Title lifted from an Ebay auction with Pink Floyd
search terms.

I've read a little about quad LPs recently. If my understanding
is correct, the encoding is a little like FM stereo in that there
is information encoded in HF above the audible range, and it's
something to do with sum-and-difference, that when decoded,
yields the rear channel information. How close am I?

I was surprised when I read that, since I assume a lot of that
information is above 20kHz, and quad was en vogue back in the 70s.
Were cartridges and preamps of the time capable of picking up signals
way in excess of 20kHz accurately?

Let's suppose I were to buy a Floyd DSOTM quad LP from Ebay:
Is there any equipment now that could take the output from
my cartridge and decode it? (Kinda irrelevant in fact because
I only have 2 speakers and a stereo amp, but... In the bottom of
my heart...) I would imagine that you can't easily get that kind
of gear any more.

Also, since the groove modulation extends to much higher frequencies
than it does with normal stereo, would playing it with a normal
cartridge/stylus be likely to cause it great (brain) damage (I use a
Sumiko Blue Point Special Evo III)?

How different was the back-channel information from the main
front signal? Could you, in practice, have 4 completely
different signals (vocals, for example) coming from the 4
speakers with very little crosstalk? Would the strident
tintinnabulation starting "Time" be different from each corner?

Yours, not-old-enoughly-but-very-nearly,


Martin

"Fleetie" wrote in message
...
Title lifted from an Ebay auction with Pink Floyd
search terms.

I've read a little about quad LPs recently. If my understanding
is correct, the encoding is a little like FM stereo in that there
is information encoded in HF above the audible range, and it's
something to do with sum-and-difference, that when decoded, yields the
rear channel information. How close am I?


I was surprised when I read that, since I assume a lot of that
information is above 20kHz, and quad was en vogue back in the 70s.
Were cartridges and preamps of the time capable of picking up signals
way in excess of 20kHz accurately?

Let's suppose I were to buy a Floyd DSOTM quad LP from Ebay:
Is there any equipment now that could take the output from
my cartridge and decode it? (Kinda irrelevant in fact because
I only have 2 speakers and a stereo amp, but... In the bottom of
my heart...) I would imagine that you can't easily get that kind
of gear any more.

Also, since the groove modulation extends to much higher frequencies
than it does with normal stereo, would playing it with a normal
cartridge/stylus be likely to cause it great (brain) damage (I use a
Sumiko Blue Point Special Evo III)?

How different was the back-channel information from the main
front signal? Could you, in practice, have 4 completely
different signals (vocals, for example) coming from the 4
speakers with very little crosstalk? Would the strident
tintinnabulation starting "Time" be different from each corner?

Yours, not-old-enoughly-but-very-nearly,


Martin

There were three basic encoding methods for quad records in the 70s. SQ and
QS were matrixed systems which means that the rear channels were encoded
essentially by phase differences in with the front channels. This would make
it reasonably compatible with normal stereo and almost compatible with mono,
important for broadcasting which was mostly on AM in those days.

The third encoding method, which you allude to in your post was CD4, which
was a carrier system similar to FM in that the front plus rear channels were
mixed in the base-band, and the front-rear encoded on a carrier and then
decoded on playback in a similar way to FM stereo. The great advantage of
CD4 was that separation between front and back was much greater than a
matrix system, and stereo and mono compatibility was excellent. The great
disadvantage of CD4 was that it didn't work! Under laboratory conditions,
with clean, unworn records it would work fine, but in the real world, with
records of varying cleanliness and wear, it would just collapse. Cartridges
had to track up to 45 kHz to recover the carrier and sidebands, and even one
playing by the blunderbuss cartridges fitted to most record players those
days would render the LP unplayable as a quad LP.

Different labels adopted different encoding standards. DSOTM was on EMI and
they used the SQ system as did CBS and, I think, did Decca, so a quad DSOTM
on EBay would be SQ encoded and you wouldn't need to worry about recovering
an HF carrier. If it's in good condition for stereo, the quad encoding
should also be in good condition.

Decoding SQ these days is pretty much impossible unless you can find a good
decoder dating back to the 70s. Sony made a few good ones the best probably
being the Tate decoder. As standard, the SQ system has only something like
7dB of crosstalk between L and R and only 3 dB between front and back if I
remember my matrix co-ordinates correctly. The later decoders had logic
steering to improve the subjective separation at the expense of wavering
positions.

The combination of three incompatible systems that never really worked
properly (there was a fourth, Dr. Duane Cooper's UD4, which however never
came to Europe in significant numbers)and the need for four loudspeakers
arranged in a square around the listener meant that the quadraphonic craze
only lasted a few years in the mid '70s.

