SDI vs Analogue Cable

[pete10uk]

We have recently been looking at a new vision mixer, and the question of cable type has cropped up.

 

We were told that SDI used a different size pin than analog BNC and the use of standard analog cable in SDI would over time splay the connections and make a bad connection, however I can't find this information anywhere in spec sheets. I always thought a BNC connector was a Standard and the different size BNC connectors available were just to take into account the different cable diameters?

 

I know the use of RG6 is the preferred for SDI and RG59 was a poor mans choice but I always though it like the old DMX / Mic cable argument!

 

Any thoughts / spec sheet links would be gratefully appreciated.

 

Cheers

[sameness]

There is a pin size difference between 50 Ohm and 75 Ohm connectors.

 

The 50 Ohm has a bigger pin crimp area.

 

EDIT: Too early to write coherently. Thanks Tim for clarifying what I managed not to.

[timsabre]

There is a pin size difference between 50 Ohm and 75 Ohm connectors.

 

The 50 Ohm has a bigger pin. However as you use 75 Ohm for video you shouldn't be finding issues

 

This is not true, the mating bit of the pin (the bit that goes into the socket) is the same. Otherwise there would be all sorts of trouble as people connected the wrong connectors together.

 

The end you crimp/solder inside is a different size - the 75 ohm pin is the same diameter all the way through but the 50 ohm pin gets larger at the back to maintain the impedance (apparently)

 

http://www.milestek....tors-explained/

 

 

It's not true that all BNC connectors are the same, just made for different cable diameters. The internal construction of the 50/75 ohm connectors are different to give the correct impedance to the connection as described in the link. HDSDI is pretty resilient to cable type unless you are running significant distances.

[alistermorton]

This is not true, the mating bit of the pin (the bit that goes into the socket) is the same. Otherwise there would be all sorts of trouble as people connected the wrong connectors together.

 

Interestingly. I had also been told (back in the 90s) that there was a pin size difference between the different impedance BNC connectors. I wonder where the idea came from if, as you say (and I've no reason to doubt you) it's untrue.

 

I even heard the guys at LWT repeat it when I was occasionally working there. Dire warning about using network cables as video patch cables and vice versa.

[timsabre]

This is not true, the mating bit of the pin (the bit that goes into the socket) is the same. Otherwise there would be all sorts of trouble as people connected the wrong connectors together.

 

Interestingly. I had also been told (back in the 90s) that there was a pin size difference between the different impedance BNC connectors. I wonder where the idea came from if, as you say (and I've no reason to doubt you) it's untrue.

 

I even heard the guys at LWT repeat it when I was occasionally working there. Dire warning about using network cables as video patch cables and vice versa.

 

Apparently it is true for N connectors - the pins really are a different size on those.

According to wikipedia (so it must be true) "The different versions are designed to mate with each other, and a 75 ohm and a 50 ohm BNC connector which both comply with the 2007 IEC standard, IEC 60169-8, will mate non-destructively."

 

The visual difference is that the 50R has dielectric up the inside of the plug, but the 75R does not

 

http://www.wecusurveillance.com/files/891125/uploaded/507hohm.jpg

[alistermorton]

Interesting. Thanks.

[Brian]

And from Canford.

[pete10uk]

Thanks for the replies.

 

You all seem to be indicating as I thought that a 75ohm BNC has a standard mating surface, the sales rep was actually trying to say that a 75ohm BNC for Analog video had a larger pin size than a 75ohm BNC used for SDI, I can't find any reference to this!

 

The only thing I can find is that Van-Damm sell RG59 cable for both Analog and Digital SDI signals as a separate items 268-306-000 vs 268-275-000, both link to the respective BNC crimps one referred to as Standard 339-003-000 and one referred to as HD-SDI BNC cable male RG59/U 339-275-300. Even though they both reference different SMPTE standards and state different Gb/s ratings (1.5 vs 3) and the centre pin looks different, my personal thought would be the pin is the same diameter, just not as tapered giving iy a larger surface contact area, hence the better performance. It would seem mad to change the pin on the same connector if they are not fully compatible.

[timsabre]

You all seem to be indicating as I thought that a 75ohm BNC has a standard mating surface, the sales rep was actually trying to say that a 75ohm BNC for Analog video had a larger pin size than a 75ohm BNC used for SDI, I can't find any reference to this!

 

Unfortunately sales reps are well known for sounding convincing on subjects where they actually have no idea what they are talking about.

 

As for van damme, they produce a lot of similar cables with different names to fulfill system specifications where a particular cable is asked for. I am sure the "digital" variety is priced higher just cos its digital and better quality innit.

The "Analog" cable you show above doesn't say it is RG59, just that it fits RG59 type connectors.

[pete10uk]

You all seem to be indicating as I thought that a 75ohm BNC has a standard mating surface, the sales rep was actually trying to say that a 75ohm BNC for Analog video had a larger pin size than a 75ohm BNC used for SDI, I can't find any reference to this!

 

Unfortunately sales reps are well known for sounding convincing on subjects where they actually have no idea what they are talking about.

