
How to pack a stereo signal in one record groove - soundsop
http://www.vinylrecorder.com/stereo.html
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jonsen
If you listen carefully to a vinyl record playback you can hear, very faintly,
the start of a track at little before it actually starts. A 'pre-echo' that
starts exactly one turn of the table before. The master record is cut in a
relatively soft material. Copper, I think. The force of cutting removes
material from the groove but also compresses the remaining material somewhat.
This distortion may spill over into the neighbor groove just cut one turn
earlier. The neighbor groove is silent between tracks. So in this silence you
may be able to hear a faint copy of the modulation one turn later. This ghost
signal gets more volume the tighter you pack the groove spiral, which you may
want to do tighter to obtain longer playing time.

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anigbrowl
Interesting! I wonder if this has been deliberately exploited by dance
producers...at 45 rpm and a bpm of 135, this will yield a 3-beat delay which
sits nicely with a 4-beat base rhythm. Dance music production techniques owe a
lot to reggae and dub, and as record producers working with low-quality
equipment in Jamaica found ways to creatively abuse their gear.

For example, there's a trick whereby you can turn a regular mixing desk into a
low or high pass resonant filter by feeding the same signal to two channels
and inverting one (so they cancel out), and then EQing the other channel
aggressively. So if you can't beat the 'echo' resulting from the manufacture,
you might as well look for ways to incorporate it.

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zs
Another reason for turning it by 45 degrees is backward compatibility. If you
play a stereo disc on a mono player you want to get a "mix" of the channels
instead of one of the channels.

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nazgulnarsil
that doesn't seem like it would work.

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jonsen
_In a stereo recording, the two channels are arranged to drive the record
cutter head at an angle of 45° to the vertical, effectively encoding each
channel in the left and right V-shaped walls of the record groove. This system
worked well, since it provided full compatibility with a monaural pick-up, so
stereo records could be played on older mono equipment._

From:
[http://en.wikipedia.org/wiki/Magnetic_cartridge#Stereo_repro...](http://en.wikipedia.org/wiki/Magnetic_cartridge#Stereo_reproduction)

See drawings of pick-ups above on that page. The vertical movement of a
monaural pick-up stylus will be the sum of the two stereo movements. Each
channel contributing with an sqrt(2)/2 amplitude of its 45° amplitude.

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nazgulnarsil
thanks, I was trying to work the math in my head and was making it WAY harder
than it needed to be.

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daydream
This is why, on most vinyl albums, the low end/bass sounds are panned dead
center.

Lots of low end panned hard left or hard right can result in the needle
jumping out of the groove, and can result in unnecessary compromises in
mastering.... there's one album in particular that doesn't have hardly
anything above 5k on it, and it's because they decided to pan the bass hard
left.

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tricky
that's very interesting. I had learned that no one bothers to pan bass
frequencies because IRL they are difficult to localize. For example, no one at
a live show could tell you if the bass rig is on stage right or left. So, a
recording with hard panned bass would just sound wrong.

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tricky
I always thought that's why most systems have a single subwoofer. What's the
album with the hard panned bass? I'd love to hear it (or know why they chose
to do that)

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daydream
Electroputas "3" is the record I was thinking of. There's basically no high
end on it. It was so noticeable that I actually emailed the label about it,
thinking I got a bad press or something, but to their credit they admitted
that the bass panning made it impossible to master the vinyl cleanly.
(Electroputas are a really good band, by the way. Not sure if they're still
around.)

Stories like this abound. The more bass you want on a record, the wider the
grooves need to be, which gives you less time per side; the more playing time
you want on each side, the narrower the grooves need to be, and you don't get
as much dynamic range, loudness, or frequency bandwidth.

Another example: there's some Yo La Tengo record, I forget the name (it's been
a while), but it's a one sided 12". The running time on that one side was
extreme for a rock record - something like 28 minutes or so - and as a result,
it sounds terrible.

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glhaynes
Super cool!

But I didn't understand the last panel ("Maximum Stereo or Mono and 1 channel
180° phased")... and, also, how did they do quadraphonic?

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anigbrowl
Inverting the phase of a mono signal and feeding it to one side of a stereo
channel (with the original on the other) isn't true stereo, but flipping the
phase of one channel fools the brain into believing it has a spatial dimension
- the sound seems louder and more vibrant although in reality all that's
happening is that the left and right channels are interfering with each other.

It was a trick used on a lot of old record when the music industry was moving
from mono to stereo recording, but still effective if you're stuck with a mono
sound source (quite often the case in film and TV) and you need to beef it up
without changing the timbre of the sound substantially.

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joe_bleau
Wait, isn't that the same effect as a mono signal with one speaker wired
backwards? If so, that 'head-swimming' sound that results is _awful_ , and the
out of phase drivers will kill much of the bass response.

I'm very surprised that this trick was ever used in industry; after all, if
someone listened to that old film on a single speaker TV, the signals would
sum to zero and you'd have no audio at all! (And yes, our local cable company
has managed this very stunt from time to time...flip to mono and the audio
goes away.)

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anigbrowl
Yeah, it can go wrong and it does sound pretty awful. It's good practice to
check the final mix out in mono for just that reason but stereo is ubiquitous
now and people often don't bother any more.

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poronski
In the next episode - "How to use sparks to dial a phone number" :)

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anigbrowl
I used to have a party trick whereby I'd dial a number by tapping the receiver
of a phone on a table to emulate pulse dialing. It probably still works on
landlines, but it's so long since I had one that I don't know if they support
pulse any more.

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Luc
That actually worked? I thought you had to tap the line switch (the switch the
receiver pressed down on when not in use), not the receiver. Which would make
sense, to reduce interference from sound coming through the receiver while
dialling.

Supposedly pulse dialling still works on (most?) new telephone equipment.
That's what the vendors of classic phones I've been looking at are saying,
anyway.

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anigbrowl
It does (or at least did) - try it if you know someone with a landline. I was
surprised too, as I just tried it on a whim one day. I grew up with pulse
telephones and particularly loved the chunky sounds of old models so I used to
play with the phone a lot as a kid.

You had to have a good feel for the timing of the pulses or the trick didn't
work. Taps and clicks broadband noise with a very sharp transient so it would
have to be a very noisy environment indeed for the ambient noise to interfere.
I've never played with any analog exchange equipment but the electronics to
detect and count pulse dialing doesn't need to be very sophisticated.

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Luc
But the electronics would be different if you did it by tapping the line
switch, than if you did it by generating a sharp noise on the line. After all,
pulse dialing is also called 'loop disconnect dialing':
<http://en.wikipedia.org/wiki/Pulse_dialing>

"The pulses are generated through the making and breaking of the telephone
connection (akin to flicking a light switch on and off); the audible clicks
are a side effect of this."

It's confusing.

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anigbrowl
Well, if you think of it in terms of voltage rather than switching, the audio
transient will cause similar spikes in the voltage amplitude. I _think_ that
the analog circuits in exchange hardware are only interested in the shape
rather than the origin of the pulse but as I said I've never looked at such
equipment directly.

I have a modular synth here and have managed to rig up a patch that counts
audio pulses (incoming signal goes through a rectifier and an envelope
follower and on to an accumulator) and generates an appropriate tone for the
counted number after a ~1/4 second pause, and doesn't much care whether I'm
opening and closing the circuit or just tapping the front of the microphone.
It's not a very complex circuit so maybe analog exchange switching works on a
similar principle.

