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

On Thu, 28 Mar 2013 17:02:12 +0000, John Williamson
wrote:

On 28/03/2013 10:49, Java Jive wrote:

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.

Exactly.

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.

Maybe, but for some reason I remembered them as being better.

This sort of knowledge has come rather back into prominence recently
with the recent misguided myths that somehow old audio technology was
better. A few years ago, the absurdity of these myths led me to
research a page for my site debunking analogue recording technology,
and I think I may subsequently have confused some of my TC-330 specs
with best of breed, which it definitely wasn't.

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.

That's rather unexpected. As I've already admitted, my memory of this
stuff is getting hazy, but ISTR that the bias couldn't be recorded.

Let's do some work ...

From the specs above, the bias was 85kHz, so at 19cm/s, each
wavelength of bias was 0.19 / 85000 = 2.2 m-6, so, according to
information theory, the size of field to be discriminated would have
to be half that, or 1.1 microns.

http://en.wikipedia.org/wiki/Magneti...alog_recording

"The commonly used magnetic particles are Iron oxide particles or
Chromium oxide and metal particles with size of 0.5 micrometers."

So that's ok. Now head gap width ...

http://en.wikipedia.org/wiki/Tape_head

"The desirability for a narrow gap means that most practical heads are
made by forming a narrow V-shaped groove in the back face of the core,
and grinding away the front face until the V-groove is just breached.
In this way, gaps of the order of micrometres are achievable."

Note micrometres in the plural. Your claim sounds unlikely to me. Are
you sure it was the bias you were hearing?
--
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Please always reply to ng as the email in this post's
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Terrestrial Switchoff - sorry to labour the point but...
 

On Thu, 28 Mar 2013 20:13:25 +0000, Johny B Good
wrote:

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

Would say it was worth a lot more than tuppence!

For replay, the effective head gap needs to be no greater than about
67% of the shortest wavelength you wish to record.

Doesn't that rather go against information theory, which would say
that it has to be no greater than half? Or perhaps the effective
width of a gap of 67% is actually about 50%?

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.

Yes. I really can't believe that people can seriously think that the
old analogue techniques produce better sound. Right now I'm listening
to Robin Dransfield's wonderful rendition of 'The Rigs Of Rye' from
his originally vinyl album 'Tidewave', which was included as a free CD
in 'A Lighter Touch'. The sound is authentic, but there are
absolutely no scratches, no vinyl noise, not even tape hiss (that I
can hear from here, which is surprising considering that it almost
certainly would have been from an original analogue master).

And actually I'm not even listening to the original CD, which is still
with the others in boxes in my front-room, not yet unpacked. Before
selling my last house, I backed them all up onto my two NAS servers,
one of which I took with me in the car and the other went into
storage, and from the first onto my new laptop. Hence, for a similar
volume and a bit more weight as about two full 19cm tape spools, which
would have held about 3hrs of music at maximum quality, I have all my
CDs, DVDs, photos, etc, and can navigate round all of them as the mood
takes.

Wonderful!
--
================================================== =======
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...
 

On 28/03/2013 23:03, Java Jive wrote:

Note micrometres in the plural. Your claim sounds unlikely to me. Are
you sure it was the bias you were hearing?

Yes. I recorded originally at 7 1/2 ips, then played back at 15/16 ips.
The recording was made with no input, as I was trying to work out what
the whistle was.

--
Tciao for Now!

John.

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

On Thu, 28 Mar 2013 23:40:34 +0000, Java Jive
wrote:

On Thu, 28 Mar 2013 20:13:25 +0000, Johny B Good
wrote:

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

Would say it was worth a lot more than tuppence!

Yes, I only realised this when I got about halfway through my
missive...


For replay, the effective head gap needs to be no greater than about
67% of the shortest wavelength you wish to record.

Doesn't that rather go against information theory, which would say
that it has to be no greater than half? Or perhaps the effective
width of a gap of 67% is actually about 50%?

A good question which made me pause for thought. Having sketched a
sinewave on a scrap of paper to help me visualise the mechanism, I've
concluded that I was essentially correct.

