Replace off-topic parts of the desciption by an architecture overview

Remove useless (re)wording of what are audio(4) and midi(4), add a
quick description of sndiod as intermediate layer. Try to focuse on
what the sndio library and sndiod do instead of how they do it. As
suggested by schwarze, use the word "device descriptor" instead of
"device name" to avoid the common confusion with the device files.

Remove information already given in same sentence;

Tweaks from jmc@

libsndio/sndio.7

@@ -21,57 +21,54 @@
 .Nm sndio
 .Nd interface to audio and MIDI
 .Sh DESCRIPTION
-The
+Programs access audio and MIDI hardware using the sndio library.
-.Nm sndio
+It allows both access through the
-audio and MIDI system provides access to audio and MIDI hardware and
-to services provided by
-.Xr sndiod 8 ,
-summarized below.
-.Pp
-Hardware
-.Xr audio 4
-devices correspond to peripherals.
-Only one application may use any device at a given time.
-Generally a limited number of encodings, sample rates and channel numbers are
-supported by the hardware, which may not meet the requirements of
-audio programs.
-.Pp
-To overcome hardware limitations and to allow multiple applications
-to share the hardware,
 .Xr sndiod 8
-can be used.
+server and raw access to the hardware.
-It exposes one or more software sub-devices backed by the underlying hardware,
+The audio device or MIDI port, as well as the access method,
-while doing all necessary conversions on the fly.
+are designated by the sndio descriptor.
-It can mix multiple streams or split the hardware into
+It is provided by the user with the program device selection method,
-multiple sub-devices, to allow programs to use the hardware
+or with the
-concurrently.
+.Ev AUDIODEVICE
+and
+.Ev MIDIDEVICE
+environment variables if there's no device selection method.
 .Pp
-Hardware MIDI ports correspond to serial connectors provided by the
+Most programs connect to the
-.Xr midi 4
+.Xr sndiod 8
-driver.
+server, which does the hardware access for them because
-They are typically used to access MIDI hardware (synthesizers, keyboards,
+direct access to the raw hardware is exclusive and
-control surfaces, etc.), but they do not allow applications to exchange
+requires additional privileges.
-information using the MIDI protocol.
+The
+.Xr sndiod 8
+server supports multiple connections at a time, allowing multiple programs to
+use the hardware concurrently.
+It performs the necessary audio processing on the fly to
+overcome any incompatibility between software and hardware.
+Connections to
+.Xr sndiod 8
+may be established through the network, including from virtual machines.
 .Pp
-Software MIDI thru boxes allow one application to send MIDI data to other
+The
-applications connected to the thru box (for instance a software sequencer
+.Xr sndiod 8
-can send events to multiple software synthesizers).
+server exposes
-There's no hardware involved: thru boxes are created by
+.Em MIDI thru
-.Xr sndiod 8 .
+ports, allowing one program to send MIDI data to other programs,
+for instance to allow a sequencer to send events to a synthesizer.
 .Pp
 Additionally,
 .Xr sndiod 8
-exposes a MIDI port used to control and monitor audio streams
+exposes a MIDI port used to control audio programs using
-in real time using MIDI.
+standard MIDI Machine Control (MMC), MIDI Time Code (MTC),
-.Sh DEVICE NAMES
+and master volume messages.
-From the user's perspective every audio interface, MIDI port, and
+.Sh AUDIO AND MIDI DESCRIPTORS
-.Xr sndiod 8
+From the user's perspective every audio device or MIDI port
-service has a name of the form:
+has a descriptor of the form:
 .Bd -literal -offset center
-type[@hostname][,unit]/devnum[.option]
+type[@hostname][,servnum]/devnum[.option]
 .Ed
 .Pp
-This information is used by audio and MIDI applications to determine
+This information is used by programs to determine
 how to access the audio device or MIDI port.
 .Bl -tag -width "hostname"
 .It Ar type
@@ -91,7 +88,7 @@ port.
 Audio device exposed by
 .Xr sndiod 8 .
 .It Cm midithru
-MIDI thru box created with
+MIDI thru port created with
 .Xr sndiod 8 .
 .It Cm midi
 MIDI port exposed by
@@ -103,7 +100,7 @@ Default audio device or MIDI port (see below).
 The hostname or address where the remote
 .Xr sndiod 8
 server to connect to is running.
-.It Ar unit
+.It Ar servnum
 The number of the
 .Xr sndiod 8
 server to connect to, corresponding to the integer specified using the
@@ -133,17 +130,19 @@ For example:
 .Pp
 .Bl -tag -width "snd/0.rear" -offset 3n -compact
 .It Li rsnd/0
-First hardware audio device.
+Raw access to first audio device.
 .It Li rmidi/5
-Hardware MIDI port number 5.
+Raw access to MIDI port number 5.
 .It Li snd/0
-First audio device exposed by
+Audio device referred by first
+.Fl f
+option of
 .Xr sndiod 8 .
 .It Li snd/0.rear
 Sub-device registered with
 .Fl s Fa rear .
 .It Li midithru/0
-First MIDI thru box created with
+First MIDI thru port created with
 .Xr sndiod 8 .
 .El
 .Sh DEFAULTS
@@ -181,14 +180,12 @@ machines with no MIDI hardware by default, e.g. a MIDI player
 could use a software synthesizer with no manual configuration
 required.
 .Sh AUTHENTICATION
-If a shared
+For privacy reasons only one user may have connections to
 .Xr sndiod 8
-server is running, for privacy reasons only one user may have
+at a given time.
-connections to it at a given time
-(though the same user could have multiple connections to it).
 Users are identified by their
 .Em session cookie ,
-which is automatically generated by audio or MIDI applications
+which is automatically generated by audio or MIDI programs
 upon the first connection to the server.
 The cookie is stored in
 .Pa "$HOME/.sndio/cookie"
@@ -199,20 +196,18 @@ can connect to the server using the same cookie.
 .Sh ENVIRONMENT
 .Bl -tag -width "AUDIODEVICEXXX" -compact
 .It Ev AUDIODEVICE
-Audio device that
+Audio device to use if the program offers no audio device selection method.
-.Xr sio_open 3
-uses if the application provides no device chooser.
 .It Ev MIDIDEVICE
-MIDI port that
+MIDI port to use if the program offers no MIDI port selection method.
-.Xr mio_open 3
-uses if the application provides no MIDI port chooser.
 .El
 .Pp
 These environment variables are ignored by
 .Nm
 if the program has the set-user-ID or set-group-ID bits set.
 .Sh FILES
-.Bl -tag -width "/dev/audioNXXX" -compact
+.Bl -tag -width "~/.sndio/cookie" -compact
+.It Pa ~/.sndio/cookie
+User's session authentication cookie.
 .It Pa /dev/audioN
 Audio devices.
 .It Pa /dev/rmidiN
@@ -221,6 +216,7 @@ MIDI ports.
 .Sh SEE ALSO
 .Xr mio_open 3 ,
 .Xr sio_open 3 ,
+.Xr sioctl_open 3 ,
 .Xr audio 4 ,
 .Xr midi 4 ,
 .Xr sndiod 8