 

As for van damme you will note the superior pricing on the "HD-SDI" RG59 cable compared to the "Analog" RG59 cable. Need I say more.

 

 

I would concur on both accounts, however the rep was actually a technical guy from the manufacture and seemed knowledgable, he thought me a few things about SDI I didn't know, even though that’s not that hard!

[timsabre]

I would concur on both accounts, however the rep was actually a technical guy from the manufacture and seemed knowledgable, he thought me a few things about SDI I didn't know, even though that's not that hard!

Mmmm... was any of it actually true though

 

(while you were replying I changed my comment about Van Damme - as I don't think the 2 cables are the same specification.)

[Brian]

There is no dimensional difference in the mating parts of any 75 Ohm BNCs. Period.

 

However, where there are differences are those bits 'behind the scenes' which make one connector better at digital stuff. The 75 Ohm bit of a BNC is it's impedance (ie AC resistance) measured at quite a low frequency. For a connector to pass a digital signal undistorted it needs to have a wide bandwidth so that it passes the higher frequencies needed by a digital signal.

 

To do this requires that the mated connector pair continue to be 75 Ohms from DC all the way up.

 

Impedance changes, especially sudden ones, are bad news and cause reflections of the signal.

 

To achieve this it means that the ratio of the inner conductor diameter to the outer conductor diameter needs to stay constant through any given insulation (dielectric) material, be that air, polythene or PTFE (being the three most likely candidates).

 

If you visualise a cross-section through a mated BNC plug and socket you'll see that the various diameters change as they go along and that different insulators come and go. However, what is important is the ratio.

 

Higher quality BNCs, designed for digital use, maintain better control of this, often adding in extra bit of different insulators. Which may, or may not, justify the extra price.

 

 

If you look at Tim's post #5 you can see extra dielectric material in the 50 Ohm connector. This is to lower the impedance at that point whilst maintaining the dimensions.

[sunray]

So when did N, BNC and TNC specs change? In the old days I simply looked at the diameter of the pin, or hole, to establish whether is was 50 or 75 ohm and NO they never used to be compatible, in fact there was very little possibility of damage NOT occurring at the first incorrect insertion of a 50 ohm plug. The 0.32" tube was constant for every variation of the N, TNC & BNC style of connectors and was continued into 92 ohm (earlier digital, I'm reluctant to say SDI that far back) and fibre. The pin diameter WAS dictated by impedance and was the same spec across the range per impedance. Or at least they were 40 years ago when I first started playing with them. The removal of the insulator in the pin mating area is a recent innovation which I think occurred with the introduction of 92 ohm (RG62) systems and I find the 'tube' becomes compressed and therefore loose as a result of being frequently replugged. I try to avoid crimped connectors for 'hire stock' as I find the screen fails prematurely despite all attempts to add additional strain relief.

 

If you look at Tim's post #5 you can see extra dielectric material in the 50 Ohm connector. This is to lower the impedance at that point whilst maintaining the dimensions.

High quality connectors tend to have this 'insulator' in all impedance versions to add support to the metal sleeve.

[timsabre]

So when did N, BNC and TNC specs change? In the old days I simply looked at the diameter of the pin, or hole, to establish whether is was 50 or 75 ohm and NO they never used to be compatible

 

After a bit of googling, in 1978 apparently a standard IEC 169-8 was introduced to make them compatible with each other.

I've been using BNC's since the 1980s and never seen any different ones.

 

I don't think you are correct about the dielectric (insulator as you have termed it) in the centre of 75R plugs. It is there in the 50R to get the correct impedance and I cannot find any that have it in the 75R, high quality or otherwise- look at Neutrik, Amphenol, Samtec, none of them have dielectric in the 75R plug.

[sunray]

I googled for: RG59 BNC plug. there were 5 images and 2 mentioned 75 ohmh,http://uk.rs-online.com/web/p/products/1963328/?grossPrice=Y&cm_mmc=UK-PLA-DS3A-_-google-_-PLA_UK_EN_Connectors-_-RF_And_Coaxial_Connectors-_-PRODUCT+GROUP&matchtype=&gclid=EAIaIQobChMI5uvRlPXP1gIV7Z3tCh2ATg35EAQYAyABEgJUEvD_BwE&gclsrc=aw.ds and http://uk.rs-online.com/web/p/products/2127387/?grossPrice=Y&cm_mmc=UK-PLA-DS3A-_-google-_-PLA_UK_EN_Connectors-_-RF_And_Coaxial_Connectors-_-PRODUCT+GROUP&matchtype=&gclid=EAIaIQobChMI5uvRlPXP1gIV7Z3tCh2ATg35EAQYAiABEgJ6fPD_BwE&gclsrc=aw.ds Both clearly show insulators but the second is a bit vague as to whether it is correctly assembled. I have been assembling BNC's since late 60's age 11ish and within FT employment with a national company from 72 to 94 Clamp type). Crimping large quantities in AV racks 95-02 and initially ALL had the full size insert. The first batch of crimped type for SDV style cable (from coax connectors) had reduced inserts and since then have found the majority of cheaper plugs are the same. I'll check my remaining stock and report back.