SECTION 2 |
16
AUDIO TRANSPORT VIA MPEG OR OPTIONAL APTX® ENHANCED
Transmit
Set the desired codec type, bitrate, and stereo/mono mode for this channel.
Use the Encoder Type dropdown to select the method of MPEG encoding you wish to use. Along with several
options for AAC, MP3 and MP2 encoding, you can also choose linear (uncompressed) audio or GPIO only. If the
optional aptX® Enhanced encoder is licensed, you will also see the options for Enhanced 16 and 24 bit encoding.
The
Protocol type
may be UDP, TCP or multicast There are tradeoffs in this choice that are only fully understood
with knowledge of this aspect of IP networks. Here is a simple overview: UDP is the usual choice because it offers
lower delay than TCP and is not impaired by TCP’s flow/rate control. However, with UDP, there is no network
recovery of lost packets as there is with TCP, so the codec’s concealment is used to reduce the audibility of these
events.
With AAC, up to around 10% random packet loss can be effectively concealed. This is one of the strengths of AAC.
Some other codecs also have concealment, but it will not be as effective.
Network links with guaranteed Quality of Service are a plus, but can lead to a false sense of security. The main
weakness of UDP is that it does not recover lost packets, whereas TCP does. QoS only reduces the variance of
network latency for the higher-priority traffic, thus allowing the use of less buffering at the receiving end. QoS does
not prevent packet loss in general.
DSCP Class of Service
refers to Differentiated Services Code Point, which sets the 8-bit Differentiated Services
Field (more commonly, the DS field) in the IP header for packet classification purposes. This is where you can
fine-tune the management of network traffic and determine the Quality of Service. Settings range from low-latency
for streaming media to best-effort for non critical services.
Unless you are very familiar with DSCP, we recommend you keep this set for
46 Expedited Forwarding.
The
Output Configuration
section allows you to specify the IP of the main output, as well as up to 3 more replicated
streams.
The
Enable SHOUTcast protocol
option is used for sending a SHOUTcast compliant stream to an external SHOUT-
cast server.
If you do elect to use the SHOUTcast stream, the next section is where you set it up. For more information, read
Chapter 6: Streaming.
GPIO
Each codec channel has a robust set of options for GPIO. They are described in Chapter 3: GPIO.
