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

"Andy Wade" wrote in message
...
On 21/03/2013 18:58, Woody wrote:

\pedant mode on

There were five channels in Band 1 and eight channels in Band
III. They were only about 6.5MHz wide as against 8MHz used today.

\pedant mode off

A c t u a l l y , dear pedant, I think you'll find that 405-line system A
used 5 MHz channelling, or perhaps I should say 5 Mc/s.

European 625-line is (or was) System B in the VHF bands and used 7 MHz
channels, not 8. This was never used in the UK of course. 8 MHz
channelling only appears at UHF, System G for the mainland and System I
(with 6 MHz sound and originally a wider VSB) for the UK, also used in
South Africa.

DVB-T (and -T2 AFAIK) have a 7 MHz channel option to allow use in the VHF
bands. Do any countries use this?

Andy,

Australia does. They appear to have all their major networks on VHF with
DVB-T in 7 MHz channels. Also Potsdam in Germany.

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

"Ian Jackson" wrote in message
...
In message , Stephen
writes

"Ian Jackson" wrote in message
...
In message , Stephen
writes

If they sold off the whole of UHF, we could go back using VHF for TV if
we
just forgot about the regional news. Freeview only needs 6 channels for
single frequency networks and these 6 should be available on VHF Bands I
and III.

Only the regional TV variations need to be relegated to satellite, cable
&
internet.

The VHF bands are much less attractive to mobile phone operators because
of the much longer aerials required (particularly for Band I) but they
are
perfectly good for terrestrial TV, and a new VHF TV aerial would be
cheaper than a dish, and much easier to align. We might return to the
days
of H
aerials, X aerials and Band III Yagis, but with digital TV on VHF
instead
of 405 lines.

There are only eight or nine 8MHz VHF TV channels available (3 in Band 1
and 6 or 7 in Band 3) - and it would be extremely difficult
comprehensively to provide the whole of UK with 6 digital MUXes in the
same way as the two analogue channels were provided. At best, the full 6
MUXes could probably only be provided in well-separated centres of large
populations.

I don't think this is the case. They should be able to use the same 6
channels at all transmitters like National DAB. A VHF network for Freeview
could be designed alongside an upgrade to DAB/DAB+, using the same
transmitting aerials and the same transmitter sites.

It may only need 5 multiplexes as there would be no need to duplicate the
4
main channels (BBC1 HD etc) in Standard Definition as is done at present.
No
duplication of DAB radio on Freeview VHF would save some more space. The
system could use the European 7 MHz channel standard to integrate with DAB
which uses the same standard.

It might work out something like this: Channels E6, 7, 8, 9, 10 for
DVB-T2,
and Channels E5, 11, 12, 13, 14 for DAB+.

Because of the need to prevent co-channel interference, it took five Band
1 and eight Band 3 channels to provide two nationally-available 405-line
analogue channels. OK, digital may be more resistant to the effects of
co-channel, but it seems extremely optimistic that five adjacent channels
at the bottom end of Band 3 could provide five MUXes nationally. If it
were possible, surely they would already be doing this in Band 4 (say
channels 21 to 25), leaving the rest of Bands 4 and 5 for 'other things'?
--
Ian

AFAIK DVB is designed for single frequency network operation but it wasn't
implemented initially. I think they intend to use it for some future
mulitplexes.

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

In message , Peter Duncanson
writes:
[]
Somewhere I have a pair of headphones with "laser" in the name. They do
not send beams of light into the ears!

No; with some of the target customer base, it might come out the other
sides ... (-:
--
J. P. Gilliver. UMRA: 1960/1985 MB++G()AL-IS-Ch++(p)[email protected]+H+Sh0!:`)DNAf

"So, I take it you've ... been with a man before?" "I'm a virgin. I'm just not
very good at it." Topper Harley & Ramada Thompson (Charlie Sheen & Valeria
Golino), in "Hot Shots!" (1991).

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

In article , Java Jive
writes
the top end of AC is limited
by the slow tape speed to about 16kHz or so

It's inversely proportional to the replay head gap. Tape speed
theoretically doesn't have anything to do with it (until you get down to
1/2 wavelength ~ particle size), although it does affect the noise
characteristics.

The recording head gap doesn't matter much (within reason) as recording
happens as the tape leaves the head's field, not in the gap itself.

