A few weeks ago, a friend posed what seemed like a couple of simple questions in one of my technical mailing lists: What's up with subcarrier audio over satellite these days? What exactly does SCPC mean? How did we get to this state of the art? I was in Korea, sitting in a hotel room checking my mail after finishing up some work there, and figured I would take afew minutes to reply. The answers turned into a writing project that filled up the rest of my afternoon in Seoul. After a bit more thought, the answers turned into this brief history of satellite audio networking. I hope youfind it enjoyable.
Audio transmission over satellite began with analog aural subcarrier with video carriers -- TV programming needed to have sound, and conventional TV had long used an FM subcarrier to deliver it. People soon realized that one could add an extra subcarrier or two to a video channel, and radio networking began to re-emerge as a programming source after its near deathwith the advent of high-fidelity FM broadcasting in the early 1960's. (WhenFM began to become popular, the traditional networks, still transmitting over telephone lines, didn't sound good enough for the new FM listeners,and were relegated to short newscasts and sports programming.) With satellite subcarriers, one could get FM's 15 kHz quality, and music programming began to reappear at the network level. The first of these new radio networks is still one of the biggest - the Satellite Music Network, founded by JohnTyler and now a division of ABC Radio.
It wasn't too long before entrepreneurs began to put up satellite signalst hat contained no video at all - just audio (and quickly, FSK data) subcarriers riding a top full-transponder FM carriers. Named "FM Squared"by its champion, United Video / SpaceCom Systems, and commonly called FM/FM by other service providers, this type of analog audio transmission was very efficient in its day. Programmers could backhaul their programming to a central satellite transmission hub, where many programs would be combinedonto one FM carrier. Receivers were easy to manufacture, and relatively inexpensive, since acquisition of the powerful analog FM carrier required a simple design and manufacturing process. Since the cost of the full transponderwas shared by many users, the service was perceived as relatively inexpensive, especially when a very large base of receivers was being served.
Shortly after this type of service was inaugurated, the USA's National Public Radio Service developed a different type of analog audio satellite network. NPR's requirement that programming be available from a varietyof locations inspired the deployment of analog SCPC, or Single Channel Per Carrier, technology. Analog SCPC enabled NPR to deploy many smaller, relatively inexpensive satellite uplinks around the country. Each SCPCFM carrier could transmit a single channel of audio in a relatively small amount of satellite bandwidth -- rather than requiring a full or nearly-fulltransponder -- which enabled many uplink locations to share a single satellite resource without interfering with each other's transmissions. FM SCPC delivered up to 15 kHz audio fidelity per carrier, with two carriers required for stereo audio transmission. Signal to noise performance was acceptable with companding techniques and maintenance of high carrier to noise signal levels. Partly because of the high C/N requirements, C band satellites were chosen for analog SCPC networks because of their inherently low rain-fade susceptibility.
In the early 1980's, digital SCPC satellite audio transmission became popular with the adoption of Scientific Atlanta's DATS technology by several major commercial American radio networks. DATS utilized a " straight" PCM analog to digital format, and could transmit 15 kHz audio channels at T1, or 1.544 mbps, data rates. These large digital carriers utilized a new form of modulation in the private satellite industry, BPSK, or Bi-polarPhase Shift Keying, to transmit in a true digital format the ones and zeros that contained the audio information. DATS still used the lower-frequency C band satellite frequency band, but due to the digital format, had much greater audio signal to noise performance than analog FM/FM or analog SCPC services. In addition, low speed data services could be easily multiplexed into the audio data stream. The DATS service still required a large sharedhub, and therefore programming had to be delivered to the hub before it was uplinked to the satellite network affiliates.
During this time, a small southern California company, now known as Titan Linkabit, began developing a revolutionary new satellite transmission system -- the digital PSK (phase shift keying) Ku band VSAT, or Very Small Aperture Terminal. Led by Dr. Andrew Viterbi, Linkabit envisioned a new, all digital, Ku-Band satellite network architecture that would permit large businesses to establish low-cost low to medium speed data communications enterprise networks linking thousands of locations across the country. The original Linkabit VSAT network concept survived and grew large, from about 150 digital VSAT's in 1986 to over 150,000 installed worldwide only 10 years later. These TDM/TDMA VSAT networks are now manufactured by many leading space and microwave technology companies.
A small Colorado startup company in 1988, the National Supervisory Network Ltd, adopted this then-new Ku band VSAT technology to implement a broadcast radio transmitter data acquisition system. At the same time, NSN engineers began experimenting with the transmission of ADPCM compressed digital audio over their early low speed BPSK digital VSAT network. Non-real-time results were acceptable, but a real-time method of transmitting CD quality audio over the VSAT's relatively low data rates was not available at that time.
At about the same time NSN was experimenting with digital audio over VSAT, several former Linkabit engineers who left the company after mergers with new owners had founded a new manufacturing company in San Diego. These alumni of the original Linkabit called their new firm the ComStream Corporation,and quickly became known for their high-quality Ku-band digital PSK satellite modems and small VSAT earth stations. In 1989 / 1990, ComStream introduced the world's first Ku band real-time CD quality digital satellite audio system, using a new form of psycho acoustic digital audio bit rate reduction named apt-X. Apt-X delivered CD quality stereo audio within a 256 kbps data stream -- one quarter of the old DATS rate of 1.544 mbps. The driving customers behind ComStream's apt-X development were ABC Radio Network sand Gannett Broadcasting, both of whom needed a small, inexpensive meansto originate satellite programming from multiple locales.
