From: kludge@grissom.larc.nasa.gov ( Scott Dorsey)
Subject: Re: Uncorrected errors on digital transfer
Organization: NASA Langley Research Center and Reptile Farm
Date: Fri, 31 Jul 1992 12:20:41 GMT

In article <712557375snx@black.demon.co.uk> tchannon@black.demon.co.uk (Tim Channon) writes:
>Any uncorrected error at any point in a mastering chain may cause a hard 
>coded error. I have a lot of anecdotal evidence that this is a common 
>occurance and in my opinion can be responsible for some of the bad sounds 
>which some of our 'golden ear' friends do not like about CD. On top of that 
>there are replay errors on the CD which are yet again audible.
>

True enough.  If there is an error in one sample, which is a fairly common
occurance (happens two or three times a minute on the DAT decks I played
with), the system will interpolate a reasonable value for the sample and
replace the value in error with the interpolated one.  (While it's possible
for an error to go undetected, it's amazingly unlikely; probably a once in
a lifetime sort of thing).

Now, I can't hear any change when this happens, because the interpolation
algorithm is pretty good, and more importantly the single sample represents
such a short amount of time.

>It is high time that the issue was fully addressed so what better than 
>starting to shout about the problem.

It's not that big an issue, since it only happens a few times a minute
(worst case), and isn't an ongoing thing.  On extremely damaged tapes, the
error interpolation can cause a noticeable garbling, but this is because 
of the enormous number of interpolations performed.

>Here is an example of the problem:
>A recording is mastered on DAT. The recording is then digitally transfered to 
>some other system. 
>Unfortunately DAT (in common with all other media) does produce occasional 
>uncorrectable errors. Now, if you listen to the DAT analog output you will 
>probably hear nothing unusual because the DAT interpolates over the error -- 
>** this is not true of the digital output ** where the sample will be flagged 
>as unreliable but will _not_ be interpolated.
>I know of no DAT that interpolates digital output or equipment which 
>interpolates on digital input.
>If you listen on equipment connected to the digital link you may hear bad 
>noises.

No, not really, since the equipment connected to the digital link should do
the interpolation also.  Remember that the errors are occurring very seldom.
More seriously I would worry about the fact that many DDD recordings are made
on a digital recorder, mixed in the analogue domain and converted onto
another digital recorder, which is played back over an analogue link into
the mastering recorder.  On each A/D and D/A conversion, the amount of
group delay increases exponentially.  This is much more responsible for
bad sound.

>I recently designed commercial equipment which happens as a side issue to 
>indicate flagged errors on the digital link. Watching this is educational and 
>even CD players can show a great deal of bad data being output.

You can tie an LED to the error lines on your CD player and watch them flash
from time to time.  I have a number of CDs whose coating was not very well
sputtered (holes as large as 5 mil in diameter in the coating), and a large
number of errors occur in sequence on some tracks.

>A cure for some of this on CD is to clean the disk! Honest! 
>The advice of 'do not touch the face of the disk' is good.

True enough.  Scratches, by the way, can be polished out with car body wax.

>The transfer of material to a CD mastering plant on UMatic is a potential 
>source of these errors. I have spoken to some plants (sic) and there is 
>either no admitted knowledge of the problem or a policy is implemented. The 
>policy seems to be to make a decision as to whether to reject the tape. I am 
>unconvinced that all errors will be caught even if monitoring has taken place 
>at the CD plant.
>Then there is the matter of producing a UMatic tape containing perfect data.

This is also true, but I think you are neglecting the fact that the individual
interpolations aren't too detectable, and the fact that the number of total
uncorrected errors is (generally) very small.  While it's true that tapes
exist that have a high error rate, there are also a lot of analogue tapes out
there with azimuth skew.  Proper maintenance will eliminate these problems.
A lot of studios think that because they have gone to digital systems that
they don't ever have to realign their equipment anymore, and this is quite
patently false.
--scott