Introduction 3
significantly better distortion and signal-to-noise performance.
4.Lower noise. Because they are digital, ISDN lines are not susceptible to induced noise. Analog lines are exposed to a variety of noise and impulse trouble-causers as they snake across town on poles and through your building. Hum is the main problem, given most lines’ proximity to pole transformers and AC power wires, but there is also impulse noise from motors, switches, and other sources. Digital lines convey the bits precisely and accurately from the network to your studio equipment without any perturbation – so the audio remains clean. Even when the caller is using an analog phone line, the digital connection on the studio side makes for noticeably lower noise and better overall quality.
5.Higher gain and reduced feedback during multi-line conferencing. When conferencing is required on analog circuits, hybrids are needed to separate the two audio paths in order to add gain in each direction. When the gain around the loop exceeds unity, the unpleasant result is feedback. Since the conference path in a studio on-air interface usually includes four
AGC functions, the hybrids must be sufficiently good to cover the additional gain that may be dynamically inserted by these AGCs. With digital telephone lines, the hybrid function is more effective – and more reliably so across a variety of calls. That means more gain can be inserted between calls before feedback becomes a problem.
6.Digital call setup and supervision. Analog lines use a strange mix of signaling to convey call status. Loop-current drop signals that a caller has disconnected and blasts of 90 Volts at 20 Hz mean someone wants you to answer. Should we be using a mechanism designed to bang a hammer against a metal bell to transmit network status information in the 21st Century?
ISDN uses a modern digital approach to controlling calls and conveying status information about them. ISDN call set-up times are often only a few 10s of milliseconds, enhancing production of a fast-paced show. Perhaps more importantly, when a caller disconnects while waiting on hold, the ISDN channel communicates this status change instantly. This contrasts with the usual 11-second delay on most analog lines. One of the most common complaints of talk hosts is that they go to a line where they expect a caller to be waiting, only to be met with a blaring, annoying dial tone. The chance of this happening with an ISDN line is nearly zero.
Another common error is the condition where a talent punches-up a line that looks free, but which actually is just about to begin ringing and connects to a surprised caller. This condi- tion, called glare, results from the delay in the ring signaling, which comes from the nature of the analog line’s ringing cadence. ISDN eliminates the ambiguous status period.
We don’t know why the condition of picking up a line which was about to ring and getting an incoming call instead of dial tone is called glare. Perhaps it’s because the looks air staff give engineers when this happens too often! If you find out, be sure to let us know.
7.Better Line Monitoring, Call Setup and Supervision. Since there is a full-time connection between the central office and the terminal on the D channel, it is possible to detect when a line is not working. On an analog line, one discovers a problem only from a failed attempt to use the line. With ISDN, you can quickly detect when a line is a not working.
Even when ISDN Basic Rate Interface (BRI) lines are not routinely available, ISDN Primary Rate (PRI) lines are. BRIs have 2 channels per line, while PRIs have either 23 or 30. PRIs are well entrenched for use with large corporate PBXs, so are likely to be around for a long time. There are devices on the market that convert from PRI Telco lines to BRI for internal use, so this is a way that the Nx12 could be used long into the future on ISDN.
