest, BNC connector is better than RCA" is wrong.
The statement: "110Ω AES balanced plug is reliable but with limited bandwidth" is wrong.
The statement: "Those optical TA&T is an idealistic interface as its transmitter & receiver are the root cause of jitter (20 times of BNC)" can be highly misleading as well.
In short, ONE OF THE MANY mechanisms that may cause jitter has to do with the transmission of the digital audio. The mechanism is based on BOTH signal rise time and signal droop, which is in fact the upper frequency and the lower frequency of the transmission bandwidth.
The lower frequency is usually determined by the AC coupling characteristics such as AES transformer (mostly AES) or Capacitive coupling time constant (Mostly SPDIF).
In fact, very often, the high frequency capability of the link is higher then the signal rise time, making it a low frequency transmission capability (limitation) issue.
The transmission of signals requires understanding of electromagnetic fields, which takes some studying. Anyone with an appropriate exposure to the fundamentals of electronics (Maxwell’s equations and what they represent), understands the details of such transmission. It is unfortunate the people without a clue, feel free to “spit out garbage” straight out of thin air!
One of the common errors made by those that “spit out garbage”, is to view electronics as collection of parts, each part with “stand alone” characteristics. True, each part has it’s own characteristics, but it is the INTERACTION between parts that counts most.
It is possible that the “garbage maker” saw an application where say 75 Ohms had more bandwidth then 110Ohms, but making conclusions based on one case is ridicules! Everything needs to be put in context. You can find high speed internet cables such as Cat 5 cables with 100MHz bandwidth, CAT6 with 250MHz and the proposed CAT7 with 600MHz. They are all 110Ohm, twisted and unshielded. They are not 75Ohms.
A standard RG58 50 Ohm coax has more bandwidth then RG179 75 Ohm coax.
A standard RG59 75 Ohm coax has more bandwidth then RG178 50 Ohm coax.
The difference is dimensions, not impedance. The RG58 and RG59 are much thicker then RG178 and RG179, mostly skin effect issue. And there are times when it counts, especially for long cables. But it has nothing
to do with a connector, which comes into play at more then a few hundred MHz, when the signal rise time (bandwidth) gets into physical dimensions comparable to the connector dimensions (we are talking about around 150psec per inch, which is over 2GHz)... All of those connector considerations are way above digital audio!
The fact is: Take a typical AES or SPDIF signal, run it through say 100 feet of 110Ohms cable with an AES connector, or 100 foot 75Ohms cable, and in either case the bandwidth is more the 99.9% due to the cable, less then 0.01% due to the connector!
The connector alone has nothing to do with jitter. The cable alone has nothing to do with jitter. The issues making jitter are, as I mentioned already, the lower and upper cutoff of the transmission link, the termination tolerance, the cable impedance tolerance and more. The biggest potential issue is the skin effect (non issue at very short distances), which has to do with the dimensions of the cable, who much copper, how many wire strands and so on.
XLR, RCA and BNC are all circular connectors, all very simple structures. From signal transmission standpoint, one looks at it as a very short transmission path with a certain very small series resistance, small series inductance, and some small parallel capacitance. By adjusting the dimensions and the insulation material of a connector, one can get the desired impedance (somewhere between say above 30 Ohms, and if you separate the distance between conductors between here and the moon as high as 300Ohms).
For ideal cables: The impedance of a cable, or connector is the square root of the L divided by C where L is the inductance per unit length, and C is the capacitance per unit length… and so on.
Skin effect mucks things up at long cable length. I have a tutorial on this forum, under tutorial section…
I have some work to do, but will try and find time soon to post some material about signal transmission and cables. There is so much BS floating around, it begs for some reality.
Meanwhile, one is well advised to realize that BS is an international phenomenon.
Regards
Dan Lavry
