(Work-in-progress)
- Ariff Abdullah <ariff@FreeBSD.org)
- Joel Dahl <joel@FreeBSD.org)

Finally commit the once-a-year mega-patch to the sound system.  Changes
includes:

1 Volume Per-Channel (vpc).  Provides private / standalone volume control
  unique per-stream pcm channel without touching master volume / pcm.
  Applications can directly use SNDCTL_DSP_[GET|SET][PLAY|REC]VOL, or for
  backwards compatibility, SOUND_MIXER_PCM through the opened dsp device
  instead of /dev/mixer.  Special "bypass" mode is enabled through /dev/mixer
  which will automatically detect if the adjustment is made through
  /dev/mixer and forward its request to this private volume controller.
  Changes to this volume object will not interfere with other channels.

  Requirements:
    - SNDCTL_DSP_[GET|SET][PLAY|REC]_VOL are newer ioctls (OSSv4) which
      require specific application modifications (preferred).
    - No modifications required for using bypass mode, so applications
      like mplayer or xmms should work out of the box.

  Kernel hints:
    - hint.pcm.%d.vpc (0 = disable vpc).

  Kernel sysctls:
    - hw.snd.vpc_mixer_bypass (default: 1).  Enable or disable /dev/mixer
      bypass mode.
    - hw.snd.vpc_autoreset (default: 1).  By default, closing/opening
      /dev/dsp will reset the volume back to 0 db gain/attenuation.
      Setting this to 0 will preserve its settings across device
      closing/opening.
    - hw.snd.vpc_reset (default: 0).  Panic/reset button to reset all
      volume settings back to 0 db.
    - hw.snd.vpc_0db (default: 45).  0 db relative to linear mixer value.

2 High quality fixed-point Bandlimited SINC sampling rate converter. The
  main motivation for this was that FreeBSD lacks a high quality Sample
  Rate Converter (SRC).  Available SRC's are either too restrictive
  (licensing), have a floating point implementation (you can't do that in the
  kernel), lack definitive filter design program (those coefficients suddenly
  appear out of nowhere) or are too convoluted to comprehend.

  I've created a native implementation for FreeBSD, based on Julius O'Smith's
  Digital Audio Resampling - http://ccrma.stanford.edu/~jos/resample/.  It
  includes a filter design script written in awk.

  A few notes:
    - 100% 32bit fixed-point, 64bit accumulator.
    - Possibly among the fastest (if not fastest) of its kind.
    - Resampling quality is tunable, either runtime or during kernel
      compilation (FEEDER_RATE_PRESETS).
    - Quality can be further customized during kernel compilation by
      defining FEEDER_RATE_PRESETS in /etc/make.conf.  At its simplest form,
      defining FEEDER_RATE_PRESETS="8 16 32 64 128" will create
      resampler that will be filtered through 8, 16, 32, 64 or 128 taps FIR.
    - FEEDER_RATE_PRESETS.
      * "8 16 ..." simplest form, define taps per-filter.
      * "100:32 ..." stopband-attenuation:taps.
      * "100:32:0.95 .." stopband-attenuation:taps:rolloff/bandwidth.
        Caution: defining too many presets, or a preset with large taps will
        bloat the kernel!
      * Default presets: "100:8 100:16 100:32:0.7929 100:64:0.8990 100:128:0.9499"

  Kernel sysctls:
    - hw.snd.feeder_rate_quality.
      0 - Zero-order Hold (ZOH).  Fastest, bad quality.
      1 - Linear Interpolation (LINEAR).  Slightly slower than ZOH, better
          quality but still does not eliminate aliasing.
      2 (and above) - Sinc Interpolation(SINC).  Best quality.  SINC quality
        always start from 2 and above.

  Rough quality comparisons:
    - http://people.freebsd.org/~ariff/zresampler/z.jpg

3 Bit-perfect mode.  Bypasses all feeder/dsp effects.  Pure sound will be
  directly fed into the hardware.  Audiophiles will love this.

4 PCM Software Equalizer (Bass/Treble mixer).

5 Transparent/Adaptive Virtual Channel.  Now you don't have to disable vchans in
  order to make digital format pass through.  It also makes vchans more dynamic
  by choosing a better format/rate among all the concurrent streams, which means
  that dev.pcm.X.play.vchanformat/rate becomes sort of optional.

6 Exclusive Stream, with special open() mode O_EXCL.  This will "mute" other
  concurrent vchan streams and only allow the channel with O_EXCL set to keep
  producing sound.

Manual page updates are on the way.

Tested by:	joel