The problem with analogue recording is that the magnetic modulation
is longitudinal to the direction of tape travel (an aspect that I
always thought was far less than ideal - but transverse
(perpendicular) magnetic recording techniques only became of interest
to the HDD makers over the last decade).

The best way to visualise why a recorded tone with a wavelength equal
to the effective replay head gap gives zero output (first extinction
frequency on a response chart for a frequency swept test tone
recording that spans wavelengths going from several gap widths down to
one third or even one quarter wavelength) is to cut out a square
window in a piece of card which can be placed over another piece of
card that has 3 or more cycles of a sinewave with a wavelength equal
to the width of the cutout in the first piece of card.

The first card with the window represents the gap of the replay head
whilst the other represents a segment of tape with a recorded
wavelength equal to the effective gap. In this case, plotting a line
between the points of intersection with the sinewave on each side of
the window will produce a line parallel to the X axis.

Whilst this line shows an up and down shift as you slide the window
along the X axis, this does not represent an output response. What
does represent an output response is, in fact, the varying slope of
the line joining the interesection points on each side of the window.
When such a plot line merely bobs up and down with no tilting, you
have zero output. It is the magnetic equivilent of a balanced line
feeder being subjected to common mode interference where the unwanted
signal induction cancels itself out at the recieving end.

For all lengths of a wave less than the effective gap width, you will
obtain an output. You will also obtain output for shorter wavelengths
that aren't an exact multiple of the gap width. Since a replay
response that includes one or more nulls is unsuitable for recording a
spectral continuim intended to faithfully represent an audio frequency
range after suitable frequency equalisation is applied, we can safely
ignore the sub-gap length wave region option.

ISTR that the shortest practical wavelength worth considering was
somewhere around 67% of the effective replay head gap width.
Certainly larger than 50% and almost certainly less than 80%. By the
time we've reached such short wavelengths, we're already in enough
trouble from declining S/N and increased distortion to say that's as
far as it's worth stretching the system's top end frequency response.


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.

Yes. I really can't believe that people can seriously think that the
old analogue techniques produce better sound. Right now I'm listening
to Robin Dransfield's wonderful rendition of 'The Rigs Of Rye' from
his originally vinyl album 'Tidewave', which was included as a free CD
in 'A Lighter Touch'. The sound is authentic, but there are
absolutely no scratches, no vinyl noise, not even tape hiss (that I
can hear from here, which is surprising considering that it almost
certainly would have been from an original analogue master).

Having googled "'Tidewave' album" to get some idea of when it would
have been recorded, I see that it was originally released in 1980 so
almost certainly recorded to magnetic tape before the final stereo mix
was committed to an acetate disk for its vinyl release. There's every
chance that the CD rip originated from a magnetic tape master
(possibly even remixed from the studio multitrack tape).

If we could all have experienced the playback quality enjoyed by the
studio staff as a matter of routine in our homes, there wouldn't have
been any motivation to develop the audio CD technology in the first
place.


And actually I'm not even listening to the original CD, which is still
with the others in boxes in my front-room, not yet unpacked. Before
selling my last house, I backed them all up onto my two NAS servers,
one of which I took with me in the car and the other went into
storage, and from the first onto my new laptop. Hence, for a similar
volume and a bit more weight as about two full 19cm tape spools, which
would have held about 3hrs of music at maximum quality, I have all my
CDs, DVDs, photos, etc, and can navigate round all of them as the mood
takes.

Wonderful!

Quite! Also, you've nicely illustrated the argument against investing
any more of our hard earned cash in optical disk based systems.
Speaking for myself, I for one won't be wasting money on such short
lived fads as Blue Ray Disk (now, lazily, refered to as BD). I've
learnt my lesson with CD and DVD.

The Smart Money has been on server based storage systems for some
time now (at least since the days of recordable DVD media),
supplemented by flash media storage options to satisfy our
"Sneakernet"(tm) requirements.
--
Regards, J B Good

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

On Thu, 28 Mar 2013 23:50:07 +0000, John Williamson
wrote:

On 28/03/2013 23:03, Java Jive wrote:

Note micrometres in the plural. Your claim sounds unlikely to me. Are

It's worth noting that Akai's "One Micron" gapped replay head first
appeared in their 4000DS model in the early 70s.

you sure it was the bias you were hearing?

