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CSS - Aural Media


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A web document can be rendered by a speech synthesizer. CSS2 allows you to attach specific sound style features to specific document elements.

Aural rendering of documents is mainly used by the visually impaired. Some of the situations in which a document can be accessed by means of aural rendering rather than visual rendering are the following.

  • Learning to read
  • Training
  • Web access in vehicles
  • Home entertainment
  • Industrial documentation
  • Medical documentation

When using aural properties, the canvas consists of a three-dimensional physical space (sound surrounds) and a temporal space (one may specify sounds before, during, and after other sounds).

The CSS properties also allow you to vary the quality of synthesized speech (voice type, frequency, inflection, etc.)

Here is one example:

<style tyle="text/css">
<!--
h1, h2, h3, h4, h5, h6 {
    voice-family: paul;
    stress: 20;
    richness: 90;
    cue-before: url("ping.au")
}
p.heidi { azimuth: center-left }
p.peter { azimuth: right }
-->
</style>

This will direct the speech synthesizer to speak headers in a voice (a kind of audio font) called "paul", on a flat tone, but in a very rich voice. Before speaking the headers, a sound sample will be played from the given URL.

Paragraphs with class heidi will appear to come from front left (if the sound system is capable of spatial audio), and paragraphs of class peter from the right.

Now we will see various properties related to aural media.

  • The azimuth property sets where the sound should come from horizontally.

  • The elevation property sets where the sound should come from vertically.

  • The cue-after specifies a sound to be played after speaking an element's content to delimit it from other.

  • The cue-before specifies a sound to be played before speaking an element's content to delimit it from other.

  • The cue is a shorthand for setting cue-before and cue-after

  • The pause-after specifies a pause to be observed after speaking an element's content.

  • The pause-before specifies a pause to be observed before speaking an element's content.

  • The pause is a shorthand for setting pause-before and pause-after.

  • The pitch specifies the average pitch (a frequency) of the speaking voice.

  • The pitch-range specifies variation in average pitch.

  • The play-during specifies a sound to be played as a background while an element's content is spoken.

  • The richness specifies the richness, or brightness, of the speaking voice.

  • The speak specifies whether text will be rendered aurally and if so, in what manner.

  • The speak-numeral controls how numerals are spoken.

  • The speak-punctuation specifies how punctuation is spoken.

  • The speech-rate specifies the speaking rate.

  • The stress specifies the height of "local peaks" in the intonation contour of a voice.

  • The voice-family specifies prioritized list of voice family names.

  • The volume refers to the median volume of the voice.

The azimuth property:

This property sets where the sound should come from horizontally. The possible values are:

  • angle: Position is described in terms of an angle within the range -360deg to 360deg. The value 0deg means directly ahead in the center of the sound stage. 90deg is to the right, 180deg behind, and 270deg (or, equivalently and more conveniently, -90deg) to the left.
  • left-side: Same as '270deg'. With 'behind', '270deg'.
  • far-left: Same as '300deg'. With 'behind', '240deg'.
  • left: Same as '320deg'. With 'behind', '220deg'.
  • center-left: Same as '340deg'. With 'behind', '200deg'.
  • center: Same as '0deg'. With 'behind', '180deg'.
  • center-right: Same as '20deg'. With 'behind', '160deg'.
  • right: Same as '40deg'. With 'behind', '140deg'.
  • far-right: Same as '60deg'. With 'behind', '120deg'.
  • right-side: Same as '90deg'. With 'behind', '90deg'.
  • leftwards: Moves the sound to the left and relative to the current angle. More precisely, subtracts 20 degrees.
  • rightwards: Moves the sound to the right, relative to the current angle. More precisely, adds 20 degrees.

