BAS transition to digital
Jan 20, 2007 8:00 AM
One of the most overlooked aspects of broadcast operations is the actual technology required for the origination of content. Studio shows, remote feeds and telecine conversions are considered generic content sources. Yet, the actual transmission of this content either from the field or from the studio to the transmission site is sometimes taken for granted.
Collectively, these activities are categorized as Broadcast Auxiliary Services (BAS). The FCC defines BAS as an RF system used by a radio or TV station that is not part of its direct broadcast to listeners or viewers. These are channels not intended for consumer viewing, but rather to get signals to (or from) a transmitter or a remote OB vehicle.
Microwave frequencies in the 2GHz, 7GHz, 13GHz, 18GHz and 23GHz band are used in the U.S. These video channels are reserved in each DMA for BAS terrestrial microwave communications, with frequency coordination typically handled by the local SBE chapter.
BAS can be broken down into categories:
- Studio-Transmitter Link (STL)
- Transmitter-Studio Link (TSL)
- InterCity Relay (ICR)
- Remote Pickup Unit (RPU)
- Electronic Newsgathering (ENG)
Studio-transmitter links (STL) and transmitter-studio links (TSL) have been in use since the earliest days of broadcasting. With a two-way microwave link, a studio operator can monitor remote sites and conduct problem diagnosis. The ATSC has issued CS/TSG-696r1, "Candidate Standard: ATSC Automatic Transmitter Power Control Data Return Link Standard," which outlines key operating and control parameters for such links.
Intercity Relay (ICR) is used for relaying broadcast TV signals between two points, such as a main studio and an auxiliary studio.
A Remote Pickup Unit (RPU) is used to send program material from a remote location to the broadcast station or network.
Electronic Newsgathering (ENG) uses point-to-point terrestrial microwave signals in the 2GHz band to backhaul the remote feed to the studio. ENG is generally done using a specially modified truck or van. Terrestrial microwave vehicles usually have masts that can be extended up to 50ft to allow line-of-sight communications.
BAS goes digital
As with terrestrial program broadcasts, BAS systems benefit from digital technology. Perfect transmission until the digital cliff is hit ensures the pristine quality of remote content backhauled to the operation center and of program content sent to the transmitter site.
A digital BAS system consists of an MPEG-2 encoder, digital COFDM modulator and digital upconverter. VSB and QAM digital modulation is also often used, and some equipment vendors offer an MPEG-4 AVC compression option.
A COFDM signal works down to -94dBm as compared to -85dBm for analog threshold receiver levels. Even with a lower received signal, however, COFDM typically out performs analog for both signal capture and multipath rejection.
A perfect COFDM spectrum would be rectangular, but operation of amplifiers and mixers in nonlinear regions produces noise on the sides of the signal. This noise is called sideband regrowth. Regrowth levels can be 25dB-40dB, which can cause interference problems.
An in-depth discussion of digital microwave systems can be found at vendor Web sites:
"COFDM and Narrowband FM vs. 17MHz FM Technology"
Replacing Analog Links with Digital Links for STL or Intercity Relay
COFDM versus QAM and VSB in ENG/HD-ENG.
The transition to digital BAS also includes services in the 7GHz and 13GHz bands. Channel bandwidth has been reduced to 12MHz from 17MHz. The center frequencies have also been changed. Digital modulation will help stations comply with the new FCC requirements. This has prompted stations that have not moved to digital operation to do so when they switch their operating frequencies.
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