On Mon, 14 May 2001, Dr Alan McIvor wrote:
> Hi,
>
> I recently had to grab some video from VHS videotape using a BT878 and
> found it necessary to alter the brightness and contrast settings in the
> BT878 input section to get a decent picture. Even then, the quality
> wasn't particularly good. Having little experience digitizing from
> videotape, I would like to know if this is something that normally has
> to be done in such situations, or is indicative of problems with the
> videotape or VCR.
>
> Thanks
>
> Alan
>
VHS introduces some degree of loss on the original
video by nature. In fact all the modulation techniques
do so. PAL/NTSC also do this with any RGB stream.
Cable and Antenna video signals are QAM-VSB modulated.
They need to be demodulated into a composite video stream
to be recorded using Beta/VHS record methods.
Beta is almost dead. VHS is more widespreaded now.
Both methods use a technique similar to PAL over NTSC,
e.g., phase-shifting by line.
In simple terms: they record chroma using AM into
a carrier that is FM modulated w/luminance. Smart tech.
Audio and sync are completed separated.
On the chroma signal:
Beta uses 2 pass phase-shifting (1H comb-filter).
VHS uses 4 pass phase-shifting (2H comb-filter).
VHS is an improvement over Beta in a way similar
as PAL is an improvement over NTSC.
These modulation techs. rely on a device
composed by a "delay line" as a "comb-filter"
to minimize "crosstalk" among adjacent regions
of the frequency spectrum and between adjacent
tracks of the magnetic tape. Smart approach.
The "brightness" you refer is the luminance
level (FM strength (or bias) ) impressed on
the tape. But colors also suffer this.
Anyway you can expect inferior result from the
original always. You can sum all losses: PAL/NTSC
modulation + Beta/VHS modulation + the magnetic
intrinsic one.
However all this stuff allow high density
recording using Beta/VHS with some degree of
controlled loss (2~8 hours) into some dozen
meters of 1/2 inch tape using helicoidal sweep.
Tons of video fields at *REAL* 30fps (the
video head is in sync @60Hz).
In fact what you see on the monitor/TV screen
is a bit different from what is really on the
tape. There are several steps of De/Modulation
involved between them and the control is really
Eletro-Magneto-Mechanical and not "visual".
You can be shure that they are different and chances
are that they will look different in different
devices with different EMM characteristics (bias).
As a suggestion:
- avoid modulated signals, e.g., use RGB when
possible over composite video or Cable/Antenna
- avoid slow speeds on VCRs. Tape speed increases
the "distance" over adjacent tracks thus minimizing
crosstalk.
- S-VHS is an improved version of VHS with already
separated signals on the wiring. If you can, use it.
- Use high quality magnetic media.
- Clean your video heads.
Hope this helps.
Abracos
PauloCastro
trap error: Excuse my bugs and typos...
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