@@ -706,43 +706,6 @@ <h3 id=mixing-props-voice-balance><span class=secno>5.2. </span>The
706706 match in the SSML markup language < a href ="#SSML "
707707 rel =biblioentry > [SSML]<!--{{!SSML}}--> </ a > .
708708
709- < ul >
710- < li > When user-agents produce audio via a mono-aural sound system (i.e.
711- single-speaker setup), the ‘< a href ="#voice-balance "> < code
712- class =property > voice-balance</ code > </ a > ’ property has no effect.
713-
714- < li > When user-agents produce audio through a stereo sound system (e.g.
715- two speakers, a pair of headphones), the left-right distribution of audio
716- signals precisely match the authored values for the ‘< a
717- href ="#voice-balance "> < code
718- class =property > voice-balance</ code > </ a > ’ property.
719-
720- < li > When user-agents are capable of mixing audio signals through more
721- than 2 channels (e.g. 5-speakers surround sound system, including a
722- dedicated center channel), the physical distribution of audio signals
723- resulting from the application of the ‘< a
724- href ="#voice-balance "> < code
725- class =property > voice-balance</ code > </ a > ’ property must be performed
726- so that the listener perceives sound as if it was coming from a basic
727- stereo layout. For example, the center channel as well as the left/right
728- speakers may be used altogether in order to emulate the behavior of the
729- ‘< code class =property > center</ code > ’ value (zero, on the
730- [-100,100] scale of the ‘< a href ="#voice-balance "> < code
731- class =property > voice-balance</ code > </ a > ’ property).
732- </ ul >
733-
734- < p class =note > Note that sound systems may be configured by users in such a
735- way that it would interfere with the left-right audio distribution
736- specified by document authors. Typically, the various "surround" modes
737- available in modern sound systems (including systems based on basic stereo
738- speakers) tend to greatly alter the perceived spatial arrangement of audio
739- signals. Some users may even configure their system to "downgrade" any
740- rendered sound to a single mono channel, in which case the effect of the
741- ‘< a href ="#voice-balance "> < code
742- class =property > voice-balance</ code > </ a > ’ property would obviously
743- not be perceivable at all. The rendering fidelity of authored content is
744- therefore dependent on such user customizations.
745-
746709 < dl >
747710 < dt > < strong > <number></ strong >
748711
@@ -793,13 +756,90 @@ <h3 id=mixing-props-voice-balance><span class=secno>5.2. </span>The
793756 the resulting number to ‘< code class =css > 100</ code > ’.</ p >
794757 </ dl >
795758
759+ < p > User agents may be connected to different kinds of sound systems,
760+ featuring varying audio mixing capabilities. The expected behavior for
761+ mono, stereo, and surround sound systems is defined as follows:
762+
763+ < ul >
764+ < li > When user-agents produce audio via a mono-aural sound system (i.e.
765+ single-speaker setup), the ‘< a href ="#voice-balance "> < code
766+ class =property > voice-balance</ code > </ a > ’ property has no effect.
767+
768+ < li > When user-agents produce audio through a stereo sound system (e.g.
769+ two speakers, a pair of headphones), the left-right distribution of audio
770+ signals can precisely match the authored values for the ‘< a
771+ href ="#voice-balance "> < code
772+ class =property > voice-balance</ code > </ a > ’ property.
773+
774+ < li > When user-agents are capable of mixing audio signals through more
775+ than 2 channels (e.g. 5-speakers surround sound system, including a
776+ dedicated center channel), the physical distribution of audio signals
777+ resulting from the application of the ‘< a
778+ href ="#voice-balance "> < code
779+ class =property > voice-balance</ code > </ a > ’ property should be
780+ performed so that the listener perceives sound as if it was coming from a
781+ basic stereo layout. For example, the center channel as well as the
782+ left/right speakers may be used altogether in order to emulate the
783+ behavior of the ‘< code class =property > center</ code > ’ value.
784+ </ ul >
785+
786+ < p class =note > Note that sound systems may be configured by users in such a
787+ way that it would interfere with the left-right audio distribution
788+ specified by document authors. Typically, the various "surround" modes
789+ available in modern sound systems (including systems based on basic stereo
790+ speakers) tend to greatly alter the perceived spatial arrangement of audio
791+ signals. The illusion of a three-dimensional sound stage is often achieved
792+ using a combination of phase shifting, digital delay, volume control
793+ (channel mixing), and other techniques. Some users may even configure
794+ their system to "downgrade" any rendered sound to a single mono channel,
795+ in which case the effect of the ‘< a href ="#voice-balance "> < code
796+ class =property > voice-balance</ code > </ a > ’ property would obviously
797+ not be perceivable at all. The rendering fidelity of authored content is
798+ therefore dependent on such user customizations, and the ‘< a
799+ href ="#voice-balance "> < code class =property > voice-balance</ code > </ a > ’
800+ property merely specifies the desired end-result.
801+
796802 < p class =note > Note that many speech synthesizers only generate mono sound,
797803 and therefore do not intrinsically support the ‘< a
798804 href ="#voice-balance "> < code class =property > voice-balance</ code > </ a > ’
799805 property. The sound distribution along the left-right axis consequently
800806 occurs at post-synthesis stage (when the speech-enabled user-agent mixes
801807 the various audio sources authored within the document)
802808
809+ < p > Future revisions of the CSS Speech module may include support for
810+ three-dimensional audio, which would effectively enable authors to specify
811+ "azimuth" and "elevation" values. In the future, content authored using
812+ the current specification may therefore be consumed by user-agents which
813+ are compliant with the version of CSS Speech that supports
814+ three-dimensional audio. In order to prepare for this possibility, the
815+ values enabled by the current ‘< a href ="#voice-balance "> < code
816+ class =property > voice-balance</ code > </ a > ’ property are designed to
817+ remain compatible with "azimuth" angles. More precisely, the mapping
818+ between the current left-right audio axis (lateral sound stage) and the
819+ envisioned 360 degrees plane around the listener's position is defined as
820+ follows:
821+
822+ < ul >
823+ < li > The value ‘< code class =css > 0</ code > ’ maps to zero degrees
824+ (‘< code class =property > center</ code > ’). This is in "front" of
825+ the listener, not from "behind".
826+
827+ < li > The value ‘< code class =css > -100</ code > ’ maps to -40
828+ degrees (‘< code class =property > left</ code > ’). Negative angles
829+ are in the counter-clockwise direction (the audio stage is seen from the
830+ top).
831+
832+ < li > The value ‘< code class =css > 100</ code > ’ maps to 40 degrees
833+ (‘< code class =property > right</ code > ’). Positive angles are in
834+ the clockwise direction (the audio stage is seen from the top).
835+
836+ < li > Intermediary values on the scale from ‘< code
837+ class =css > -100</ code > ’ to ‘< code class =css > 100</ code > ’
838+ map to the angles between -40 and 40 degrees in a numerically
839+ linearly-proportional manner. For example, ‘< code
840+ class =css > -50</ code > ’ maps to -20 degrees.
841+ </ ul >
842+
803843 < h2 id =speaking-props > < span class =secno > 6. </ span > Speaking properties</ h2 >
804844
805845 < h3 id =speaking-props-speak > < span class =secno > 6.1. </ span > The ‘< a
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