8/19/2023 0 Comments Bbc binaural soundSince the audio processing is happening in the browser, we can allow users to select their preferred headphone option without needing to provide multiple different streams. This is the way that surround sound sources are conventionally processed for listening on headphones (or a stereo loudspeaker system). We are also offering a stereo down-mix, where the surround and front-centre channels are added to the front-left and front-right signals to create a two-channel stereo version, as shown in the image below. In this experiment we have made two different binaural processing options available, with impulse responses measured in different acoustic environments. The Web Audio node graph for creating the binaural headphone surround For more background on binaural techniques, you can read this previous R&D blog post or this BBC News article. The image below shows the signal flow within the Web Audio API to create this effect, it was created using Firefox Web Audio Editor. T hese measurements are used to process each channel of the surround sound signal and the results are mixed together to form a two-channel signal for headphones. The resulting effect should be that of the original surround sound mix coming from five virtual loudspeakers, outside of the listener's head. We have measured impulse responses from a surround sound loudspeaker system using a dummy head microphone, these are two-channel measurements describing the transfer function from the sound source to each ear. Using binaural impulse responses, measured using a dummy head microphone, the convolution process can be used to create the impression that a sound source is coming from a specific point in space, when listening on headphones. The Web Audio API specification provides a helpful informative section on convolution. This allows a developer to convolve an audio signal with a chosen impulse response. The most important feature of the API for headphone surround is the ConvolverNode interface. We can then process this sound source through a Web Audio node graph. To create the headphone surround sound experience, we use the MediaElementAudioSourceNode to grab the multichannel audio signal coming from the HTML5 audio element. BBC R&D's Matthew Paradis is co-chairing with the W3C 's Audio Working Group, which is working on the specification. It has a range of applications, such as music creation and games, or even recreating the sounds of the Radiophonic Workshop. The new headphone surround sound feature is possible thanks to a web technology called the Web Audio API. This client-side script API introduces capabilities for audio signal processing in web applications. After the broadcast, the play will be available on-demand for 30 days. This Saturday we are again streaming the play online in 5-channel surround, this time using our MPEG-DASH player, which works directly in a page on the Radio 4 website. This production was mixed in surround sound and at the time a 5.1 stream was made available to our audience. This week's Saturday Drama on Radio 4 will be the highly-acclaimed 2003 production of Dylan Thomas' Under Milk Wood, in celebration of the centenary of the Welsh writer's birth.
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