Terrestrial Switchoff - sorry to labour the point but...
 

In message , SpamTrapSeeSig
writes
In article , Clive
writes
In message , Jim Lesurf
writes
The recording side is rather different as the 'trailing edge' of the field
being applied tends to 'wipe' what other parts previously imposed. Hence
the angled offset 'cross field' recording systems can have a smaller and
more controlled effective gap.
As I remember it, audio will not leave an imprint on the tape because
of the hysteresis which is needed to keep the recorded item on the
tape. A high frequency waveform is applied to the tape very much the
same as a Carrier wave carries signal from one place to another , the
only difference being that the carrier is modulated in an A.M. Radio,
whereas bias is used to "punch" the signal into the tape pass the
hysteresis boundary and is at a constant level set by the
characteristics of the tape.
I don't know where you got that lot from, but Jim's quite correct.
Bias is to make the transfer characteristic as linear as possible, and
I'm fairly certain machines without bias were used in the early days of
magnetic recording. They worked, just not very well. I think Marconi
Stille machines didn't use bias, but I may be wrong about that.
Hysteresis is a physical property of the recording medium, not
something you do to it.
I agree with your last sentence, hysteresis is a property of the
magnetic medium, without it as soon as you removed the polarising field,
the field in the medium would go as well. The bias does exactly the
same to cold tape as heat does when copying at high speed and a high
temperature when it exceeds it's Curie point only whilst copying and
dropping below it to retain the magnetic image. I can remember a tape
recorder that had a DC magnet as the bias, it was appalling.
--
Clive

Terrestrial Switchoff - sorry to labour the point but...
 

In message , charles
writes
and usually known as "Hanover Bars"
Perhaps it's known as different names depending on where you were when
learning, these names tend to stick.
--
Clive

Terrestrial Switchoff - sorry to labour the point but...
 

In article ,
Clive wrote:
In message , charles
writes
and usually known as "Hanover Bars"
Perhaps it's known as different names depending on where you were when
learning, these names tend to stick.

you mentioned Fnat, so I assume you learned things at the BBC. I don't
think anyone else used it - or even knew what it was.

--
From KT24

Using a RISC OS computer running v5.18

Terrestrial Switchoff - sorry to labour the point but...
 

On Fri, 29 Mar 2013 19:40:31 +0000, SpamTrapSeeSig
wrote:

In article , Clive
writes
In message , Jim Lesurf
writes
The recording side is rather different as the 'trailing edge' of the field
being applied tends to 'wipe' what other parts previously imposed. Hence
the angled offset 'cross field' recording systems can have a smaller and
more controlled effective gap.
As I remember it, audio will not leave an imprint on the tape because
of the hysteresis which is needed to keep the recorded item on the
tape. A high frequency waveform is applied to the tape very much the
same as a Carrier wave carries signal from one place to another , the
only difference being that the carrier is modulated in an A.M. Radio,
whereas bias is used to "punch" the signal into the tape pass the
hysteresis boundary and is at a constant level set by the
characteristics of the tape.

I don't know where you got that lot from, but Jim's quite correct.

Bias is to make the transfer characteristic as linear as possible, and
I'm fairly certain machines without bias were used in the early days of
magnetic recording. They worked, just not very well. I think Marconi
Stille machines didn't use bias, but I may be wrong about that.

Hysteresis is a physical property of the recording medium, not something
you do to it.

Originally, an actual DC bias was used to overcome the hysteresis
effect which caused a form of distortion analogous to that exhibited
by a sticking loudspeaker voice coil[1] where signals below a certain
threshold would fail to record (just as very quiet levels would fail
to be reproduced by a loudspeaker suffering a sticking voice coil)
with higher levels producing a very gritty sounding distorted parody
and very loud levels producing an almost distortionless recording due
to the much stronger recording masking out the effects of the
distortion.

This worked but at the cost of increased noise and reduced dynamic
range. The noise increase was the result of two seperate mechanisms.
The first being simply due to turning a random collection of magnetic
particles into a random collection of magnets. The second being down
to scrape modulation now having a permanently magnetised source to
work with.

Although erase typically took the form of a swingaway permanent
magnet to remagnetise to saturation in the opposite sense to the DC
bias, you still had to avoid reversals of magnetic flux by the
combined effects of the DC bias and recording signal, effectively
halving the amplitude range that was potentially available.

Ultrasonic AC bias and erase, provided the solution to all of the
above shortcomings. Although an AC signal approximately ten times the
maximum recording signal level isn't normally regarded as being a bias
of any sort in any other system, the name stuck by virtue of its
original function in magnetic recorders and an absence of a ready to
hand substitute word to replace it.

The only concession to the use of ultrasonic AC instead of DC being
the addition of the AC suffix to distinguish it from its precursor.
Nowadays, hearking back to the 60s, any references to tape recording
bias are made on the assumption that the original DC bias technique
has long since been obsoleted by the use of AC bias and the AC suffix
is generally discarded as being superfluous. The assumption being that
any mention of bias is a reference to AC bias unless, exceptionally,
ancient recording machines are being discussed when it is assumed the
DC prefix will be applied to avoid any confusion.


[1] Alternatively the other analogy commonly used was that it was akin
to the 'crossover distortion' that would ensue in a class B amplifier
lacking the necessary bias required to avoid the 'dead zone'.
--
Regards, J B Good

Terrestrial Switchoff - sorry to labour the point but...
 

