Adding TV-Link (Red -Eye) interface to KGuard CCTV DVR

Adding TV-Link (Red -Eye) interface to KGuard CCTV DVR

Postby NigelJ » Tue Aug 23, 2011 2:36 pm

Using the Red-Eye TV-Link to control the KGuard (SHA 108.v2) CCTV DRV.

The attached picture shows a home brew TV-Link (Red-Eye) decoder, which converts RC5 IR control signals to KGuard DVR IR remote control signals.
The following details the development of this interface unit.

I have connected my KGuard DVR video output to a Maplin video modulator, which injects the PAL modulated RF signal into the house TV distribution amplifier. This arrangement allows the CCTV images from the KGuard to be displayed on both the home PC network AND any of the non networked television receivers in the house.

The KGuard PC network software allows control of the CCTV pictures display format, selecting between octal, quad or single image. Unfortunately the only methods of selected the video output display format is via the DVR control panel or the DVR Infra Red remote control interface. This is a little inconvenient when viewing the CCTV images on one of the house television receivers which are not co-located with the KGuard DVR.

It was therefore necessary to find a method for remote controlling the DVR from any room in the house. Consideration was given to using a radio remote control, and this may yet be implemented using a secure ZIGBEE control interface, but for now it was decided to use the Red-Eye TV-Link interface which is designed to allow remote control of home satellite TV receiver boxes from secondary home TVs.

The TV-Link modules work by converting the local TV IR control signal into an RF signal that can be transmitted, via the TV coax aerial cable, back to a remote satellite receiver, which may be positioned in another room, maybe next to the the main home entertainment centre. Note, most TV aerial distribution amplifiers systems, such as “Loft Box", support TV-Link communication between TV ports and supply the necessary short circuit protected, 12 volt supply to each of the TV aerial ports.

While the TV-Link transmitter modules are readily available from many high street stores the same cannot be said of the associated receiver decoders, which are usually built into some satellite receivers, such as those made for Sky satellite reception. A reasonable amount of time was spent searching for TV-Link decoder circuits, with very little success. It is likely that someone reading this report, will write in to explain how such a decoder maybe produced using a simple single IC circuit, however without this advance knowledge it was necessary to designed the decoder from first principles. This involved analysing the TV-Link module output signal and the KGuard IR remote control signal and designing a circuit to convert between the two.

Since there is some concern that this project might infringe BSB, KGuard or TV-Link patents, the implementation details described in this posting have been limited.

The TV-Link module works by converting the 38KHz IR modulation pulses into an On Off Key modulated RF signal at 7.2MHz. For the satellite receiver application the OOK pulses are converted directly back to a digital signal that are decoded by the receiver box and used to control it.

In this application, the TV-Link is used to encode RC5 IR signals, which are then sent as an RF signal to the new Link Receiver. These signals are demodulated and then decoded by a PIC processor and then encoded in KGuard IR format. This translation process ensures the generated KGuard signals are perfectly formatted and limited to only safe commands which will not change the DVR configuration. RC5 was selected for the input IR format as control handsets are readily available for this format and can be chosen so as to not interact with other local IR controlled equipment, such as the local TV and video recorder. In the case of this project, a number surplus Hauppauge Win TV IR handsets were readily available from old computers. The KGuard commands so far implemented are; selection of command mode by pressing zero, picture format, i.e. octal, single image using the numbers 0 to 9 and full screen, and audio mute on/off.

The new Link receiver provides the option for IR signals to be regenerated without being decoded, allowing any handset protocol to be transferred transparently, however it has been discovered that the RF modulation/demodulation process degrades the timing of the regenerated signals, making them more likely to be rejected by the more discerning DVR or other connected appliance.

The circuit of the new Link receiver consisted of a three transistor RF front end, matched to the 75ohm aerial input impedance. **see note below. This is implemented with a tuned common gate, J310, JFET preamp, NPN RF amplifier and emitter follower buffer. The buffer output is connected to a SA602 double balanced mixer, which uses a crystal controlled local oscillator. The resulting low IF mixer output signal is filtered and passed to a high frequency quad op-amp. This amplifiers and rectifiers the RF signal. The detected signal is then low pass filtered, by a second order Sallen and Key filter, before being further amplified and passed to an OOK demodulator, all implemented in the same quad OP-amp package.

