At 06:41 AM 2010-01-21, Tom Fine wrote:
>Ted, I understand the point of knowing about an error. But, does the
>tape-drive-to-computer method CORRECT the error for the purpose of
>USING the audio? That's the whole point of error-correction in the
>DAT machine design (and in CD players) -- to make the audio stream
>usable to the average listener. What is the point of transfer if the
>audio can't be used? So far no one has confirmed that the
>direct-to-computer method provides as good error correction
>abilities as just playing the DAT.
Please do not confuse error CORRECTION and error CONCEALMENT. DAT,
audio CD, CD-ROM, and DDS have a layer of error CORRECTION. Audio DAT
and audio CD have an error CONCEALMENT layer AFTER the error
CORRECTION to compensate for errors that were beyond the scope of the
error CORRECTION. DDS and CD-ROM, I believe, both have an extra layer
of error CORRECTION. However, this is not implemented on reading the
In other words, both the DDS drive and the DAT player will have the
same level of error CORRECTION for the DAT (audio) tape. The DAT
player will add error concealment which is not necessarily what we
want when making a preservation master.
>As to Shai's point about multiple passes -- I too have had success
>once in a while re-playing what had been a dropout. It doesn't work
>all the time, but often enough that I'll do it with problem tapes.
Yes, that can either get better or worse -- and cleaning can also help.
>And yes, problem tapes can have the "error" indicator light flashing
>constantly or on for long periods of time yet recovered music stream
The "error" indicator comes on at some threshold within the
capability of the error correction protocol in most machines. The
Panasonic SV3800s that I have provide a four-digit error readout.
Many machines have a two-digit error readout which is essentially the
two left digits of the four-digit readout on the Panasonic. I believe
this is a hexadecimal readout, but I'm not sure at the moment.
However, on most playback of normal tapes, I get "error" readings up
to let's say 02xx and those are all correctable. Where I start to
hear problems are around 07xx and higher. When I made the tapes, I
rarely saw errors as high as 0100. So any errors below that would not
even show on a two-digit machine. I think I remember being upset when
I saw an error into the 004x region. It stays for long stretches of
time at 0000.
>One other point -- you have the same mechanical issues with a
>computer drive as a DAT machine, no? It's the same method of a
>rotary head recovering data from a magnetic tape, isn't it?
It's the same method of recovery, but the computer drive may (or may
not) be more robust than the audio drive/transport.
If I recall properly, there were two different drum sizes that were
somehow made to be compatible. The porta-DATs had a drum of half the
diameter of the studio/home machines. I'm keeping my D8 DAT Walkman
as an alternative for recovering some tape in the future where this
might be a benefit.
At 04:14 AM 2010-01-21, Shai Drori wrote:
>Okay, that I understand, but I am thinking about correctable errors.
>We are then assuming that both systems will correct the errors the
>same way since both use the schemes implemented. How do we know
>which system has fewer errors over the other? My experience with
>rotary head systems is that sometimes second or third reading
>yielded better results, I think due to "cleaning" actions of the
>previous playing. Maybe we should compare five readings of the same cassette?
As I understand it, the DDS tape readout software will flag any
uncorrectable error in a log file (which I've also referred to as a
printout). I believe it logs it by A-time.
I also recall that the DDS playback method will fail if there is no
ATIME on the tape.
As to your comment about correctable errors -- who cares where there
were correctable errors as they were corrected. All hard drives today
have error correction and it's used regularly in normal usage.
Also, don't confuse analog playback with digital playback. With
digital, all you need to do is keep the error level within the
correctable range and you're done. With analog, there is NO error
correction and you need to get everything absolutely as good as you
can. The Drop Out COMPENSATORs in analog (video) machines are the
same as the error CONCEALMENT protocols in digital machines.
In audio, our equivalent of "drop out compensators" was increasing
tape surface area per unit time. In pro video formats there was
little option for changing track width or tape speed. In some
respects, that is why 15 in/s 1/2-inch 2-track with IEC equalization
ended up being close to the ideal stereo mastering format for analog
audio. That seems to be the point where cost, performance,
low-frequency response, high-frequency response, noise, and azimuth
wander are all optimized. 0.2 inch x 15 in/s is 3 sq in/s of tape per
track. Compare that to DAT or analog cassette...you do the math <smile>.
Richard L. Hess email: [log in to unmask]
Aurora, Ontario, Canada (905) 713 6733 1-877-TAPE-FIX
Detailed contact information: http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.