General Information; Scope; Overview - JPS QMT-1B Installation And Operation Manual

1.1 Scope

This instruction manual provides the information necessary to install and operate the QMT-1B
IP Backhaul Remote. This manual assumes a basic understanding of the SNV-12 voter, analog
voter fundamentals, and the SVM-3 site voter module. See also the manual for the SNV-12 and
any manual addenda relative to the SVM-3 module.

1.2 Overview

The SNV-12 Voter has long been the leader in analog receiver voting. Prior to the introduction
of IP backhaul made possible by the QMT-1B working in concert with the SVM-3 modules, all
audio input into the SNV-12 was strictly synchronous, interfaced to SVM-2 modules. The
QMT-1B/SVM-3 pair allow voting receiver audio to be transported via non-synchronous IP.
To gain a good understanding of the system, it's worth considering some of the technical
roadblocks that had to be overcome to make it work:
• An analog voter can function properly only if audio from all unsquelched voting
receivers is time-synchronized when it reaches the voter. Unfortunately for this
function, IP data transfer is not concerned with timing.
• Synchronization is needed because analog voters chose the best site by comparing FM
noise levels within a standard audio spectrum – at the same frequencies that carry
speech. High frequencies within the speech content alter the noise measurement, but all
balances out if audio is synchronized.
• Also, the voter may need to be able to switch to a new voted site during a field
transmission, and it must be able to do so without the listener experiencing any echoes
or missed syllables as the voter transitions from one site to another.
• Most codecs do a poor job of accurately transferring FM noise, simply because they are
not optimized to do so. This means that an SNV-12 module would not be able to do as
accurate a measurement on digitized RX audio as it does on analog RX audio
The SVM-3/QMT-1B pair resolves these potential problems:
The QMT-1B digitizes the audio for transport to the SVM-3 module, but also performs a pair
of other functions – it performs a Signal Quality (FM Noise) measurement on the analog audio
before digitization and also adds a time stamp.
This information is sent as metadata along with the digital audio packets. The SVM-3, in
concert with the SNV-12 voter, returns the audio to analog, uses the timestamp to
resynchronize the audio, and compares Signal Quality measurements to determine (and vote)
the best site.

1 General Information