The demodulate signal is then passed to a MicroChip PIC processor, where the RC5 protocol is decoded and translated to KGuard command signals. These are modulated by the PIC, onto a 38kHz carrier, using the PIC internal PWM module, before being applied to an AC coupled P channel enhancement mode MOSFET IR transmitter driver. The new Link receiver has a local IR transmit LED on the front panel and a jack socket at the rear, into which an IR LED cable may be plugged, disconnecting the front panel IR LED in the process.
It should be noted that the PIC decode software has been designed to accommodate the timing degradation associated with the TV-Link RF modulation/demodulation process.

Two other visible light LEDs are mounted on the front panel. A red LED is connected to the output of the demodulator and indicates the presents of the OOK signal. A green LED is connected to the PIC and indicate the KGuard modulation pulse output. From these it is possible to determine if a remote IR signal is being received and if this is being translated into KGuard commands.

The circuit is designed to operate from the 12 volt CCTV camera supply PSU and the unit contains a non-switched, DC supply, pass through connector on the rear panel for this purpose.

The units internal electronics is protected, from a reversed supply voltage by a series diode, and from short circuits by a panel mounted, mA rated, cartridge fuse. The RF front end operates directly from the 12 volt supply. An internal voltage regulator supplies 5 volts to the mixer, op-amp and PIC circuits.

The units power toggle switch is the centre off type, switching high to select transparent IR pass through mode and low for KGuard translation mode.

Provision has been made for future expansion of the PIC interface, by providing both a serial data and I2C I/O port. The current KGuard software does not allow PTZ CCTV camera control from the IR handset, so the new Link serial port maybe used to implement an RS485 interface to provide the missing PTZ control when viewing CCTV on remote television receivers.

The TV-Link interface has the following limitations. The 7.2MHz RF signal it uses is in the very active 40m radio band. Reception of external signal on this band are thus a source of interference to the TV-Link control signal if the TV distribution system is not designed to reject these signals at the aerial amplifier.
The new Link interface has been designed to reject this interference within the limitations of component cost and circuit complexity, however, the current design could be further improved by replacing the RC IF network filter with a narrow bandpass ceramic filter, working at a higher IF. The current receiver has a poor IF image rejection performance, which degrades the channel signal to noise ratio. While 455kHz ceramic filters are ubiquitous, low cost, components, it is proving difficult to source the small number required to build the improved mark two version of this unit.
That said, the mark one version has so far worked very reliably, but has yet to be tested in RF lift conditions, where 40m band breakthrough might be a problem.
**(Note, the new Link receiver connects to the aerial cable using a standard TV aerial splitter.)

(Information update 11/1/2012.)

The stack of equipment shown on the left of the attached picture is, from top to bottom, as follows:

1) A TV video audio microwave receiver, used to connect to a mobile CCTV camera, linked into KGuard.
This receiver is remote on/off controlled by a radio switched power plug.
2) The new TV-Link receiver, as described above.
3) A Maplin TV PAL modulator, used to send CCTV pictures to all the TVs in the house.
4) A home made CCTV camera fuse and filter box, used to protect the camera PSU from attack on the
cameras or wiring.
5) A Quad Video processor, used as a back-up display to the KGuard DVR.
6) The Maplin KGuard SHA 108v2 DVR.
Just shown in the bottom right hand side of the picture, the IR LED link from the TV-Link Receiver to
the KGuard DVR.

I am happy to report that after six months of operation the TV-Link box has proved very reliable.
Despite the mark one IF filter limitations, the IR control function has always worked as required, this is due to the design of the OOK decoder, which is very good at rejecting break through interference signals.

It is still my intention to design a mark 2 version, with a narrow band ceramic filter in the IF, just as soon as I can source the necessary five or six pole ceramic filter, maybe from one of this year's armature radio rallies. It is my intention to publish the full circuit details of the mark 2 version.
RedEye Decoder
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