--
SimonM

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

I've forgotten now whatever hardware level theory I once knew, it's
all a jumble of bias frequency and tape hysteresis, but ...

On Thu, 28 Mar 2013 09:51:33 +0000, SpamTrapSeeSig
wrote:

In article , Java Jive
writes
the top end of AC is limited
by the slow tape speed to about 16kHz or so

It's inversely proportional to the replay head gap.

I remember gap width as being important as well. When I replaced a
set of worn heads on an old TC-330 the improvement in the top end was
noticeable.

Tape speed
theoretically doesn't have anything to do with it (until you get down to
1/2 wavelength ~ particle size), although it does affect the noise
characteristics.

Whatever the theory, in practice I'm right about the tape-speed
determining the top cut-off point (and actually these figures are much
worse than I remembered them, wish I'd kept the manual when I threw
out the machine):
http://www.ebay.com/itm/Original-Fac...em19 d8b33ae0

Open-reel section
30Hz - 18KHz at 19cm/s
30Hz - 13kHz at 9.5cm/s
30Hz - 7kHz at 4.8cm/s

Audio-cassette section
50Hz - 10kHz

The recording head gap doesn't matter much (within reason) as recording
happens as the tape leaves the head's field, not in the gap itself.

My experience above of replacing knackered heads leads me to disagree
with this as well.

Cue Jim Lesurf?
--
================================================== =======
Please always reply to ng as the email in this post's
header does not exist. Or use a contact address at:
http://www.macfh.co.uk/JavaJive/JavaJive.html
http://www.macfh.co.uk/Macfarlane/Macfarlane.html

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

In message , Java Jive
writes
Whatever the theory, in practice I'm right about the tape-speed
determining the top cut-off point (and actually these figures are much
worse than I remembered them, wish I'd kept the manual when I threw
out the machine):
Indeed you are, it's a combination of tape head width and tape speed,
higher speed and smaller tape head gap mean better frequency response,
track width is the one that seriously affects noise, increasing by 3dB
for halving of the track width.
--
Clive

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

In article , Java Jive
writes
I've forgotten now whatever hardware level theory I once knew, it's
all a jumble of bias frequency and tape hysteresis, but ...

On Thu, 28 Mar 2013 09:51:33 +0000, SpamTrapSeeSig
wrote:

In article , Java Jive
writes
the top end of AC is limited
by the slow tape speed to about 16kHz or so

It's inversely proportional to the replay head gap.

I remember gap width as being important as well. When I replaced a
set of worn heads on an old TC-330 the improvement in the top end was
noticeable.

TC330s had notoriously soft material for the heads, and a very narrow
replay gap to increase the frequency response (I think there was a
_very_ pronounced dovetail shape to the gap, too. They didn't stay
fettled for very long as a consequence, and I always smile when I see
the occasional one come up on eBay.

I can't understand what punters think that type of thing is going to do
these days -- it's like the penchant for germanium transistor circuitry
in studios: "move on, nothing to see (or listen to) here." Someone was
recently trying to sell bits of a mono Longden desk for £13,000. I can't
deny that JL did a brilliant job creating them, but using it now?

As I get older I realise the world is far weirder than I could possibly
have imagined as a teenager.

S.


--
SimonM

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

On 28/03/2013 10:49, Java Jive wrote:
I've forgotten now whatever hardware level theory I once knew, it's
all a jumble of bias frequency and tape hysteresis, but ...

On Thu, 28 Mar 2013 09:51:33 +0000, SpamTrapSeeSig
wrote:

In article , Java Jive
writes
the top end of AC is limited
by the slow tape speed to about 16kHz or so

It's inversely proportional to the replay head gap.

I remember gap width as being important as well. When I replaced a
set of worn heads on an old TC-330 the improvement in the top end was
noticeable.

That's because as the head wears, the gap becomes wider and also loses
its straight edges. The effect is mich more noticeable on a head used
for playback than on a head used to record.

Tape speed
theoretically doesn't have anything to do with it (until you get down to
1/2 wavelength ~ particle size), although it does affect the noise
characteristics.

