Multichannel Webcasting
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Multichannel broadcasting is not very common in the internet and one should have patience to find anything at all. It is mainly because it requires wider bandwidth and most systems nowadays are optimised for 2 channel stereo with some exceptions on HDTV. At this point it is still regarded as technically challenging. The movie theatres and home cinema rely on mainly 2 systems – Dolby Digital and DTS. Or at least on one of their versions. Technology is constantly developing and there are multiple similar file types used in different fields of audio playback.Multichannel webcasting can be done with formats supporting multichannel audio, such as Ogg Vorbis, MPEG AAC, AAC Plus, Neural Surround or Windows Media. All these formats are lossy.
So how does it work?
We can divide the stream into 4 parts:
- Input signal – DVD Player, Software player
- Encoder – encoding the input signal
- Streaming Server – takes encoded signal and distributes to clients
- Output signal – Software player or hardware outputs
The chain is the same for a regular stereo broadcast.
Let’s see who, where and how have used the internet to broadcast spatial heaven.
Success story 1
As part of CESNET research program Miloš Wimmer experimented with surround sound broadcasting using Ogg Vorbis- compressed audio using VLC as a server. So lets go through what they did and how it works.
They have previously designed and implemented 2-channel internet streams using Ogg Vorbis compressed audio for Chech Radio stations to provide for high quality Internet broadcasting. With home cinema systems becoming more and more popular + more and more multichannel recordings become available – their study dwelled into the unknown world of multichannel audio streaming. Like said before – the whole process is divided into 4 parts. So lets brake it down below.
The Input Source – None of regular input signal sources can be used for multichannel streaming as they only support stereo. One would need to use a dedicated audio interface if they require to get the data into the computer. As they didn’t need to acquire the input from an external source they used an audio file on the computer as you would on a regular stereo playlist-based internet radio station. As they prefer open formats they converted the originally dts encoded file into an Ogg Vorbis.
The Producer’s part is taken over by the VLC application reading the input file in an infinite loop and forwarding the data as a stream to VLC’s output module used for actual broadcasting, ie. delivering the stream to individual Clients. It is important to note that they had to use a playlist and not just a single file to stream the file as otherwise an internal command would cause the server to stop and restart between playbacks. Basically disconnecting anyone trying to listen to the stream. The streaming servers part was taken by VLC’s streaming module. A streaming server could be used if it ran the same instance of VLC receiving the stream and distributing it among the clients. Because theirs was just a simple test – the need for that wasn’t there so they didn’t have to have a dedicated server and used just the VLC app itself.
Not a lot of clients or in plain english – audio players support multichannel audio. Among the audio players available to them at the time it was only VLC that supported Ogg Vorbis encoded multichannel audio. The whole project worked as they planned! They achieved – high quality and the stability of the production system!
Success story 2
Prix Europa opening concert in 2006 This is the first time an event was broadcast live in 5.1 over the internet. Prix Europa has a number of technical firsts in the 5.1 field and in time they will get to our pages.
They turned to P2P solutions, as it has a proven to work over and over again. Peer-to-Peer is being used to download music, films, video clips and our own beloved youtube is a good example of success story. It has also been used for live streaming. Eurovision Song Contest was webcast over the internet in May 2006 with some 70’000 unique users. They’ve used Octoshape – an instance of P2P technology. The system is reliable and flexible for their purpose. The other component in use is the AAC Plus codec thats is promoted by Coding Technologies. Being part of standardised MPEG-4 system – it is used in both radio and television sound. As covered in previous articles – it is in use in XM Satellite Radio, HD Radio in the USA, in DRM and DMB in Korea and being considered for mobile communications applications.
Their Input signal originated at Radio-Berlin Brandenburg. It was encoded into APTX format ( it is a WorldNet Oslo encoder/decoder. Being modular, multiple channel audioplexer it was designed to transport high-quality content over various networks. It is used by many EBU broadcasters to provide an audio link between the studio and transmitting sites). The signal was then sent via E1 and Eurovision Fibre Optic Network to the Eurovison premises in Geneva. Decoded from APTX and encoded to AAC Plus at 160 kbps.
Producers part was in this instance taken by a software encoder by Orban. Because it was still a beta when they tried it out it proved to be a little problematic. In the end however – it worked fine although limited to 160 kbps (which by certain Golden Ears might condemn to be insufficient).From there it was uploaded to shoutcast server and Octoshape in Copenhagen.
Finally the signal was distributed across the Internet using P2P system making it available for people to listen with WinAmp or VLC.
Overall the purpose was to integrate a multichannel HE AAC- encoded sound programme with P2P streaming server and then webcast across the internet. It was a success.
Hopefully we’ll be publishing some further success stories here shortly.
Producer, engineer, composer and a surround fanatic based in North London, UK.
