Cable problem - Page 8 - AVS Forum
Forum Jump: 
Reply
 
Thread Tools
post #211 of 218 Old 12-17-2013, 03:31 PM
AVS Special Member
 
Glimmie's Avatar
 
Join Date: Sep 2000
Location: Los Angeles, CA
Posts: 7,806
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 50 Post(s)
Liked: 173
Quote:
Originally Posted by DonH50 View Post

And the latest 9451 spec is 45 ohms for whatever silly reason.

Interesting that just happens to be the specified impedance of USB cable. I just built a small USB matrix for a project at work and the spec sheet for the USB approved analog switch chips went into this in detail.

http://www.analog.com/library/analogdialogue/archives/40-01/usb_switch.html

Glimmie's HT Page
Being redone - comming soon!

Glimmie is offline  
Sponsored Links
Advertisement
 
post #212 of 218 Old 12-18-2013, 05:05 AM
AVS Addicted Member
 
arnyk's Avatar
 
Join Date: Oct 2002
Location: Grosse Pointe Woods, MI
Posts: 13,878
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 435 Post(s)
Liked: 1059
Quote:
Originally Posted by Glimmie View Post

Quote:
Originally Posted by DonH50 View Post

And the latest 9451 spec is 45 ohms for whatever silly reason.

Interesting that just happens to be the specified impedance of USB cable. I just built a small USB matrix for a project at work and the spec sheet for the USB approved analog switch chips went into this in detail.

http://www.analog.com/library/analogdialogue/archives/40-01/usb_switch.html

Notice that 45 ohms is the "single ended impedance" while USB is actually, like HDMI and DVI and based on balanced transmission.

"Differentially, the D+ and D– lines should have a mutual impedance of 90 ohms, that is, the impedance between D+ and D– should be 90 ohms".

To expand on this, DVI and HDMI are based on TDMS which in turn is based on transmission lines with mutual impedances of 100 ohms. While I'm not recommending intentionally putting mismatches into a high frequency system, 90 and 100 ohms are close enough so that for the line lengths involved in home A/V, some slight mismatching is at least tolerable and generally indistinguishable. To complete the picture SPDIF is based on 75 ohms unbalanced while AES/EBU and AES 3 are based on balanced lines with 110 ohm mutual impedance.
arnyk is online now  
post #213 of 218 Old 12-18-2013, 12:11 PM
AVS Special Member
 
DonH50's Avatar
 
Join Date: Feb 2010
Location: Monument CO
Posts: 5,928
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 97 Post(s)
Liked: 235
Hmmm... I don't know, Arny. And I mean that literally. The datasheet I linked from Belden just says "nominal impedance" but it is a twisted pair, why list single-ended impedance? Not saying that is not what they did, would not really surprise me. I am used to more complete impedance specs that include both signal modes' (even and odd) impedance plus common-mode impedance. But, this is an audio cable, for heaven's sake, so I am not surprised at the dearth of RF data. Still think the capacitance is high, still think it doesn't matter for the vast majority of systems. smile.gif

A 10% impedance change (e.g. 100 to 90 ohms) is only about 1.11 VSWR and loss (from just that) is only a tiny fraction of a dB (<0.1 dB).

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
DonH50 is offline  
post #214 of 218 Old 12-18-2013, 02:34 PM
Senior Member
 
Skytrooper's Avatar
 
Join Date: Nov 2010
Location: Baden, Pa.
Posts: 430
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 9 Post(s)
Liked: 77
I always thought you had to use 600 ohm cable for low level audio signals. but reading other posts I see most people use 75 ohm for their subs. Some one commented that was old school thinking.

(LCD - Sony KDL - XBR4) (Receiver - Sony STR-DA4ES)(Blu Ray - Oppo BDP-83) (PS3)( Dish Hopper DVR With Sling) Speakers (L & R - Paradigm Studio 20) (Center -Paradigm CC-470) (Surrounds & Back Surrounds - Paradigm SA-15R in walls) (Subwoofer 1 - Sunfire HRS-12) (Subwoofer 2 - Paradigm PW-2100)
Skytrooper is online now  
post #215 of 218 Old 12-18-2013, 02:49 PM
AVS Addicted Member
 
arnyk's Avatar
 
Join Date: Oct 2002
Location: Grosse Pointe Woods, MI
Posts: 13,878
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 435 Post(s)
Liked: 1059
Quote:
Originally Posted by Skytrooper View Post

I always thought you had to use 600 ohm cable for low level audio signals. but reading other posts I see most people use 75 ohm for their subs. Some one commented that was old school thinking.

Audio signal wiring is not impedance matched. That includes line level audio signals, speaker signals, and phono cables. Neither the sources nor the loads nor the cables are impedance matched. It is a bridging system.

