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| Section 2
Which codec type should I use?
There are tradeoffs among those available in the iPort, with each having advantages and
disadvantages. That’s why we give you the choice. Here are some guidelines:
♦
AAC is the best all-round codec for bitrates of 96kbps and above (stereo). It has
excellent packet-loss concealment.
♦
AAC-HE (AAC+) should be used at rates under 96kbps. It has good audio quality
at 64kbps, and is pretty good even down to 48kbps. It also has good packet-loss
resilience, but not as good as AAC.
♦
AAC-HEv2 is the most efficient codec for stereo. It has a new “parametric stereo”
function that kicks-in at low bitrates. Rather than sending the left/right channels
discretely, it sends a core mono signal together with steering control. This makes
reasonable quality stereo possible down to 32kbps, and useful stereo even to 24kbps.
♦
AAC-LD has the lowest delay of the codecs, so is the choice when inter-activity is
important, such as for intercoms. It has about 30% less efficiency than AAC, which
means that for equal quality, you would need to use 30% higher bitrate. Its packet-
loss concealment is good, but not as good as AAC.
♦
MP3 (MPEG layer 3) is not as efficient as AAC and has the worst packet-loss
concealment. It is included mostly for compatibility with codecs and software players
that only support MP3.
Will you be including other codec types in future software releases?
Maybe. Please let us know your needs.
Where can I learn more about TCP and UDP?
Any good network engineering book would explain these in detail. One of our favorites is
Computer Networking by Kurose and Ross. There is a section in our Introduction to Livewire
that introduces networking concepts to audio engineers, including a discussion of TCP and
UDP. If a copy was not included with your iPort, you can download one from our website.
Indeed, the axia.com and telos.com sites have a number of papers and other resources that
could be useful to you.
I need to calculate the actual network bitrate. There will be packet overhead, right?
Yes. The network rate is higher than the codec rate owing to the headers for the IP packets
taking some additional bandwidth. MPEG streams are very efficient in this regard, however.
The overhead varies with the specific codec, but should be under 10%.
Will the iPort work over the public Internet?
That depends. There are no guarantees of any kind on most Internet connections. This is
certainly true when multiple ISPs are involved, since nobody can take full responsibility for
the entire link. If you are lucky, all could be well. When you choose AAC as your codec, the
iPort provides quite good packet-loss concealment up to 10% random loss. That’s pretty
good, and would probably allow many Internet links to work reasonably well. Higher buffer
time helps, of course - but at the expense of delay.
