DARA ATV Web Pages
Jones Road ATV Repeater Link - Linking Dayton to Columbus with Live ATV!
Table of Contents
1. General Information
3. Springfield, Ohio Video Input
4. Audio Coordination
5. Link Repeater Operation
6. Hardware Design and Theory of Operation
7. Control Software Theory of Operation
8. Troubleshooting FAQs
The concept of linking the Dayton (DARA) and Columbus (ATCO) amateur fast-scan television repeaters has been the subject of many lunch meetings dating back to at least 2002. The road to realizing this dream has had it's fair share of pot-holes, but as of this writing, we have finally installed an operating link at the Jones Road site located just North of Interstate 70 near South Vienna. A map of the actual location is here. The purpose of these web pages is to document the installation and operation of the repeater, and to serve as a body of corporate knowledge for operators at both the Dayton and Columbus ends. A side benefit of these pages is the inspiration it might serve to others who seek to link ATV sites between remote QTHs.
The general problem of sharing video between Dayton and Columbus is one of distance. Even with diffraction effects taken into account, ATV QSOs directly between Dayton and Columbus are rare, and depend on good propagation effects. The separation between the DARA and ATCO ATV repeater sites exceeds 80 NM. A suitable mid-point repeater site was chosen at a fairle elevated site (approximately 1300 MSL) that lies approximately midway between the ATV repeaters. Experimentation had shown that reliable paths on the UHF ATV bands could be realized with approximately 80 foot antenna elevation at the link site. Of course, the task was simplified by the outstanding elevation of the ATCO ATV repeater (atop the State Government building downtown Columbus, approximately 600 AGL) and the 165 foot AGL tower situated on the DARA 1000 MSL ATV site at Beyer's Road south of Dayton. The repeater is approximately 1/4 mile southwest of the intersection of I-75 and I-675 (map). Back to Top
Due to band limitations imposed by the crowed 420-450 MHz band, UHF frequencies were selected to establish the 2-way links from the Jones Road site to the DARA and ATCO ATV repeater systems. The under-utilized 900-MHz amateur band was selected for 2-way transmission from DARA to Jones Road. Experimentation revealed high levels of fast-hopping spread-spectrum interference in the upper portion of the 902-928 MHz allocation. For this reason, an 8 MHz band centered on 910 MHz was selected to achieve the best interference immunity, yet stay clear of the weak-signal work down around 902 MHz. We found that 10 watts was not quite enough to overcome the spread-spectrum noise, which created brief but objectionable white flashes across the NTSC video picture. We noted that the interference lasted approximately 5-30 milliseconds, which, when compared to the 64 microsecond period of an NTSC scan-line, served to "white-out" many lines of video at once. DARA was able to find a pair of Ericsson 120 Watt UHF amplifiers designed for the Cell band service that operated at full output on the 910 MHz band. Suitable interference rejection was achieved at approximately 30-60 watts, depending on band conditions.
The 1200-1300 MHz band was selected for the link from ATCO to Jones road. This choice was driven mainly by the hardware currently in use at the ATCO site. ATCO ATV UHF operation on the 1200 MHz band is already well established, with input on 1280 MHz and output on 1250 MHz (both vertically polarized). Click here for more ATCO ATV Repeater Details. Thus, the Jones road site transmits video to ATCO on 1280 MHz and receives video from ATCO on 1250. Each signal occupies approximately 8 MHz after bandpass filtering. Back to Top
Springfield, Ohio Video Input
An additional feature of the central location of the Jones Road site is its proximity to the greater Springfield area. With this in mind, we have designed the ATV link repeater with the capability to receive video on 439.25 MHz (horizontal polarization, upper sideband VSB please, to avoid conflict with AMSAT operations) and transmit that video out to both ATCO and DARA repeaters simultaneously. Thus, if you live within 20-30 miles of the link site, you should make it into both repeater systems with minimal power. Please note that the link repeater does not have the capability to transmit on 439.25 MHz to the Springfield area. The higher power operation of the ATCO and DARA ATV repeaters offers better chance of reception at your QTH on any of several operating frequencies (DARA ATV repeater info is here, while ATCO ATV info is here). Back to Top
Audio coordination for the DARA ATV repeater is conducted on 2-meters simplex 144.34 MHz, vertical polarization, while 2-meter coordination for the ATCO site occurs on 147.45 MHz simplex vertical. The lack of a direct and reliable audio communication path between DARA and ATCO presents a challenge, and the audio subcarrier of the ATV channel provides a necessary solution. Although seldom used when operating within either ATV repeater area autonomously, the audio received on 144.34 or 147.45 MHz is received by the respective ATV repeater and retransmitted on the audio sub-carriers of the ATV frequencies. These sub-carriers are linked along with the video carriers at the Jones road site to allow clear voice communications between operators at either end of the link. Additionally, remote control of the Jones Road link may be achieved via DTMF on a yet to be determined control frequency. This control is established by way of a dedicated receiver located at the Jones Road site, fed by an omni vertical antenna at approximately 50 feet AGL on the tower. Back to Top
Link Repeater Operation
In most circumstances, the operation of the link repeater is automatic and unattended. Several additional features augment the operation of the repeater, and aid in testing the integrity of the link. These additional features are described in detail in the Theory of Operation Section below. A concise, printer-friendly listing of DTMF commands is located here.
