A Centralised Public Alerting Solution for CBC/Radio-Canada’s Radio Networks

Over the course of the past year, public alerting has become an issue of high importance for Canadian broadcasters and for radio broadcasters in particular. Providing emergency alerts to the Canadian public is critical to protecting the public from natural disasters and other threats to human life. The National Alert Aggregation & Dissemination System (NAADS) has been operational for over three years but, for a variety of reasons, its alerts have not been broadcast by the vast majority of Canadian broadcasters.

As you may well know, the Canadian Radio-television & Telecommunications Commission (CRTC)[1] has imposed a condition of licence upon CBC/Radio-Canada requiring that emergency alerts from the NAADS be distributed on the Corporation's radio transmitters upon meeting certain criteria. Alerts required to be broadcast must:

  • Be relevant to an area served by one of CBC/Radio-Canada's transmitters;
  • Be designated for immediate broadcast by the alert originator;
  • Be in the language of the CBC/Radio-Canada service; and
  • Contain audio content.

For those not familiar with the NAADS, these last two criteria will significantly limit the number of alerts required to be broadcast. Environment Canada[2], far and away the biggest alert contributor to the NAADS, does not include audio content in its alerts. However, the implementation of Text-to-Speech (TTS) generators in alerting equipment enables broadcasters to generate an audible alert from NAADS alerts without audio content.

Additionally, some provincial and territorial Emergency Management Organisations (EMOs) are not issuing alerts in both official languages. Given that ignoring a tornado alert because it did not have an audio file attached and it was not in the language of the radio service would be extremely irresponsible on the part of CBC/Radio-Canada, the Corporation will broadcast alerts even if they do not meet the last two criteria.

NAADS BAckground

The NAADS is an emergency alert distribution system that allows Last Mile Distributors (LMDs), such as broadcasters and telecommunication companies, to monitor one source for emergency alerts for almost the entire country (Alberta has its own provincial system). The NAADS is managed by Pelmorex Communications, Inc., and its alert servers are located in Oakville, ON, and Montreal, QC. Emergency alerts used by the NAADS meet the Common Alerting Protocol – Canadian Profile (CAP-CP) standard, which is based on Extensible Markup Language (XML)[3].

The NAADS servers receive their alerts from authorised government agencies, such as provincial/territorial EMOs, Environment Canada, police forces, as well as other organisations given authorisation by the EMOs. In the context of public alerting, these authorised government agencies are referred to as alert originators or simply originators.

To provide an example of how the NAADS works, consider a hypothetical tornado in southern Manitoba. Given that the alert would be weather-related, Environment Canada would be the alert originator, prepare the alert with the appropriate information, and submit it to the NAADS servers.

After verification, the alert is made available to LMDs. The alert-decoding equipment will see that a new alert is available and compare the information contained within the alert to determine whether the alert should be relayed. An internal file known as a filter contains the criteria that must be met for an alert to be forwarded down the audio chain. If all of the filter criteria are met, the encoded audio in the alert is decoded and output from the alert decoder. If no audio is contained within the alert, the decoder will use the text found in the XML code, and use a TTS generator to create an audible alert. The output alert is then played on the LMD's transmitter(s).

Centralised/De-Centralised Implementation Comparison

Broadcasting all emergency alerts matching the aforementioned filter criteria will be carried out on the four CBC/Radio-Canada networks: Radio One, Radio Two, ICI Radio-Canada Première, and Espace Musique. The various audio streams associated with these networks are sent via a satellite distribution system to 719 AM and FM radio transmitters that are spread across the country. A de-centralised approach would involve the installation of alert-decoding equipment at the transmitter sites for each of these 719 transmitters, which would be a very costly proposition. Figure 1 is representative of a typical installation at a single transmitter site, using the de-centralised approach.

Figure 1 – CAP-CP De-centralised System Implementation Diagram

Currently, the 49 audio streams associated with Radio One and the fifteen audio streams associated with Radio Two are packaged and uplinked to satellite in Toronto, at the CBC/Radio-Canada Toronto Broadcast Centre (TBC). Likewise, the 45 audio streams for ICI Radio-Canada Première and the fourteen audio streams for Espace Musique are packaged and uplinked to satellite at Maison Radio-Canada (MRC) in Montreal. The centralised model involves the installation of alert-decoding equipment on the 64 English radio streams emanating from the TBC and on the 59 French radio streams emanating from MRC. The centralised approach would require the installation of 123 (64 plus 59) single alert decoders and associated equipment at these two locations. Since a manufacturer offers public alert-decoding boxes that can each handle four audio streams, the number of decoders required using a centralised model can be further reduced to 31 (123 divided by four).

The key advantage of the de-centralised approach over a centralised approach is that it offers superior alert targeting, since the equipment is installed at the transmitter site and affects only the coverage area of that specific transmitter. The disadvantages include an increase in overall cost (719 single decoders vs. 31 quad decoders), difficult and costly access to equipment due to the remote geographical locations of these sites, and a lack of Internet connectivity at most of these locations.

