Telos NX12 user manual System Signal Flow and Audio Processing, Send Audio Processing

detailed configuration & reference  71

5.11 System Signal Flow and Audio Processing

ISDN

Telco

Adaptive FIR

X	Fdbk
	Reduce

Send

AGC

	A/D
HP	AES
	SRC
Filter

Analog

Telco

Analog

Hybrid

-

Duck

Control

X	Receive
	AGC

DDEQ

Input Level

Output Level

SRC

LW

D/A

AES

100BT

Send Audio Processing

The Nx12’s send-to-caller audio processing consists of the following functions: ♦♦ Sample rate conversion

♦♦ High-pass filter ♦♦ Frequency shifter ♦♦ Send AGC/Limiter

Sample Rate Conversion

AES and Livewire sources are sample rate converted to the Nx12’s internal sample rate of 8kHz. All inputs are re-clocked to the Nx12’s internal clock (derived from the ISDN clock if at least one ISDN BRI is present).

High-Pass filter

A high pass filter with a 300Hz break frequency improves hybrid performance and enhances intelligibility by removing unnecessary low-frequencies from the input audio. This function is always enabled.

Frequency Shifter

A Telos innovation, the frequency shifter (pitch shifter) inserts a small, unnoticeable, shift in frequency to the send audio to prevent feedback buildup when the system is used with open speakers.

Send Automatic Gain Control / Limiter

This	AGC helps maintain consistent audio levels to the caller. This function is always enabled.
This	benefits from a carefully-crafted algorithm utilizing a feed-forward topology with near in-

audible artifacts below the limit threshold. At moderate levels it is ‘AGC-like’ while at high peak levels it is ‘limiter-like’. In addition to making levels more consistent to callers, this performs the protection limiting required to meet Telecom regulations.

Receive (Caller) Audio Processing

The Nx12’s receive audio processing consists of the following functions: ♦♦ High-pass “hum” filter