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What's New in Program Distribution?
About IP Multicast
Our prior two examples of network distribution used multiple unicast streams over IP networks. This is fine if you intend to serve a limited number of clients from one encoder; but what happens when you want to serve dozens, hundreds, or even thousands of clients from one source? The typical resources of a radio station, or network, would not support that, and clearly, it seems that there must be a better way. And, there is: It's known as IP Multicast.
IP unicast is a one-to-one connection between client and server (or audio encoder and decoder in this context). Recall that an IP "broadcast" is a message sent out on a LAN segment meant to be read by all hosts on that segment - like an ARP request. An IP multicast message is one meant to be read by some of the hosts on the network - those that have joined a multicast group. Each host that wants to decode messages of a particular group is told to listen for messages that are sent to a particular destination address by the originator. That destination address will be in the multicast range of IP addresses 184.108.40.206 through 220.127.116.11. The Internet Group Management Protocol (IGMP) runs between hosts and their immediately neighboring multicast routers; the mechanisms of the protocol allow a host to inform its local router that it wishes to receive transmissions addressed to a specific multicast group.
So, to make use of IP multicast to distribute network programming, it's clear that the IP network you intend to use must support multicast from end to end. You would work with your network provider to determine this. Now let's take a look at a couple of examples where this is being done.
CKHL (YL Country) and CKKX (KIX 106, hot AC format) each have multiple transmitters located at six separate sites near Peace River, AB (about 250 miles northwest of Edmonton). The end-to-end physical links are wireless, and the local ISP is Wispernet. (Wispernet is built on top of the wider facilities of Alberta Supernet, which you might find interesting in and of itself.) The handoff at each location is a Layer-2 switch providing a 100BaseT connection.
The encoder used for both the county and hot AC formats is the single rack unit SupriMax from Musicam USA. This is a modular system, and it can support up to four IP audio modules. The modules themselves are derived from the Suprima line; they can generate multicast (as in our current example) or unicast streams. The integral Ethernet port (per module) is used for both the stream output and management. Each module can be controlled via a Web browser, or the CCS management software (which allows for simultaneous control of multiple Musicam codecs). Each module has stereo analog input/output connectors by way of a DB15 connector; AES inputs/outputs by way of a DB9. The card comes with the following algorithms: PCM, G.722, Musicam MPEG1/2 Layer II/III, MPEG2/4 AAC LC, MPEG4 AAC LD, and Apt-x (Standard and Enhanced 16, 20 and 24 bits). The unit will pass RS-232 data up to 9600 baud, and includes four configurable GPIO inputs/outputs. It has a jitter buffer that will correct for up to 500ms of jitter in the IP connection; the frame size is configurable to give flexibility when adjusting the delay and bandwidth used; and it comes with a test tool that allows the user to check bandwidth, delay and jitter of the connection. This information can be used to adjust the streaming parameters and so on, to obtain the best quality in the real-time audio reception.
At the far ends, Musicam Suprima codecs are used to decode the multicast streams, delivering the program audio for subsequent transmission.
Scalability - a system's potential to grow in terms of the number of users, but without a change in its basic architecture - is very evident in our next and final example.
- continued on page 4
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