Crossfield Heads?

[William Balmer]

My wife regularly brings home "for sale" ads that are posted where she works
if she thinks they would interest me. Yesterday she brought home one for a
Roberts 770X Crossfield reel to reel tape machine. After a quick look on
the net, I don't think I'm interested in this beast, but my curiosity has
been piqued. The little I've seen has referred to a separate head that
introduces a "crossfield" bias. Can someone explain how this configuration
worked? Also, any reason why this machine would be worth looking at?

Thanks,

Bill Balmer

[Scott Dorsey]

William Balmer > wrote:
>My wife regularly brings home "for sale" ads that are posted where she works
>if she thinks they would interest me. Yesterday she brought home one for a
>Roberts 770X Crossfield reel to reel tape machine. After a quick look on
>the net, I don't think I'm interested in this beast, but my curiosity has
>been piqued. The little I've seen has referred to a separate head that
>introduces a "crossfield" bias. Can someone explain how this configuration
>worked? Also, any reason why this machine would be worth looking at?

The idea is that the bias and audio are supplied by seperate heads, which
was supposed to reduce intermodulation effects.

In reality it doesn't do much, and the Akai transports were really godawful.
But if you need parts, I have a bunch of spare parts from when Akai dropped
support on these.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[William Sommerwerck]

> Ordinarily a high frequency bias current is applied to record heads
> along with the audio signal to linearize the recording process. In the
> crossfield process, the magnetic field that this bias signal would
> create in the record head, is instead created by a separate head
> on the other side of the tape. The benefit is that the record head
> only has to carry the audio signal, and not the bias signal.

That is not my understanding of the benefit. Crossfield bias supposedly causes
less erasure of short wavelengths, allowing better slow-speed recording. Many
Akai crossfield machines could record to 13kHz at 1.875 ips, a remarkable
performance for an open-reel machine whose record head was probably optimized
for 7.5ips. (Yes, I said record head, not play head.)

[Arny Krueger]

"William Sommerwerck" > wrote in message

>> Ordinarily a high frequency bias current is applied to record heads
>> along with the audio signal to linearize the recording process. In
>> the crossfield process, the magnetic field that this bias signal
>> would create in the record head, is instead created by a separate
>> head on the other side of the tape. The benefit is that the record
>> head only has to carry the audio signal, and not the bias signal.

> That is not my understanding of the benefit. Crossfield bias
> supposedly causes less erasure of short wavelengths, allowing better
> slow-speed recording.

That was a benefit as well. Less self-erasure and greater linearity were
generally stated as benefits for crossfield recording.

>Many Akai crossfield machines could record to
> 13kHz at 1.875 ips, a remarkable performance for an open-reel machine
> whose record head was probably optimized for 7.5ips. (Yes, I said
> record head, not play head.)

Right. The area were recording actually takes place is far narrower than the
gap, enabling a head designed for high speeds to also operate at low speeds.

[Arny Krueger]

"William Balmer" > wrote in message

> "Arny Krueger" > wrote in message
> ...

> >In the crossfield
>> process, the magnetic field that this bias signal would create in
>> the record head, is instead created by a separate head aligned on
>> the other side of the tape.

> By "the other side of the tape", do you mean the back-side? So the
> record head and the bias head faced each other?

Yes and yes.

[William Sommerwerck]

I'm not sure -- but I don't think the gap in the crossfield head directly faced
the record-head gap. I think it was ever so slightly offset. CMIIW.

>> By "the other side of the tape", do you mean the back-side?
>> So the record head and the bias head faced each other?

> Yes and yes.

[William Sommerwerck]

Crossfield heads had nothing to do with crosstalk.

> I think you're correct. The idea was to limit crosstalk between the two
> recording tracks by cancelling the crosstalk out, theoretically giving one a
> much better separated recording. Crosstalk was generally higher noise than
> hiss in most recordings in those days. I don't think it worked all that
> well, as I recall, but hey, not a bad idea.

[Arny Krueger]

"Roger W. Norman" > wrote in message

> I think you're correct. The idea was to limit crosstalk between the
> two recording tracks by cancelling the crosstalk out, theoretically
> giving one a much better separated recording. Crosstalk was
> generally higher noise than hiss in most recordings in those days. I
> don't think it worked all that well, as I recall, but hey, not a bad
> idea.

Not at all. The crossfield head was for applying bias.

[Roger W. Norman]

Oh well, they say memory is the first thing to go.

--


Roger W. Norman
SirMusic Studio

301-585-4681




"Arny Krueger" > wrote in message
...
> "Roger W. Norman" > wrote in message
>
> > I think you're correct. The idea was to limit crosstalk between the
> > two recording tracks by cancelling the crosstalk out, theoretically
> > giving one a much better separated recording. Crosstalk was
> > generally higher noise than hiss in most recordings in those days. I
> > don't think it worked all that well, as I recall, but hey, not a bad
> > idea.
>
> Not at all. The crossfield head was for applying bias.
>
>

[P Stamler]

>Oh well, they say memory is the first thing to go.

No, it's the second thing. I forget what the first thing is.

Peace,
Paul

[Philip Perkins]

These decks were very common in the early-to-mid 1970's. Their
awfulness was part of the reason that Sony and TEAC became successful.

Philip Perkins