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Microphone Month


Implementing an ISDN network for long-distance recording sessions.

Today’s Tech Tip question comes from David in Canada: “Thanks to Sweetwater for the really informative inSync™ articles… I have a question: How does a recording studio implement ISDN? What hardware/software is needed? I heard that more than one line is required to get 20Hz-20kHz bandwidth. Why is this so? How does one set-up a long-distance session? How do you compensate for delay? How do you retrieve files? What about a stereo pair? Is ‘cable’ (as in cable-tv) an alternative solution or does this not work in the same way? Thank-you!” Well that’s actually about eight questions, but they all have to do with one subject, so we’ll have a go at it. Several studios worldwide have installed Dual ISDN lines and special equipment to make ISDN calls and encode/decode the digital audio in (for all practical purposes) real time. It’s popular for voiceover talent, and also has been successfully used to record instrumentalists and vocalists. A notable example of its use was in the recording of Frank Sinatra’s Duets album. In practice, ISDN doesn’t work specifically like the web does… you make an ISDN “call” (literally dialing up another similarly-equipped machine at another location). An ISDN call requires a terminal adapter (TA) and a Dual ISDN line – “dual” in this case meaning twice the throughput of a single ISDN line, either of which is transmitted over the normal phone wiring. To establish a Dual ISDN link, two channels have to be dialed separately, which may have been what was confusing to you. On the TA these channels are referred to as ports. If you are making a stereo studio-to-studio link, you will dial up both ports. If you’re handling a mono narrow bandwidth radio broadcast commentary, you will only require one port. You can also synchronize over long distances if your TA has the means with which to transmit Time Code or receive Chase sync signals.A number of companies have developed audio- and broadcast-friendly TAs capable of using ISDN lines to relay high quality audio. Also, TAs are available that store ISDN numbers in resident memory and dial up both channels automatically (manual connection can be a complex process). These hardware codecs convert the analog audio signal into a compressed digital audio data format that can be transferred over an ISDN line to be decompressed at the receiving machine. Unfortunately, there is no common standard for this process and as a result there are at least six systems available, each using a different coding technique, and each one is incompatible with any of the others. Nevertheless, there are networking services that allow for incompatible codecs to work interchangeably if you purchase their services; one ISDN networking service purports to feasibly offer up to 10 simultaneous connections or 6 concurrent audio channels, incompatible codecs notwithstanding.Without a networking service, however, the limited transmission rate of Dual ISDN (128 kbits/s) is such that the highest fidelity for two simultaneous tracks of audio will differ according to the codec used. But the advantage of ISDN over current versions of seemingly faster connection technologies like Cable, DSL and FireWire networking (in the not-too-distant future) is that ISDN allows a more reliably constant, uninterrupted data throughput, which you would want if you were trying to conduct a professional recording session. It remains to be seen if the up-and-coming technologies will develop in a way that supplants ISDN in the near future, although it is known that any new [non-ISDN] method of conducting long-distance broadcast and recording won’t be able to interface with the established network of ISDN equipped studios and their current ISDN-based equipment. For more information about terminal adapters and ISDN audio networking, visit the Broadcast ISDN User Guide and Directory where you’ll find more ISDN information, and links to manufacturers and distributors of TAs.

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