IPB

Welcome Guest ( Log In | Register )

Coaxial SPDIF cable?
bennetng
post Nov 25 2012, 15:19
Post #1





Group: Members
Posts: 224
Joined: 22-December 05
Member No.: 26587



We were told only use 75 ohm cable for SPDIF purpose. So what is the impedance of typical analog RCA audio cable?

Another question is I have a 75 ohm TV antenna cable, is it suitable for SPDIF purpose?

Some people said that wrong impedance may not cause problem if the cable is short enough, if it is true, then what cable length will cause problem?

I ask these because I just made a SPDIF cable with two spare RCA plugs and a 75 ohm TV antenna cable (3 feet), I tested it using RMAA and found no problem (+- 0dB freq. response, -146dB noise level, 0% THD and so on) and I can't hear any problem as well. Does it mean that I don't need to spend money to buy a standard cable?

Thanks.
Go to the top of the page
+Quote Post
 
Start new topic
Replies
Glenn Gundlach
post Nov 25 2012, 18:17
Post #2





Group: Members
Posts: 366
Joined: 19-April 08
From: LA
Member No.: 52914



QUOTE (bennetng @ Nov 25 2012, 06:19) *
We were told only use 75 ohm cable for SPDIF purpose. So what is the impedance of typical analog RCA audio cable?

Another question is I have a 75 ohm TV antenna cable, is it suitable for SPDIF purpose?

Some people said that wrong impedance may not cause problem if the cable is short enough, if it is true, then what cable length will cause problem?

I ask these because I just made a SPDIF cable with two spare RCA plugs and a 75 ohm TV antenna cable (3 feet), I tested it using RMAA and found no problem (+- 0dB freq. response, -146dB noise level, 0% THD and so on) and I can't hear any problem as well. Does it mean that I don't need to spend money to buy a standard cable?

Thanks.


I just tried searching "transmission line impedance mismatch" and got a lot of pages including 2 Wiki pages with a lot of math that will tell the average person nothing useful. What happens with mismatched lines is the signal doesn't get absorbed on either end of the cable and reflects back and forth on the line. This shows up as pulse distortions depending on the length of the cable vs the length of the pulses involved. Get enough distortions and you may reach the point where the digital information is _sometimes_ wrong. More distortions and more data lost.

I did find a few pages to show the effect and this one is quite good. While the pulse times are faster than SPDIF, the principle is the same.

http://www.avtechpulse.com/appnote/techbrief12/

The simplest way to avoid problems is to use proper transmission line and connectors.

An important thing to remember is there is no such thing as a 'digital' signal on a cable. What it is is an ANALOG _representation_ of a digital signal. All the problems with analog signals on a cable (loss, HF roll off, reflections, etc) apply to 'digital' signals. Keep the analog signals clean and you'll have no problems.

G

This post has been edited by Glenn Gundlach: Nov 25 2012, 18:30
Go to the top of the page
+Quote Post

Posts in this topic
- bennetng   Coaxial SPDIF cable?   Nov 25 2012, 15:19
- - Speedskater   a] The longer the cable the more important the 75 ...   Nov 25 2012, 17:41
|- - greynol   QUOTE (Speedskater @ Nov 25 2012, 08:41) ...   Nov 25 2012, 18:27
||- - Arnold B. Krueger   QUOTE (greynol @ Nov 25 2012, 12:27) QUOT...   Nov 26 2012, 02:53
||- - Soap   QUOTE (Arnold B. Krueger @ Nov 25 2012, 20...   Nov 26 2012, 03:00
||- - Arnold B. Krueger   QUOTE (Soap @ Nov 25 2012, 21:00) QUOTE (...   Nov 26 2012, 14:31
||- - Soap   FWIW the fraction of my interior wiring done by th...   Nov 26 2012, 14:37
|- - Soap   QUOTE (Speedskater @ Nov 25 2012, 11:41) ...   Nov 25 2012, 18:36
- - pdq   Analog audio cables may not be closely matched to ...   Nov 25 2012, 18:00
- - mzil   Besides the wire itself, the plug used on the end ...   Nov 25 2012, 18:14
|- - pdq   QUOTE (mzil @ Nov 25 2012, 12:14) F-pin a...   Nov 25 2012, 21:52
|- - mzil   Both exist: http://en.wikipedia.org/wiki/BNC_conn...   Nov 25 2012, 22:49
|- - bennetng   QUOTE RG6 and RG59 look the same in photos, but RG...   Nov 26 2012, 03:51
||- - mzil   Your cable is marked with a JIS (Japanese Industri...   Nov 26 2012, 05:40
|- - 2Bdecided   QUOTE (mzil @ Nov 25 2012, 21:49) Both ex...   Nov 26 2012, 13:52
- - Glenn Gundlach   QUOTE (bennetng @ Nov 25 2012, 06:19) We ...   Nov 25 2012, 18:17
- - bennetng   Thanks all. Although I don't have enough knowl...   Nov 25 2012, 18:45
- - washu   I've found SPDIF rather resilient when run ove...   Nov 25 2012, 19:03
|- - Arnold B. Krueger   QUOTE (washu @ Nov 25 2012, 13:03) I...   Nov 26 2012, 03:01
- - greynol   Are you sure your speakers aren't being fed by...   Nov 25 2012, 19:08
|- - washu   QUOTE (greynol @ Nov 25 2012, 13:08) Are ...   Nov 26 2012, 03:28
- - bennetng   Thanks mzil!   Nov 26 2012, 08:56
- - Dynamic   You should get away with a lot with S/PDIF because...   Nov 26 2012, 20:39


Reply to this topicStart new topic
1 User(s) are reading this topic (1 Guests and 0 Anonymous Users)
0 Members:

 



RSS Lo-Fi Version Time is now: 22nd August 2014 - 20:55