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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 + URL: + http://rockbox.haxx.se/mail/archive/rockbox-archive-2004-04/0814.shtml + + 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 + mono. + + 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) |