Solent Club for Amateur Radio & Television

Thursday, February 23, 2012

Last Updated on Friday, 27 May 2011 16:27 Written by M6PJD

A GUIDE TO HOW GB3IV WORKS – Part 1

Although GB3IV is constantly evolving, it is important to remember that despite this its default mode should always provide a basic ATV repeater which responds to any valid analogue or digital 23cm inputs without any need for DTMF etc (which is needed just to operate additional facilities or options) so access with simple gear will continue to be straightforward.

The heart of the repeater is a software system developed over several years by the late Dave Wheeler (G0GMK) to meet our specifications and at the time of his untimely death, many of the advanced features were being finalised (but they can never be implemented because some key commands etc sadly died with him). However Dave's software was so good that we devised a compromise set of "workarounds" so that we could continue to use the heart of the original system.

Modular construction is used wherever possible together with fail-safe techniques and a good example of these is that if the 25 way connector between the main and "BB" racks falls out, IV still works in default 23cm analogue mode!

This block diagram shows the assemblies in the Rx/Logic, Tx and Backup Modules (which are all in the Main Rack) followed by explanations of the intended functions and commands together with some tips.

Part 2 will follow and will explain the functions of the BB Rack.

(Please Click image to resize)

The 23cm Rx Aerial (Antenna to Americans and entomologists)! feeds a mast head GAsFET pre-amp driving a 7 pole inter-digital filter (which has 110dB out-of-band rejection), then via the Main/Backup relay to a Gali-39 pre-amp/splitter to the self-contained FM Rx plus the digital Rx in the BB Rack.

The 23FM Rx has a Comtech front end together with soft-squelched audio, envelope ALC providing a constant 1V p-p video output irrespective of signal characteristics, plus a Sync Detector. The latter provides the primary instruction to the G0GMK CPU and AV Switch to change from Beacon to Repeat Mode (but this can also come from the BB Rack).

Re Signal to Noise Ratio: As the FM Rx's output is constant, an apparently noisy signal (looking like weak RF) can be due to under-deviation (because your modulation isn't filling the Rx's 16MHz passband enough to keep the noise out), so try adjusting your deviation.

The AV Switch is solid state and controls video plus audio on a prioritised basis as follows: The slide show from the DPV has the lowest priority and this is overridden by the FM Rx when the Rpt/Bcn line goes low. The camera video can override these (if the CPU's requirements are met) and the highest priority is given to a request to select the "Ext" input, which is a looped-back signal from the BB Rack (and therefore the other Rxs).

The AV Switch also does the secondary functions of bypassing the TBC, selecting colour bars and muting (to deal with interference or possible misuse).

The CPU communicates with the graphics generator and GMK DTMF decoder via I2C and also has an internal clock to provide the Idents at 15 minute intervals. All camera plus Pan & Tilt commands come from this source but a couple of inbuilt anomalies should be mentioned:- The CPU switches the camera steering off when video is received, so this prevents remote control of the camera/dish on 23cms (which is why DTMF is on 70cms), an annoying side effect which happens occasionally is that if "the box" is opened by anything (e.g. a Dx carrier) whilst you are steering, you will drop out of Steering Mode! – (So send *0# to re-select).

You should also be aware that while the CPU is switched to "Single Digit Mode" (for camera/dish steering) it will not accept any other commands until you DTMF a "Zero" to restore normal operation; however the CPU resets the circuits that it controls when it sends the Station Ident every 15 minutes.

The Graphics Generator provides the captions and "S" Meter Bar (via an ADC plus software in the CPU). The "S" Meter progressively indicates analogue signals in 3 bar increments.

3 additional DTMF modules are fitted and module "A" handles User commands, "BB" selects signal routes plus aerial options and "C" handles the essential system commands such as Main/Backup selection and shutdowns. - (More detail is given under "Commands").

To check that your DTMF is working correctly: A single 'pip' is superimposed on the 23cm audio every time that a valid DTMF (dual) tone is received, so you can check that all of your buttons work by trying them in turn – if they don't, you probably need to adjust your deviation.

The DTMF Protection Timer had to be fitted because the DTMF-8 Modules are susceptible to a wrong digit during the first 4 seconds after switching on or after a Reset, the timer therefore isolates these for the first 6 seconds.

The Standby Timer sensed when the power was off and it could then trigger (if previously enabled by a "C" DTMF command) to switch IV back on for about one minute after the last valid signal. – BUT it has now been replaced by the new A2* function.

The Main Timer removes power from the transmitters and other units, saving about 75% of the potential power bill; it has also proved to be very useful during lift conditions - particularly because we share the output frequency with other South Coast repeaters. (G4KLB achieved some ATV Dx records due to this feature).

Slow regular pips are superimposed on the 23cm audio for 2 minutes prior to the timer switching to Standby, you may then DTMF A5* to extend the working hours – (then A5# when you have finished).

The Main Tx and Backup Rig units probably don't need explanations but please ask a member of the Tech Team if you have a query.

COMMANDS:

Module 0 - GMK CPU CAMERA/DISH COMMANDS (which reset to defaults at each Ident sequence): It is important that camera selection is carried out before selecting the (single digit) Steering Mode (see "CPU" above).

*6# - Selects camera video. And *7# cancels camera video)

*0# - Selects Steering Mode (Indicated by: "GB3IV - Live Camera"! - which requires single digits).

4 - Pan Left, 6 - Pan Right, 2 - Tilt Up, 8 - Tilt Down, 1 - Zoom In, 3 - Zoom Out.

0 - MUST be sent after use (to cancel Single Digit Mode and restore multi-digit operation).

Module A - USER DTMF COMMANDS (which reset at 15 minutes passed each hour or if DTMF A6 is received):

A tip to save you over-burdening your memory!: Any command which finishes with * is cancelled by replacing the * with a #

A1* - Transfers the Tx from the Solent 23cm PAL FM analogue exciter to the SR Systems MPEG2 23cm digital source, (and A1# restores back to the analogue default).

A2* - (As 23cm digi/analogue Rx selection is now automatic) A2* now disables 24/7 instant access by digital inputs on 23cm (and soon on 70cm as well) - and A2# will restore 24/7 digital access. – This facility may be needed on occasions to prevent Dx signals or QRM from interrupting existing traffic but hopefully it will encourage more Dx working.

(Unfortunately we found that our excellent range and sensitivity makes 24/7 analogue access impractical due to QRM but this facility is still available "out of hours" via the A5* command)

A3* - Bypasses the TBC (and A3# re-activates it). - The TBC can only re-process the video if inputs approach the correct characteristics (esp. the correct video/sync ratio of 7:3, which sets the AGC/Contrast level), therefore this bypassing facility can be useful for testing and also if a distorted signal is being repeated through IV.

A4* - Tells the TBC to produce colour bars (which are at the correct levels for test purposes) and A4# restores normal operation.

A5* - Overrides the Main Timer to provide continuous operation outside normal hours - and A5# restores normal operation.

A6 - Sends a Reset pulse to the "A" and "BB" modules so that all defaults are re-selected, (this duplicates the timer's xx:15 automatic reset) but can be a useful shortcut e.g. when you wish to restore analogue settings after a digital session. Resets can be recognised by 4 seconds of rapid pips on the 23cm audio.

A7 - Pan Left }

A8 - Pan Right} Are always available twin momentary commands, enabling you to pan the steerable dish whilst watching the results of your 10GHz or 70cm transmission on your look-through monitor (or via the stream if you allow for the delay).

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