Brian Willoughby <[email protected]> wrote: > On Jan 7, 2011, at 16:27, Declan Kelly wrote: ... >> As the human hearing can't really tell direction with lower >> frequencies, >> it's not as essential. This same shortcut is why most movie "surround >> sound" systems have only one sub bass channel. > > In this case, you have been misled by a common misconception in the > consumer audio industry. ... > The reason why most consumer electronics experts get this wrong is > because of the standard techniques use in studio recording. Most > music is recorded as multiple channels, e.g., 16, that are each > monophonic. These channels are played back through a mixing console, > and a simple pan pot is used to artificially place them in a > location. Because the pan pot only effects the volume, not the phase > difference or time delay, this means that a studio recording is going > to have no directionality at low frequencies.
The reality is more complicated than this. When a pair of speakers are in front of the listener and separated by 60 degrees or less, because each ear hears both speakers, low-frequency amplitude differences between the speakers are converted to phase differences between the ears. This means that pan-potted studio recordings have directionality at low frequencies even though the recording will contain only amplitude differences. Human hearing is discussed briefly in the Ambisonic Surround Sound FAQ at: http://members.tripod.com/martin_leese/Ambisonic/faq_latest.html#SECTION12 The conversion of low-frequency amplitude differences at the speakers to phase between the ears is nicely explained at: http://uod-true-multi-channel-mixing.wikispaces.com/Blumlein+Stereo As the article points out, Blumlein described all this in his 1930s patent. The reason why movie surround systems only have one "sub bass channel" is also complicated. The key is to note that the correct name for the .1 channel is the Low Frequency Effects (LFE) channel. It is there only for special effects such as explosions and dinosaur footsteps. The channel's most useful characteristic is therefore its extra 10 dB of headroom. It is likely that humans localise very low frequencies, but they probably use things like chest cavities and not ears. However, this is usually irrelevant because, in most listening rooms, localisation of low frequencies is screwed up by standing waves. And this is why the typical listening room only needs one sub-woofer. As Brian pointed out, the other channels in 5.1 (or whatever) are all full range. If every system had effective bass management, the LFE channel would be redundant for music (with the exception of the cannons at the end of the 1812 Overture). Whether the sub-woofer is redundant or not for music depends on the bass handling of the main speakers. Regards, Martin -- Martin J Leese E-mail: martin.leese stanfordalumni.org Web: http://members.tripod.com/martin_leese/ _______________________________________________ Flac-dev mailing list [email protected] http://lists.xiph.org/mailman/listinfo/flac-dev