In most circumstances, a user will transmit into his respective ATV repeater in Columbus or Dayton, or directly into the Jones Road site on 439.25 MHz VSB. If the user comes through the DARA ATV repeater, the link site receives video on 910 MHz FM and retransmits on 1280 MHz towards ATCO. The signal is then broadcast from the ATCO repeater to Columbus area users. The reverse occurs if a user keys the ATCO repeater: FM ATV video is transmitted to Jones Road on 1250 MHz, and then delivered to the DARA ATV repeater on 910 MHz, then to be re-broadcast via the DARA ATV repeater to Dayton Area users. It is important that ATV users have the capability to receive subcarrier audio from their local ATV repeater. It is unlikely that users will be able to achieve direct simplex 2-Meter coordination comm on either 144.34 or 147.45 MHz. Finally, users in the Springfield area are encouraged to enter the system by direct input to the Jones Road site on 439.25 MHz USB VSB. However, these users will need a separate antenna directed to either the ATCO or DARA ATV repeaters in order to receive video from the link users. Another option might be to attempt reception via sidelobes from the 910 MHz or 1280 MHz link antennas. However, these antennas have very good directivity and sidelobe levels are fairly low. Note also that either of these antennas will only provide "one side" of the link. That is, reception via 910 MHz sidelobe will allow viewing only of ATCO traffic, and vice versa. Back to Top
Hardware Design and Theory of Operation
Users are encouraged to read the hardware theory of operation, but a thorough understanding is not required for operation of the link. The link repeater is comprised of 2 transmitters, 3 receivers, a video switcher, a colorbar video generator, and a main CPU control module. All components are mounted in a single equipment rack within the Jones Road blockhouse. At any given time, only one of the three receivers is actively receiving a signal, and either one or two transmitters serve to repeat the received signal. The outputs of the three receivers are routed to the 8x2 matrix video switcher. Additionally, the colorbar generator and other external video signals (tower cameras, for example) are fed as inputs to the matrix switcher. The video switcher has the capability to switch, under computer control, up to 8 inputs into 2 outputs. Output 1 is fed to the DARA 910 MHz ATV transmitter, while output 2 is fed to the ATCO 1280 MHz transmitter. In normal operation, video received on the ATCO channel is piped to the DARA transmitter, and vice versa. However, in the case where the sync detector of the Springfield 439.25 MHz receiver is activated, the Springfield video input is routed to both the ATCO and DARA outputs simulaneously. Additionally, software control of the switcher and transmitters allows colorbars or other video inputs to be transmitted to either ATCO, DARA or both. For example, by bringing up the 910 MHz transmitter and commanding the switcher to enable an input which is connected to an optional tower-mounted camera, the tower-cam video may be broadcast to the Dayton repeater system.
The UHF 900 and 1200 MHz band FM ATV transceivers were constructed using modules produced by Comtech for the wireless security market. No pre or de-emphasis circuits are used between the repeaters. An example picture of one of the receiver modules are located here.
The audio circuit is significantly different than the video portion. No switching is performed between audio inputs. Instead, all of the receiver voice channel outputs are tied together in an audio buss arrangement. Receivers that do not detect an active video sync squelch their audio output. In this way, only the active receiver places audio on the buss. This audio is ported to both transmitters and transmitted when activated. Power to the FM receiver modules is disconnected on transmit of the respective transmit module by way of a switching relay in order to prevent audio feedback.
The CPU is a PIC Microchip Based 16F877 micro-controller programmed using a standard C compiler. Source code for the controller is located here. Operation of the software is described below. Back to Top
Control Software Theory of Operation
The following discussion is presented to clarify operation of the software used to program the controller installed at Jones Road. The controller is a PIC Microchip 16F877 based computer that allows great flexibility of operation. Users are encouraged to read the software theory of operation, but a thorough understanding is not required for operation of the link.