By 1992, ComStream had abandoned the apt-X algorithm, and adopted anewer MPEG digital audio compression standard. MPEG provided the same stereo audio fidelity of apt-X in a 128 kbps data stream, using a newer QPSK (quadrature phase shift keying) modulation and sequential error correction format. The National Supervisory Network, which had recently adopted the new name NSN Network Services, and which had been a ComStream distributor since 1991, installed the first MPEG digital audio network in the USA in early 1993.
The impact of MPEG Ku VSAT technology on the radio broadcast industry has been nothing short of revolutionary. This new MPEG QPSK digital transmission required only 200 kHz of satellite bandwidth, at minimal power levels --typically under 18 dB/W Ku band satellite downlink EIRP -- to deliver 20kHz CD stereo audio, 9600 bps ancillary data, digital relay control signal sand even over-the-air satellite network management signals with 99.9% network availability. Satellite networks at this power and bandwidth could be operated at unheard-of low costs -- under $1,500 per month for nationwide coverage.(Older FM/FM analog costs averaged some $30,000.00 per month for lower quality service!) Uplinks could be located anywhere, using antennas as small as 1 meter in diameter, and licensing was greatly simplified since the Ku band required no prolonged terrestrial interference studies. No central shared hub or backhaul was needed.
Within 3 years, NSN and ComStream had sold and installed over 300 MPEG digital VSAT audio networks, with thousands of low-cost integrated receiver/MPEG decoders worldwide. Most of these new mini and micro networks are usedby broadcasters who previously would not have been able to afford networking at all, yet today originate daily programming and production from various sites across the country and around the world with ease.
In 1995, National Public Radio opted to abandon its obsolete analog SCPC system and adopt Digital SCPC MPEG transmission on its C band satellite transponders. At about the same time, most commercial radio networks previously using analog FM/FM or DATS shared hub services began to update their services to new digital MPEG PSK subcarriers, or to partial or full transponder shared MCPC (multi-channel per carrier) services. Other satellite manufacturers,such as Wegener and International Data casting, began to deliver QPSK digital audio product. ABC Radio and Scientific Atlanta adopted a proprietary digital audio compression format, termed SEDAT, to increase space segment availability and lower transmission costs.
At the close of 1996, the satellite term SCPC no longer means " single channel per carrier " in the old audio or data sense. Today's SCPC audio systems do contain one digital data stream, but this digital streamtypically contains two audio channels, coding and decoding information, ancillary and control data, and network identification data. So why call it SCPC? Habit -- and the fact that in most cases, the data content all belongs to a single format or customer.
" Digital FM Squared" is a real misnomer, since the former FM subcarriers are now PSK digital subcarrier signals riding atop a conventional FM main carrier.
Digital MCPC, used by background and commercial-free subscription music programmers like DMX, multiplexes over 100 CD quality audio channels into a single 30 mbps aggregate full transponder carrier. With the full-transponder digital modulation format, very inexpensive receivers are possible to manufacture, offsetting the high satellite transponder time charges over very largereceiver and subscription bases.
Narrowband digital MCPC is also becoming popular, since less than full-transponder segment and power may be used, resulting in lower hub operations costs, while keeping receiver costs somewhat less than narrowband SCPC prices.
Today's technology offers a good solution for almost every network application. Smaller networks (under 500 sites) choose SCPC digital technology withits low uplink earth station installation and operation costs and very low recurring spacetime charges. Slightly higher receiver costs are morethan offset by savings on recurring satellite spacetime access charges,and the overall quality of the network technology is known to be reliable and stable.
Very large commercial networks and background music services now typicallyopt to use digital subcarrier and true multiplexed MCPC shared hub services, since the receivers used by these formats are less expensive than SCPC digital format units and therefore easier to deploy in great quantities.The higher satellite access and hub service costs associated with the shared hub formats are easily absorbed across these customers large user base.
Original analog subcarrier audio transmission is still very popular, particularly with broadcasters who want to reach the millions of individual listeners still using older TVRO backyard satellite receivers. Analog SCPC technology is fading away fast, since it cannot compete with the newer digital services in cost, quality, or programming security, and isn't compatible with the "backyard" TVRO receiver market in most cases.
NSN's industry analysts think that the majority of the domestic USA broadcast network marketplace has now been retrofitted with digital technology,and with the exception of some as yet unexploited broadcast capability on high-powered DBS television satellites, there will be little movement within this market over the next five years, when compared to the previous five years.
Internationally, digital SCPC services and narrowband MCPC sales are expected to continue to increase on par with the deregulation and privatization of broadcasting worldwide. Full-transponder MCPC will probably not grow as rapidly as narrowband digital MCPC and SCPC, due to the higher space time access cost of international satellites and more fragmented markets served.International satellites tend to cover many countries with differing cultures, languages and customs, all of which combine to limit the total installed base across which these higher costs may be absorbed.
©1996 Bill Sepmeier All Rights Reserved