Next Steps in Surround


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Creating surround content

With monitoring, appropriate infrastructure in place and overcoming the challenges involved with delivering surround content, the next challenge is creating content itself.

There are two primary approaches in creating surround content. The first approach is placing microphones on each instrument and mixing that instrument into the surround sound stage. This method is useful for popular music. An engineer can create a mix that has instrumentation all around the listener, essentially putting the listener “in the band.” The second approach to creating surround content is capturing the event so that it sounds like you are in the natural acoustic space in which the performance is occurring, or said another way, from an audience perspective.

There are challenges in creating accentuated content as all radio broadcast surround technology systems use some version of a downmix in their structure. (Note: A downmix is the process where multiple channels of surround audio content are converged together into a reduced number of audio channels, typically stereo, labeled Lt/Rt. Upmix is the reverse, where multiple channels of surround audio content are derived from a reduced number of audio channels.) Many of these challenges can occur from the in-the-band scenario as well; however, since they are more prevalent from the audience perspective, they will be primarily discussed from that context.

Conventional spaced omnis, ORTF or other standard stereo recording techniques may be utilized for creating surround content. However, they have one drawback: They do not utilize the center front channel of the surround field. Center images would become phantom center images just like they are in standard stereo. With most downmix algorithms, the original phantom center image may be recreated in the center channel only, sometimes with different intensity. Therefore, use of standard stereo microphone techniques should not be the primary means of capturing surround content and surround microphone methods are recommended.

Multiple methods

There are now several surround microphone arrays, methods and systems available. When choosing, pay particular attention to one detail that is often overlooked: interchannel crosstalk. To understand interchannel crosstalk, you must first understand how most surround microphone array and methods work.

With almost all the microphone arrays and methods available, there are three microphones assigned (left, center, right) to the front-left, center and front-right of the surround sound playback system. In some of these systems, the placement of these microphones is relatively close. Interchannel crosstalk arises when multiple phantom images (just like the phantom center that occurs with stereo microphone techniques) arise from the front three or more microphones.

Figure 1

Figure 1. Phantom images created in surround.
Click to enlarge image

For example, if three microphones were placed across the front of a stage and assigned to front-left, center and front-right, it is possible that a phantom image would be generated for each pairing of microphones for a single sound source on the stage. A phantom image would occur in the playback system from the front-left and center microphones. Another phantom image will occur from the front-left and front right and another from the center and front-right microphones. These three phantom images compete auditorally. This interchannel crosstalk, smears the location of the sound-source, reducing the localization of that sound source in a surround playback system. The stronger the phantom image for each pairing that competes with one another in the surround playback system, the more interchannel crosstalk.

Interchannel crosstalk may not seem to be much of an issue in discrete-channel recording, however, just as a human will have to deal with the multiple phantom images emerging out of the speakers, a surround sound broadcasting technology will have to deal with this as well. For example, when high amounts of interchannel crosstalk are applied to a downmix algorithm, it challenges the system, often producing comb-filtering (unless the system has adaptive filters to address this issue), loss of fidelity and can produce localization errors in the both the downmix and the upmix.

Furthermore, high amounts of interchannel crosstalk can produce undesirable phase relationships in downmix algorithms that can increase L-R. With some technologies, unpleasant IMD can occur in the recreation of the surround material. Some technologies can deal with these issues much better than others, but regardless of the surround broadcast technology employed, interchannel crosstalk challenges the systems, sometimes producing undesirable effects.

As seen in Figure 1, for a single sound source, there would result three different phantom images in the surround playback system.



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