Benefits of ISDN
So much for the technology; what we really want to know is "What’s it going to do for me?" The answer is multifold:

• ISDN lines are inherently 4-wire, maintaining separation between send and receive audio signals and improving hybrid performance
• Better digital-analog conversion quality
• Lower noise
• Better, faster call set-up and supervision
• Higher gain and reduced feedback during conferencing
• Caller ID
• Line monitoring capability

ISDN Lines are Inherently "4-wire"
Analog lines use a single pair of wires (hence are a "2-wire circuit") for both signal directions, mixing the announcer and caller audio. This causes the famous "leakage" problem–where the announcer’s audio is present on the interface output, where we desire that there only be caller audio. The announcer audio, having been affected by the phone network is filtered and phase-shifted. When it combines with the original caller audio in the on-air mix, the result can be a significant distortion of the announcer’s voice.

A "4-wire" circuit has two wire pairs, and therefore two independent audio paths. Some telephone networks are actually implemented that way–the military’s AUTOVON being the major example in the USA. Digital circuits inherently offer independent and separated signal paths because it is not possible to have bits moving in both directions without separating them somehow. Though a digital circuit may today not use wires at all, but rather fiber, or microwave radio, or satellite, telephone engineers, bowing to tradition, continue to refer to all separated speech paths as being "4-wire."

While the application of DSP to the problem of separating the signals–used in digital hybrid interfaces–has made a dramatic improvement over analog systems, ISDN enables a yet further improved performance. That is because it offers a fully independent path for each speech direction. In the case where both ends of a connection are digital, there is no mixing whatsoever. In the call-in application, the far-end from the studio will still be 2-wire, so the audio paths will not be fully independent and we will still need a digital hybrid function to cancel residual leakage. Moving the studio side connection away from mixed analog can help tremendously because it is a much better starting point.

Better Digital-Analog Conversion Quality
The codec (Coder/Decoder) conversion chips used in telephone central offices are not very good compared to the converters commonly used in audio equipment. Fidelity is not an important consideration when designers choose parts for this function. And much of the problem is due to telephone standards: these codecs have 8-bit digital inputs and effectively only 13-bit analog outputs.

In a professional interface for studio application, we are able to afford to design-in much better converters than available in the phone company’s equipment. Noise-shaping functions permit a larger word-length converter to provide significantly better distortion and signal-to-noise performance.

In all-digital installations, the phone interface can maintain a digital path all the way. AES/EBU can be provided on the interface to accomplish the connection to the studio gear.

Lower Noise
Being digital circuits, ISDN lines are not susceptible to induced noise. Analog lines are exposed to a wide variety of noise and impulse trouble-causers as they move across town on poles and through your building. Hum is the main one, given the lines proximity to transformers and power lines, but there are also sources of 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.

Call Setup and Supervision are Better
Analog lines use a strange mix of signaling to convey call status. Loop current drop and returned dialtone signal that a far-end caller has disconnected; blasts of 100 volts at 20 Hz mean someone wants you to answer. Why should we be using a mechanism designed to bang a gong against a metal bell to transmit network status information in the 90s? ISDN uses a modern digital approach to controlling calls and conveying status information about them. The sophisticated transactions on the D channel are able to keep both ends of a call accurately informed about what is happening.

For starters, ISDN call set-up times are often a few 10’s 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 dialtone. The chance of this happening with an ISDN line is reduced to near zero.

Another common error is the condition where a talent goes to punch-up a line that looks free, but which actually is just about to begin ringing and connects to a surprised caller. This condition results from the delay in the ring signaling which comes from the nature of the analog line’s ringing cadence. This is much less likely with ISDN because the ambiguous status period is eliminated.

Higher Gain and Reduced Feedback During Multi-line Conferencing
When conferencing is required on 2-wire circuits, very good 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, there is the possibility of feedback "singing." Since the conference path usually includes four AGC functions, the hybrid must be sufficiently good to cover the additional gain that may be dynamically inserted.

Because of the 4-wire nature of ISDN, 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.

Line Monitoring
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 a not working. On an analog line, one discovers a problem only from a failed attempt to use the line.

• •