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Volume control in amixer v. alsamixer

I am trying to understand how to use volume control in amixer and alsamixer.  The name of the control parameter for volume in amixer and alsamixer in my version of Xbian is "Digital" instead of "PCM", so my commands look like this

0db point, no attenuation or amplification:

xbian@xbian ~ $ amixer sset Digital 81%
Simple mixer control 'Digital',0
Capabilities: pvolume pswitch
Playback channels: Front Left - Front Right
Limits: Playback 0 - 255
Mono:
Front Left: Playback 207 [81%] [0.00dB] [on]
Front Right: Playback 207 [81%] [0.00dB] [on]

And reducing volume in 6 db steps would be like this?

xbian@xbian ~ $ amixer sset Digital 76%
Simple mixer control 'Digital',0
  Capabilities: pvolume pswitch
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 255
  Mono:
  Front Left: Playback 194 [76%] [-6.50dB] [on]
  Front Right: Playback 194 [76%] [-6.50dB] [on]

xbian@xbian ~ $ amixer sset Digital 71%
Simple mixer control 'Digital',0
  Capabilities: pvolume pswitch
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 255
  Mono:
  Front Left: Playback 182 [71%] [-12.50dB] [on]
  Front Right: Playback 182 [71%] [-12.50dB] [on]

xbian@xbian ~ $ amixer sset Digital 67%
Simple mixer control 'Digital',0
  Capabilities: pvolume pswitch
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 255
  Mono:
  Front Left: Playback 171 [67%] [-18.00dB] [on]
  Front Right: Playback 171 [67%] [-18.00dB] [on]

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