Practical SDI And IP - Part 1

SDI has been and continues to be a mature and stable standard for the distribution of video, audio and metadata in broadcast facilities. From its inception in 1989 to the modern quad-link 12G-SDI available today, it has stood the test of time and even with the advent of IP and Ethernet, it shows no sign of waning.

This article was first published as part of Essential Guide: Practical SDI and IP - download the complete Essential Guide HERE.

IP is making a significant impact in broadcast facilities throughout the world and is starting to show its true worth. It’s flexibility and scalability empower engineers to future proof their facilities and meet the growing demands of new formats, especially as we move to 4K and 8K.

HDR, WCG and higher frame rates are all contributing to the immersive experience and broadcasters are looking for methods to integrate these formats into their facilities. SDI is more than capable of this, but IP also has much to offer.

Although we speak fondly of SDI and are quite happy to discuss the challenges of IP, SDI’s ease of use has not always been a given. Early adopters would regularly find new challenges that they were not aware of when building their facilities. Cable types, lengths and equalizers all seemed to conspire against the engineer. A lack of test equipment further exasperated the challenges and made SDI integration a challenge.

We often forget this when considering IP installations. There will be a point where IP is as easy to install as SDI, and it’s fair to say we’re not there yet, but the benefits IP has to offer far outweigh the challenges we experience. Prior to SDI we used three cables for RGB distribution in graphics and VT areas to maintain the highest possible video standards and making the cables exactly the same length to stop RGB registration issues was a challenge in itself.

Through the adoption of ST-2110, broadcasters are able to gain unprecedented flexibility. This isn’t just about increasing the size of a facility, but also the number of formats that can be combined in one network. Abstracting away the video, audio and metadata from the underlying transport stream, as with ST-2110, gives us the opportunity to distribute, combine, and process video, audio and metadata with great innovation.

Interoperability wasn’t always a given, even with SDI. Prior to SMPTE’s specifications for inserting audio into the “spare” blanking data, several vendors implemented their own methods of achieving this. These solutions were often proprietary and completely inoperable. Inserting audio into an SDI stream using one vendor’s solution could not be guaranteed to work with another.

The big advantage of IP is that it is software driven. This will allow early adopters to future proof themselves so that as interoperability progresses, then so will their own facilities as many of the solutions will be software upgrades.

SDI and IP both have their place in broadcast facilities and provide solutions for many different workflow scenarios and in these articles, we look deeper into the solutions SDI and IP both offers.

Broadcasters seem to be faced with a bewildering choice between SDI and IP. Is one better than the other? Are there specific applications for IP or SDI? In this Essential Guide, we take a deeper look at SDI and IP to answer these questions, and more.

Although SDI is now considered a robust, reliable, and interoperable format, it hasn’t always been that way. When video was first digitized, we used SMPTE’s ST-125 to distribute digital 4:2:2 component 525 and 625 video over parallel interfaces.

The SMPTE ST-125 format was effectively the predecessor to SDI as the sample rates and formats were similar. The luma component was sampled at 13.5MHz and each of the Cb and Cr signals were sampled at 6.75MHz, both with 10bit depths. A 27MHz oscillator was used as the base sample clock giving a total data rate of 27MWords per second. In this context a word was 10 bits and laid the path for 270Mbit/s SDI.

Parallel Digital Video

The physical connectivity of ST-125 consisted of a D-Type 25 pin connector. Balanced pair ECL logic provided the ten data signals for each of the bits as well as a separate 27MHz clock signal. Due to the resistance, capacitance and inductance in the connecting cable, data skew and HF attenuation was inevitable, resulting in limited cable lengths.

SMPTE’s ST-259M SDI specification provided a method to serialise ST-125 and pass the signal over installed coaxial cables. This greatly improved reliability and the transmission distances were much greater.

This may have solved the initial challenges around distribution, but the interoperability was still to be addressed, especially when vendors started to insert audio into the ancillary areas of the data stream. Originally, some vendors used the available data in the line syncs and while others used the vertical blanking. It wasn’t until SMPTE released ST-272 that audio insertion and extraction was formalized and became truly inoperable.

SDI Early Adopter Challenges

It’s not been an easy ride for SDI and any early adopters will remember the challenges faced, even when trying to find the right cable. The primitive equalizers at the time severely restricted the cable available giving rise to high costs and restricted availability.

RGB signals use 4:4:4 color subsampling, that is full bandwidth chroma. SDI uses 4:2:2 color subsampling giving half horizontal and vertical chroma bandwidth. This provided some interesting challenges in the early days of SDI as chroma keyers preferred to use 4:4:4 to derive the optimal key, however SDI would only provide 4:2:2. This was soon dealt with by vendors and chroma keyers were able to provide high quality keying in 4:2:2 SDI.

RGB signals use 4:4:4 color subsampling, that is full bandwidth chroma. SDI uses 4:2:2 color subsampling giving half horizontal and vertical chroma bandwidth. This provided some interesting challenges in the early days of SDI as chroma keyers preferred to use 4:4:4 to derive the optimal key, however SDI would only provide 4:2:2. This was soon dealt with by vendors and chroma keyers were able to provide high quality keying in 4:2:2 SDI.

Even chroma keying had its issues. Before SDI, full bandwidth RGB was fed directly from the camera to the chroma key processor. However, as SDI uses YCbCr, the color bandwidth is limited causing challenges for the chroma key processor. Instead of relying on full bandwidth 4:4:4 color RGB signals, it had to compromise with 4:2:2 YCbCr color bandwidth.

Interoperability is another phrase that is often banded around, but some caution must be exercised when referring to SDI. For example, HD-SDI is definitely not compatible with SD-SDI, the data-rates and bit stream definitions are worlds apart. There are further divisions when we start to look at Dual and Quad 3G, or 6G, or 12G. Although these are all SDI standards, interoperability cannot be assumed.

This is the crux of the challenge when considering IP. We tend to look at SDI through the historic lens of our rose- tinted spectacles. Over thirty years have passed since SMPTE released the first ST-259M specification, longer if we consider ST-125. We’ve forgotten about the pain we collectively went through all those years ago. It’s not that SDI is perfect, by any stretch of the imagination, it’s just that we are now more aware than ever of its strengths and weaknesses.

IP Enabling Technology

IP has exciting potential for broadcasters. Although IP is only a transport stream, it’s an enabling technology that further improves efficiencies and delivers new opportunities for program makers.

It is the advent of SMPTE’s ST-2110 suite of standards that has really enabled IP to form the core part of a broadcast infrastructure. In effect, ST-2110 has abstracted away the video, audio, and metadata essence away from the constraints of the underlying hardware. This allows us to process the essence streams independently of each other and multiple formats can be mixed on the same network without any special configuration.

The inbuilt-clocking mechanism of SDI makes it robust and easy to use. Connecting a camera to a monitor is an easy and straightforward task as the pictures will instantly appear, without further configuration. However, this strength is also a weakness as it makes systems rigid, it’s often difficult to mix 30/1.001fps and 25fps systems on the same network or connect HD-SDI to SD-SDI. It can be done - but involves processing electronics and it’s not a standard method of operation.

Deciding between SDI and IP is a seesaw compromise between flexibility with complexity, and rigidity with ease of use. SDI, although relatively static and rigid is easy to use, but only because it’s a well proven and understood technology. Whereas IP is new to us and we’re still trying to solve many of the challenges we don’t yet know we have.

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