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Tech Breakfast: Developments in SDi Cabling for 12G phil.crawley@jigsaw24.com @IsItBroke on Twitter http://www.root6.com/author/phil

Developments in SDi including 12G and fibre A history of digital video transport and SDi Some of the variations/adaptions SDi is used for Physical layer considerations – making measurements What cable does to the signal; closing of the eye! Options – fibre optics allowing long distance working 12G – how does it even work?

History of SDi A family of digital video interfaces first standardized by SMPTE (The Society of Motion Picture and Television Engineers) in 1989. ITU-R BT.656 and SMPTE 259M define digital video interfaces used for broadcast-grade video. Related standard SMPTE 292M; this provides a nominal data rate of 1.485 Gbit/s. SMPTE ST-2083 specified a max data rate of 24 Gbit/s and allows for 4096x2160 at 120 fps. This seems like a bit of revisionism as none of these standards define the physical layer – BNC etc. which Sony brought to the party in 1991.

History of SDi cont. Engineers of a certain age will remember this – I’m looking at you ……!

History of SDi cont.

History of SDi cont. One or more coaxial cables with BNC connectors, Nominal impedance of 75 ohms, Signal amplitude at the source is 800 mV (±10%) peak-to-peak, Uncompressed digital component signals encoded in NRZI format, Linear feedback shift register is used to scramble the data, Self-synchronizing and self-clocking, Framing is done by detection of a sequence of ten ones followed by twenty zeroes (twenty ones followed by forty zeroes in HD); this bit pattern is not legal anywhere else within the data payload.

History of SDi cont. Dual link SDi - SMPTE 372M allowing for bit-rates of 2.970 Gbit/s, over two wires. These bit-rates are sufficient for 1080p video at 60fps or 4:4:4:4 RGBA at 24p ASI - transmission of an MPEG Transport Stream (MPEG-TS), usually as part of its run within a transmission facility. SDTI - allows compressed (i.e. DV, MPEG and others) video streams or faster-than-realtime (2x, 4x,...) video transmission. SMPTE 297-2006 defines an optical fibre system for transmitting bit-serial digital signals So it seems that SDi has been a remarkably expandable and adaptable interface.

Physical layer considerations Fundamental electrical parameters Shunt capacitance per unit length, in farads per metre. Series inductance per unit length, in henrys per metre. Series resistance per unit length, in ohms per metre. Derived electrical parameters Characteristic impedance in ohms (Ω). The complex impedance Z of an infinite length of transmission line. Where R is the resistance per unit length, L is the inductance per unit length, G is the conductance per unit length of the dielectric, C is the capacitance per unit length, and s = jω = j2πf is the frequency.

Physical layer considerations – the eye pattern

Physical layer considerations – the eye pattern

The damage that 100m of quality coax does… ends up thus → ↓ starts like this ↓ 1.5 Gbit/sec 4:2:2 100m of Belden 1694A coax.

It’s not all about cable length – Return Loss Return loss is the loss of signal power resulting from the reflection caused at a mismatch with the terminating load or with a device inserted in the line. It is usually expressed as a ratio in decibels (dB); Return loss is related to both standing wave ratio (SWR) and reflection coefficient. Return loss is a measure of how well devices or lines are matched. A match is good if the return loss is high. A high return loss is desirable and results in a lower insertion loss. In the case of newer budget HD/SDi equipment return losses can be as bad as 12dBs (I've measure Blackmagic boxes thus) whereas the spec for 3G is 16dBs and in the case of proper broadcast manufacturers 18dBs or better is often measured (Sony, Tektronix). RL(dB) is the return loss in dB, Pi is the incident power and Pr is the reflected power Sometimes the effect of the reflected signal can be minimized by using a longer connecting cable as this allows a longer distance for the reflection to be dissipated over; that time at ITV Manchester with the fibre->SDi->HDMI – a longer bit of coax made it work.

Formalised testing of cabling for 3G

Formalised testing of cabling for 3G cont. Although all the measurable parameters – jitter and attenuation are completely out of spec and the eye-pattern is unrecognisable at 160m pictures can be recovered (sometimes!)

Formalised testing of cabling for 3G conclusions At 3G using coax specified for HD 60m seems to be the workable cable length before attenuation becomes an issue and the eye closes below 400mV. SD coax goes about half the distance - this seems counter-intuitive as most SD coax has a notional analogue bandwidth (+/- 6dBs) of 360Mhz - three octave less than HD coax. The variation between the best (most expensive) and worst cable at HD before the signal becomes sub-optimal (i.e. worse than 3dBs attenuation) is less than 10m with Belden 1694 coming out on top. The Tek was still able to recover a signal at 150m with Belden but only 120m with the Draka DC DVC13C. At these length the mean time between corrupt video frames would be unacceptable. The Condufil 1694-equivelent tracks the more expensive Belden cable very well. As mentioned this was really a test of run-lengths for a practical guide to cabling TV facilities. In the bulk of the tests we used the correct BNC crimp connectors and the proper tools for the brands of BNCs (attached by an experienced wireman) - We did try and provoke jitter by mixing up connectors with cable but it seemed to make scant difference. It does seem for 3G HD video the newer style 4.5Ghz are to be preferred over the original HD-type cable. In the end I suspect that these results will represent the best possible world as Tektronix gear is known to drive a coax line optimally and has excellent return loss on its inputs. Other manufactures are less so and if our experience with 1.48G 4:2:2 HD is anything to go by the massive variation in the quality of line-drivers and receivers will make these results meaningless. Nobody (particularly in these hard economic times) builds a facility with only Sony and Tektronix equipment.

The main event! The 12G physical layer results Using our Leader LV5490 test set (which includes 12G test signals and 12G eye pattern) we have repeated our 3G tests using the current crop of cables widely advertised as being 12G ready Although these results are copyright Jigsaw24 they are presented to assist film and TV engineers in building reliable installations.

SMPTE 2082-1 parameters for 12G SDi

Same 2m length at Belden 1694A at 1.5G and 12G amp: 772.0mv rise: 51ps timing Jitter: 58ps overshoot: 2.6% amp: 780.3mv rise: 36ps timing jitter: 10ps overshoot: 2.8%

The best vs. the worst SD73 – AKA Belden 7731 SD05 – AKA Belden 1855, Image 360

The hairy-edge; 36m of Image-360 cable This was the tenth screen grab to show the occasional “splat” – although pictures were stable most of the time this connection is not usable.

Practical limits for different cable types 640mV SD05 – AKA Belden 1855, Image 360 SD10 – AKA Belden 1694A, SD50 – AKA Belden 1505, Image 1000 SD73 – AKA Belden 7731

Do the gold connectors make any difference? The difference between 3G and 12G crimp’able BNCs is modest. Not quite in the same camp as Audiophile gold-plated mains plugs, but nearly….!

Reliable fibre transmission of 12G Our favourite fibre/multiplexing/switching manufacturer have recently added 12G parts to their family of products. So long as your optics support it fibre is more than able to carry 4k/60P video.

Further Reading Tektronix: Eye & Jitter Measurements https://www.tek.com/document/how-guide/sdi-eye-and-jitter-measurements “Digital Signal Processing” – Kenton Williams http://116.58.21.27/articles/1856176231.pdf “The Art of Digital Video” – John Watkinson http://www.root6.com/blog http://philtechnicalblog.blogspot.co.uk I’m showing the third edition cover as it’s much better than the current version which looks like an accident in DTP factory c. 1991…!

Thank-You! phil.crawley@jigsaw24.com @IsItBroke on Twitter http://www.root6.com/author/phil Thank-You!