Whatever the theory, in practice I'm right about the tape-speed
determining the top cut-off point (and actually these figures are much
worse than I remembered them, wish I'd kept the manual when I threw
out the machine):
http://www.ebay.com/itm/Original-Fac...em19 d8b33ae0

Open-reel section
30Hz - 18KHz at 19cm/s
30Hz - 13kHz at 9.5cm/s
30Hz - 7kHz at 4.8cm/s

Audio-cassette section
50Hz - 10kHz

All about average for the period, limited by both the playback EQ curves
as laid down in the standards and the width of the gap in the playback head.

The recording head gap doesn't matter much (within reason) as recording
happens as the tape leaves the head's field, not in the gap itself.

My experience above of replacing knackered heads leads me to disagree
with this as well.

The most important thing about the recording head is the straightness of
the trailing gap edge. I used to own an Akai X-IV portable with a
seperate bias head that would play back the bias tone from the
crossfield head at audible frequencies if you reduced the tape speed enough.

It used the same head for recording and playback, so would have been
capable of recording frequencies of more than 20KHz at 7 1/2 ips if the
electronics had let it. I really regret letting that recorder go.

--
Tciao for Now!

John.

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

On Thu, 28 Mar 2013 11:07:06 +0000, Clive
wrote:

In message , Java Jive
writes
Whatever the theory, in practice I'm right about the tape-speed
determining the top cut-off point (and actually these figures are much
worse than I remembered them, wish I'd kept the manual when I threw
out the machine):
Indeed you are, it's a combination of tape head width and tape speed,
higher speed and smaller tape head gap mean better frequency response,
track width is the one that seriously affects noise, increasing by 3dB
for halving of the track width.

Isn't topic drift amazing? Having asked that rhetorical question, I'm
going to add my tuppence worth:

Analogue magnetic tape recording presented quite a large number of
technical challenges which needed to be addressed before useful
devices could be marketed.

The obvious ones being speed stability (both absolute and dynamic),
frequency response and signal to noise ratio which applied to both
vinyl and magnetic systems. Speed stability is a fairly trivial issue
of good mechanical design which, for quality products, was solved
fairly early on in the development cycle. Frequency response and
signal to noise issues took longer to solve but eventually achieved
their best practical performance limits in the 70s.

With magnetic tape, several compromises needed to be made to get the
least objectional performance out of the system (eg even studio tape
decks running at 76cm/s could produce IMD products as high as 30% -
it's rather fortuitous that IMD is nowhere near as obnoxious as all
the other forms of distortion we know of).

Magnetic tape materials were improved over the years since the 40s
(paper backing tape and iron oxide coatings bonded with a suitable
binding material going to plastic backing tapes and rare earth
magnetic coatings with more sophisticated binders).

Tapes became more consistent in their performance, both in terms of
less 'dropouts' and higher replay output/better S/N ratio and dynamic
range. In the early 60's, it was the shiny side that was the back of
the tape (the dull side being the magnetic coating). By the seventies,
for quality Japanese made open reel tapes, this was reversed (eg
Maxell UD35-180B where the letter B suffix stands for 'Back Coated').

I've no doubt Maxell (and TDK) manufactured non- Back Coated varients
compatable with older open reel decks which relied upon the use of
pressure pads to hold the tape against the heads during replay/record
but I couldn't vouchsafe this since I was using an Akai GX630DB and an
ancient Akai M8 (of Crossfield Bias 'fame') tape recorder, both of
which relied upon back tension and head layout to maintain tape to
head contact to eliminate pressure pad induced scrape modulation
noise, so I would have been ignoring any such offerings that may have
been around at that time.

The back coating on these modern tapes offered a twofold advantage.
The first being that it acted as a balancing coating to the magnetic
coating on the acetate tape base with the second being an improvement
in high speed spooling performance (less weave due to the lubricating
effects of the film of air being trapped between the outer layer and
the rest of the tape pack under high speed spooling windage
conditions).

Even so, the back coating couldn't prevent tape weave during fast
forward or reverse winding on either the GX630DB or the M8 so the
advice "Never rewind your tape until just before use" still stands.
IOW, only store the tape after it has played through to the end, even
if the machine is blessed with a slow forward or rewind spooling
option (which none of my tape recorders/decks ever had).

