Section 24: Connection Setup
T
ieline
T E C H N O L O G Y
Page 220
Of particular relevance to
T
ieline
codec users is the ability to encode at
lower sample frequencies. This enables mono and stereo audio to be
encoded using fewer bits and requiring less connection bandwidth.
24.9.2.
Artifacts and Cascading, what should I be aware of ?
Artifacts are the enemy of the codec broadcaster. Artifacts are to a certain
extent introduced by quantization in data compression and may be inaudible
at low levels. When audible they can sound ’ hissy’, ‘grainy’, ‘metallic’ or like
‘warbling’. Artifacts may become audible when caused by a variety of factors
such as:
Overly aggressive compression rates;
Packet errors or loss of packets from a bit stream;
Loss of sync (i.e. AES/EBU connections); or
Cascading the audio signal.
Cascading is when an audio signal is processed through more than one
codec. The effects of cascading through multiple codecs may evidenced by
increases in artifacts and their audibility. This is particularly noticeable with
‘lossy’ codecs, which reduce source audio data by nature.
The best way to avoid cascading artifacts is to choose the highest bitrates
possible for your codec connection. In addition, plan for your end-to-end
broadcast chain to include the least amount of signal compression and
cascading as possible.
To listen to an example of quantization noise, visit
http://en.wikipedia.org/wiki/Quantization_noise and download the audio file
available from this website.
24.9.3.
Which Algorithm is best for my Broadcast Situation?
In general, it is a question of what connection quality can be achieved
depending on the connection bit rate available. It may also be a question of
trying to be compatible with other equipment. In this section we will provide
you with some rule-of-thumb settings for how to configure your codec –
depending on the broadcast situation you are faced with. Here is your
connection checklist:
What is the connection bandwidth available on my connection?
How stable is the bit rate likely to be on this connection over time?
Will I be sending music, voice, data or a combination of these
elements?
Will I be broadcasting live and if so will I be requiring communications
with the studio?
The algorithm you connect with will be dependent on the program audio you
are sending and the connection bit-rate you are able to achieve. For example,
Music
is generally the preferred POTS algorithm setting if your program
content contains music.
Voice G3
is generally best to use for a POTS
connection if your connection bit-rate is 16,800 bps or lower.
