Telos NX12 High-pass ‘hum’ Filter, Receive Automatic Gain Control, Noise Gate, Ducking System

72  Section 5

♦♦ Automatic Gain Control/Platform leveler ♦♦ Noise gate

♦♦ Dynamic equalization ♦♦ Sample rate conversion

High-pass ‘hum’ Filter

This filter removes hum and other unwanted low frequency noise from the caller audio. This filter has a break frequency of 100 Hz.

Receive Automatic Gain Control

This AGC serves to improve consistency of caller audio levels. This function is very important to effective hybrid performance as caller levels can vary by as much as 30 dB. The Nx12 offer a new approach to AGC to accommodate the fact that loud callers have become just as great a prob- lem as low-level callers. The AGC is reset for each caller to a neutral position. to most-rapidly accommodate both low and high-level callers. The AGC has a range of up to +/- 12 dB. The AGC is an advanced dB- linear feed-forward topology providing a consistent sound indepen- dent of drive level. Its ‘smart’ operation normalizes levels while retaining the natural dynamics of the caller’s voice. The default setting is FULL.

An important additional feature of this AGC is that it is cross-coupled to other sections of the Nx12 and can therefore reliably distinguish between caller audio and hybrid leakage. This allows a more aggressive gain control for bringing up low-level callers while still preserving excellent hybrid performance.

Noise Gate

Turning on the noise gate enables the built-in downward expander. The downward expander reduces low level line noise when no caller audio is present. And reduces low level leakage thereby improving hybrid performance. This function is cross-coupled with the AGCs and the ducking system.

Digital Dynamic Equalizer

Telephone audio frequency response varies widely as many different factors can effect it (we’ve measured the response on a number of calls and the results were revealing). Consequently, some form of receive equalization is desirable.

The Digital Dynamic EQ process used on the Telos is the most sophisticated equalizer avail- able in a broadcast telephone interface. All processing is performed in the digital domain. The Receive EQ selection controls the type of equalization applied to the receive telephone audio as follows:

♦♦ Off - The caller audio is passed without modification. ♦♦ Fixed - This is a simple manual equalizer mode.

♦♦ Adaptive (Digital Dynamic) - This is a three band dynamic equalizer. We’ve chosen frequency breakpoints, time constants, and other characteristics to optimize the tonal quality of varied telephone callers. The user-entered EQ values are used as ‘target levels’. This allows you to customize the caller spectral characteristics, but still get a consistent spectral characteristic from caller to caller.

Ducking System

The ducking function occurs primarily on the caller audio. This function is important for several reasons:

♦♦ Provides an “aesthetic” control over the caller that many programmers prefer. When the announcer speaks, the caller is ducked, or reduced in volume dynamically. This is particu-