Just What the Heck Is a Codec?

| September 11, 2013

By Thomas R. Ray, III CPBE, AMD, DRB
Tom Ray Consulting
Technical Editor

NEW YORK — Got a call recently from someone inquiring about IP codecs and looking for an explanation as to exactly what they are.  I was puzzled at first because to me, it’s an easy-to-understand topic.  But not everyone is as tech savvy as I am.

So first, let’s look at our old friend ISDN – you know, the thing that’s eventually going away (in New York at present, you cannot order an ISDN line from Verizon – they are no longer installing them).

We use the term ISDN to describe…an ISDN codec.  These are devices that have two main parts – the audio coding section and the transport section.

The audio coding section does two things.  First, it takes an audio input (your microphone), digitizes the audio, and turns it into a real time audio stream using a “codec” of your choosing.  Codec is short for “enCOder, DECoder.”  The codec is the method used to take the digitized audio and package it up to send to Western Walla Walla for that remote broadcast.

There are numerous “flavors” of codecs.  You’re probably familiar with MP3, MP2, G.722 and AAC.  These are codecs.  The codec looks at the audio and determines, based on a mathematical model of human hearing, what your ear will not be able to tell is missing – then gets rid of that little bit of audio.  Human ears are not high definition.  They can be tricked easily (which is why PPM data can be mixed into the audio and the listener does not hear it – but that’s another article).  This gets the size of the audio package down to something manageable that will pass through a narrow pathway.

So the encoder part of a codec pares the audio down to something that is easy to transport.  The decoder takes this digitized, packaged audio and recreates, as closely as it can, the original audio.  Most people cannot tell they are listening to data reduced audio and your remote hits the air.

After the codec does its thing, you need to take the data stream and send it to the transport section because, frankly, it won’t do you any good sitting in the device.  In the case of an ISDN codec, we are using an ISDN line from the phone company, which gives us two 64 kiloBit data paths, or 128 kiloBits total, to transport our digitized, packaged audio from point A to point B.  But that data rate is severely limited – you only have 128 kiloBits to play with.

Enter the IP codec.  What is an IP codec?  It’s an ISDN codec that does not use ISDN, but an IP service as the transport medium to get the audio data from point A to point B.  It’s that simple.  Everything I said above is valid for IP codecs, until the packaged audio gets to the transport section.  There, it travels over an IP line, most likely the public internet, to get your remote on the air.

The big advantage to IP codecs is that, with most internet services, you have much higher bit rates to play with.  My internet service at home gives me a consistent 20 MegaBits down and 4 Megabits up.  So I can put out more data, which will make the audio sound better.

And yes, sometimes dealing with the internet can be a scary thing.  That being said, the public internet is generally not the problem when there are issues with an IP codec.  The problem is generally the “last mile,” or between your location and the internet provider’s facility.  I’m finding these problems are more and more rare.

Look at the history of remote broadcasting – equalized lines, AT&T Long Lines, then devices like the Comrex 2 line frequency extender, then ISDN codecs, and now IP codecs.  All were scary transitions.  And sometimes there were wise guys like me who tried various things – like using a 2 line Comrex with 2 cell phones – successfully.

We can’t be afraid of the technology, because, frankly, technology will keep marching on.  If we intend to keep up remote broadcasting, we need to embrace and learn to use the new technology.  Sometimes, it takes a leap of faith.


Thomas R. Ray, III CPBE, AMD, DRB is president of Tom Ray Consulting and Technical Editor of TALKERS. He can be phoned at 845-418-5065 or emailed at tomray@tomrayconsulting.com.  His website is www.tomrayconsulting.com

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Category: Technical