When the lights dim in a theater or we flip on our AV receivers at home, we take for granted that we’re going to hear a great surround sound experience, but many people don’t realize that you can get this same great experience with spatial rendering over headphones as well.
This has opened up a world of possibility for audio on portable devices. Transmitting surround sound to cinemas and home theaters is fairly straightforward. Whether D-Cinema, broadcast or streaming, bandwidth constraints for audio are virtually nonexistent. But, how do we get the same mix to the bandwidth-constrained world of mobile devices?
Most audio for mobile devices, whether streamed as audio only or associated with video, is transmitted at 128kb/s and below, if we’re lucky. Sometimes it’s as low as 32kb/s. Surround codecs cannot fit down such narrow pipes. Even if larger pipes were occasionally available, adaptive streaming concerns could force the content from a surround capable codec (192kb/s+) into a stereo codec. This would cause a major shift in the audio image. Matrix formats, such as SRS 5.1 found in Expression Encoder, by Microsoft, enable content providers to stream surround over any stereo codec. Devices with spatial rendering, such as those from HTC and Samsung, will create a theatrical experience over headphones.
So what is a matrix format and how does it work? Prior to the advent of discrete surround codecs, nearly all broadcast and theatrical surround was delivered by a matrixed surround format. By taking the surround content from the mixing room and summing the channels in a specific manner during the downmix to stereo, difference information is preserved between the channels that cues the decoder where to extract the objects during playback for surround. In the absence of a decoder, matrix encoded signals playback as stereo. When a decoder is present, in a home theater, multi-speaker renderer, or in TVs and mobile devices, with two-speaker spatial renderer, the surround mix is restored.
During this era, most surround content was mastered and archived in a matrix format. This same master was used for delivery to theaters or broadcast providers. Because it was a stereo compatible, Left total/Right total (Lt/Rt) master, the mix was preserved all the way to the final destination. Where mono delivery was required, a standard mono sum of the two channels delivered mono compatibility. With the emergence of discrete surround codecs for theatrical and broadcast mixes, most facilities delivered both a 6-channel (5.1) and stereo compatible mix, matrixed (Lt/Rt) or standard stereo (Left only/Right only – Lo/Ro). With the transition to HDTV in the U.S. complete, many mixes are only created in 5.1, leaving it up to the end-point of the codec (TV for example) to handle any downmixing.
Adding the matrix
Within Expression Encoder, this downmix technique is moved upstream to the content ingest workflow to allow for a stereo compatible surround mix. The principle is the same as used in the early era of surround from broadcast and cinema. The SRS 5.1 tools provide streaming content providers the ability to ingest 5.1 audio into their Expression Encoder workflow. Utilizing the batching tools provided in expression encoder, multiple profiles can be created to export various file types. Where higher audio bandwidths are available, discrete codecs can be used. When compressing for stereo codecs, the user can enable SRS 5.1 encoding to down mix a stereo compatible matrix. Any of the stereo codecs supported by Expression Encoder can carry the resulting output. Because a matrix format is stereo compatible, it is also supported within Silverlight-enabled players and browsers. Listeners can decode SRS 5.1 for playback over 5.1 speaker systems, using an AVR, or over two speaker or headphone rendering on virtual surround enabled TVs and mobile devices.
When 5.1 discrete codecs took over the air, many people abandoned matrix formats, but, with the emergence of streaming content to mobile devices, the need for a stereo compatible surround format has resurfaced. When rendering surround over headphones, the benefits of discrete codecs are lost. Matrix formats provide these renderers with the information they need to present a cinematic experience on any mobile device, regardless of the limitations of today’s bandwidth-constrained mobile world. In short, you don’t actually have to abandon surround just because you’re limited to a stereo transmission path, matrix systems still work very well for this type of environment.