The centralised model offers an estimated sevenfold cost reduction, easy access to equipment, rapid restoration of service due to 24/7 staffing, seamless insertion of alert messages, IP connectivity, and audio logging as mandated by the CRTC. However, its major disadvantage is the lack of granularity when transmitting an alert message; i.e., a wider target area and, thus, an increase in the number of residents who will receive the emergency-alerting message. Figure 2 is representative of the installation at the TBC and at MRC using the centralised approach.

Figure 2 – CAP-CP Centralised System Implementation Diagram – CBC Radio

Figure 3 – Radio One Presentation Display

The insertion of an emergency message will occur upon receipt of a valid alert, matching the filter criteria previously uploaded into the alert decoder unit. Once detected, a dry contact closure activates the routing switcher, substituting the programming stream being uplinked to satellite with the received audio message or text-to-speech audio, if audio was not included with the alert. This action is immediately displayed on the screens located in the presentation centres in Montreal or Toronto along with an audible alarm for the operators on duty. Once the alert has been transmitted, the routing switcher returns to its normal configuration, substituting the alert audio with its programmed audio stream. Figure 3 shows the presentation display while an alert message is being inserted on the Sydney and Mulgrave Radio One feeds, as indicated by the blue bars above the audio meters.

Alert Filtering

At this point, as mentioned above, incoming alerts must meet criteria set by an internal filter file. However, that still leaves the issue of how the filters are to be set. There are several parameters to be set in the alert decoder filter; these include the affected locations, the urgency of an alert (i.e., immediately/future), the severity of an alert, the certainty (i.e., observed, likely, possible, etc.), and the alert event. Deciding which types of alerts to be broadcast is up to the LMD, and different settings will greatly affect the number of alerts being relayed by the alert decoder. CBC/Radio-Canada will only be relaying broadcast-intrusive alerts, those that are deemed to alert of an immediate threat to human life. There is a table of alerts that are eligible to be designated as broadcast-intrusive. These are the most serious alerts, such as those warning of fires, earthquakes, tornadoes, or amber alerts, amongst others.

Filtering for broadcast-intrusive alerts only simplifies the filtering process, as the only other filter setting left to be determined is the location setting. Location filtering is done by overlaying the desired alerting region (usually the signal coverage) over a map of Statistics Canada's Standard Geographical Classification (SGC)[4] divisions or subdivisions. These divisions and subdivisions are those used for the Census, and while the divisions are static, the subdivisions can change from year to year.

There is no one correct way of setting the location filtering; it is up to the LMD to decide the locations for which it will provide alerts. The LMD may decide to play alerts for only the areas that are covered by its official CRTC contours, or it may want to use its terrain-sensitive coverage to determine the locations for the decoder filter. It is also reasonable to include areas just outside of an LMD's coverage to alert listeners travelling in vehicles towards an event or disaster. CBC/Radio-Canada has decided to use its official CRTC contours as the basis for determining the locations to be used in its alert filters.

Figure 4 – CBC Radio One Feeds in Nova Scotia

As mentioned in the previous section, CBC/Radio-Canada will be using a centralised solution for inserting alerts onto its radio networks. Since multiple transmitters will be receiving the same feed, alerts for all transmitters using the same feed will be played on all of the transmitters, even if the given alert affects only one transmitter. Figure 4 below shows five different Radio One feeds in Nova Scotia mapped over the Census divisions. The map is colour-coded in such a way that the coverage contours of the transmitters broadcasting the same feed are in the same colour.

Figure 5 shows a close-up of the Radio One feed serving the Yarmouth, NS area. Since the zones with SGC codes 1201, 1202, 1203, 1204, and 1206 are either partially or completely intersected by the yellow contours, the filter for the Yarmouth Radio One feed must play alerts for all of these zones. Using this location filter setting will mean that, according to our official coverage contours, even though Chester, NS is not served by the Yarmouth Radio One feed, all broadcast intrusive alerts for Chester will be played on our Liverpool, Shelburne, and Yarmouth transmitters.

Figure 5 – Radio One Feed Serving Yarmouth, Nova Scotia

This over-alerting can be considered as a “quantisation error” of sorts. The over-alerting can be mitigated by filtering for Census subdivisions, but their quasi-dynamic nature would result in a considerable amount of work to re-map all of the filters for all feeds of all services when the subdivisions are updated. For this reason, CBC/Radio-Canada will use the less granular method of involving the Census divisions for its alerting filters. This lack of granularity is more problematic in areas where the Census divisions are large, or where a large geographical area is served by the same network feed, but the relatively low number of broadcast intrusive alerts means that over-alerting will not be excessive.


This article has highlighted the advantages and disadvantages of using a centralised public alerting solution, specifically for CBC/Radio-Canada's radio services. Aside from the obvious cost savings, having the alerting equipment in the studios in Toronto and Montreal allows both physical accessibility and network connectivity to the alerting equipment, enabling constant monitoring, and rapid troubleshooting should the need arise. In the opinion of CBC/Radio-Canada, these advantages outweigh the occasional over-alerting that might occur by implementing the alert decoding on a per-feed instead of a per-transmitter basis.


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