Video and digital audio lines are impedance matched, but given the short cables that are commonly found in home audio systems, there is no need for hyper accurate matching. The wire from your TV antenna or cable trunk in your backyard or on your room is long enough for impedance matching to be a good idea.

Looking at cables, I know about 45 ohm cables, 50 ohm cables, 75 ohm cables, 100 ohm cables, 110 ohm cables, 300 ohm cables and 450 ohm cables. I don't know about any 600 ohm cables.

Impedance is based on conductor spacing and conductor size.

Knowing what 300 and 450 ohm cables look like, 600 ohm lines would be composed of two fairly small wires fairly widely spaced. I'm guessing several inches. Know of any cable like that? Current telephone lines are composed of approximately 24 gauge twisted pair whose impedance I would put at around 100 ohms. These pictures are about 1/4 actual size:



300 Ohm 18g, 0.25 inch separation
450 Ohm 18g, 0.75 inch separation

I googled 600 ohm cable and found just tools and devices, but zero pictures of cable!

The impedance matched 600 ohm systems were dominated by the phone system. The sources and the loads were 600 ohms. However, the telephone lines themselves were generally not 600 ohms, either. Various impedance matching devices, some called loading coils were used to draw the systems together.
arnyk is online now  
post #216 of 218 Old 12-18-2013, 03:14 PM
AVS Special Member
 
DonH50's Avatar
 
Join Date: Feb 2010
Location: Monument CO
Posts: 5,928
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 97 Post(s)
Liked: 235
Audio XLR (mics and such), most older (not sure about newer), and of course old phone lines were based upon 600-ohm impedance. Twisted pairs could hit around 600 ohms "depending". I know of no current systems that use 600-ohm cables nor that really care about impedance with the possible exception of some transformer-based isolation boxes.

Here's a historical link, Google: https://www.fmsystems-inc.com/index.cfm?tdc=dsp&page=publication_detail&pid=94

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
DonH50 is offline  
post #217 of 218 Old 12-18-2013, 04:12 PM
AVS Special Member
 
Glimmie's Avatar
 
Join Date: Sep 2000
Location: Los Angeles, CA
Posts: 7,806
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 50 Post(s)
Liked: 173
Up to the 1980s radio and TV broadcast audio systems were "power matched" +8db at 600 ohms. CBS also used 150 ohm systems for long runs in their large network centers - NY and LA. Oh yes, and they used pin 3 hot in their XLR connectors.

The music recording industry started out as 600 ohm power matched systems as well but made the transition to +4db voltage matched systems in the late 1960s or thereabouts. These systems were low source impedance - about 60ohms and 10K receiver impedance. Bridging was allowed as well because at audio frequencies and the distances in an average building, cable impedance is not relevant. And they used PIN 2 as hot on their XLR connectors.

When stereo television came about in the early 1980s many TV plants decided to do a complete audio rebuild as most systems were from the 1960s and at that point they decided to follow the music industry and go with a +4db voltage matched system and the change to XLR pin 2 hot. This was further promoted by the transition to one inch video tape and the greatly improved audio performance. Old 2 inch VTRs had rather poor audio performance. This era also experienced the explosion of home video and it's demand for higher TV audio quality - Beta HiFi and LaserDisk to start with.

AES-1985 started out as a 250ohm source impedance and a 110 ohm sink impedance.. The idea was you could bridge two receivers from a single source. The well known reflection problems at 6mhz soon brought chaos to the AES standard. So AES-1992 changed the spec to a 110 ohm power matched system that remains today. AES3 also allows 75ohm coax in a power matched system just like analog video is distributed.

Glimmie's HT Page
Being redone - comming soon!

Glimmie is offline  
post #218 of 218 Old 12-18-2013, 04:24 PM
AVS Special Member
 
Glimmie's Avatar
 
Join Date: Sep 2000
Location: Los Angeles, CA
Posts: 7,806
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 50 Post(s)
Liked: 173
Quote:
Originally Posted by Skytrooper View Post

I always thought you had to use 600 ohm cable for low level audio signals. but reading other posts I see most people use 75 ohm for their subs. Some one commented that was old school thinking.

Consumer audio cable never had an impedance specification because it never mattered. Pro audio had impedance specification as I noted above but the cable was still about 60 ohms and again this did not matter if under a few thousand feet in length.

The 75ohm sub woofer idea came from the DIYers and is a bit of a kludge to try and reduce Hum on subwoofers. RG6 has superior shielding to audio cable or so they think. If you want to run your sub woofers over 25 feet use a balanced system. That will ensure you don't get hum.

Glimmie's HT Page
Being redone - comming soon!

Glimmie is offline  
Reply Audio theory, Setup and Chat

User Tag List

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off