To develop HD Surround, XM partnered with Neural Audio, a leader in innovative audio signal processing. Neural came up with Neural Surround, a way to downmix a 5.1-channel recording to two channels, then upmix those channels back to 5.1.
It sounds simple, but in fact the technical challenges are enormous. The signal has to be compatible with XM's normal broadcast chain and with all the existing XM stereo receivers. And it has to provide a good surround experience when reproduced through receivers with XM HD Surround.
Neural addressed those issues with ingenious spatial compression using digital signal processing and psychoacoustics. The 5.1-channel signal is analyzed for inter-channel intensity differences and coherence, and this information is embedded in a downmixed two-channel signal using an inaudible watermark. The two-channel signal goes through the entire XM broadcast chain the same as any other signal. But when it reaches an HD Surround receiver, the watermark is used to re-synthesize the original 5.1 channels. The result, Neural says, is "an impression of the original source 5.1 content." Everything needed for reconstruction is in the audio signal.
The watermark can survive many types of analog and digital processing. For example, if you receive XM via DirecTV, you can use an HD Surround receiver to listen to the multichannel broadcasts. In the world of satellite radio, Neural processing is exclusive to XM, but it's compatible with any stereo signal path, so you can expect to see it used elsewhere.
Broadcasting surround adds some steps to XM's usual production methods. The programmers go through the DVD-Audio and SACD libraries, rejecting anything they consider of poor artistic quality or just gimmicky. Once they've made their choices, they work with the disc's high-resolution uncompressed (or losslessly compressed) channels rather than the Dolby Digital mixes. The recordings are put through Neural processing, and the encoded recordings are auditioned again, to make sure the surround sound is properly encoded and that the mixes still sound right when played back in stereo. The recordings are then added to the library, ready for broadcast.
Of course, all the broadcast technology in the world isn't much good without a receiver to listen to it - which is why companies like Yamaha, Pioneer, Onkyo, and Denon now offer receivers with HD Surround built in. These components carry "XM-Ready" and "Neural Surround" logos, showing that the necessary software has been added to their digital signal processing (DSP) chips. XM subscribers can use either an XM Connect-and-Play home antenna or the upcoming XM Passport cartridge to receive the XM HD Surround signal.
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