On Sun, 1 Nov 1998, Rodney Arne Karlsen wrote: > I think if you converted the audio to 1bit information like in an audio CD > that would bring down the size. 1 bit vs. 16 bit at same quality. I am not > shure how the 1 bit DAC works, baybe somone in the group could explain it to > us. I'm not sure about how 1-bit DAC works, either, although I suspect that it is something that decreases a manufacturer's costs more than it increases sounds quality. I think they put it on portable CD players for purely marketing reasons, eg. "Now with 1-bit DAC and Fritzen Jammin circuits!" In any case, CDs still store the audio information in 16 bit per sample per channel, 44,000 (or is it 48,000? I forget) samples per second. Sum total: about 10MB per minute of music. There are only two ways to decrease the storage requirements for sound that I know of. The first is the easiest. You simply decrease the sample size and/or rate. This has the obvious effect of decreasing quality as well. However, in a wearable, CD-quality audio is probably not the biggest concern, and telephone-quality audio would be acceptable for verbal alerts and such, and is relatively low bandwidth. The other way to decrease your storage requirements is to go the route of mp3s (MPEG-II, layer three, to be precise), which uses perceptual encoding. This is the process of removing the parts of the sound that we know from experience that a human won't be able to hear. To use a slightly flawed example, if you recorded the sound of a pin dropping on a table, you'd be able to hear that in the playback; and if you recorded a brick dropping on the same table, you'd be able to hear that in the playback as well; but if you recorded the pin and the brick dropping at the same time, you probably wouldn't be able to hear the sound of the pin over the sound of the brick. So, why encode the sound of the pin in the first place? To achieve this, mp3 encoding breaks up the signal into 40 different frequency bands, and cuts the sound into 70 (or is it 72?) frames a second. It performs this pin-vs-brick comparison on all the frequency bands in each frame. The problem with perceptual encoding is that it is processor-intensive. As has been noted on this list, it would take a very fast processor to encode CD-quality sound at realtime, and it takes at least a Pentium-100 or so to decode the same in realtime. Not to mention patent problems with the Fraunhofer Institute. I hope I didn't bore those of you who already know this. I wanted to get it out there for everyone, just in case. Paul Archer ---------------------------------------------------- A key to the understanding of all religion is that a god's idea of amusement is Snakes And Ladders with greased rungs. -- Terry Pratchett, "Wyrd Sisters" ---------------------------------------------------- -- Subcription/unsubscription/info requests: send e-mail with subject of "subscribe", "unsubscribe", or "info" toWear-Hard Mailing List Archive (searchable): http://wearables.ml.org
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