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AIR Studios - Technical - Technical Library - ISDN FAQ

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Codecs

DolbyFax
Audio quality similar to Dolby Digital (it uses AC2 encoding, a forefather of AC3)
Uses 4 ISDN channels (2 lines).
Supports timecode with additional ColinBroad IS-1 unit.

APT-X
Audio quality depends on the number of ISDN channels used.
2 Channels (1 Line) - 7KHz stereo or 15KHz mono
4 Channels (2 Lines) - 15KHz stereo
6 Channels (3 Lines) - 20KHz stereo
Supports timecode as standard.

Other codecs available on request.

ISDN Line Numbers
We prefer to do a connection test at least an hour before any session.

Line 1: +44 (0)20 7794 7390 (2 channels)
Line 2: +44 (0)20 7794 8017 (2 channels)
Line 3: +44 (0)20 7794 7828 (2 channels)
Line 4: +44 (0)20 7794 5722 (2 channels)

Please check before the session which lines you are attached to.

Good Practice

Do not add the signal we send you in with what you are sending us, e.g. if you are sending us your mix do not sum our signal into the mix as well. The result is a slap-back echo that really confuses the artists. This is easily avoided by feeding us a separate mix from the auxiliaries of your desk.

Do not have an open mic in the control room or the same slapback echo will result. Talking to us should be via a talkback button that turns on the mic and cuts the speakers. The separate aux mix mentioned above facilitates this. If this is not possible then monitoring has to be done via headphones at your end.

Terminology

CODEC
Each end of the ISDN session requires a matching system (enCOder-DECorder). APT-X or DolbyFax are the most popular for audio but the CDQ Prima is also used. There is a daily rental charge for the codec.

LINES & B-CHANNELS
Each ISDN line has two channels - known as B1 and B2. These are usually used as a pair and have the same phone number. Not to be confused with the number of channels of audio you need.

BANDWITH & QUALITY
Each B-channel has a guaranteed bandwidth of 64Kpbs. The codecs digitised the audio and apply compression. The less compression the better the audio but you need more bandwidth therefore more B-channels to cope.

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