Matters Concerning Audio - A Practical Guide to Video and Audio Compression - page 161

7.4.8 Removing Redundancy to Simplify Coding

Reducing the complexity of the sound so that it compresses better is virtually impossible
to do in the analog world. This is something that modern codecs do automatically. Then
at the decoding stage they reintroduce some harmonic content based on information that
was coded in the bit stream.
The AAC Plus codec uses spectral band replication ( SBR ) to simplify the encoded sig-
nal and then recreate a very close approximation of the original sound artificially.
Other opportunities occur when there is a loud transient sound that masks all the
others. Selectively removing the fine detail when a transient occurs will reduce the work
that the encoder has to do.

7.5 Software Encoders Versus Hardware Encoders

Software encoders must be fast enough when processing audio data. Hardware encoders
are useful in situations that require low-latency encoding. That is important in video con-
ferencing, which requires zero latency.
If you have the time to carry out multiple passes, then hardware encoding does not
provide any significant gains since you are in a non real-time scenario.
Your choice of hardware or software codec is likely to be mandated more by the
video coding and what you need from that in terms of performance.

7.6 Source Audio Formats

Some of the issues you must deal with will relate to the setup of your studio. Routing ana-
log audio around the system is optimal if your mixing desk is analog. If you are building
a new and modern installation from scratch, it is very likely you will be totally digital with
analog only at the input and output of the system. Appendix K has more information on
the diversity of audio formats you will encounter.

7.6.1 Analog Audio

Analog audio is presented in a lot of different levels and formats. There are quite a few
well-known connector standards for plugging things together, with minor variants on
those providing some interesting interconnection problems.
Connecting digital equipment together is quite a bit simpler. It is a good idea to gather
a collection of adapters and cables for interconnecting different pieces of equipment.

7.6.2 Balanced and Unbalanced Audio

Balanced audio is used in studios and in professionally constructed sound systems. This is
designed to be very noise-immune because the cable carries two phase-opposed versions