[vorbis] RipTrax Update Available

[John Lundy]

David,

----- Original Message -----
From: "David K. Gasaway" <dave at gasaway.org>
To: <vorbis at xiph.org>
Sent: Thursday, February 14, 2002 11:22 PM
Subject: Re: [vorbis] RipTrax Update Available

<p>> John,
>
> I'm a bit confused by your description of the new TruTrax feature.
> You seem to be describing a feature which improves the all-around
> quality of audio extracted from a CD.  But, you say:
> "With TruTrax functionality, the user can position the read heads to
> the exact sample where a track begins and extract the exact number of
> samples intended."
> Which suggests that I, through my own will, would be meticulously
> positioning the laser somehow.  I'm just supposed to check the box
> and see if it works, no?
>

At this moment, both yes and no. I'll explain. A CD drive, when told to read
track 5 (which is sector 12345) will position the head pretty close, but not
exactly at sector 12345. Unlike a data CD, the positioning system for audio
is based on timing rather than data information (I know, it's a combination
of both, but for our discussion this works). So, due to inaccuracies in
drive mechanics and electronics, if you tell the drive to go to sector
12345, a disk will actually position its heads over sector 12345 with an
offset of 635 samples; instead of balls-on 12345 offset 0. So, with TruTrax,
if you know the positioning error (in our example, 635 samples), you tell
the TruTrax settings this value, tell it to use the value in the ripping
process, and shazam; you rip the track just as it was intended to be read
(the exact data that was writen).

If you think about it this all makes sense. The CD industry forceably puts 2
seconds of silence after every music track (except for some concert CDs and
the like). This is the reason for the 2 second slop; drives are not exact,
just close.

Now, in RipTrax the measure button forces a test of the center track on an
audio CD and measures the time from the start of a track (as recorded in the
TOC) and when the first non-silence begins; in samples. Try it. Select a
non-concert audio CD in a drive and measure the amount of silence. Now, put
that same CD into another drive and measure it. Most likely the two drives
will return different measured values. Now here is the problem area. The
measured value is all silence. What we don't know is how much was intended
silence and how much was due to drive position error. And this is a
difficult problem. However, there is a solution. It turns out that EAC has
resolved these offset values for many, many CD drives. The values used by
EAC will work, without modification as the TruTrax offset. I'm working on a
technique to accurately measure this value but it is next to impossible to
know what is deliberate silence and what is error.

I hope that helps a bit. This option is really for extremists that just
absolutely *must* have only the song they want without garbage. And they
have valid reasons. It has to do with recording off of recordings of
recordings of original CDs. If at every step of the process the drive error
offset is considered in the ripping and duplication process, the last CD
made will be as good as the first CD made. No accumulated errors.

John

<p><p>--- >8 ----
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