In message
,
Curtis Interruptus writes
I get the impression that you think that you understand PAL (and by
definition NTSC), so I'll keep it simple for you
Thank you
Hanover lines are caused, in simple PAL, by a phase shift on alternate
lines leading a "venetian blind" effect on the image
The phase shift occurs to the chroma signal, (two in quadrature, giving
one resultant sub-carrier) whose phase is concurrent with the desired
colour has rotated about it's axis because of sideband attenuation in
either or both of the two colour difference signals. So where in NTSC
you'd get a colour shift that you can partially correct with a hue
control, in simple PAL you would get a two line structure (each line
being either side of the required colour giving narrow lines (bars if
you like) crawling up the screen .
Alternate lines,
when a phase error is present, swing alternately towards the red and
blue axes (as seen on a vectorscope), a delay line system will
effectively add the errors and display the difference between the two
(but given the way that the summation takes place, the saturation will
decrease at the cost of displaying the correct chroma phase).

The dot patterning to which I assume that you refer, is caused when
there is a fixed mathematical relationship between the CSC and line
frequencies. (283.75 in the original 625/50 PAL). Older readers may
know this frequency as (F)nat. The precision offset of 25Hz was added
resulting in (F)csc. The septics got round the same issue by changing
the Frame rate by 1% (hence 525/59.940.
The dot structure to which I refer can be seen at it's clearest on
colour bars between Magenta and Green stripes.
Thank you again for keeping it simple for me.
--
Clive

Terrestrial Switchoff - sorry to labour the point but...
 

In message , Johny B Good
writes
[1] Alternatively the other analogy commonly used was that it was akin
to the 'crossover distortion' that would ensue in a class B amplifier
lacking the necessary bias required to avoid the 'dead zone'.
Indeed, your reference to a class B amplifier is very apt, but I would
change crossover distortion which implies insufficient transistor base
bias to the output pair, to no base bias giving only the peaks positive
and negative and a flat or null signal crossover region, the harmonics
of which are dire.
--
Clive

Terrestrial Switchoff - sorry to labour the point but...
 

In message , charles
writes
you mentioned Fnat, so I assume you learned things at the BBC. I don't
think anyone else used it - or even knew what it was.
I'm sorry, but you've wrongly attributed this to me.
--
Clive

Terrestrial Switchoff - sorry to labour the point but...
 

In message , Clive
writes
In message , Johny B Good
writes
[1] Alternatively the other analogy commonly used was that it was akin
to the 'crossover distortion' that would ensue in a class B amplifier
lacking the necessary bias required to avoid the 'dead zone'.
Indeed, your reference to a class B amplifier is very apt, but I would
change crossover distortion which implies insufficient transistor base
bias to the output pair, to no base bias giving only the peaks positive
and negative and a flat or null signal crossover region, the harmonics
of which are dire.

With no DC bias, class B push-pull audio amplifiers have dire distortion
at small signals levels. The % distortion decreases as the signal level
is increased (as the waveform spends most of its time in the linear
region).

The distortion of a low-level signal (on one frequency) decreases when a
second signal (on a different frequency) is added (particularly if it
has a higher level) as this keeps the low-level signal in the linear
region for most of the time. In this respect, the effect is somewhat
similar to that of using AC bias for a record head.
--
Ian

Terrestrial Switchoff - sorry to labour the point but...
 

In message , Ian Jackson
writes
With no DC bias, class B push-pull audio amplifiers have dire
distortion at small signals levels. The % distortion decreases as the
signal level is increased (as the waveform spends most of its time in
the linear region).

The distortion of a low-level signal (on one frequency) decreases when
a second signal (on a different frequency) is added (particularly if it
has a higher level) as this keeps the low-level signal in the linear
region for most of the time. In this respect, the effect is somewhat
similar to that of using AC bias for a record head.
I agree.
--
Clive

Terrestrial Switchoff - sorry to labour the point but...
 

"Johny B Good" wrote in message
...

Originally, an actual DC bias was used to overcome the hysteresis
effect which caused a form of distortion analogous to that exhibited
by a sticking loudspeaker voice coil[1] where signals below a certain
threshold would fail to record (just as very quiet levels would fail
to be reproduced by a loudspeaker suffering a sticking voice coil)
with higher levels producing a very gritty sounding distorted parody
and very loud levels producing an almost distortionless recording due
to the much stronger recording masking out the effects of the
distortion.

This worked but at the cost of increased noise and reduced dynamic
range. The noise increase was the result of two seperate mechanisms.
The first being simply due to turning a random collection of magnetic
particles into a random collection of magnets. The second being down
to scrape modulation now having a permanently magnetised source to
work with.

Although erase typically took the form of a swingaway permanent
magnet to remagnetise to saturation in the opposite sense to the DC
bias, you still had to avoid reversals of magnetic flux by the
combined effects of the DC bias and recording signal, effectively
halving the amplitude range that was potentially available.

Ultrasonic AC bias and erase, provided the solution to all of the
above shortcomings. Although an AC signal approximately ten times the
maximum recording signal level isn't normally regarded as being a bias
of any sort in any other system, the name stuck by virtue of its
original function in magnetic recorders and an absence of a ready to
hand substitute word to replace it.

Reminds me of my old Grundig Cub recorder, with spool drive, DC bias and
permanent magnet erase, which worked surprisingly well, for speech at any
rate. I think that there were some really cheap cassette recorders that used
DC bias and erase.

--
Max Demian