The basic function of the control program is to provide simplex video and audio repeat functionality between Dayton (DARA) and Columbus (ATCO). As an added function, the system allows receive only capability at the repeater site to collect ATV signals from the greater Springfield area. However, no transmit capability is included to the Springfield area. As of the creation of this code, the frequencies in use are:
1250 MHz: Transmit from Columbus to Jones Road
1280 MHz: Transmit from Jones Road to Columbus
910 MHz: Transmit from Dayton to Jones Road and Transmit from Jones Road to Dayton
439.25 MHz: Receive only at Jones Road from surrounding area
The Main loop of the code scans the sync enable lines of the three receive frequencies (1250 MHz, 910 MHz, 439.25 MHz) and turns on the opposing transmitter as long as a received signal remains present. There is a capability to disable sync lines, in the case that spurious signals are erroneously triggering the repeater. However, the sync enable circuits are designed only to become active after detecting valid NTSC signals, and not common RF interference. For example, if the sync detect from Columbus goes high on the 1250 MHz input, then the code will configure the switcher to monitor video input from Columbus, and output this video to the 910 MHz transmitter aimed at Dayton. The transmitter will remain active as long as the sync enable is active, or TIMEOUT (15 minutes), whichever occurs first. At this point, the transmitter will drop and begin the search for other active sync lines and begin the process again. At the end of each transmission, the link ID and colorbars are transmitted for a duration of 5 seconds (default). This appended ID can be disabled using keypad or DTMF A80 if desired.
There are a series of DTMF codes that can be transmitted that allow diagnostic transmissions or alternate video source selection. For example, the system may be temporarily set up to transmit the colorbar ID video that is connected to video input #4. This signal can be transmitted to Dayton, Columbus or both, depending on the configuration of the switcher.
The video 8x2 MUX switcher can be configured to switch any of the eight (8) video inputs to either of the two (2) output banks. The output of Bank 1 is fed to the 910 MHz Dayton transmitter, while the output of Bank 2 is fed to the 1280 MHz transmitter to Columbus.
The inputs are as follows:
Input # Video Source
1 Dayton (910 Receive)
2 Columbus (1250 Receive)
3 Springfield Area (439.25 Receive)
4 Colorbar / Video signal generator
5 Stationary Blockhouse Camera (Future Upgrade)
6 Tower Mounted Rotating Camera (Future Upgrade)
Note that audio is not switched. Both the voice audio channel is connected in a common buss arrangement. If a sync line for a particular channel is not active, its audio is squelched and does not contribute to the buss. Note that the audio channel is fed by received remote control audio from a separate receiver tuned to the control frequency. This allows control of the repeater remotely.
DTMF Character Decoding Routine:
All DTMF codes follow the same template. The first two characters must be "#" codes to "wake up" the repeater. The next character is alpha (A,B,C or D). The next two characters are numeric (0-9). The final termination character is a "*". If characters are entered in a sequence other than the above format, the code will emit an audible error on the voice buss and exit the DTMF capture sequence. As an example, the code: ##A41* is a valid code and will be processed by the program. However, #D34* will return an error tone since the first two characters need to both be "#" characters. Additionally, a five (5) second "shot clock" timer is incorporated. If the user begins to enter digits, he has up to 5 seconds between keystrokes to complete the entered code sequence. If this timer expires, the DTMF subroutine will exit with an audible error code on the audio buss. You must be able to receive transmitted audio via the voice subcarrier to hear these tones (i.e., an active transmitter must be "pointed your way"). Note that status of the sync detect, transmitter enable and video ID append can be queried by the A90 command, and heard via an active transmission channel.
Note: The local keypad mimics the functionality of DTMF tones.
DTMF Codes (Excluding initiator(s) and termination chars):
A00 Reset to defaults (power on reset)
A11 910 MHz Sync Enable (Dayton)
A10 910 MHz Sync Disable
A21 1250 MHz Sync Enable (Columbus)
A20 1250 MHz Sync Disable
A31 439.25 MHz Sync Enable (Springfield)
A30 439.25 MHz Sync Disable
A41 910 MHz Transmitter Enable
A40 910 MHz Transmitter Disable
A51 1280 MHz Transmitter Enable
A50 1280 MHz Transmitter Disable
A61 All Sync Detects Enabled
A60 All Sync Detects Disabled
A71 Both Transmitters Enabled
A70 Both Transmitters Disabled
A81 Append Colorbar/ID to end of transmissions
A80 No Colorbar/ID at end of transmission
A90 Query Sync Detect and Transmitter Status
A99 Break the link (disable all sync and TX)
Bxy Transmit colorbars to "x" for duration "y"
x: Dayton = 1, Columbus = 2, Both = 3
y: 0 = 30 sec, 1-9 = Minutes
Example: B28 -> Transmit bars to Columbus for 8 min
Note 1: software responds to A00 (CPU reset) from within this routine in case of lock-up
Note 2: software responds to B99 (routine EXIT) to end transmission earlier than timeout
Note 3: the active video input may be switched from the default (channel 4 - colorbars) to any of the valid inputs by using the switcher command outlined below.
