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41.   Codecs I Have Known, Part 1: APT-X   7/18/13 by Dave Immer

I have been using ISDN audio codecs since 1993 and IP codecs since 2001. In this series the term “codec” refers to both hardware and software as well as the underlying algorithm doing the actual coding. All legacy models are still in use today.

One model, the Tokyo, no longer made, or supported by Worldcast, is a rebranded Mayah Centauri featuring 4 ISDN S/T jacks for bit-rates up to 512kbs. It has wide compatibility with MPEG and APT-X systems. Although the front panel and menu system is a mediocre design, the free remote control Windows PC software that came with it is excellent, making this one of my favorite ISDN codecs.

The APT-X algorithm delivers the highest resolution waveform and, in my opinion, sounds the best. It does this at the expense of a higher bit-rate. Transmitting via ISDN, 6 calls (384kbs) are required to send/receive 20kHz Stereo in standard APT-X mode. Maintaining the 3 ISDN BRI circuits to do this is somewhat expensive. Whereas 384kbs over broadband internet, with software and hardware IP codecs, avoids that expense.

APT-X is a predictive coding algorithm, resulting in very low latency. Extremely low delay combined with its great audio quality makes the APT-X algorithm perhaps the best choice for IP audio codecs used for remote production where a conversation is required between both ends.