Yes. I recorded originally at 7 1/2 ips, then played back at 15/16 ips.
The recording was made with no input, as I was trying to work out what
the whistle was.

For a bias frequency of 85KHz at 7 1/2 ips that would have produced a
'whistle' frequency just in excess of 10KHz at a replay speed of 15/16
ips which I calculate to have a wavelength of 2.24 microns.

However, I think the bias frequency is more likely to have been 60KHz
which would have produced a wavelength of 3.17 microns or a frequency
of 7.5KHz at 15/16 ips.

As long as the effective gap was not too close to that 3.17 micron
figure or a multiple thereof, it would have produced some output. An
effective gap width of 5 microns could even have produced a healthy
output for a 7.5KHz tone at the 15/16 ips speed.

Having worked the calculations, those 1 micron heads of Akai seems to
be a real Tour de Force in 'Overkill'. One micron corresponds to a
first extinction frequency of 95KHz for a 3 3/4 ips speed. They were
certainly dead set against coming anywhere near the 67% effective gap
width wavelength region!
--
Regards, J B Good

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

In article , 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:


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.

As ever, its more complex than just one factor, as the discussion has
already shown.

The Tandberg decks I've used all employ a 'cross field' head to allow the
recording head to work better. And the replay head can have a smaller
effective gap. The recording head can record shorter wavelengths than its
gap might imply because the remaining imposed magnetisation is set by the
'trailing edge' of the apllied fields.

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

As below, various reel decks could fo better than that. Although how much
it mattered for home recording is questionable in an age when home
equipment probably didn't give much useful around 18kHz upwards! I suspect
most people used the nicer 'hifi' tape decks to record from the radio or
their friend's LPs. FM radio means you don't want 16 kHz. Most LPs and
cartridges wouldn't have produced much that was useful by 20kHz.

Audio-cassette section 50Hz - 10kHz

A number of cassette deck designs could do rather better than that. Naks,
Revox, etc. They easily provided a pretty flat response to 20kHz. ...at a
cost, of course. :-)

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?

As usual: Devil in all the details. The 'hard' heads developed later on in
the life of analogue tape both resisted wear and allowed for smaller gaps.
Also higher field levels. Ealier designs wore down, particularly if used
with tapes that were more abrasive. So measured well in a review, but
deteriorated swiftly in use.

FWIW I still have a Nak C2 that works nicely. But I only use it to play old
tapes to transfer to digital. I'd be doing the same for reel to reel. But
I've not yet sorted out the problems with my ancient Tandbergs.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.audiomisc.co.uk/index.html
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html

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

In message , SpamTrapSeeSig
writes:
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

I don't think that's _completely_ true - see below.

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.

Agreed.

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.

I have the feeling that there will be _some_ pickup of signals whose
wavelength is much less than the replay head gap - just you'll get
strange aliasing-like nulls in the frequency _response_. Say, for
example, there are three half-wavelengths across the gap, then I think
you'd get the same amount of output as from a gap one half-wavelength
wide. On the way up to that frequency, though, you'd pass through a
frequency where the gap (well, allowing for edge effects and other such
field anomalies) was exactly one wavelength, at which I think you'd get
no output.

I think it's like aerials: a three-half-wavelegnth one will give the
same output as a one-half-wavelength one; since it uses three times as
much metal and takes up three times the space, there's no advantage to
it. It's the _difference_ between the ends that matters.