Cxy Switcher x: Bank 1 (to DARA), y: Bank 2 (to ATCO)
Eg. C34: Bank 1 = Input #3 (Springfield->DARA) , Bank 2 = Input #4 (Colorbars->ATCO)
Dxx Serial Control Decoder (future upgrade)
xx is a two digit number ranging from 00 to 99 that is directly sent to the serial line output (DB9 connector). D99 is a special command that momentarily enables a control line that will perform a hard reset for a personal computer. The intent of this upgrade is to allow connection of a personal computer via standard 9600 baud serial communicaion cable. The remote commands can be programmed to allow a range of functions (weather stations, remote cameras, etc) to be controlled via DTMF from remote users.
Eg. D51 - sends the ASCII characters 51 followed by a carriage return and a line feed to the PC. D99 performs a hard reboot in case of CPU lockup on the PC.
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The following FAQs attempt to remedy all of the expected problems encountered by link users and administrators.
Q. How do I remotely control the Jones Road Link?
A. First of all, ask yourself "do I really need to control the link?" In most circumstances, the answer is no, since normal operation is automatic and unattended. If you have a valid need, you can send a link command by pressing # twice, then the alpha numeric code, (eg: B12) followed by the terminator character *. A common "non-administrator" type of command might be the immediate deactivation of a sync detect line, which might be inadvertently triggering due to interference.
Q. Why can't I contact the link repeater using DTMF on the established control frequency?
A. There are several causes that might cause the controller to appear non-responsive. First, be sure that you can make it into the link repeater with your antenna. If you have a directional beam, ensure that it is pointed at Jones Road, for example. It is possible that a power failure has occurred at Jones Road. If the backup battery connected to the DTMF control receiver has failed, it is probable that the receiver has recovered to its default frequency of 146.01. If required, contact should be attempted on this frequency (for example, to break the link in a required situation). Sending B10 (Dayton users) or B20 (Columbus users) will bring up the ID (colorbars) from the link repeater for 30 seconds to check link operation.
Q. The link doesn't seem to be working. What is going on?
A. Several failures may occur that might cause the entire link to appear dead. For example, the 910 MHz transmitter might fail, causing an apparent total link failure to DARA users. Thus, overall link operation should be confirmed with Columbus users. More likely, however, is that the link is simply "broken". The software is programmed to "break the link" (same as sending an A99 command) in cases where one of the inputs has stayed on for longer than the timeout period (currently 15 minutes). In this case, the software disables sync detect on that channel to prevent continuous keying of the entire repeater system. In such cases, the sync detects can be re-enabled by using the A11, A21, A31 or A61 commands. It is also possible that someone disabled one of the two transmitters with the A40 or A50 commands. These can be enabled using A41, A51 or A71.
Q. I can't hear audio from the current opposite end link user, what can I do?
A. First, ensure that you are equipped to receive the audio subcarrier of the ATV repeater you are working. If you are receiving on the UHF VSB channel, chances are good that you are using a down-converted TV set or similar, and all you need to do is turn up your volume. If you are using one of the UHF FM channels (for example DARAs 1258 MHz output), you need to ensure that you are properly equipped to decode the 6.0 MHz subcarrier. Note that you will not be able to hear the audio on your local 2-meter simplex system, since these systems are not audio repeaters.
Q. How do I query the status of the sync detect lines, along with the other status signals?
A. The A90 command executes the system status query routine. In order to hear the returned morse code characters (O = on, F = off), you must first establish an active communication path to your repeater. Turn up your audio volume, then send the command B10 (for DARA users) or B20 (for ATCO users). This will bring link colorbars up for 30 seconds. Then, key in A90. You should then hear the following sequence of morse code characters: xxx yy z, where xxx are the three sync detects, DARA, ATCO and Springfield. yy are the transmitters, DARA then ATCO. Finally, z is the video colorbar append. As an example, OOF OO F decodes to mean: all of the sync detects are enabled except for Springfield, both transmitters are enabled, and the 5 second colorbars are not appended to the end of each transmission.
Q. The repeater seems to be coming on repeatedly via the link video channel. What can I do?
A. It depends on weather you can isolate the source of the transmitter that is activating the link. If you suspect that interference on 439.25 MHz is triggering the Springfield input and activating both the ATCO and DARA systems repeatedly, it might make sense to disable the Springfield sync detect temporarily using A30. Of course, this will not allow Springfield area users to access the input, so this should be used with discretion. The same procedure may be used to isolate the other two offending channels. Note that the link can be broken (all sync detects and transmitters disabled) quickly by using A99. The A00 command is handy to then bring the system back to default (all sync detects and transmitters on, 5-second video colorbars appended).
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This page created by N1GX on 5 July 2005.
Last update: 12 July 2005 by N1GX