If you want to find out more about quadraphonics in general, there's
http://www.quadraphonicquad.com/ which has information on the systems, a
forum etc.

S.

--
http://audiopages.googlepages.com

"Fleetie" wrote in message
...
Title lifted from an Ebay auction with Pink Floyd
search terms.

I've read a little about quad LPs recently. If my understanding
is correct, the encoding is a little like FM stereo in that there
is information encoded in HF above the audible range, and it's
something to do with sum-and-difference, that when decoded, yields the
rear channel information. How close am I?

I was surprised when I read that, since I assume a lot of that
information is above 20kHz, and quad was en vogue back in the 70s.
Were cartridges and preamps of the time capable of picking up signals
way in excess of 20kHz accurately?

Let's suppose I were to buy a Floyd DSOTM quad LP from Ebay:
Is there any equipment now that could take the output from
my cartridge and decode it? (Kinda irrelevant in fact because
I only have 2 speakers and a stereo amp, but... In the bottom of
my heart...) I would imagine that you can't easily get that kind
of gear any more.

Also, since the groove modulation extends to much higher frequencies
than it does with normal stereo, would playing it with a normal
cartridge/stylus be likely to cause it great (brain) damage (I use a
Sumiko Blue Point Special Evo III)?

How different was the back-channel information from the main
front signal? Could you, in practice, have 4 completely
different signals (vocals, for example) coming from the 4
speakers with very little crosstalk? Would the strident
tintinnabulation starting "Time" be different from each corner?


There were several different "Quadraphonic" systems, and I don't know which
Dark Side of the Moon used. The original quadraphonic system was the CBS
"SQ" system. This, like most of the others, (such as Sansui's "QS" and the
BBC's "Matrix H") was a "matrix" system in which four original audio tracks
were combined into two by a process of phase-shifting and combining, and
then separated again using phase-sensitive circuitry and gain-controlled
amplifiers. The most commercially successful of these systems is the Dolby
pro-logic system. Though this has been marketed as a surround-sound system,
not a quadraphonic one, it is essentially the same idea.

There was also the RCA "FM4" system, which seems to be the one you are
thinking of. This used a frequency-modulated carrier around 20kHz to carry
the band-limited rear channels.

With the matrix systems channel separation is very poor, hence the use of
gain controlled amplifiers which "enhance" the separation by reducing the
gain of the speaker channels into which the wanted signal has bled. Clearly
here it is NOT possible to carry 4 completely different signals.

With the RCA system in theory crosstalk should be very good (at least no
worse than normal disc stereo), but as you realised there were serious
problems with the sub-carrier at the top of the audio band. I had no
practical experience of this system, but I do remember that it didn't do
well commercially. If a record of this type is anything other than pristine
I would guess that the sub-carrier would be seriously reduced in amplitude
from it's new condition.

David.

Thanks, Serge.

Interesting stuff. I kinda suspected that the records with
the HF encoding wouldn't've worked really work well with cartridges
of the time.

I was trying to decide whether it was worth buying the quad
version from Ebay, but you suggest what I suspected, which
was that it's basically impossible to get the quad info off
the vinyl and into my room these days.

I'll probably pass on it.


Thanks again.


Martin

Thanks David.

Yeah, you seem to be confirming what I suspected. Even these
days, I think it'd be hard to recover signal above 20kHz from
vinyl with good performance, so I'm still surprised they tried
it in the 70s and expected it to work *in*the*field* with the
domestic blunt-knitting-needle styli (for that's how it's spelt,
Jim Lesurf :-) ) of the day.

I don't think I'll bother buying the quad LP from Ebay.


Martin

"David Looser" wrote in message
...

There was also the RCA "FM4" system,

Correction: "CD4", as Serge Auckland correctly stated. My fault for relying
on my 30-year old memories rather than looking it up!

David.

In article , Serge Auckland
wrote:

"Fleetie" wrote in message
...


There were three basic encoding methods for quad records in the 70s. SQ
and QS were matrixed systems which means that the rear channels were
encoded essentially by phase differences in with the front channels.
This would make it reasonably compatible with normal stereo and almost
compatible with mono, important for broadcasting which was mostly on AM
in those days.

The third encoding method, which you allude to in your post was CD4,
which was a carrier system similar to FM in that the front plus rear
channels were mixed in the base-band, and the front-rear encoded on a
carrier and then decoded on playback in a similar way to FM stereo.

IIRC There was more than one 'ultrasonic subcarrier' system. UD-4 also
comes to mind.



