Conversion of an analog voting system to IP transport is a common mode of system upgrade being successfully done in many locations. Careful consideration must be given to several aspects of the change in the system configuration in order to achieve good results.
The first consideration is planning the network topology to have minimal bandwidth constrictions and packet delays. Ideally, packets moving across the network carrying audio will need to have the most logically direct path possible from port to port…ideally an isolated flat network. The network should be designed with enough bandwidth so that all lines of the IP-223s should be able to function simultaneously without restrictions.
It is necessary to configure network timing parameters so that all receiver audio traffic arrives at the voter with nearly the same propagation time, regardless of which receiver site it originates from…something on the order of plus or minus 10 ms. Some latency is acceptable, as long as it is virtually uniform, from all sites.
Obviously the audio level characteristics from each receiver site would also need to be matched as closely as possible. Analog audio levels need to be very consistent from site to site.
Example: GE Analog Voter system conversion to IP based audio:
A typical GE “valley voted” system uses a 1950 Hz status tone to determine when receive traffic is present, and that the receiver and transmission line are functional. The status tone is continuous when no signal is present. The voter cards also use the status tone for line level compensation to provide consistent audio levels regardless of which line is in use.
Status Tone Transmission
A continuous status tone cannot be used with the IP-223, as packets from receiver to receiver will become largely out of sync over time. Also the CODECs are designed for voice audio rather than a continuous tone and thus a tone will have distortion. One possible workaround is to generate the 1950 Hz locally, just ahead of the voter, and gate it with received audio from the IP-223. A circuit would have to be installed at the voter that generates the status tone the voter is looking for, and passes voice audio during receive activity. Such devices are available from third party vendors.
The second change needed to the voter system would be to re-configure it for “vote and hold”, rather than continuous voting during mobile PTTs. The first receiver voted locks in until the mobile PTT drops. This avoids problems that would occur with continuous voting, which can create echoes from audio streams not being in sync.
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