Know-how

A single timing reference isn’t always a virtue. Signals with different clock sources may need to be treated asynchronously. The new XSDI board addresses this problem intelligently. Two minor changes have a major effect

Over the last two years or so, the NEXUS SDI interface board has become a standard component in radio-broadcast installations. Unsurprising, since it handles all 16 audio channels defined by the SDI format intelligently and practically – and this is considerably more than most audio embedders offer. The new XSDI 02 version also features integrated integrated sample-rate converters and optical transceiver modules that can be replaced at any time. At first glance these changes might appear rather unspectacular but closer consideration reveals a multitude of new options, for example, in the OB Truck or the main control room.

SDI: Movie Mayhem

Serial Digital Interconnect or SDI is a popular international standard for transmitting digital video with embedded audio and ancillary data. The synchronous transmission channel for video and audio data over a single line provides solid benefits in the real world, for example, for satellite transmission or when routing via SDI video routers. However, embedding of digital audio into the SDI stream is not defined in detail by the standard, thus raising compatibility issues between SDI devices by different manufacturers. Therefore, designing a universal embedder/deembedder solution is a non-trivial task! The structure of the serial SDI video stream is based on the - now obsolete - assumption that digital video data is created by direct A/D conversion of analog video signals. Thus, similar to analog video, the SDI stream contains horizontal as well as vertical blanking with no image information. Consequently, these areas within the data stream are available for the transmission of ancillary data. For example, timecode is written into the vertical blanking¹  following each field while the horizontal blanking², which occurs after each line in the SDI standard, might carry audio signals. However, there is no data block explicitly reserved for audio. In fact, ancillary or control data might equally well be embedded into the same area. The standard specifies up to four data blocks, each containing four channels with 20-bit resolution - two AES/EBU signals plus ancillary data. So, up to 16 audio channels might be transferred in the four blocks. However, incompatibility begins to creep in as many of today’s standard SDI components, such as vtr’s, will only process the first of these four audio blocks – i.e. just four channels. If you want to transfer with 24-bit resolution, some cunning is needed to store the missing four bits in an extra data block. Here too, caution is advised as not all deembedders support this distribution of audio signals: It is possible that the receiver will ignore the last four bits.

Audio Processing Included

Of course, the XSDI board for NEXUS provides all these options – and many more. It performs as a simultaneous deembedder and embedder for all 16 of the audio channels supported by the SDI standard and is compatible with all common devices. Each of the four audio groups within the SDI stream can be processed individually. The audio signals can be read out, re-embedded into the SDI signal, existing audio data deleted or replaced with new channels received from the router. Of course the video content in the SDI signal will not be affected, it will be transparently forwarded to the output of the same SDI board. One of the real strengths of the XSDI board is its ability to de-embed and embed in parallel. It allows audio to be extracted from the incoming SDI signal, processed within the NEXUS router or via a connected mixing console and then immediately re-integrated into the SDI stream in place of the original data – and all of that in real time. Consequently, the audio interfaces of the board are displayed as 16 inputs and 16 outputs on the Matrix user interface; these resources can be used in exactly the same way as the router’s standard audio I/O’s. The XSDI board’s ability to act as a simultaneous embedder and de-embedder is especially interesting if the board is connected to an SDI router instead of simply inserting it into an incoming or outgoing SDI line. In a similar manner to audio lines with NEXUS, this allows versatile configurations to be achieved without rewiring. OB Trucks in particular, face a variety of tasks on a daily basis and, without the routable XSDI-board, each configuration would require different SDI hardware. For example, a broadcasting signal yet to be dubbed may be routed to the XSDI-board input using an SDI router. Subsequently, the outgoing SDI signal – now including audio – is then output from a second video crosspoint directly to the broadcasting line.

Two Part Harmony

Until now, to be compatible with the XSDI board, the passing SDI signal had to be synchronous with the NEXUS Base Device clock. However, if external SDI signals are received in a control room or OB Truck, these signals will not be synchronous with the local clock and thus not with the NEXUS either. A frame store is often employed as a synchroniser in such cases but this only supports image synchronisation. Due to their modus operandi, many of these devices are not transparent to embedded audio signals – here, the audio simply falls by the wayside. Actually, this is no surprise since cached frames are repeated or dropped as required to adapt the incoming video signal to the local clock. This results in audible crackling in the audio, making frame synchronisers an insuperable obstacle for the audio embedded in an SDI signal. Therefore, STAGETEC are breaking new technological ground with their XSDI variant: The implementation of new audio sample-rate converters on the XSDI board creates a “clock boundary” between the asynchronous SDI signal and the locally clocked NEXUS, or, in other words, between the video and the audio part of the board. Thus, the SDI signal does not necessarily need to be in sync with the NEXUS. The part of the board where the SDI signal passes is completely autonomous! Obviously, the SDI-signal clock will not be affected by the audio sample-rate conversion but will remain asynchronous on the output side of the board, too. The samplerate converters are an optional feature of the XSDI board. Thus, the client only pays for options he really needs.

No Connection?

The latest generation of XSDI boards includes one more significant improvement. Until now, the customer had a choice of three connector options: BNC, singlemode- fibre, or multimode-fibre connectors. However, the new board will be available in a version with BNC connectors as standard and optical interfaces as an option. The benefit here is that the interface module is socketed, so the user can quickly and simply change between singlemode and multimode modules. Up until now, the modules were soldered, so the interface format had to be decided when ordering the board. Modules may be changed without even removing the board. This is a clever solution not only for the SDI board but also for NEXUS fibre-optic boards in the future.

Multitool

Possible applications for SDI embedding technology are manifold. They range from elegant “transport” of multilingual versions of a TV production via a single line to the use of audio channels for imageindependent tasks such as talkback and intercom or even transparent forwarding of control signals for external devices! Combined with the optional new audio samplerate conversion, all these functions can be used with asynchronous SDI sources, too. This makes NEXUS a real problem solver for SDI audio processing.

Footnotes:
¹ Vertical Ancillary Data, VANC
² Horizontal Ancillary Data, HANC