Components

In a NEXUS system, the XHDI board option is provided to make available the audio embedded into an SDI digital serial video stream as specified by SMPTE 259 M (SDI) and SMPTE 292M (HD). In addition, any audio existing on the NEXUS system can be written to the SDI stream. The board features a video input and a video output. This allows any video to be transparently forwarded with no need for additional hardware components. Of course, it is also possible to use the XHDI as a pure input board for extracting (de-embedding) audio from a video signal.
Unlike competing solutions, the XHDI was designed as a generic SD and HD interface. It offers a variety of options for reading audio from a video stream, or adding it to the video signal, respectively:

› De-Embedding  Any audio can be extracted from the video stream and be used and forwarded on the NEXUS audio network. As the video stream remains unaffected, the audio of the output video signal is retained.

› Embedding  Any audio signals on the NEXUS system can be embedded into the video stream. The groups and channels to be written are freely selectable. Audio data from other blocks originally contained in the video stream are retained. Channels from the same group can be reapplied.

› Replacing  With due regard to the numerous data structures possible in the SMPTE specifications, the XHDI board also allows new audio to be written to existing data structures. This will embed only the pure audio data. This mode provides a high degree of compatibility even for non-standard streams, for example, if only two audio channels are defined for a group.

› Deletion Audio groups already contained in the video signal or marked for deletion are entirely deleted and/or overwritten.

High Flexibility

The XHDI board handles signals compliant with the SMPTE 259M (4:2:2, Component, 270 Mbps) and SMPTE 292M (4:2:2, Component, 1485/1483.5 Mbps) standards. The standard format and any sub-formats under it are automatically detected.

Supported video formats include:

Video Standard Video Format SD/HD Test-tone Generator SMPTE 296M 1280x720 / 60p HD (SMPTE 292M) yes 1280x720 / 30p - 1280x720 / 50p yes 1280x720 / 25p - 1280x720 / 24p - SMPTE 274M 1920x1080 / 60i HD (SMPTE 292M) yes 1920x1080 / 30p (PsF) - 1920x1080 / 30p yes 1920x1080 / 50i yes 1920x1080 / 25p (PsF) - 1920x1080 / 25p yes 1920x1080 / 24p (PsF) - 1920x1080 / 24p yes SMPTE 260M 1920x1035 / 60i HD (SMPTE 292M) - SMPTE 125M 1440x487 / 60i SD (SMPTE 259M) yes 1440x576 / 50i yes

SD Mode

The board fully supports the SMPTE-272M-AC (audio, SD) standard specifying a maximum of four groups, each comprising four 20-bit or 24-bit audio channels. Single or multiple channels can be de-embedded from, or embedded into, the data stream at the same time.
The SMPTE-272M-AC standard supports various types of embedding audio data into the video stream. This fact can cause problems in extensive signal chains and might render additional incremental embedding operations impossible. In order to avoid this problem, the board allows existing data structures including the audio to be completely rearranged and rewritten.
A FIFO cache is used for reconstructing the necessarily isochronous audio stream. The cache buffer can be set to 30, 40, or 60 samples, and is thus adjustable to suit the input signal.
The NEXUS system can be synchronised to the video signal.

HD Mode

The board fully supports the SMPTE 299M-AC (audio, HD) standard specifying a maximum of four groups, each comprising four 20-bit or 24-bit audio channels. Single or multiple channels can be de-embedded from, or embedded into, the data stream at the same time.
With SMPTE 299M (audio, HD), there are no restrictions regarding embedding. No compatibility issues resulting from distinct embedding algorithms will arise.
In HD mode, the XHDI board provides a special function: It
achieves identical latency for all blocks. However, this function can only be implemented in the de-embedding/embedding modes (I.e. not in replace mode) and requires the existing embedded audio to be of identical structure. This functionality is useful in multichannel applications where the maximum number of 4 channels per audio block is exceeded. Using only the embedding mode ensures identical latency.
The NEXUS system can be synchronised to the video signal and, with asynchronous audio, to the audio wordclock contained in the data stream.
The (AES-3 compliant) ancillary data contained in the audio stream is made available for software evaluation. On the sending side, the ancillary data can either be transparently adopted from AES-3 compliant signal sources or be regenerated by the XCPU board.

Signal Processor

The XHDI board features a DSP for level adjustment and signal processing without relying on other boards. The input and output levels can be separately adjusted. Reverse-polarity signals can be corrected using a phase-inversion function.
The standard audio resolution within NEXUS is 24 bit. However, if audio has to be output in 20-bit or 16-bit format, the signal quality can be kept at the highest level by performing the re-quantisation with the dithering function enabled. In addition, a noise-shaping function moves the noise energy up into frequency ranges less perceivable to the human ear. The noise reduction achieved in the range below 1 kHz is approximately 15 dB.

Error Detection

To ensure reliable operation, the input-signal status is frequently
checked. This includes “Video PLL Lock”; signal condition; video-format detection; audio/video sync; EDH/CRC statuses; audio-block availability, errors and formats etc.
Two front-panel LEDs indicate the board status. They show sync errors, the bypass mode, HD/SD mode, test mode and the status of the I/O PLL.
At input-signal failure, the board automatically switches to test-generator mode. The output-signal format can be set using the control software. This can be either a fixed format or the format of the recently present input signal. In addition, colour bars or BlackFrame signals are available.
At critical system component failure (e.g. the XCPU board), the board automatically enters a bypass mode, ensuring that the video stream is not interrupted.

Incompatible Data

With incompatible formats, it is often desirable to retain existing structures in the data stream. For this purpose, the board offers a replace mode where only the pure audio samples are replaced while keeping the existing structures. This strategy is especially helpful with SD applications where format incompatibilities occur time and again.
If the video stream contains data blocks that cannot be handled by the interface, these can still be transparently forwarded to the output.

Ancillary Data

AES-3 compliant ancillary data in the data stream can also be read by the XHDI board. This data is available on the NEXUS system and can be output using suitable boards (e.g. XET AES/EBU-output boards).
Ancillary data can also be applied to the audio contained in the output video stream. This allows data from AES-3-compliant sources to be adopted (for example, XER AES/EBU-input boards) or for having the XCPU main-controller board generate new ancillary data from scratch.

Ports

The XHDI board is equipped with BNC and optical ports (LC, single mode). The board provides a BNC input and two independant BNC outputs. An SFP module featuring an LC terminal is used as optical I/O port.
The implementation of various optical modules can be carried out onsite.
Format conversions are also supported.