Application Note Considerations when Choosing a Signal Splitter for DAB ETI Streams

Transcription

1 Application Note Considerations when Choosing a Signal Splitter for DAB ETI Streams Document Reference APPN EK-DSTR-SPL Date: 19 November 2018 Document prepared by on behalf of and in partnership with Somerdata Ltd. The Old Weighbridge Underwood Business Park Wookey Hole Road Wells BA5 1AF UK Tel: +44 (0) dabstor@somerdata.com Registered in England No VAT Reg. No GB

2 Notices Copyright 2018 Somerdata Ltd. and PARBROOK Business Consulting. This publication is protected by copyright and all rights are reserved. No part of it may be reproduced or transmitted by any means or in any form, without prior consent in writing from PARBROOK Business Consulting. The information in this document has been carefully checked and is believed to be accurate. However, neither Somerdata Ltd. nor PARBROOK Business Consulting assumes responsibility for any inaccuracies that may be contained in this publication. In no event will Somerdata Ltd. or PARBROOK Business Consulting be liable for direct, indirect, special, exemplary, incidental, or consequential damages resulting from any defect or omission in this document. In the interest of continued product development, Somerdata and PARBROOK Business Consulting reserve the right to make improvements in this publication and the products it describes at any time, without notice or obligation. All product names mentioned herein are used for identification purposes only and may be the trademarks or registered trademarks of their respective companies. 19 November 2018 Page 2

3 Application Note Contents Page Notices 2 Contents 3 Introduction 4 Technical Considerations 4 ETI Signals vs Composite Video Signals 5 Conclusion 6 Appendix A. ETI Splitter Application Examples 7 Appendix B. ETI (ITU G.703, 2048kbits/s) Signal Format Summary 10 Appendix C. DABSTOR ETI Splitters Block Diagrams 12 1:4-Way Splitter 12 1:8-Way Splitter 12 2 x 1:4-Way Splitter 13 4 x 1:2-Way Splitter November 2018 Page 3

4 Application Note Introduction For Digital Audio Broadcasting (DAB/DAB+), ETI (Ensemble Transport Interface) is a standardised (ETSI ETS ) method for the distribution of ensemble multiplexer outputs over communications networks to the inputs of transmitters. ETI splitters provide a convenient way to duplicate one or more ETI data streams to support redundancy in distribution networks, as well as monitoring, analysis and recording. Unlike some communications signals, the ETI stream physical form is quite specific and can be sensitive to incorrect termination, transmission or cable lengths. It is, therefore, important to be aware of the choices when considering the technical requirements for splitting/duplicating ETI bitstreams. Technical Considerations ETI is a 2048kbits/s HDB3-encoded bipolar signal waveform that follows a precise wave shape as defined in ITU recommendation G.703. ITU G.703 recommendation was originally adopted for DAB Ensemble Transport Interface to ensure that multiplexer outputs could use standard telecommunications E1 (and T1) connections on Synchronous Data Hierarchy (SDH) networks for the distribution of ETI signals to transmitter sites. For maximum robustness against electrical interference and attenuation, the signal envelope must fall within the pre-defined G.703 pulse-mask, for both amplitude and timing. When splitting ETI streams, Somerdata s experience with G kbits/s signals is that complete termination using correct transformer coupling, line interface units configured for line length & impedance, and regeneration of the ETI signal chain is the only way to ensure error-free performance. For some organisations, it may be tempting to use analogue video switches for splitting ETI bitstreams. At first sight, analogue video switches can appear to have sufficient bandwidth, BNC connections, appropriate switching options and, maybe, transformer coupling. However, the use of analogue video switches for ETI bitstreams carries the likelihood of an uncertain and unpredictable outcome typically, any of the following situations: it works it appears to work it works sometimes it does not work at all 19 November 2018 Page 4

