by Steve Church
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by Steve Church |
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For decades, radio studios were pretty much the same: Analog
consoles were connected to audio sources with good ol’ Belden 8451. You had
your basic FM/AM combo with two control rooms and a couple of production
studios. The biggest innovations were slide pots and punch blocks. How the world
has changed! Now most audio is likely to come from a computer and there may well
be 8 – or 80 – studios in one place. Mixing consoles at new installations
are likely to be digital, and will soon be universally so. The one stick-in-the-mud has been the connection to telephone
lines for call-in use. Few question the power of having real people’s voices
on the air, and dial-up telephone is the way to get this done. There are maybe a
billion phones in the world – and all of them can become an audio source with
a few button punches. The massive and ubiquitous telephone switching network as
an extension of your studio. Nearly every facility has some way to hook into the
phone network, almost all using the analog lines invented in A. G. Bell’s
time. Think about it… blasts of 100 VAC to signal an incoming call? How long
has it been since you listened to clanging metal to know you had a call? Yet,
that is probably the way your studio gear knows someone wants to talk. This despite the fact that the telephone network is nearly all
digital, with only the “last mile” connection from the Central Office being
analog. You may be familiar with the Telco’s digital possibilities
because of your use of MPEG codecs over ISDN. In that case, the network provides
a simple 64 kbit/sec end-to-end digital path. MPEG encoding is powerful, in
particular MP3 and the new MPEG AAC, so it’s possible to convey full-fidelity
stereo using ISDN’s two channels. ISDN works all over the world with standard
and familiar dialing procedure because the ubiquitous voice network is
inherently comprised of 64 kbit digital pipes. We are so used to using ISDN for codecs that we forget that this
is not what it was created for. The original idea was that a user would have a
voice call going on one channel while a computer data connection was on the
other channel. ISDN was designed primarily to hook into the standard voice
network and interwork with the enormous quantity of analog phones out there. In
addition to providing data service, ISDN was to offer a way for Telcos to give
users a variety of modern call processing capabilities. For example, you can
connect up to eight phones on a single line, with each having its own directory
number for incoming calls. For normal voice telephone calls, a coding method much simpler
than MPEG is used. Phone engineers invented it in the 60’s when processing
power was not so available and cheap as it is today. This is called instantaneous
companding and allows the natural 8-bit depth to represent approximately 13
bits dynamic range. The sampling rate is 8 kHz, with the usual top audio
frequency being 3.4 kHz. While not high-fidelity, this is digital audio
– indeed it was the first application for digital audio technology. 
Modern radio studios are digital, computer-assisted, and often
consolidated. Modern telecom connections are digital and often high capacity,
bearing little resemblance to the analog lines of old. Today’s studio
telephone interface gear must be ready to link these worlds, delivering audio
quality and control flexibility. This article outlines the available digital
telephone connection scenarios and describes requirements for a system that will
capably serve today’s multi-studio facilities.
Digital Voice Telephone Services
ISDN for Voice
Fig. 1: ISDN
interworks with analog POTS lines. The Telco CO provides the required
conversion. The studio hybrid separates the send/receive as usual, and provides
sample rate conversion and other functions.
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