30 Nov
1998
30 Nov
'98
10:45 p.m.
> Possibly. But I distinctly recall that when we
recorded tapes for sale
> using the tape deck from my Sanyo stereo (not on a PET BTW - this was a
BBC
micro) we
found that Dolby noise reduction had to be disabled for it to
work...
Did you try recroding with Dolby and then replaying on a machine with
Dolby noise reduction (turned on), or replaying on the normal cheap
cassette recorder that you use with computers? If the former, then I am
not suprised it didn't work - the frequency response would have been
rather odd. The latter should have worked, though. I would be suprised if you couldn't make a CD that
could be loaded. I
can't try it because I have no way of writing to a CD.
I never meant to imply that you couldn't make a CD that could be loaded.
What I meant was you probably have to be more sophisticated than old
cassette -> digitised audio -> audio CD. I'd recommend old cassette ->
signal restoration -> digital signal (0s and 1s sampled at some highish
speed) -> possibly prefilter to pre-emptively undo the CD player's output
filter -> digitised audio -> audio CD.
For PET (and family) tapes a C2N would probably make a good cassette
machine for playing it initially, since it does some of the signal
restoration itself.
Philip.
1 Dec
1 Dec
7:55 a.m.
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12:58 p.m.
At 03:19 PM 12/1/98 -0800, Cameron wrote:
The question is, does source => Dolby encoding =>
Dolby decoding == source?
Dolby encoding, AFAIK, increases the volume of sounds >10KHz, and decoding
does the reverse.
Dolby compresses the sound track in the high end causing high frequencies
to get a lot higher. Generally static is thus moved out of the reproducible
range of the tape and when the signal is decompressed the "hiss" is gone.
And yes, it alters the frequency spectrum of the signal source in an effort
to remove "noise".
--Chuck
1:40 p.m.
New subject: Dolby Enc/Dec: Tangent to List Topic
On Tue, 1 Dec 1998, Chuck McManis wrote:
At 03:19 PM 12/1/98 -0800, Cameron wrote:
Okay... This (I think) is at least *close* to the topic of this
List, since many of our machines are designed with cassette tape I/O
as secondary storage...
HOWEVER: The above posts reflect some general mis-conceptions re:
just what those "D"s do to signals on/in a medium of storage or
transmission.
Humble Disclaimer:
I am a complete novice concerning many aspects of the hobby of the
care and feeding of elderly computers... but professional and cinema
audio is how I have made my living for most of my life, and for the
last almost ten years it has been as the Chief Engineer of M.G.M.
Studios.. I *think* I can help y'all with how Dolby A, B, C, SR, and
HQX work... since I have designed and built and use every day
systems with these encoding and decoding schemes, as well as dbx,
Telcom C-64, Ultra*Stereo, NatSemi DNR..... etc.
First, with most small cassettes, we are talking Dolby 'B' or 'C';
these DO NOT 'alter' the frquency (ie the pitch) of the signals...
they do not shift the spectrum in any way.. [Cameron is closer in his
post], but each uses band-limited compression and decompression to
reduce the overal signal in given frequency bands, store or transport
the signal across a noisy medium, then decompress at the destination.
Any noise *existing* in the original will be reproduced, but noise
The question is, does source => Dolby encoding
=> Dolby decoding == source?
Dolby encoding, AFAIK, increases the volume of sounds >10KHz, and decoding
does the reverse.
Dolby compresses the sound track in the high end causing high frequencies
to get a lot higher. Generally static is thus moved out of the reproducible
range of the tape and when the signal is decompressed the "hiss" is gone.
And yes, it alters the frequency spectrum of the signal source in an effort
to remove "noise".
--Chuck
from the media will be reduced. These systems can also
add distortion
of various kinds when Things go wrong... an understanding of the
nature of the processes can help in restoration of data media made
by older systems and (cheap) cassette decks.
I have detailed info (many hundreds of pages) on each of the
common Noise Reduction schemes, and I would be glad to correspond
PRIVATELY with anyone interested in the real poop on how NR is
implemented in audio systems. I am also working on a document for
the Archives (maybe a FAQ supplement) on exactly how the common NR
schemes work as they specifically relate to cassette-based computer
data storage.
At least I hope this will be of use to some of us who have old
tapes to restore and preserve..... and the physics, chemistry, and
electronics of magnetic recording in general is a fascinating and
quite beautiful topic in and of itself... IMHO.
Cheers
John
PS: Help Prevent Topic-Drift! E-mail me privately unless it's list
oriented... thanx!
3 Dec
3 Dec
2:39 a.m.
> Possibly.
But I distinctly recall that when we recorded tapes for sale
> using the tape deck from my Sanyo stereo (not on a PET BTW - this was a
BBC
micro) we
found that Dolby noise reduction had to be disabled for it to
work...
Did you try recroding with Dolby and then replaying on a machine with
Dolby noise reduction (turned on), or replaying on the normal cheap
cassette recorder that you use with computers? If the former, then I am
not suprised it didn't work - the frequency response would have been
rather odd. The latter should have worked, though. I can't
remember if we recorded with and played back with - I imagine that would
work
Should work. Because what Dolby does to the sound it undoes afterwards and
you theoretically have a recording closer to the original than normally
possible - without the extra hiss due to the recording process.
Theoretically you shouldn't lose any hiss from the original. But, even if
you did, the computer doesn't really need it.
- but we definitely couldn't get it to work recording without and
playing back with, although this actually works quite
well for music.
I've noticed exactly the opposite. Recording with and playing without gives
a 'crisper' sound.
But, doing any of these two would be altering the sound.
I would be
suprised if you couldn't make a CD that could be loaded. I
can't try it because I have no way of writing to a CD.
I never meant to imply that you couldn't make a CD that could be loaded.
What I meant was you probably have to be more sophisticated than old
cassette -> digitised audio -> audio CD. I'd recommend old cassette ->
signal restoration -> digital signal (0s and 1s sampled at some highish
speed) -> possibly prefilter to pre-emptively undo the CD player's output
filter -> digitised audio -> audio CD.
Possibly a better result. If you know
what you are doing.
Nasos
9363
days inactive
9365
days old
5 comments
6 participants
participants (6)
-
a.kotsenos＠rca.ac.uk -
ard＠p850ug1.demon.co.uk -
ckaiser＠oa.ptloma.edu -
cmcmanis＠freegate.com -
jpl15＠netcom.com -
Philip.Belben＠pgen.com