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Television station WSLS-10 in Roanoke Virginia recently presented an in-depth investigative report on the perils of digital trunked radio used by some public safety agencies. The report describes many of the issues related to using digital radio in critical, life threatening situations. Click the start arrow to watch the video.

The WSLS report indicates that digital technology is the best way to meet the FCC’s mandate for narrowband (12.5 KHz) operation prior to January 1, 2013. I disagree with this conclusion and recommend that public safety users opt for narrowband analog operation instead. Radios using 12.5 KHz narrowband do not need to be digital to operate well. Click here for more information.

Digital radios can work fine under some circumstances, however digital has let many users down in mission critical, life saving and life threatening situations. There are several reasons that digital fails for the user. One reason is the well known shortcoming of digital related to the choice of the vocoder. Vocoders are the circuits that convert human speech into the digital realm for transmission and reconvert the digital signal back into a voice on reception. The P25 vocoders can only deliver a rough representation of a human voice.

The vocoders in use today don’t have a way of truly replicating voice masked by noises, such as sirens, barking dogs, engine noise and loud music. The best they can do is attempt to filter out the non-speech sounds before transmission. This usually doesn’t lend itself to an understandable sound from the speaker when the transmitting user is next to a loud machine, in a moderate breeze or talking through a respirator.

Newer, improved vocoders are available but cannot be retrofitted into present radios. The present radios will need to reach their end-of-life before replacement brings the newer vocoder equipped units. Many of the existing radios are only part way through their life expectancy with some being less than one year old. It’s difficult to justify dumping only partially used $5000 units for new $5000 units. Albeit, the price of radios is coming down as long as you’re not stuck in a proprietary system from one of the major manufacturers.

Another caveat to digital is the way a signal propagates in, through, and around various terrain and physical surfaces. It’s commonplace for a receiver to hear the same signal coming from multiple directions at the same time. This is called “multipath.” When a signal bounces from an object, or several objects, it will take longer for that reflected signal to reach a receiver than the part of the signal that went directly to the receiver. When each of these signals arrive at a particular receiver, they can add together or subtract from each other in many ways. This results in distortion and sometimes less signal available for the vocoder to work with. The vocoder doesn’t know what to do with this type of interference and may not be able to recover the audio. The user may hear extremely garbled audio, or no audio at all. When analog signals are received with multipath interference, the human brain can usually “filter” the noise and recover a usable voice.

This is not to say all digital is bad. The technology is improving, however we may be handicapped by an attempt to prematurely set standards for digital technology. The P25 standard is not easily extensible, which limits the deployment of newer, improved digital technology because of the need to remain compatible with older equipment and equipment made by different companies. At this time, the best technology for interoperability is analog.

Note: This article was inspired by BobC who posted a very similar article on the WSLS website. I don’t know BobC, however it is quite evident that he has a comprehensive understanding of the issue.