The great advantage of CD4 was that separation between front and back
was much greater than a matrix system, and stereo and mono compatibility
was excellent. The great disadvantage of CD4 was that it didn't work!
Under laboratory conditions, with clean, unworn records it would work
fine, but in the real world, with records of varying cleanliness and
wear, it would just collapse. Cartridges had to track up to 45 kHz to
recover the carrier and sidebands, and even one playing by the
blunderbuss cartridges fitted to most record players those days would
render the LP unplayable as a quad LP.

I think the modulation survived moderatey well with one or two carts like
the one Shure did specially for CD4/UD4. But even then I have my doubts
about how many times the end of side could be played before becoming
undecodable. Systems like this seemed doomed from the start to me given the
struggle to play even modest hf levels with LP. Plus, of course, the way
companies at the time couldn't be bothered to take any care when pressing
LPs. Wonder how many LPs would have even had the subcarrier on the walls
when they popped out of the press. ;-

Different labels adopted different encoding standards. DSOTM was on EMI
and they used the SQ system

Not checked, But the 'quad' LP I used as a test disc for the work in the
webpage I put up today is an EMI one using SQ. So confirms the above.


The combination of three incompatible systems that never really worked
properly (there was a fourth, Dr. Duane Cooper's UD4, which however
never came to Europe in significant numbers)and the need for four
loudspeakers arranged in a square around the listener meant that the
quadraphonic craze only lasted a few years in the mid '70s.

Ah, as I recalled above.

IIRC Hi Fi News also did a double LP showing off 'quadrophony' by using a
different method for each LP side. No idea if anyone who bought it was
every able to play all four sides. :-)

Friend of mine at the time got keen on 4-channel (sic) so bought various
LPs, but only the SQ and QS types. But I was unimpressed, so stuck with
stereo.

IIRC There is now a surround-sound version of DSOTM on DVD. No idea how it
compares with the LP, and don't have a multichannel AV system so can't
check.

Slainte,

Jim

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

In article , David Looser
wrote:
"Fleetie" wrote in message
...


With the RCA system in theory crosstalk should be very good (at least no
worse than normal disc stereo), but as you realised there were serious
problems with the sub-carrier at the top of the audio band. I had no
practical experience of this system, but I do remember that it didn't do
well commercially. If a record of this type is anything other than
pristine I would guess that the sub-carrier would be seriously reduced
in amplitude from it's new condition.

I can also recall at least one JAES paper discussing how to cope with the
large levels of intermod distortion between the audible and ultrasonic that
risked making the results such that decoding would fail, or be dreadful to
endure!

With the SQ and QS I imagine that someone could now write a computer
program that did the channel seperation in software, and you could then
playback or make a '4-channel' output. But I have no idea how many working
SQ/QS/H/etc decoders still exist in working order. IIRC one of the CD
lables (Unicorn?) did also produce matrix encoded CDs... May have one or
two somewhere...

Slainte,

Jim

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

"David Looser" wrote in message
...
"Fleetie" wrote in message
...
Title lifted from an Ebay auction with Pink Floyd
search terms.

I've read a little about quad LPs recently. If my understanding
is correct, the encoding is a little like FM stereo in that there
is information encoded in HF above the audible range, and it's
something to do with sum-and-difference, that when decoded, yields the
rear channel information. How close am I?

I was surprised when I read that, since I assume a lot of that
information is above 20kHz, and quad was en vogue back in the 70s.
Were cartridges and preamps of the time capable of picking up signals
way in excess of 20kHz accurately?

Let's suppose I were to buy a Floyd DSOTM quad LP from Ebay:
Is there any equipment now that could take the output from
my cartridge and decode it? (Kinda irrelevant in fact because
I only have 2 speakers and a stereo amp, but... In the bottom of
my heart...) I would imagine that you can't easily get that kind
of gear any more.

Also, since the groove modulation extends to much higher frequencies
than it does with normal stereo, would playing it with a normal
cartridge/stylus be likely to cause it great (brain) damage (I use a
Sumiko Blue Point Special Evo III)?

How different was the back-channel information from the main
front signal? Could you, in practice, have 4 completely
different signals (vocals, for example) coming from the 4
speakers with very little crosstalk? Would the strident
tintinnabulation starting "Time" be different from each corner?


There were several different "Quadraphonic" systems, and I don't know
which Dark Side of the Moon used. The original quadraphonic system was the
CBS "SQ" system. This, like most of the others, (such as Sansui's "QS" and
the BBC's "Matrix H") was a "matrix" system in which four original audio
tracks were combined into two by a process of phase-shifting and
combining, and then separated again using phase-sensitive circuitry and
gain-controlled amplifiers. The most commercially successful of these
systems is the Dolby pro-logic system. Though this has been marketed as a
surround-sound system, not a quadraphonic one, it is essentially the same
idea.

