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Application NoteAN-30b

copyrightAugust, 2016

An HDMI Switch for a Digital TV RepeaterJim Andrews, KH6HTV

www.kh6htv.com

While developing a new combination analog/digital Television repeater for Boulder,Colorado, the need was found to be able to switch easily between different HDMIsources. While such devices are available for broadcast studios, they are very expensive.I wanted a very low cost switch that had KISS (Keep It Simple Stupid) technology thatdid not require computer control, nor tearing into the guts of other devices and reverseengineering.

A search on the internet for HDMI, multiple input switches results in many "hits" for lowcost switches. Purchasing some of these showed that most all of them had one feature incommon. They were all Auto Selector switches. Whenever a new HDMI source wasturned on, the switch automatically moved to this input. This meant that they could notbe reliably controlled by their front panel selector switch button, nor their supplied IRremote control. Also any "glitch" in a selected HDMI input, and the switchautomatically moved on to the next active HDMI input in sequence. This wasunacceptable for reliable operation in a remotely located, unattended, TV repeater.

I discovered a low cost, no modification, solution to this auto switch dilemma. This wasto exercise positive DC power control up-stream of the various HDMI sources beforethey got to the multi-input, auto, HDMI switch. If I only allowed a single HDMI sourceinput to the auto switch to be active at any one time, then the auto switch locked onto itand stayed there, outputting the desired video. What I found worked quite well was touse a low cost ($10), 1 in x 2 out HDMI splitter as my DC controlled switch. Thussimply turning on or off the +5Vdc to the splitter accomplished my control of thatparticular HDMI source. The block diagram on the next page shows the concept.

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Fig. 1 Block Diagram of KISS, 5 input x 1 output, DC controlled HDMI Switch

The key control requirements for this HDMI switch is to (1) Never allow more than onesource to be active at any one time and (2) always switch off the active source, beforeswitching on the next desired active source.

In actual use, I found this worked quite well when the various switches SWx-b werecontrolled by mechanical switches, SWx-a. Trying to control the dc power switchingusing an Arduino micro-computer driving solid-state switches for SWx-a lead toproblems. Apparent "glitches" in the Arduino output lead to indeterminate behaviour ofthe auto switch, SW6. The solution found was to have the Arduino do a 2 second powerreset for the master +5V power to all of the above switches.

The example shown in Fig. 1 would be for a typical DTV repeater. It will work with anyHDMI sources. As shown, sources 1, 2 and 3 are from 70cm, 23cm and microwave,DVB-T receivers. Source 4 is the Video ID Generator to satisfy FCC identificationrequirements. This could be from a local TV camera with call sign super-imposed, aDVD player running a continuously auto-looping DVD slide show or video show, or acomputer playing an .mp4 file continuously. Source 5 shown is how to inject compositeanalog video and audio into the HDMI switch. This uses an inexpensive ($30), DCcontrolled Composite to HDMI converter.

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After purchasing various, miscellaneous HDMI components on the internet to build myHDMI Switch Matrix, I found that they are not all equal, contrary to the advertising. Ihave been burned on several purchases, that either didn't perform as advertised, orworked but were not reliable and were prone to "glitching". The HDMI components thatI did find to work well for me are listed below:

Fig. 2 HDMI 1x2 splitter (left) & 5 x 1 auto switch (right)

5 IN x 1 OUT HDMI SWITCH: Monoprice model 8203, $25https://www.amazon.com/Etekcity-Video-Audio-Converter-Adapter/dp/B00MNFWYBI 1 IN x 2 OUT HDMI SPLITTER: Swees model SP27695-1, $16https://www.amazon.com/Splitter-Amplifer-Support-Compatible-Blu-ray/dp/B01DY29REA

The major drawback to my switch design, compared to broadcast studio switches, is theconsiderable time delay in accomplishing the switching. I measured it to beapproximately 12 seconds. Acceptable for ham TV. Not acceptable for broadcast TV.

Fig. 3 W0BCR TV repeater HDMI Switch Matrix

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Fig. 4 HDMI Switch mounted in TV Repeater relay rack

HDMI SWITCH MATRIX: The actual HDMI Switch Matrix, Figs. 3 &4, which I designed and built for the Boulder, Colorado, W0BCR, TV repeater is a bitmore complex than shown in the block diagram of Fig. 1. It also included (3) HDMI tocomposite video converters, a composite to HDMI converter, and a Raspberry Pi micro-controller. The Raspberry Pi was used as the source for the Video ID. It played in acontinuously looping mode, an .mp4 video file. The video file is a slide show of about 30slides describing and showing the TV repeater, it's shack, tower, etc. Each slide had thecall letters, W0BCR, super-imposed on it for ID purposes. The idea of using aRaspberry Pi and the R-Pi program was supplied by Ken Rawlings, KV5Y, and DonNelson, N0YE. The detailed schematic diagram for the switch matrix is shown in Fig. 5on the next page.

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Fig. 5 Detailed schematic diagram of W0BCR-TVR, HDMI Switch Matrix