Coded Transmitters

VEMCO coded transmitters use a single frequency, 69 kHz, coding scheme called R64K¹. By using a single frequency approach, VEMCO has been able to provide a family of low cost, long life submersible receivers and compatible long life tags. The single frequency approach allows all VEMCO receivers to detect all VEMCO tags enabling researchers to share equipment and data the world over.

VEMCO coded tags come in a variety of sizes, power outputs, battery life and sensor options. The following table provides a quick comparison of what's available to help you choose the most appropriate tags for your study. For more details on options available, click on the links in the table below to see the individual datasheets for each tag family.

R64K Coding
Coded tags emit a series of pings called a pulse train which contains ID and error checking information. This allows the user to individually track multiple fish. Currently, this coding scheme supports up to 192,000 unique IDs. The time between pulse trains is varied randomly about a nominal point and can be 10s of seconds to multiple minutes long. The off time, or Delay as it is called, is required to ensure that other transmitters have a chance to be detected by the receivers. The R64K pulse train contains eight pulses, making seven periods between pulses. After the Delay is completed, the cycle repeats. An example is shown below.

Collisions and Selecting the Delay
"Collisions" happen when two or more tags transmit all or part of their pulse train at the same time. When this happens, the pings overlap and neither transmission can be detected by the receiver. Although collisions are inevitable, tag Delay is randomized eliminate the possibility of any two tags continuously colliding with each other. Assuming the Delays are set long enough and the fish stay in range for a long enough period of time, randomization of Delay ensures that the receiver will eventually detect all the tags. Therefore the Delay choice is critical to the success of a study and should be chosen to optimize performance based on the maximum number of fish expected to be resident around the receiver at one time. We encourage users to choose the longest Delay time acceptable to their study as a longer Delay period will result in fewer collisions. As well, a longer Delay will provide substantially longer battery life for the tag allowing for longer study periods. In some cases, multiple years is possible. The table below shows expected time to detect multiple tags for varying numbers of tags present within the detection range of the receiver and selected Delay times.

Optional Dormancy Periods
VEMCO coded tags can be programmed with preset dormancy periods. During dormancy, the transmitter goes to sleep to conserve power and is scheduled to turn back on at a later date. This feature has enabled scientists to study the behaviour of young fish in rivers and estuaries and then continue to study the same fish several years later when they returned to the river to spawn. The tags are capable of being programmed with multiple on/off phases.

Options like dormancy periods are possible due to advanced processing capabilities existing in VEMCO coded tags. Other custom variations can be considered as well. Please contact the VEMCO sales team to discuss your specific needs.

Sensor Tags and S64K Coding
VEMCO coded tags are also available with sensor options. Our standard products are available with temperature and/or pressure sensors for most sized tags. VEMCO is also continually working with researchers to uncover other useful sensor technologies and integrating them into our transmitters. For information on current developments with environmental and physiological sensors please see our Experimental Transmitters page.

Sensor tags use a variant of the R64K coding scheme, called S64K. S64K combines the previous S256 coding with R64K coding to allow us to provide many thousands of uniquely coded sensor tags. With S64K coding, the transmitter emits a pulse train containing an ID code between 1 and 256 followed by 8 bits of sensor data. Then the transmitter emits a R64K pulse which contains a unique ID used to identify the tag. The researcher will use the two IDs to map the sensor data to a particular tag. See the FAQ on S64K coding for an explanation of how this works.

¹ R64K replaces previous coding schemes R256 and R04K. These became obsolete as they no longer provide a sufficient number of ID codes. See the article on Unique ID Codes in VEMCO's October 2006 Newsletter article for more information.

² Battery life will be lengthened or shortened depending on various options chosen.

³ Coded tag Delay is defined as the time between pulse trains emitted by the transmitter. This value varies randomly +/- 50% about the nominal value.

⁴ 100% of pingers present, 95 times out of 100.