Switched Digital Services

The use of switched digital services to connect from remote venues to the NOC in Washington, DC, has become widespread. These digital links have almost completely replaced analog audio lines as the best and cheapest means to deliver live programming to the NOC.

These links are available from the telephone company as either ISDN (Integrated Services Digital Network) or Switched-56 services. The two types will interoperate under certain circumstances, but ISDN is the better choice.

ISDN services are sold as either Basic Rate Interface (BRI)—which gives the customer up to two "bearer" ("B") channels of 64 kilobits per second (kbps) each for the transmission of customer data (audio), plus a "delta" ("D") 16 kbps channel for signaling that is used for accessing other circuits; or Primary Rate Interface (PRI)—which has 23 "B" channels at 64 kbps each and one "D" channel at 64 kbps. Most applications of ISDN for broadcast audio use BRI circuits. ISDN allows higher bit rates through the use of what are known as BONDING (named for the Bandwidth On Demand Interoperability Group) protocols that allow for the aggregation of multiple "B" channels to be used as a single, high-speed data link of 112 or 128 kilobits. Multiple ISDN lines can also be aggregated through a device known as an Inverse Multiplexer (or IMUX) to achieve higher audio fidelity at bit rates of 256 kbps (2 ISDN BRI's) or 384 kbps (3 ISDN BRI's). Compatibility of the IMUX units on each end of the circuit is a critical factor when combining multiple ISDN lines for higher bit rates-different IMUX units from different manufacturers may not operate together.

Switched-56 service uses a digital telephone circuit that operates at a "speed" of 56 kbps. This type of circuit is less flexible than ISDN and is pretty much limited to lower fidelity, monaural audio requirements. In some areas of the country, however, Switched-56 is the only digital service available. All of the uplinks that offer switched digital services are ISDN-capable.

Both ISDN and Switched-56 circuits permit dial-up access to the network via digital service units (a Terminal Adapter "TA" for ISDN or a CSU/DSU for Switched-56). Devices called codecs (for coder/decoder) are connected to the digital service units. New Codecs may have the TA built-in, so an "outboard" TA might not be needed. Codecs take an analog audio signal (or signals, for stereo), and digitally encode the audio into a data stream for transmission. These units employ data compression to reduce the amount of data that needs to be transmitted. This allows for high-quality audio transmission at low bit rates.

There are several different coding algorithms typically used, including:

G.722: This is an older algorithm that allows a monaural audio signal to be transmitted over a 56 or 64 kbps data channel, with a frequency response of up to 7.5 kHz. It is best used for speech.
ISO/MPEG Layer II (a form of which is known by the trademark MUSICAM): This algorithm allows for audio of up to 20 kHz bandwidth to be transmitted at bit rates of 56, 64, 112, 128, 192, 256, or 384 kbps. This audio can be monaural, "joint" stereo, or discrete (dual mono) stereo. Joint stereo is accomplished during the digital encoding process by removing redundant information that is found in a stereo analog audio signal. At higher bit rates, less data compression is required, and therefore the audio signal is more faithful to the original. Therefore, it is recommended that, when using Layer II encoding, mono audio should be encoded at 128 kbps or higher, joint stereo should be encoded at 192 kbps or higher, and discrete stereo should be encoded at 256 kbps or higher. The PRSS uses Layer II (MUSICAM) encoding at 128 kbps for mono and 256 kbps for discrete stereo. Joint stereo is not used.
ISO/MPEG Layer III: This algorithm also allows for high quality audio transmission. Because of more complex coding, Layer III is able to transmit a 20 kHz stereo signal, either joint or discrete, at either 112 or 128 kbps, or a mono signal with 20 kHz audio bandwidth at 56, 64, 112, or 128 kbps. This complex coding may result in some time delay (typically greater than 1/4 second).
APT-X: This system delivers 7.5 kHz audio bandwidth at 64 kilobits, 15 kHz mono audio bandwidth at 128 kbps, and 15 kHz discrete stereo audio quality at 256 kbps.

Please note: It is the producer's responsibility to deliver program audio to the NOC. This means that the link from the producer to the STC's codec is the producer's responsibility. Any failure of audio ahead of the NOC's codec equipment will not be cause for a makegood.