5 ETI Signals vs Composite Video Signals The most significant difference between ETI [ITU G.703 (2048kbits/s)] signals and composite video signals is amplitude. Composite video signal amplitude is nominally 1V peak, compared to 2.4V peak for ETI. Superficially, it may seem to be reasonable to feed a larger signal into a smaller signal input, especially as the lower impedance (50 Ohms for composite video and 75 Ohms for ETI) attenuates the ETI signal. However, this ignores the consequences of over-driving the video input with a 2.4V signal. Amplitude If fitted, coupling transformers for video signals are carefully designed to be suitable for the frequencies and amplitudes of video signals, as well as being optimised for physical size. The over-driven transformer may go into saturation, causing it to stop transforming - in other words, it cuts-off the incoming signal amplitude at some (practically indeterminate) level. While this may seem to be acceptable, the effect on the pulse-width must also be taken into consideration. Pulse-width The ITU G.703 pulse-mask includes a precise definition of the ETI pulse width. Variations in pulsewidth is referred-to as Jitter. A saturated video transformer produces inconsistent, out-of-g.703-specification jitter as the ETI signal content changes. Such jitter can have a profound effect on the signal timing. The timing effect on the ETI bit-stream can be quite subtle, for example, occasional, random biterrors, lost ETI frames or even bit-slip. These factors depend on a combination of data content, signal amplitude, existing signal jitter and electrical noise on the incoming ETI (2048kbits/s) link. Even with composite video switches that are not fitted with coupling transformers, there are other factors that can result in out-of-g.703-specification jitter. Capacitive coupling causes asymmetry in the pulse width (more jitter), while diode clamps on the video switch will also cause early signal cut-off, again leading to jitter. Video Switch Outputs While the above considerations apply to video switch inputs, it is equally important to be aware that the video switch output signals will not meet the amplitude and pulse-mask requirements of ITU G.703-compliant down-stream equipment that expect DAB/DAB+ ETI bit-streams or E1- compatible telecoms bit-streams. Use of video switches assumes, incorrectly, that the down-stream devices will work beyond their specification to correctly process the input signal, which, at best, will be only half the required amplitude. 19 November 2018 Page 5

6 Conclusion The errors created by using the wrong type of splitter for ETI (and E1) signals can be quite difficult to track down. Often, the blame is wrongly attributed to cable connections, lengths, ETI/E1 signal source or receiving devices, etc. Worse still, the system may appear to work for some time, before becoming unreliable. A summary of the reasons for Choosing a Somerdata ETI Splitter over a Video Splitter are: Support for 2.4V peak signal level to ensure the correct (ITU G.703) waveform amplitude and pulse-mask Use of telecoms coupling transformers that are optimised for 2.4V peak amplitude (per ITU G.703) 19 November 2018 Page 6

7 Appendix A. ETI Splitter Application Examples Multiple Distribution Networks Multiple Transmitters Point-to-Point Distribution 19 November 2018 Page 7

8 Distribution Network with ETI Monitoring & Logging Redundant Multiplexers and Distribution Networks with ETI Monitoring & Logging 19 November 2018 Page 8

9 Redundant Multiplexers, Switches and Distribution Networks with ETI Monitoring & Logging 19 November 2018 Page 9

10 Appendix B. ETI (ITU G.703, 2048kbits/s) Signal Format Summary ETI Pulse Mask ETI Signal Parameter Pulse shape (nominally rectangular) Load impedance Nominal peak voltage of a mark (pulse) Peak voltage of a space (no pulse) Nominal pulse width Ratio of the amplitudes of positive and negative pulses at the centre of the pulse interval Ratio of the widths of positive and negative pulses at the nominal half amplitude Maximum peak-to-peak jitter at an output port Specification All marks of a valid signal must conform with the mask (see above) irrespective of the sign. The value V corresponds to the nominal peak value. 75 ohms resistive 2.37 V 0 ± V 244 ns 0.95 to to 1.05 Refer to clause 5.1 of [ITU-T G.823] 19 November 2018 Page 10

11 Jitter Maximum permissible jitter at traffic interfaces Measurement Bandwidth -3 db frequencies Peak-to-Peak Amplitude 20 Hz to 100 khz 732 ns Maximum permissible jitter at Output interfaces Observation Interval t (seconds) Maximum Time Interval Error Requirement (µs) 0.05 t t 0.2 t t t 64 t November 2018 Page 11

12 Appendix C. DABSTOR ETI Splitters Block Diagrams 1:4-Way Splitter 1:8-Way Splitter 19 November 2018 Page 12

13 2 x 1:4-Way Splitter 4 x 1:2-Way Splitter Product Brochure 19 November 2018 Page 13