Here is one example:

<style tyle="text/css">
<!--
h1   { azimuth: 30deg }
td.a { azimuth: far-right }          /*  60deg */
#12  { azimuth: behind far-right }   /* 120deg */
p.comment { azimuth: behind }        /* 180deg */
-->
</style>

The elevation property:

This property sets where the sound should come from vertically. The possible values are:

  • angle: Specifies the elevation as an angle, between -90deg and 90deg. 0deg means on the forward horizon, which loosely means level with the listener. 90deg means directly overhead and -90deg means directly below.
  • below: Same as '-90deg'.
  • level: Same as '0deg'.
  • above: Same as '90deg'.
  • higher: Adds 10 degrees to the current elevation.
  • lower: Subtracts 10 degrees from the current elevation.

Here is one example:

<style tyle="text/css">
<!--
h1   { elevation: above }
tr.a { elevation: 60deg }
tr.b { elevation: 30deg }
tr.c { elevation: level }
-->
</style>

The cue-after property:

This property specifies a sound to be played after speaking an element's content to delimit it from other. The possible values are:

  • url: The URL of a sound file to be played.
  • none: Nothing has to be played

Here is one example:

<style tyle="text/css">
<!--
a {cue-after: url("dong.wav");}
h1 {cue-after: url("pop.au"); }
-->
</style>

The cue-before property:

This property specifies a sound to be played before speaking an element's content to delimit it from other. The possible values are:

  • url: The URL of a sound file to be played.
  • none: Nothing has to be played

Here is one example:

<style tyle="text/css">
<!--
a {cue-before: url("bell.aiff");}
h1 {cue-before: url("pop.au"); }
-->
</style>

The cue property:

This property is a shorthand for setting cue-before and cue-after. If two values are given, the first value is cue-before and the second is cue-after. If only one value is given, it applies to both properties.

For example the following two rules are equivalent:

<style tyle="text/css">
<!--
h1 {cue-before: url("pop.au"); cue-after: url("pop.au") }
h1 {cue: url("pop.au") }
-->
</style>

The pause-after property:

This property specifies a pause to be observed after speaking an element's content. The possible values are:

  • time: Expresses the pause in absolute time units (seconds and milliseconds).
  • percentage: Refers to the inverse of the value of the speech-rate property. For example, if the speech-rate is 120 words per minute (i.e. a word takes half a second, or 500ms) then a pause-after of 100% means a pause of 500 ms and a pause-after of 20% means 100ms.

The pause-before property:

This property specifies a pause to be observed before speaking an element's content. The possible values are:

  • time: Expresses the pause in absolute time units (seconds and milliseconds).
  • percentage: Refers to the inverse of the value of the speech-rate property. For example, if the speech-rate is 120 words per minute (i.e. a word takes half a second, or 500ms) then a pause-before of 100% means a pause of 500 ms and a pause-before of 20% means 100ms.

The pause property:

This property is a shorthand for setting pause-before and pause-after. If two values are given, the first value is pause-before and the second is pause-after.

Here is the example:

<style tyle="text/css">
<!--
/* pause-before: 20ms; pause-after: 20ms */
h1 { pause : 20ms }  
/* pause-before: 30ms; pause-after: 40ms */
h2{ pause : 30ms 40ms }  
/* pause-before: ?; pause-after: 10ms */
h3 { pause-after : 10ms }
-->
</style>

The pitch property:

This property specifies the average pitch (a frequency) of the speaking voice. The average pitch of a voice depends on the voice family. For example, the average pitch for a standard male voice is around 120Hz, but for a female voice, it's around 210Hz. The possible values are:

  • frequency: Specifies the average pitch of the speaking voice in hertz (Hz).
  • x-low, low, medium, high, x-high : These values do not map to absolute frequencies since these values depend on the voice family.

The pitch-range property:

This property specifies variation in average pitch. The possible values are:

  • number: A value between '0' and '100'. A pitch range of '0' produces a flat, monotonic voice. A pitch range of 50 produces normal inflection. Pitch ranges greater than 50 produce animated voices.

