Friday 14 April 2017

BBC Radio 3 Delivering Concert Sound on the Net

Listeners able to receive studio quality signal via web browser
Last week, BBC Radio 3 and BBC R&D launched what they believed is another world-first for a classical music radio network: audio delivered directly to your web browser with completely lossless compression. Together various technologies including MPEG DASH, FLAC compression, HTML5 and the Media Source Extensions offer a bit-perfect representation of Radio 3's live output, exactly as it left the studio. Radio 3 makes considerable investment in music performance and technical excellence, and this pilot enables the most transparent listening experience possible according to the BBC.

Open, royalty-free and well supported in both software and hardware, FLAC was an obvious choice for compression for this pilot. Distributing uncompressed Radio 3 to the home would require bandwidth of around 1.5 Mbit/s, considerably more than is typically used even for high quality audio distribution over broadcast or the Internet.

To reduce the data rate to something more practical for distribution, BBC have traditionally used a compression codec, AAC, that delivers high quality audio at much lower bitrates.  Radio 3 offers a very high quality stream at a fixed rate of 320 kbit/s for internet listeners, which is a significant saving both in terms of bandwidth and storage compared to the 1.5 Mbit/s original. Experts expect that, for the vast majority of material, this should be indistinguishable from the uncompressed source.

Lossy codecs such as AAC do change the audio signal, and there is the potential for noticeable artifacts with some material, whereas FLAC will always encode the signal transparently by employing a combination of channel decorrelation, linear prediction and residual and run-length encoding.

This  FLAC stream appears to average around 550 kbit/s over an extended period, though this is highly variable, peaking up to around 1Mbit/s, and is dependant on the average complexity of the input. This is significantly lower than the uncompressed source, but some way off even the high bitrate AAC version.

Several modern web browsers now feature a built-in FLAC decoder. Mozilla have implemented the FLAC encapsulation specification and this is publicly available from Firefox 51 upwards, on the desktop. Other browsers are considering support for FLAC in ISOBMFF. BBC hopes that in time they will follow suit, and that mobile devices will also add the necessary functionality.

BBC is also in discussions with internet radio manufacturers, many of whom have expressed an interest in supporting lossless streaming.

Read the full story by BBC Research & Development