The BBC's Matrix H was yet another matrix system, but one that came out of
Michael Gerzon et. al's work on ambisonics, and which was evaluated for
broadcast. The BBC did some test transmissions in Matrix H, but it never
went into full service as it didn't have full mono compatibility (something
the BBC was somewhat paranoid about at the time) and anyway, the whole quad
thing had pretty much gone away by then.

There was also the RCA "FM4" system, which seems to be the one you are
thinking of. This used a frequency-modulated carrier around 20kHz to carry
the band-limited rear channels.

You're thinking of CD4, which was the system used by RCA, JVC, Denon and
others. It was developed by JVC.

With the matrix systems channel separation is very poor, hence the use of
gain controlled amplifiers which "enhance" the separation by reducing the
gain of the speaker channels into which the wanted signal has bled.
Clearly here it is NOT possible to carry 4 completely different signals.

With the RCA system in theory crosstalk should be very good (at least no
worse than normal disc stereo), but as you realised there were serious
problems with the sub-carrier at the top of the audio band. I had no
practical experience of this system, but I do remember that it didn't do
well commercially. If a record of this type is anything other than
pristine I would guess that the sub-carrier would be seriously reduced in
amplitude from it's new condition.

David.


As one who lived and worked through the Quadraphonic era, it was a very
interesting time, and might have succeeded if there hadn't been three
competing and incompatible systems (UD4 was the fourth system, but it never
came to Europe). SQ and QS worked acceptably well for classical music and
jazz if ambiance only was recorded in the rear channels, giving a much
better impression of the venue. For rock music, I thought it worked less
well, given that producers wanted to use the surround for effect, swinging
instruments not only side to side, but also front-back and diagonally. With
the earlier non-logic decoders, crosstalk was so bad as to prevent accurate
localisation, and with the later logic-steered decoders, pumping effects
could be unpleasant. It's a great pity that Ambisonics never caught on, it
was being sponsored by the state-run NRDC at the time, with the predictable
result when marketing is handled by civil-servants. Of the competing
systems, SQ was probably the most successful commercially, but CBS, EMI and
others lost interest and never fully developed the potential.

Dolby later realised that what didn't work for music could well work for
movies, and launched the analogue matrixed Dolby Surround, which then led to
the discrete Dolby Digital we know and love.

S.

--
http://audiopages.googlepages.com

"Serge Auckland" wrote in message
...


The BBC's Matrix H was yet another matrix system, but one that came out of
Michael Gerzon et. al's work on ambisonics, and which was evaluated for
broadcast. The BBC did some test transmissions in Matrix H, but it never
went into full service as it didn't have full mono compatibility
(something the BBC was somewhat paranoid about at the time)

As I remember it a fair number of programmes were transmitted in Matrix H, I
still have a Matrix H off-air recording of "The Tempest" (the version
staring Paul Schofield, still my favourite version of the play).

and anyway, the whole quad thing had pretty much gone away by then.

There was also the RCA "FM4" system, which seems to be the one you are
thinking of. This used a frequency-modulated carrier around 20kHz to
carry the band-limited rear channels.

You're thinking of CD4, which was the system used by RCA, JVC, Denon and
others. It was developed by JVC.

Indeed, my mistake for relying on my memory rather than looking it up!


As one who lived and worked through the Quadraphonic era, it was a very
interesting time, and might have succeeded if there hadn't been three
competing and incompatible systems (UD4 was the fourth system, but it
never came to Europe). SQ and QS worked acceptably well for classical
music and jazz if ambiance only was recorded in the rear channels, giving
a much better impression of the venue. For rock music, I thought it worked
less well, given that producers wanted to use the surround for effect,
swinging instruments not only side to side, but also front-back and
diagonally.

QS was used for the initial release of the film of "Tommy" (with the
addition of a discrete centre channel) just the job for bouncing the pinball
sound around the cinema!

It's a great pity that Ambisonics never caught on,

I agree.

it was being sponsored by the state-run NRDC at the time, with the
predictable result when marketing is handled by civil-servants.

It also came too late.

Of the competing systems, SQ was probably the most successful
commercially, but CBS, EMI and others lost interest and never fully
developed the potential.

SQ was also the worst system (shades of VHS?) and was no advert for "quad".

Dolby later realised that what didn't work for music could well work for
movies, and launched the analogue matrixed Dolby Surround, which then led
to the discrete Dolby Digital we know and love.

Though of course Dolby Stereo (as it is known in cinemas) provides L, C, R
and (mono) surround, rather than LF, RF, LR and RR, which makes things a bit
easier, and used "logic" steering from the word go, at least in cinema use.

David.