The play-during property:

This property specifies a sound to be played as a background while an element's content is spoken. Possible values could be any of the followings:

  • URI: The sound designated by this <uri> is played as a background while the element's content is spoken.
  • mix: When present, this keyword means that the sound inherited from the parent element's play-during property continues to play and the sound designated by the uri is mixed with it. If mix is not specified, the element's background sound replaces the parent's.
  • repeat: When present, this keyword means that the sound will repeat if it is too short to fill the entire duration of the element. Otherwise, the sound plays once and then stops.
  • auto: The sound of the parent element continues to play.
  • none: This keyword means that there is silence.

Here is the example:

<style tyle="text/css">
<!--
blockquote.sad { play-during: url("violins.aiff") }
blockquote q   { play-during: url("harp.wav") mix }
span.quiet     { play-during: none }
-->
</style>

The richness property:

This property specifies the richness, or brightness, of the speaking voice. The possible values are:

  • number: A value between '0' and '100'. The higher the value, the more the voice will carry. A lower value will produce a soft, mellifluous voice.

The speak property:

This property specifies whether text will be rendered aurally and if so, in what manner. The possible values are:

  • none: Suppresses aural rendering so that the element requires no time to render.
  • normal: Uses language-dependent pronunciation rules for rendering an element and its children.
  • spell-out: Spells the text one letter at a time .

Note the difference between an element whose 'volume' property has a value of 'silent' and an element whose 'speak' property has the value 'none'. The former takes up the same time as if it had been spoken, including any pause before and after the element, but no sound is generated. The latter requires no time and is not rendered

The speak-numeral property:

This property controls how numerals are spoken. The possible values are:

  • digits: Speak the numeral as individual digits. Thus, "237" is spoken "Two Three Seven".
  • continuous: Speak the numeral as a full number. Thus, "237" is spoken "Two hundred thirty seven". Word representations are language-dependent.

The speak-punctuation property:

This property specifies how punctuation is spoken. The possible values are:

  • code: Punctuation such as semicolons, braces, and so on are to be spoken literally.
  • none: Punctuation is not to be spoken, but instead rendered naturally as various pauses.

The speech-rate property:

This property specifies the speaking rate. Note that both absolute and relative keyword values are allowed. The possible values are:

  • number: Specifies the speaking rate in words per minute.
  • x-slow: Same as 80 words per minute..
  • slow: Same as 120 words per minute .
  • medium: Same as 180 - 200 words per minute.
  • fast: Same as 300 words per minute.
  • x-fast: Same as 500 words per minute.
  • faster: Adds 40 words per minute to the current speech rate.
  • slower: Subtracts 40 words per minutes from the current speech rate.

The stress property:

This property specifies the height of "local peaks" in the intonation contour of a voice. English is a stressed language, and different parts of a sentence are assigned primary, secondary, or tertiary stress. The possible values are:

  • number: A value, between '0' and '100'. The meaning of values depends on the language being spoken. For example, a level of '50' for a standard, English-speaking male voice (average pitch = 122Hz), speaking with normal intonation and emphasis would have a different meaning than '50' for an Italian voice.

The voice-family property:

The value is a comma-separated, prioritized list of voice family names. It can have following values:

  • generic-voice : Values are voice families. Possible values are 'male', 'female', and 'child'.
  • specific-voice: Values are specific instances (e.g., comedian, trinoids, carlos, lani).

Here is the example:

<style tyle="text/css">
<!--
h1 { voice-family: announcer, male }
p.part.romeo  { voice-family: romeo, male }
p.part.juliet { voice-family: juliet, female }
-->
</style>

The volume property:

Volume refers to the median volume of the voice. It can have following values:

  • numbers: Any number between '0' and '100'. '0' represents the minimum audible volume level and 100 corresponds to the maximum comfortable level.
  • percentage: These values are calculated relative to the inherited value, and are then clipped to the range '0' to '100'.
  • silent: No sound at all. The value '0' does not mean the same as 'silent'.
  • x-soft: Same as '0'.
  • soft: Same as '25'.
  • medium: Same as '50'.
  • loud: Same as '75'.
  • x-loud: Same as '100'.

Here is the example:

<style tyle="text/css">
<!--
P.goat  { volume: x-soft }
-->
</style>

Paragraphs with class goat will be very soft.


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