For replay, the effective head gap needs to be no greater than about
67% of the shortest wavelength you wish to record. When you have the
luxury of a seperate recording head, as has already been mentioned, a
much larger gap is desirable since the actual impression of the
magnetic signal into the tape coating all takes place on the trailing
edge of the gap as the tape passes through a diminishing bias field. A
small gap reduces the amount of magnetic flux that can penetrate the
magnetic coating of the tape.

This last process is a little more tricky to get a handle on,
compared to the replay process. The key to understanding being the
fact that the tape coating is being subjected to the self erasing
effects of a diminishing bias field whilst said bias field is itself
being biased by the recording signal.

What's happening is essentially the same as trying to demagnetise a
bar of tool steel whilst it is under the influence of a magnetic field
not quite strong enough to impress a permanent state of magnetism
under normal conditions (just enough that it displays induced
magnetism). The demagnetisation field becomes biased by the fixed
magnetic field so that instead of the tool steel item being subjected
to progressively smaller and smaller loops of hysteresis centred on a
neutral filed, it becomes subjected to loops of hysteresis centred on
the magnetic field induced by the external magnetic force that would
otherwise fail to induce a state of permanent magnetisation on its
own.

AFAIR, an optimum recording head gap is around 3 or 4 times the
corresponding replay head gap (but, please don't quote me on this,
ICBW, it might be even larger). The main benefit of having seperate
record and replay heads is due to the very conflicting design
requirements between the two processes rather than because you can
immediately monitor the off tape recording quality and make
adjustments accordingly (although this latter feature is undeniably
useful too).

Seperating the record replay functions allows other optimisations of
the head design such as the use of more turns on the replay head to
get a higher signal voltage to the replay amp (a narrower gap means
less signal) and less turns of thicker wire on the record head to
reduce the necessary driving voltage from both the record amp and the
bias source.

The traditional material for magnetic heads has been mu-metal right
up to the 70s. The problem with this material is that it is relatively
soft and subject to wear by the abrasive effects of the magnetic
coatings used on tape. In the case of recording studio multitrack
machines, the answer to this is to have the heads re-ground every few
hundred hours, a practical, if pragmatic, solution for a recording
studio but a little onerus to say the least for the domestic user.

Sometime in the late 60s, manufacturers, such as Akai, substituted
ferrite for mu-metal, culminating in their famous Glass Crystal
Ferrite heads which basically reduced the wear rate by two orders of
magnitude. However, this wasn't the only improvement, a major benefit
was the significant reduction in scrape modulation noise due to the
ultra smooth surface finish as well as reducing oxide shedding rates
of the older, ****-poor quality, Philips tapes I had the misfortune to
own sufficiently to allow me to replay one full side of a 7 inch reel
of 1800 foot tape without noticable HF deterioration - a feat not
possible with my M8 mu-metal headed tape recorder (but I still had to
clean up the guides, capstain and pinch roller, as well as the heads,
after each such session).

That GX630DB deck (and a later model 6 headed reversomatic GX747)
represent the epitome of home reel to reel recording excellence when
modern tape formulations of the day were used. However, analogue
magnetic tape recording technology (even professional grade kit) still
suffers several serious deficiencies as a recording method.

The introduction of digital recording techniques, which obsoleted all
forms of analogue recording methods overnight, wasn't just a matter of
novelty for its own sake, it was a serious answer to all those
troublesome shortcomings peculiar to magnetic tape (as well as, to a
lesser degree, similar limitations in the "Direct Cut" to acetate disk
method that became popular as a means to improve vinyl recording
quality by eliminating the magnetic tape recording distortions from
the recording process that had become the normal method of producing a
vinyl pressing up untill then).

The whys and wherefores of analogue tape recording techniques have
all become rather academic now that digital technology is so well
entrenched as the means to record not only sources of sound but also
video. You either had to be there, or else, heavily into nostalgia to
appreciate them.
--
Regards, J B Good

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

Johny B Good wrote:

Isn't topic drift amazing? Having asked that rhetorical question, I'm
going to add my tuppence worth:

The whys and wherefores of analogue tape recording techniques have
all become rather academic now that digital technology is so well
entrenched as the means to record not only sources of sound but also
video. You either had to be there, or else, heavily into nostalgia to
appreciate them.

By gum, there's folk on here that knows some stuff! I tek me 'at of
t'yer, lad!

Bill