|author||Daniel Stenberg <email@example.com>||2004-04-19 13:46:06 +0000|
|committer||Daniel Stenberg <firstname.lastname@example.org>||2004-04-19 13:46:06 +0000|
Added 'Q78: How do I control the recording frequency and quality?'
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@4516 a1c6a512-1295-4272-9138-f99709370657
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@@ -709,3 +709,57 @@ A77: You make the common mistake of confusing Rockbox development with that of
sufficiently useful feature, it will be written.
(That could be you.)
+Q78: How do I control the recording frequency and quality?
+A78: Linus Nielsen Feltzing replied to a similar question on April 17th 2004,
+ and this is a cut'n paste of his reply that can be read on the following
+ SAMPLE RATE
+ Controls the amount of samples per secod, basically which frequencies
+ that can accurately be reproduced buring playback. Lower frequencies
+ produce smaller files, for two reasons: 1) The amount of data to be
+ compressed is smaller and 2) the data is easier to compress, since higher
+ frequencies are not present.
+ BIT RATE
+ Controls how many bits per second that is required for accurate live
+ transmission of the compressed audio. When you compress the data harder
+ (meaning worse sound quality), the bitrate gets lower.
+ STEREO VS MONO
+ A mono file doesn't necessarily have to be smaller than a stereo file.
+ It all depends on the encoder. The MAS does produce smaller files with
+ MAS QUALITY
+ The MAS uses VBR for compression (yes always), which means that the
+ bitrate varies from frame to frame, depending on how compressable the
+ data was at that point in time. This allows for a more even quality, and
+ also smaller files if the data is easily compressed. The MAS can generate
+ frames with bit rates ranging from 32kbit/s to 192kbit/s (MPEG1) or
+ 8kbit/s to 160kbit/s (MPEG2).
+ The MAS quality setting is just a way of selecting an average bit rate
+ according to the following table (quality 0 is on the far left):
+ FREQUENCY BITRATE IN KBIT/S
+ 44100Hz stereo: 75, 80, 90, 100, 120, 140, 160, 170
+ 22050Hz stereo: 39, 41, 45, 50, 60, 80, 110, 130
+ 44100Hz mono: 65, 68, 73, 80, 90, 105, 125, 140
+ 22050Hz mono: 35, 38, 40, 45, 50, 60, 75, 90
+ (This table can be found on page 38 in the MAS3587 data sheet.)
+ MPEG VERSIONS
+ The different MPEG versions use different sample rates:
+ 44100, 48000, 32000: MPEG version 1
+ 22050, 24000, 16000: MPEG version 2
+ 11025, 12000, 8000: MPEG version 2.5 (not an official standard)