I always think of this analogy - don't know if it will help anyone: I
think of an aerial as like a bar with floats on the end, extracting
energy from water waves coming at it end on, but the output is from
something fixed to its midpoint, and only _torsional_ energy can be
extracted. So when waves come along that have a wavelength twice that of
the bar, i. e. the rod is half a wavelength, the opposite ends of the
bar will go up and down with maximum difference, so the rod fixed to the
midpoint (through which you're extracting signal) will twist by the
maximum. If waves of half the wavelength come along (wavelength _same
as_ the bar), both ends of the bar will go up and down together, so the
rod fixed to the midpoint - though it will be flapping up and down a lot
- won't _twist_ at all. Other wavelengths will have other effects: very
long ones, for example (however big), will move both ends of the bar up
and down almost together, so very little twisting output will be
obtained. Very short ones will give some twisting, if the bar isn't an
exact multiple of their wavelength, but not nearly as much as a shorter
bar would.
--
J. P. Gilliver. UMRA: 1960/1985 MB++G()AL-IS-Ch++(p)[email protected]+H+Sh0!:`)DNAf

What a strange illusion it is to suppose that beauty is goodness. -Leo Tolstoy,
novelist and philosopher (1828-1910)

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

In article , J. P. Gilliver (John)
wrote:


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.

I have the feeling that there will be _some_ pickup of signals whose
wavelength is much less than the replay head gap - just you'll get
strange aliasing-like nulls in the frequency _response_. Say, for
example, there are three half-wavelengths across the gap,

Yes. The replay side of this is a classic 'convolution problem' in physics
and maths terms. It is complicated because the sensor (head and gap) don't
have a sensitivity pattern that is rectangular. For the same reason you
also get low frequency wiggles in the response determined by factors like
the overall head size, shape, etc.

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.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.audiomisc.co.uk/index.html
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html

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

In article , John Williamson
writes
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...-Reel-Tape-Dec
k-Service-Manual-/111009807072?pt=Vintage_Electronics_R2&hash=item19 d8b


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.

Nagras 4-series could do that too, although obviously not crossfield
bias. Their bias frequency was quite low - somewhere in the 30-40kHx
range, IIRC. I never noticed the same thing with Studers, and assumed
their bias was higher frequency. I think it was chosen to match the head
design rather than the stock, but I'll be digging Jorgensen out at this
rate ;-)

--
SimonM

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

In article , Java Jive
writes
On Thu, 28 Mar 2013 17:02:12 +0000, John Williamson
wrote:

On 28/03/2013 10:49, Java Jive wrote:

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.

Exactly.

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.

Maybe, but for some reason I remembered them as being better.

This sort of knowledge has come rather back into prominence recently
with the recent misguided myths that somehow old audio technology was
better. A few years ago, the absurdity of these myths led me to
research a page for my site debunking analogue recording technology,
and I think I may subsequently have confused some of my TC-330 specs
with best of breed, which it definitely wasn't.

Too right. the Akai 4000 series and the TC330s were very popular and
equally horrible. IIRC the Akais had worse wow and flutter, but there
wasn't much else in it. You changed speed on the 4000 by use of a sleeve
over the capstan with a locknut (and an eq switch).

"The desirability for a narrow gap means that most practical heads are
made by forming a narrow V-shaped groove in the back face of the core,
and grinding away the front face until the V-groove is just breached.
In this way, gaps of the order of micrometres are achievable."

Note micrometres in the plural. Your claim sounds unlikely to me. Are
you sure it was the bias you were hearing?

It was bias, most probably.

And the Wikipedia article is a bit erroneous. You can't consider
recording and replay functions in the same way - combined function heads
were always a nasty compromise.

The reason for the rear cutaway angle in recording heads is to help
direct the flux out of the side of the pole pieces into the medium. I
think paramagnetic inserts were also used (can't remember what, though).

As I said, recording takes place as the medium leaves the influence of
the recording head, not across the pole pieces. The sharper you can make
the magnetic transition, the better the frequency response. The function
of the gap it to get the right shaped field (in 2D, perpendicular to the
tape axis), its width isn't significant, although as discussed, if the
trailing edge is ragged it will cause HF self-erasure. Assuming in all
cases the head is on-azimuth, etc.

In the case of replay, it's simpler: the narrower the gap the higher the
reproducible frequency, for a given tape speed. That's why you get the
characteristic response 'bumps' that trail off above the upper linear
limit. If 3 lambda fits in the gap, for example, you get significant
output.
--
SimonM