Telecoms, networking & broadcasting
Vaughan Taylor
Introducing FMeXtra by Vaughan Taylor, Prosound
It is estimated that there are over 100 000 radio stations ‘on-air’ around the world today. FM radio remains a commercially attractive means of delivering content to end listeners, albeit that it is a relatively ‘inefficient’ means of utilising valuable bandwidth, as one channel = one programme.
In some countries (particularly the USA), it the FMeXtra signal is an example of a multicarrier remains customary to make use of ‘subcarriers’ modulation (MCM). Other MCM signals may also - subsidiary communications authority (SCA) to be described as OFDM signals. Other familiar broadcast separate audio and/or data along MCM systems include IEEE 802.11g (WiFi), DVB-T, with the main FM stereo broadcast programme. and Eureka-147 (DAB). Typical applications have included the following Each constituent component signal in the (but not limited to); FMeXtra MCM signal spans a time interval of • Foreign language programming 1 µs, so that the baud rate of the FMeXtra signal • Bus despatching is 1 kHz. The actual number of data carriers depends upon the transmission mode. The • Radio reading services for the disabled guard interval is approximately 100 µs. • Stock market reports ‘Out-the-box’, FMeXtra includes support for • Utility load management Fig. 2: A spectrum analysis plot displaying the AAC, aacPlus (also known as HE AAC), AMR-WB, • Point-to-point and/or point-to-multipoint FMeXtra subcarrier. and AMR-WB+ audio codecs, but this may be messaging systems, and extended for other audio codecs as required. • Background ‘muzak’ channels for public The FMeXtra encoder implements a RTSP/RTP buildings. transmit additional information to its end listeners, (RFC 2326) streaming client so that pre-encoded such as programme identification (PI), programme SCAs make use of the available ‘spectrum’, audio streams, such as those generated by the type (PT) and traffic announcement (TA). which is located above the main and sub- Orban Opticodec and transported by RTSP- channel information, which is ‘frequency division This article introduces FMeXtra to the South African compliant servers such as Apple’s Quicktime, multiplexed’ (or FDM) into the audio baseband. An broadcast industry, as it offers a number of Darwin Streaming Servers and RealNetworks Helix illustration of the baseband is presented in Fig. 1. succinct advantages above its former analogue server may be directly propagated through the Traditionally, the analogue subcarriers were band- counterparts. FMeXtra system to receivers without transcoding. limited to 7,5kHz and multiplexed to lie in the FMeXtra also supports AAC/aacPlus streaming range 59,5kHz to 74,5kHz. Technical description according to the SHOUTcast and ICYcast In South Africa, the use of subcarriers has been The FM spectrum has remained under-utilised protocols. Support for the H.264 video codec is confined to the radio data system (or RDS) which for many years because digital communication planned in a later system release. was introduced in the early 1990s. Essentially the technology was not advanced enough A spectrum analysis plot displaying the FMeXtra RDS signal is injected at 57 kHz (with a mean to embed a useful number of bits into the subcarrier is shown in Fig. 2. deviation of ±2 kHz) and allows a radio station to available space. FMeXtra technology makes use of the most advanced communication FMeXtra receivers theory principles available today. The developers of the FMeXtra system, as Digital The FMeXtra signal itself is composed of many Radio Express have elected to design a specific other digital signals which are linearly combined receiving system, based around state-of-art to form the composite FMeXtra signal and then technology. The present Aruba model is offered inserted into the baseband spectrum, along as a tabletop model for home usage, although with the main programme. The signal therefore there are plans to evolve receivers for mobile includes: applications. • A reference signal FMeXtra applications • Carriers conveying system configuration and Fig. 1: Composition of the FM stereo baseband. FMeXtra can be safely used to replace the The left-hand illustration shows an equivalent data frame synchronisation information, and, applications that were formally suited to throughput of 64 kbps, while still allowing the station • A plurality of data-bearing carriers for analogue subcarrier services. A list of these to retain its stereo programming (including RDS). services. The diagram on the right shows that an equivalent was provided at the beginning of this article. throughput of 156kbps may be achieved should the The data carriers are all mutually orthogonal so station opt to transmit in mono. that they do not interfere with each other. Thus, Continued on page 68...
66 October 2008 - EngineerIT Telecoms, networking & broadcasting News
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The standard FMeXtra configuration permits two stereo channels (operating at 24 kbps, coded in AAC+) to be broadcast alongside the normal FM channel. It is also possible to broadcast four 12 kbps mono channels in the same channel occupancy. One of the greatest opportunities for FMeXtra remains that a low cost data network is realised by default. Whilst the platform allows a station to deliver additional content to a target market, a further opportunity exists in affording the delivery of data for display billboards and utility load management. Contact Vaughan Taylor, Prosound, Tel 011 675-7763, [email protected]
68 October 2008 - EngineerIT