TELOS TWOX12 USER’S MANUAL
INTRODUCTION
CHAPTER 1 3
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CURIOSITY NOTE!
The word length is what determines dynamic range–and 8 bits would only permit 48
dB were it used in standard PCM linear fashion. A primitive kind of compression is
used to stretch the dynamic range: µLaw in North America and much of Asia, and A-
law in Europe. This is a scheme that equalizes the step- size in dB terms across the
dynamic range–a smaller step- size on low- level signals reduces quantization noise
and improves effective dynamic range to the equivalent of about 13 bits. The Telos
TWOx12 automatically uses the appropriate method depending upon the ISDN
protocol setting.
While the application of digital signal processing to the problem of separating host
and caller audio – pioneered by Telos and used in all of our telephone interfaces –
has made a dramatic improvement over pure analog systems, using digital phone
lines for incoming calls further improves performance for several reasons:
ISDN Lines are Inherently “4-wire”
Analog lines use a single pair of wires for both signal directions, mixing the send and
caller audio. This causes the “leakage” problem–where the announcer’s audio is
present on the hybrid output, where we desire that there only be caller audio. A “4-
wire” circuit has two wire pairs, and therefore two independent audio paths. 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, 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 yet further improved performance.
Better Digital-Analog Conversion Quality
The analog-digital conversion chips used in telephone central offices are poor
compared to the converters used in professional audio equipment. Fidelity is not an
important consideration when most Telco equipment designers choose parts for this
function. In a professional interface for studio applications such as the TWOx12, we
can afford to design-in much better converters than available in the Telephone
Company’s equipment. Noise-shaping functions permit a larger word-length
converter to provide significantly better distortion and signal-to-noise performance.
Since we are handling the conversion, we can encode at higher levels without
concern about regulations designed to prevent crosstalk on analog lines, resulting in
improved send-to-caller levels.
In all-digital installations, the telephone interface can maintain a digital path all the
way. AES/EBU is included in the hybrid to accomplish the connection to digital
studio gear.
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